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TREATISE  OF  ELECTRO-CHEMISTRY 

Edited  by  BERTRAM  BLOUNT.  F.I.C, 


OZONE 


XRIDEAL,  M.B.E,M.A,(CANTAB),  PH.; 


CO 


A  TREATISE  OF  ELECTRO-CHEMISTRY. 
EDITED  by  BERTRAM  BLOUNT,  F.I.C.,  ETC. 


OZONE 


A  TREATISE  OF  ELECTRO-CHEMISTRY. 
Edited  by  BERTRAM  BLOUNT,  F.I.C. 

THE  MANUFACTURE  OF  CHEMICALS 
BY  ELECTROLYSIS.  By  ARTHUR  J. 
HALE,  B.Sc.,  F.I.C. 

OZONE.  By  E.  K.  RIDEAL,  M.B.E.,  M.A., 
Ph.D. 

Other  volumes  in  preparation. 


A  TREATISE  OF  ELECTRO-CHEMISTRY, 

EDITED  by  BERTRAM  BLOUNT,  F.I.C.,  ETC. 


OZONE 


E.    K^TUDEAL,    M.B.E.,    M.  A.  (CANTAB.),    Pn.D. 

PROFESSOR   OF  PHYSICAL  CHEMISTRY,    UNIVERSITY  OF   ILLINOIS 


LONDON 
CONSTABLE    &   COMPANY   LTD. 

ORANGE  STREET,  W.C. 
1920 


EDITOR'S  PREFACE. 

THE  idea  of  a  series  of  books  on  Electro-Chemistry  emanated 
not  from  me,  but  from  Messrs.  Constable.  Some  years  back 
I  wrote  for  them  a  book  called  "Practical  Electro-Chemistry," 
intended  to  cover  a  great  part  of  the  ground  of  knowledge 
then  extant.  Fortunately,  knowledge  has  a  habit  of  growing 
and  of  propagating  its  kind,  and  my  book,  in  consequence  of 
this,  became  a  "  back  number  ". 

The  subject  of  Electro-Chemistry  is  so  ramified  and 
specialised  that  it  was  impossible  for  one  man  to  make  a 
survey  of  the  whole  field.  This  fact  is  the  genesis  of  the 
present  series  in  which  those  who  have  accurate  and  intimate 
knowledge  of  the  various  branches  of  electro-chemistry  have 
undertaken  the  work  for  which  they  are  particularly  qualified. 
It  will  be  readily  understood  that,  as  the  series  of  books  was 
started  at  an  early  period  of  the  war,  many  contributors  were 
engaged  in  work  of  national  and  primary  importance,  and 
were  unable,  however  willing,  to  apply  themselves  at  the 
moment  to  exacting  literary  work.  But  this  difficulty  was 
gradually  overcome,  as  some  prospect  of  a  period  to  the 
struggle  came  within  view,  with  the  result  which  the  reader 
will  judge  with  consideration  for  the  onerous  conditions 
under  which  my  contributors  have  wrought. 

The  monographs  resulting  from  their  labours  speak  for 
themselves,  and  if  the  educational  advantages  which  I  have 
obtained  from  reading  them  during  their  passage  through 
the  press  is  shared  by  the  public,  I  believe  that  the  thorough 
and  modern  work  of  my  friends  and  collaborators  will  be 
appreciated,  and  such  faults  as  there  be  will  be  attributed  to 
the  person  ultimately  responsible — the  Editor. 


AUTHOR'S  PREFACE. 

EVER  since  the  time  of  its  discovery  Ozone  has  attracted  the 
attention  of  chemists,  physicists,  and  industrialists  alike. 
To  the  former  it  presented  the  first  example  of  a  gaseous 
allotrope  of  an  element,  differing  from  oxygen  in  many 
remarkable  ways.  The  physicist  frequently  came  in  contact 
with  the  substance  in  his  investigations  on  the  conduction 
of  electricity  through  air,  whilst  the  industrialist  was  not 
slow  to  avail  himself  of  an  oxidising  agent,  unsurpassed  in 
strength,  leaving  no  objectionable  material  in  its  wake,  and 
at  the  same  time  easy,  if  indeed  somewhat  expensive,  to 
manufacture. 

The  angle  from  which  Ozone  and  its  modes  of  prepa- 
ration was  regarded  by  these  three  different  sets  of  investi- 
gators naturally  varied,  and  an  endeavour  has  been  made  in 
the  following  pages  to  summarise  and  correlate  the  many 
different  references  which  are  to  be  found  scattered  over  a 
wide  field  of  literature.  The  merest  survey,  however,  was 
sufficient  to  indicate  that  our  knowledge  of  Ozone,  its  pro- 
perties and  modes  of  formation,  is  exceedingly  scanty.  The 
industrialist  is  ever  at  hand  with  extravagant  claims  as  to 
the  utility  of  ''electrified  oxygen";  the  evidence  as  to  the 
chemical  behaviour  and  properties  of  ozone  is  somewhat 
meagre  and  frequently  conflicting,  for  example,  the  existence 
of  the  ozonates  and  of  oxozone  still  awaits  confirmation ; 
whilst  the  hypotheses  advanced  to  explain  the  mechanism 
of  its  formation,  either  chemical,  thermal,  electrolytic,  or 
photo-chemical,  are  purely  speculative.  Ozone  is  generally 
produced  by  means  of  the  silent  electric  discharge,  the 


Vlll  AUTHOE  S   PEEFACE 

Aladdin's  lamp  of  synthetic  chemistry,  for  which  no  satis- 
factory "modus  operandi "  has  been  suggested,  synthesis 
appearing  to  result  from  a  combination  of  photo-chemical 
action  and  electron  emission. 

A  study  of  the  ultra-violet  spectrum  of  oxygen  and  its 
allotropes  gives  us  an  insight  into  the  various  photo-chemical 
actions  involved,  and  quantitative  relationships  may  be  ob- 
tained by  an  application  of  the  quantum  theory ;  at  the  same 
time  the  study  of  the  disintegration  or  synthesis  of  the 
molecules  by  electron  emission  is  as  yet  in  its  infancy. 

The  work  of  Sir  J.  J.  Thomson  at  the  Cavendish 
Laboratory  on  the  subject  of  thermionics  has  opened  up  a 
new  vista  of  electro-chemical  research,  for  it  would  appear 
that  the  elements,  including  oxygen,  can  exist  not  only  in 
the  form  of  allotropes,  but  also  as  allotropic  modifications 
possessing  electrical  charges.  It  remains  for  the  future  to 
reveal  the  influence  of  these  charges  on  chemical  reactivity. 

Thanks  are  due  to  those  who  have  been  kind  enough  to 
place  material  dealing  with  the  applications  of  ozone  at  my 
disposal,  and  if  the  following  pages  can  assist  in  stimulating 
research  both  scientific  and  technical  in  this,  one  of  the  most 
interesting  branches  of  electro-chemistry,  the  object  of  the 
writer  will  be  fully  attained. 

EEIC  K.  EIDEAL. 

UNIVERSITY  OF  ILLINOIS, 
ILLINOIS,  U.S.A.,  14£/i  November,  1919. 


CONTENTS. 

CHAPTER  I. 

PAGE 

EARLY  HISTORY  OF  OZONE  AND  ITS  GENERAL  PROPERTIES        1 

CHAPTER  II. 

THE  NATURAL  OCCURRENCE  OF  OZONE 16 

CHAPTER  III. 

CHEMICAL  PRODUCTION  .      28 


CHAPTER  IV. 
THERMAL  PRODUCTION 44 

CHAPTER  V. 

THE  ELECTROLYTIC  PREPARATION  OF  OZONE          ...       57 

CHAPTER  VI. 

PRODUCTION  BY  ULTRA-VIOLET  RADIATION  AND  BY  IONIC 

COLLISION 70 

CHAPTER  VII. 

PRODUCTION    BY  MEANS   OF  THE    SILENT  ELECTRIC  DIS- 
CHARGE      91 

CHAPTER  VIII. 

THE  CATALYTIC  DECOMPOSITION  OF  OZONE      .        .        .        .133 

CHAPTER  IX. 
INDUSTRIAL  APPLICATIONS 142 

CHAPTER  X. 
METHODS  OF  DETECTION  AND  ANALYSIS 179 

NAME  INDEX 191 

SUBJECT  INDEX  195 


ix 


CHAPTEE  I. 

OZONE. 

EARLY  HISTORY. 

IN  1783  Van  Marum,  a  Dutch  philosopher,  noticed  that  the 
air  in  the  neighbourhood  of  his  electrostatic  machine  (now 
in  the  museum  at  Haarlem,  Holland)  acquired  a  marked  and 
characteristic  odour  when  subjected  to  the  passage  of  a  series 
of  electric  sparks.  Cruickshank  in  1801  likewise  drew  at- 
tention to  the  fact  that  the  oxygen  gas  produced  by  the  elec- 
trolytic decomposition  of  dilute  acids  under  certain  conditions 
was  possessed  of  a  similar  odour. 

These  two  investigators  merely  chronicled  the  results  of 
their  experiments,  and  did  not  pursue  their  inquiries  to 
elucidate  the  origin  of  the  odoriferous  substance.  Schonbein, 
in  a  memoir  presented  to  the  Academy  at  Munich  in  1840, 
recognised  that  the  smell  noted  in  air  subjected  to  the  spark 
discharge,  and  in  the  oxygen  generated  by  electrolysis,  was 
due  to  the  presence  of  a  new  gas,  to  which  he  gave  the  name 
"  ozone  "  (ofo) — to  smell),  he  also  showed  that  ozone  was 
formed  in  certain  processes  of  autoxidation,  notably  by  the 
action  of  air  on  phosphorus,  but  failed  to  establish  the  exact 
nature  or  composition  of  this  new  substance. 

We  shall  have  cause  to  observe,  when  discussing  the  pro- 
cesses of  autoxidation,  the  development  of  Schonbein's  hypo- 
thesis in  that  ozone  or  active  oxygen  is  produced  with  its 

1 


2  OZONE 

electrical  isomer  "  antozone"  by  the  disruption  of  the  neutral 
oxygen  molecule — 

+ 
02  -»  0'  ozone  +  0  antozone. 

This  hypothesis  naturally  led  to  the  division  of  peroxides 
into  two  groups,  the  ozonides  and  the  antozonides,  and  to 
an  extended  search  for  the  two  active  electrically  charged 
forms  of  the  oxygen  atoms. 

Various  other  speculative  hypotheses  were  made  as  to 
the  composition  of  ozone,  all  unsupported  by  experimental 
evidence,  thus,  Williamson  suggested  that  it  might  be  gase- 
ous hydrogen  peroxide,  and  Baumert  considered  ozone  to  be 
an  oxidised  form  of  hydrogen  peroxide,  i.e.  H203. 

Becquerel  and  Freny  first  showed  that  oxygen  could  be 
completely  transformed  into  ozone,  thus  proving  that  ozone 
was  an  allotropic  modification  of  this  element. 

These  experimenters  effected  the  conversion  of  oxygen 
into  ozone  by  the  passage  of  a  stream  of  electric  sparks 
through  the  gas,  the  ozone  formed  being  continuously  re- 
moved by  means  of  a  solution  of  potassium  iodide.  In  this 
way  all  the  oxygen  originally  in  the  tube  ultimately  disap- 
peared. 

Andrews,  Tait  and  Soret  ("  O.K.,"  1876)  took  up  the  in- 
vestigation at  this  stage,  and  by  the  following  experiments 
proved  that  the  allotrope  was  actually  a  condensed  form 
of  oxygen: — 

A  tube  of  volume  V  connected  to  a  sulphuric  acid  ma- 
nometer and  containing  oxygen  gas  was  submitted  to  the 
action  of  the  spark  discharge  when  a  contraction  in  volume  v 
was  recorded  on  the  manometer.  On  heating  up  the  tube  to 


OZONE 


270°  C.  the  ozone  was  destroyed  and  the  gaseous  mixture  then 
occupied  its  original  volume  V. 

Soret  showed  that  no  change  in  the  volume  of  the  ozonised 
oxygen  (V  -  v)  took  place  when  the  gas  was  exposed  to 
potassium  iodide  or  metallic  silver,  nevertheless  the  ozone 
was  destroyed. 

When,  however,  the  gas  mixture  was  exposed  to  turpen- 
tine a  further  contraction  in  volume  was  observed,  the  final 


C 


Sulphuric  Acid  • 
in  Manometer. 


FIG.  1. 

volume  being  V  -  3v  where  v  was  the  volume  contraction  on 
ozonisation. 

As  a  result  of  the  experiments  Soret  came  to  the  conclusion 
that  the  molecule  of  ozone  consisted  of  three  atoms  of  oxygen, 
three  volumes  combining  to  give  two  volumes  of  ozone — 

302  =  203, 

since  the  volume  contraction  v  on  ozonisation  is  clearly  equal 
to  one-third  of  the  oxygen  converted  into  ozone  (or  one-half 
of  the  resulting  ozone),  which  is  subsequently  absorbed  by 
the  turpentine.  Further,  that  when  ozone  reacted  with 


4  OZONE 

potassium  iodide  or  metallic  silver  it  liberated  an  equal 
volume  of  oxygen  : — 

03  +  2Ag  =  Ag20  +  02. 
Soret  ascribed  the  structural  formula  0 — 0  to  the  tri- 

V 

o 

atomic  allotrope  of  oxygen,  and  confirmed  tb~  existence  of 
ozone  by  a  determination  of  its  density.  The  theoretical 
density  of  ozone  at  N.T.P.  should  be  equal  to  one  and  a  half 
times  that  of  oxygen,  and  this  value  was  obtained  by  Soret 
and  Otto  by  several  different  methods,  which  may  be  briefly 
described  : — 

A  glass  globe  of  about  one  litre  was  filled  with  pure  dry 
oxygen  at  a  determined  temperature  and  pressure ;  and 
subsequently  weighed ;  the  oxygen  was  then  displaced  by 
ozonised  oxygen  reweighed,  and  the  weight  of  ozone  in  the 
flask  determined  by  titration  with  iodine  and  sodium  thio- 
sulphate. 

If  V  be  the  volume  of  the  globe,  containing  w  grams  of 
oxygen  of  density  J,  and  w  +  w'  be  the  weight  of  the  ozonised 
oxygen  in  the  globe,  where  S  is  the  density  of  ozone,  and  v 
and  w"  the  actual  volume  and  weight  of  ozone  in  the  globe, 
then 

(i)       w  +  w'  =  &v  +  (V  -  v)A, 

(ii)  w"  =  vS, 

(iii)  w  =  JV. 

From  (i)  v(S  -  A)  =  w  +  w'  -  AV  ••=  w', 

<>     w'  „ 

hence  : —  o  K A  =  w 


=      - 

\w- 


OZONE  5 

From  two  determinations  Otto  obtained  the  values  for  the 

ratio  —A 
A 

=  1-5  -  0-0034  and  T5  +  0'0035, 

or  the  density  of  ozone  was  practically  one  and  a  half  times 
that  of  oxygen. 

DENSITY  BY  DIFFUSION. 

Soret  showed  that  the  rate  of  transpiration  through  a 
small  aperture  of  the  purest  ozone  which  he  could  obtain  was 
intermediate  between  the  values  obtained  for  chlorine  and 
carbon  dioxide.  By  applying  Graham's  law  to  the  figures 
obtained  for  the  time  of  transpiration  of  ozone  and  carbon 
dioxide,  taking  t'  as  the  time  of  transpiration  for  a  volume  of 
carbon  dioxide  of  density  A,  and  £2  for  an  equal  volume  of 
ozone  of  density  S, 


the  value  T554  was  obtained,  taking  oxygen  as  unity.  Laden- 
burg  ("  Ber.,"  1901)  at  a  later  date  obtained  the  value  1-3698 
for  an  ozonised  oxygen  containing  86  per  cent,  ozone. 

We  have  already  referred  to  Soret's  early  experiments  on 
the  comparison  of  the  volumes  occupied  by  equal  weights  of 
oxygen  and  ozone,  in  which  the  ozone  formed  from  a  known 
quantity  of  oxygen  was  removed  by  absorption  in  turpentine. 
From  the  results  of  seven  experiments  Soret  obtained  a  mean 
value  differing  by  only  2*7  per  cent,  from  the  theoretical. 

PHYSICAL  PROPERTIES  OF  OZONE. 

Ozone  possesses  a  strong  penetrating  and  characteristic 
odour  which  can  be  detected  in  concentrations  of  one  part  in 


6 

a  million  of  air.  We  may  note  that  there  is  no  Unanimity  in 
describing  this  odour,  since  it  has  been  likened  to  sulphur, 
chlorine  and  phosphorus  (presumably  undergoing  oxidation 
when  ozone  itself  would  actually  be  present) ;  other  observers 
have  compared  it  to  dilute  oxides  of  nitrogen,  whilst  De  la 
Coux  likens  it  to  lobster. 

Dilute  ozone  is  practically  colourless,  but  when  viewed 
through  a  tube  five  or  six  feet  long  it  is  found  to  give  a  sky 
blue  tint  to  the  column  of  air. 

Hautefeuille  and  Chappuis  ("C.K.,"  94,  1249,  1882)  ob- 
tained liquid  ozone  by  compressing  ozonised  oxygen  to  125 
atmospheres  at  a  temperature  of  -  103°  C.  Liquid  ozone  is 
soluble  in  liquid  oxygen,  and  Ladenburg  ("Ber.,"  31,  2508, 
1898)  obtained  a  mixture  of  liquid  ozone  and  oxygen,  con- 
taining 84'4  per  cent,  ozone  by  passing  a  current  of  ozonised 
oxygen  through  a  tube  cooled  in  liquid  oxygen,  whilst  E. 
Goldstein  ("  Zeit.  Elektrochern.,"  50,  972,  1903)  obtained 
pure  liquid  ozone  by  immersion  of  a  double  walled  quartz 
mercury  vapour  lamp  in  liquid  oxygen.  When  a  small 
quantity  of  oxygen  was  admitted  into  the  vacuous  space  it 
was  rapidly  ozonised  by  the  ultra-violet  light  emitted  by  the 
mercury  vapour  and  condensed  in  the  form  of  small  drops, 
the  pressure  rapidly  fell  and  fresh  oxygen  could  then  be 
admitted.  Dewar  likewise  obtained  practically  pure  liquid 
ozone  by  the  careful  f ractionation  of  liquefied  ozonised  oxygen. 
Liquid  ozone,  which  is  very  liable  to  explode  if  accidentally 
brought  into  contact  with  a  trace  of  organic  matter  or  if  the 
temperature  be  allowed  to  rise,  is  a  dark  blue  liquid,  opaque 
in  thickness  exceeding  2  mm.  Olszewski  ("  Monatsh.,"  8,  109, 
1887  ;  "  Ann.  der  Physik,"  3,  31,  1887)  gave  the  boiling-point 


OZONE          •  7 

at  -  106°  C.  to  -  109°  C.,  whilst  Troost  ("C.B.,"  126,  1751, 
1898)  determined  it  at  -  119°  C. 

The  formation  of  ozone  from  oxygen  is  accompanied  by 
the  absorption  of  heat  and  the  instability  of  liquid  ozone 
and  the  gas  at  ordinary  temperatures  is  doubtless  occasioned 
by  its  strongly  endothermic  nature. 

203  =  302  +  2Q. 

The  lowest  value  of  Q,  the  heat  of  decomposition  per  gram, 
mol.  of  ozone,  is  recorded  by  Hollman  in  1868  as  17,064 
calories,  later  determinations  by  Berthelot  (1876)  gave  29,600, 
Van  de  Meulen  obtained  values  between  32,600  and  36,000, 
whilst  Kemsen  gives  the  highest  value  of  36,600.  The  most 
recent  observations  of  Jahn  ("  Zeit.  Anorg.  Chem.,"  60,  357, 
1908,  and  68,  250,  1910)  give  34,000  (see  p.  45). 

Ozone  is  soluble  in  water,  but  wide  discrepancies  are  found 
in  the  published  figures,  doubtless  occasioned  by  partial  de- 
composition during  solution. 

Schone  ("  Ber.,"  6,  1224,  1873)  obtained  the  value  for  the 
solubility  coefficient  at  18°  C.  of  0'366,  McLeod  at  14°  C. 
0-2795,  Carius  ("Ann.,"  174,  30,  1874)  at  1°  C.  0'834,  Laden- 
burg  ("Ber.,"  31,  2510,  1898)  gave  the  solubility  at  12°  C.  as 
O'Ol  per  cent,  by  volume,  whilst  Mailfert  ("  C.K.,"  119,  951, 
1894)  gives  the  following  values  for  the  coefficient : — 

Temperature.  Coefficient  of 

Solubility. 

0°C 0-64 

11-8° 0-5 

15° 0-456 

19° 0-381 

27° 0-27 

40° 0-112 

55° 0-031 

60°  .  0 


8  OZONE 

about  fifteen  times  the  values  obtained  for  oxygen.  Mouf- 
gang  ("Woch  Brauerei,"  28,  434,  1911)  determined  the 
following  values  :  10  mgm.  per  litre  at  2°  C.  and  1'5  mgm.  at 
28°  C. 

In  dilute  solutions  of  sulphuric  acid  (0*03  -  0'09  percent.) 
the  coefficient  of  solubility  is  somewhat  higher,  as  is  indicated 
by  the  following  figures  : — 

Temperature.  Coefficient  of 

Solubility. 

30°  C 0-240 

33° 0-224 

49° 0-156 

57° 0-096 

Eothmund  ("  Nernst  Festschrift,"  391, 1912)  has  indicated 
that  the  above  figures  are  in  all  probability  too  low  owing  to 
the  decomposition  of  ozone  occurring  during  the  estimation 
of  the  solubility.  He  found  that  this  decomposition  was 
remarkably  small  in  O'l  N.  sulphuric  acid  at  0°  C.  and  obtained 
a  value  0*487  for  the  absorption  coefficient  at  this  tempera- 
ture; when  corrected  for  the  salting  out  action  of  the  sul- 
phuric acid  the  coefficient  in  water  would  be  equal  to  0'494. 

Ozone  is  soluble  in  acetic  acid,  acetic  anhydride,  ethyl 
acetate,  chloroform,  and  carbon  tetrachloride  (Fischer  and 
Tropsch,  "  Ber.,"  50,  765, 1917),  forming  blue  solutions  which 
are  fairly  stable.  Solutions  of  ozone  in  carbon  tetrachloride, 
in  which  the  solubility  is  seven  times  that  in  water,  do  not 
undergo  decomposition  for  twenty-four  hours. 

The  decomposition  of  ozone  (see  p.  133)  is  frequently  ac- 
companied by  a  phosphorescence  noted  by  Dewar  when  pass- 
ing ozonised  air  through  a  capillary  opening,  and  by  Otto  in  the 
action  of  ozone  on  water  containing  traces  of  organic  matter. 


OZONE  9 

A  vivid  phosphorescence  is  likewise  obtained  when  a  hot 
glass  rod  is  brought  near  the  surface  of  liquid  oxygen  contain- 
ing ozone  ("Beger.  Zeit.  Elektrochem.,"  16,  76,  1910). 

R  S.  Strutt  ("Proc.  Roy.  Soc.,"  85,  10,  1911)  has  ex- 
amined a  number  of  cases  of  phosphorescent  combustions, 
especially  marked  in  vacuum  tubes  containing  ozonised  air 
under  low  pressures.  Phosphorescence  was  noticed  during 
the  oxidation  of  a  number  of  substances  by  ozone,  amongst 
the  more  important  being  nitric  oxide,  sulphur,  hydrogen 
sulphide,  ethylene,  and  iodine.  The  spectroscopic  examina- 
tion revealed  a  banded  spectrum  in  the  majority  of  cases,  but 
occasionally  continuous  spectra  were  obtained. 

The  spectrum  of  ozone  is  exceedingly  complex  and  has 
been  the  subject  of  numerous  investigations. 

Chappuis  ("  C.R.,"  94,  858, 1882)  found  eleven  lines  lying 
in  the  region  X  =  628'5  ^  and  X  =  444  ^  in  the  visible 
spectrum,  those  lying  on  either  side  of  the  sodium  lines  being 
particularly  distinct  and  characteristic,  X  =  609'5  -  595'5  ^ 
and  X  =  577  to  560  /^.  Schone  ("  Zeit.  Anorg.  Chem.,"  6,  333, 
1894)  added  two  to  the  above  number,  whilst  Ladenburg  and 
Lehmann  ("Ber.,"  4,  125,  1906)  noticed  a  line  in  the  red 
portion  of  the  spectrum. 

J.  Stark  ("Ann.  der  Physik,"  43,  2,  319,  1914)  has  shown 
that  the  ozone  molecule  gives  rise  to  many  bands  lying  be- 
tween the  visible  green  and  the  ultra-violet,  X  =  210  fjL/j,.  The 
bands  of  long  wave  lengths  were  found  to  be  resolvable  into 
line  series. 

Certain  lines  attributed  to  the  ozone  molecule  are  fre- 
quently caused  by  other  allotropes  of  the  element,  either  of 
elementary  oxygen  in  the  monatomic  or  diatomic  form,  0  or 


10 

O2,  or  of  those  substances  when  charged.  Thus  Stark  ("  Phys. 
Zeit.,"  14,  720,  1913)  has  shown  the  existence  of  two  distinct 
arc  spectra  of  oxygen  attributable  to  the  substances  02  and 
62. 

The  line  in  the  visible  red  of  the  spectrum  noticed  by 
Ladenburg  and  Lehmann  (loc.  cit.)  is  possibly  not  due  to  ozone 
but  to  another  allotrope  of  oxygen,  viz.  oxozone,  04 ;  whilst 
the  existence  of  a  band  in  the  infra  red  or  thermal  region  at 
X  =  1040  fjifju  has  been  claimed  for  ozone  but  has  not  received 
confirmation. 

The  lines  of  the  oxygen  spectrum  at  the  negative  electrode 
of  a  discharge  tube  were  examined  by  Schuster,  Steubing  and 
F.  Croze  ("C.K.,"  153,  680,  1916)  who  gives  the  following: 
X  =  685-3  ftp,  662'5,  603*2,  564'6,  529'6  and  498.  Schuster's 
two  negative  bands  X  =  570  -  584  ^  and  X  =  601  -  596  /JL/JL 
could  not  be  resolved. 

The  lines  of  atomic  oxygen  0  are  found  in  the  examin- 
ation of  water  vapour  as  well  as  in  oxygen  submitted  to 
intense  electrical  discharges.  Fowler  and  Brooksbank  ("  Roy. 
Astron.  Soc.,"  77,  511,  1917)  have  likewise  shown  the  pres- 
ence of  lines  of  this  series,  the  third  line  spectrum  of  oxygen 
X  =  559'2  JJLJI,  and  39618  in  stars  of  the  fi  type  as  well  as  in 
Wolf  Eayet  stars. 

The  ultra-violet  spectra  of  oxygen  and  its  allotropes  are 
of  special  significance  in  the  consideration  of  their  photo- 
chemical interconversion  (see  p.  70). 

That  of  ozone  has  been  examined  by  Lenard  ("  Ann.  der 
Physik,"  i,  480,  1900),  Goldstein  ("  Ber.,"  36,  304,  1903), 
and  more  especially  Regener  ("  Ann.  der  Physik,"  20,  1033, 
1906). 


11 

The  02  molecule  gives  short  wave  length  bands  resolvable 
into  lines  between  the  region  X  200  and  X  188  fifjL  correspond- 
ing to  the  ultra-violet  fluorescence  of  oxygen.  Steubing 
noticed  five  bands  between  X  =  183*1,  and  191*1  /-&/*,  whilst 
L.  and  E.  Bloch  ("  O.K.,"  158,  1161,  1914)  isolated  two  new 
ones  conforming  to  the  Delandres  formula  at  X  =  192'3  to 
193'6,  and  194'6  to  195 '7  ^//,.  The  ultra-violet  oxygen  atom 
0  band  in  the  region  X  =  230  pp  and  X  =  340  /*//,  is  observed 
in  the  positive  column  in  pure  rarified  oxygen,  and  in  the 
decomposition  of  dissociation  of  many  oxygen-containing 
compounds.  The  strongest  band  (see  Meyerheim,  Grebe, 
Holtz  and  Fowler,  «  Proc.  Eoy.  Soc.,"  94,  472,  1918)  is  found 
at  X  =  306 '4  ftp,  and  is  usually  attributed  to  water  vapour. 

Investigations  on  the  carriers  of  positive  electricity  by 
Sir  J.  J.  Thomson  and  his  co-workers  ("  The  Carriers  of 
Positive  Electricity")  have  revealed  the  presence  of  a  great 
number  of  allotropes  of  oxygen  which  give  rise  to  their  re- 
spective band  spectra.  F.  Horton  ("  Phil.  Mag.,"  22,  24, 1911) 
has  shown  the  existence  of  carriers  of  positive  electricity  in 
oxygen  of  electric  atomic  weights,  8,  16,  32,  48  and  96. 

Becquerel  has  shown  that  the  magnetic  susceptibility  of 
ozone  exceeds  that  of  oxygen,  and  that  the  ratio  of  the 
specific  magnetic  susceptibilities  exceeds  that  of  the  ratio  of 
their  densities. 

CHEMICAL  PKOPEETIES. 

Chemically,  ozone  is  a  strong  oxidising  agent,  capable  of 
effecting  the  oxidation  of  all  the  elements  with  the  exception 
of  gold  and  some  of  the  metals  of  the  platinum  group. 

It  liberates  iodine  from  potassium  iodide  and  brings  about 


12  OZONE 

the  oxidation  of  numerous  substances  such  as  lead  sulphide, 
manganous  salts  and  ferrocyanides,  reactions  which  form  the 
basis  of  its  qualitative  and  quantitative  detection  and  esti- 
mation. 

The  general  reaction  may  be  expressed  by  the  equation  : — 

M  +  03  =  MO  +  02. 

In  some  cases,  however,  oxygen  is  not  liberated,  but  the  whole 
of  the  ozone  reacts  and  no  free  oxygen  is  evolved.  Thus 
sulphur  dioxide  is  oxidised  to  sulphuric  anhydride  by  ozone 
according  to  the  reaction  : — 

3S02  +  03  =  3S03 

(see  Brodie,  "Phil.  Mag.,"  1894,  and  Eiesenfeld,  "Zeit.  Elek- 
trochem.,"  17,  634,  1911).  In  the  combustion  of  the  organic 
matter  in  water  during  the  process  of  sterilisation  by  ozonised 
air  this  reactivity  of  the  ozone  molecule  as  a  whole  is  likewise 
noted. 

Kiesenfeld  ("  Zeit.  Anorg.  Chem.,"  85,  217, 1914)  observed 
a  similar  series  of  reactions  in  the  action  of  ozone  on  sulphur 
compounds.  Three  atoms  of  oxygen  in  the  ozone  molecule 
react  with  sodium  hydrogen  sulphite,  whilst  with  neutral 
sulphites  and  alkaline  thiosulphates  only  two  atoms  react, 
the  third  being  liberated  as  oxygen  gas. 

With  certain  peroxides,  such  as  hydrogen  peroxide,  it 
undergoes  decomposition  as  follows  :— 

03  +  H202  =  H20  +  202, 

Eothmund  ("Monatsh.,"  38,  295,  1917)  showed  that  the 
reaction  was  unimolecular  in  excess  of  hydrogen  peroxide, 
but  in  dilute  solutions  the  ozone  underwent  catalytic  decom- 
position. 


OZONE  13 

It  has  found  many  uses  industrially  as  an  oxidising  agent, 
which  will  be  detailed  in  a  subsequent  section  of  this  volume. 
Reference,  however,  may  be  made  to  the  deodorising  of  air, 
the  conversion  of  manganates  into  permanganates,  of  chlo- 
rates into  perchlorates,  and  the  "  drying  "  of  oils  in  the  pre- 
paration of  linoleum  and  varnishes. 

At  suitable  temperatures  selective  oxidation  of  undesirable 
substances  which  give  an  objectionable  colour  or  odour  to 
many  fats  and  waxes  may  be  obtained,  and  such  processes  of 
bleaching  are  receiving  extended  application.  Attempts  have 
also  been  made  to  accelerate  the  ageing  of  spirits  and  wine 
by  fractional  oxidation  with  ozone. 

Ozone  is  a  powerful  germicide,  as  was  first  indicated  by 
Frohlich.  Its  high  germicidal  activity  is  doubtless  due  to  its 
oxidising  power,  and  as  a  dual  agent  of  this  character  it  has 
been  fairly  extensively  employed  for  the  sterilisation  of  public 
water  supplies,  for  the  treatment  of  wounds  in  hospitals,  and 
for  various  purposes  of  sterilisation  and  preservation  in  in- 
dustries, such  as  hide  preservation,  cold  meat  storage  and 
the  like.  Although  ozone  in  high  concentrations  will  effect 
the  sterilisation  of  air,  yet  such  concentrations  as  are  neces- 
sary (ca.  '05  per  cent.)  are  not  capable  of  respiration  without 
damage  to  the  tissues,  consequently  its  chief  function  is  as  a 
deodoriser  and  "  freshener  "  for  air  in  confined  and  crowded 
spaces. 

In  the  realm  of  organic  chemistry  ozone  has  received  ap- 
plication in  two  directions,  firstly  as  an  oxidising  agent  of 
great  strength  which  introduces  no  foreign  matter,  and 
secondly  as  a  reagent  for  the  ethylene  linkage  -  C  =  C  -  . 

As  an  oxidising  agent  it  is  employed  for  the  preparation 


14  OZONE 

of  vanillin  on  an  extremely  large  scale.  The  production  of 
other  substances,  such  as  heliotropine,  piperonal,  and  anisalde- 
hyde,  can  also  be  accomplished  with  its  aid  (see  chap.  ix.). 
Apart  from  its  powerful  oxidising  properties,  ozone  will  react 
with  certain  substances  in  two  definite  and  characteristic 
ways  to  form  ozonates  and  ozonides. 

THE  OZONATES. 

Baeyer  and  Villiger  ("  Ber.,"  35,  3038,  1908)  state  that 
strong  ozonised  air  fumes  in  moist  air  colours  blue  litmus 
red,  and  causes  an  increase  in  the  conductivity  of  distilled 
water  when  passed  through  it.  They  therefore  regarded 
ozone  as  the  anhydride  of  an  unstable  ozonic  acid,  H204. 
According  to  these  authors,  if  due  precautions  are  taken, 
highly  coloured  ozonates  may  be  prepared  by  the  interaction 
of  ozone  and  moist  solid  alkali  hydroxides  or  concentrated 
solutions  of  the  same  at  low  temperatures. 

The  ozonates  are  usually  orange  or  brown.  If  ozone  be 
passed  into  a  cold  ammonia  solution,  it  acquires  a  dark  red 
colour  attributed  by  these  investigators  to  the  formation  of 
ammonium  ozonate,  NH4H04.  Lithium  ozonate  was  found 
to  be  least,  and  that  salt  of  caesium  most  stable. 

A  white  granular  precipitate  of  calcium  peroxide  is  formed 
on  the  passage  of  ozonised  air  into  cold  lime  water. 

According  to  W.  Manchot  ("Ber.,"  41,  47,  1908),  Baeyer 
and  Villiger's  results  are  to  be  attributed  to  the  presence 
of  small  quantities  of  oxides  of  nitrogen  in  their  ozonised 
air,  since  he  found  that  ozone  possessed  no  acidic  quali- 
ties. 


OZONE  15 

THE  OZONIDES. 

The  ozonides  are  formed  by  the  interaction  of  ozone  with 
organic  compounds  containing  unsaturated  ethylene  linkages 
according  to  the  general  equation  : — 

— C  — C— Ov 

!l  +  03     -*       |        >0 

_c  — c— cr 

Discovered  by  Harries  ("Ann.,"  343,  311,  1905;  "  Ber.,"  38, 
1195,  1905),  this  reaction  was  successfully  employed  by  him 
to  elucidate  the  composition  of  rubber  (see  p.  170),  and  has  of 
recent  years  been  frequently  utilised  to  identify  the  presence 
of  ethylene  linkages. 

Where  compounds  containing  ethylene  linkages  are 
treated  with  strongly  ozonised  oxygen  (ca.  40  per  cent.  03) 
the  ozonides  thus  formed  on  analysis  yield  more  oxygen  than 
is  to  be  expected  by  the  assumption  of  simple  saturation  of 
the  ethylene  linkage  according  to  the  equation  :— 

— C  —G—0\ 

II  +  03    ->         |        )0 

— c  —  c— cr 

According  to  Harries,  oxozonides  are  formed  by  interaction  of 
the  organic  compound  with  oxozone  present  in  the  gas  :— 

_ C  — C— 0— 0 

H  +  04     ->          |  | 

_ c  —  c— o— o 

More  recent  experiments,  however  (see  p.  184),  have  failed 
to  establish  the  existence  of  oxozone  in  ozonised  air  or  oxy- 
gen and  consequently  some  other  explanation  for  the  forma- 
tion of  oxozonides  must  be  advanced. 


CHAPTEE  II. 

THE  NATURAL  OCCURRENCE  OF  OZONE. 

THE  occurrence  of  ozone  in  ordinary  atmospheric  air  has 
long  been  a  matter  of  dispute.  C.  Schonbein  ("  J.  f.  Prakt. 
Chemie,"  73,  99,  1858),  as  early  as  1858,  showed  that  starch 
iodide  paper,  when  exposed  to  the  air,  slowly  turned  blue, 
thus  demonstrating  the  existence  of  some  oxidising  agency. 
He  noted  that  the  rate  of  liberation  of  iodine  varied  from  day 
to  day  and  attributed  this  to  an  alteration  in  the  ozone  con- 
tent of  the  air.  Cloez  and  Bineau  pointed  out  that  the 
liberation  of  iodine  from  starch  iodide  could  likewise  be 
caused  by  the  presence  of  oxides  of  nitrogen  naturally  present 
in  atmospheric  air. 

Houzeau  ("Ann.  Chem.  Phys."  IV,  27,  5,  1872,  and 
"  C.K.,"  74,  712, 1872),  as  a  result  of  over  4000  determina- 
tions with  neutral  litmus  starch  iodide  paper,  came  to  the 
conclusion  that  ozone  was  present  in  atmospheric  air  in  ad- 
dition to  the  frequent  occurrence  of  oxides  of  nitrogen.  As 
a  maximum  ozone  content  Houzeau  recorded  2 '8  mgm.  per 
cubic  metre.  Houzeau's  views  were  supported  by  Hartley 
(u  Trans.  Chem.  Soc.,"  39,  10,  111,  1881,  and  "Nature,"  39, 
474,  1889),  who  noted  that  many  of  the  dark  lines  of  the  solar 
spectrum  were  coincident  with  those  that  would  have  been 
produced  on  the  assumption  that  atmospheric  ozone  absorbed 

light  of  these  particular  wave  lengths  emitted  from  the  sun, 

(16) 


THE   NATUEAL   OCCUEEENCE    OF  OZONE  17 

thus  exhibiting  the  Frauenhofer  lines ;  which  conclusions 
were  confirmed  by  Meyer  ("Ann.  der  Physik,"  IV,  12, 
849,  1903). 

The  vivid  blue  colour  of  ozone  was  asserted  by  Hartley 
to  give  the  characteristic  coloration  to  the  summer  sky,  an 
alternative  theory  to  the  one  first  propounded  by  Lord  Bay- 
leigh  in  1871  (Hon.  S.  W.  Strutt,  "  Phil.  Mag.,"  n,  107, 1871) 
and  extended  by  Schuster  ("  Theory  of  Optics,"  p.  325)  and 
King  ("  Trans.  Phil.  Boy.  Soc.,"  A,  212,  375,  1913). 

Kayleigh  showed  that  the  intensity  of  the  light  scattered 
by  small  particles  of  dust  in  the  atmosphere  would  vary  in- 
versely as  the  fourth  power  of  the  wave  length,  i.e.  the  light 
in  the  ultra-violet  and  blue  end  of  the  spectrum  being  of  the 
shortest  wave  length  would  be  most  intensely  scattered  and 
thus  made  visible.  It  may  be  noted  that  in  Lord  Bayleigh's 
experiments  the  sky  light  appeared  somewhat  bluer  than 
anticipated  by  this  theory,  and  thus  indicated  that  absorption 
by  ozone  may  be  a  contributary  cause  to  the  colour  of  the 
sky.  C.  Fabry  and  H.  Buisson  ("  C.B.,"  156,  782,  1913),  as 
a  result  of  a  series  of  experiments  on  the  absorption  coefficient 
ozone  for  light  of  the  wave  lengths  X  =  255  fip  to  330  pp, 
showed  that  a  thickness  of  only  25  /JL  of  ozone  reduces  the 
incident  light  intensity  by  over  50  per  cent.  For  a  wave 
length  of  X  =  300  fip  the  proportion  of  transmitted  light  for 
a  thickness  of  5  mm.  of  ozone  was  only  1  per  cent.,  approxi- 
mating to  the  conditions  of  the  terrestrial  atmosphere  exposed 
to  solar  radiation.  If  uniformly  distributed  this  would  equal 
0'6  c.c.  or  1'4  mgm.  per  cubic  metre ;  this  concentration  is 
somewhat  high  for  air  at  low  altitudes,  hence  it  may  be 

argued  that  with  increasing  altitudes  the  ozone  content  rises. 

2 


18 


OZONE 


E.  Kron  ("  Ann.  der  Physik,"  45,  377,  1914)  records  X  = 
325  /uyu,  as  the  limit  of  the  effective  solar  spectrum  at  sea-level 
on  the  clearest  days.  Fabry  and  Buisson's  results  between 
the  wave  length  X  =  200  and  X  =  350  ^  are  shown  in  the 
following  graphical  form  : — 


foo 

50 

20 

^ 

\ 

1 

J 

\ 

\ 

I 

i 

\ 

0                  250                  300                 35( 

\  771  [JtfJL 

FIG.  2. 

More  recently  Fowler  and  Strutt  ("Proc.  Eoy.  Soc.,"  93, 
577,  1917)  showed  that  the  Frauenhofer  lines  between  the 
wave  lengths  X  =  319*9  ^  and  X  =  333-8  /-t/4,  the  ultra-violet 
lines  shown  by  Ladenburg  and  Lehmann  to  be  present  in  the 
ozone  absorption  spectrum,  were  present  in  the  greatest  in- 
tensity in  the  solar  spectrum  at  low  altitudes,  or  when  the 
terrestrial  air  stratum  through  which  the  light  had  to  pass 
was  greatest,  thus  again  supporting  Hartley's  contention  that 
the  atmospheric  ozone  was  an  effective  agent  in  fixing  the 
extension  of  the  solar  spectrum  in  the  ultra-violet. 

Strutt  ("  Proc.  Koy.  Soc.,"  114,  260,  1918)  likewise  showed 
that  the  limitation  of  the  solar  spectrum  to  the  lower  wave 


THE  NATUEAL  OCCUEEENCE  OF  OZONE         19 

length  of  X  =  294*8  pp  was  due  to  the  absorption  by  atmos- 
pheric ozone.  By  long  distance  experiments  on  absorption 
of  the  light  from  a  cadmium  spark  and  mercury  vapour  lamp, 
the  lower  air,  mass  for  mass,  was  found  more  transparent 
than  the  upper  air,  and  that  if  the  absorption  was  not  due  to 
dust,  the  ozone  content  would  not  exceed  0'27  mm.  at  normal 
pressure,  in  four  miles  of  air. 

Engler  and  Wild  ("Ber.,"  29,  1940,  1896)  likewise  con- 
firmed the  presence  of  atmospheric  ozone  by  the  action  of 
air  on  manganous  chloride  paper,  whilst  Schone  in  1897 
("  Brochure,"  Moscow,  1897)  obtained  as  maxima  and  minima 
the  following  values : — 

Maximum,  100  mg.  per  cubic  metre. 
Minimum,     10    ,,  ,,  ,, 

In  the  same  year,  Thierry  ("C.B.,"  124,  460,  1897)  made 
the  important  observation  from  experiments  conducted  on 
Mont  Blanc,  that  the  ozone  content  of  the  atmosphere  rose 
with  increasing  altitude,  thus,  at  1000  metres  height  he  ob- 
tained 0'039  mg.,  and  at  3000  metres,  0'094  mg.  of  ozone  per 
cubic  metre  of  air.  Similar  figures  were  observed  by  H.  de 
Varigny  (Smithsonian  College,  "  Proc.,"  39,  27),  viz.  a  maxi- 
mum and  minimum  of  0'03  and  0*01  mg.  per  cubic  metre. 

These  observations  were  continued  by  Hatcher  and  Arny 
("  J.  Amer.  Pharm.,"  72,  9, 1900),  who  determined  the  amount 
of  ozone  in  the  air  by  two  different  methods,  viz.  the  iodide 
and  arsenitic  titration  processes,  as  maxima  and  minima, 
they  observed  the  following  values  : — 

Method  of  Minimum.               Maximum. 
Estimation. 

Iodide  158           316  mg.  per  cubic  metre. 

Arsenite  34              80  „ 


20  OZONE 

Henriet  and  Bonyssy  ("C.R,"  146,  977,  1908)  showed  that 
the  ozone  content  of  the  air  at  ground  level  varied  approxi- 
mately inversely  with  the  carbon  dioxide  concentration. 

Hayhurst  and  Pring  ("J.C.S.,"  LXII,  868,  1910)  drew 
attention  to  the  wide  variation  of  the  results  obtained  by 
numerous  investigators,  and  conducted  a  series  of  observa- 
tions on  Glossop  Moor  in  Derbyshire.  They  showed,  utilising 
Houzeau's  original  method  of  estimating  both  iodine  and 
alkali  liberated  from  potassium  iodide  solutions,  a  procedure 
which  was  found  to  give  extremely  accurate  results,  both  for 
ozone  and  mixtures  of  ozone  and  nitrogen  dioxide,  that  in 
this  neighbourhood  at  least,  oxides  of  nitrogen  were  always 
present  in  the  air  up  to  an  altitude  of  8000  ft.,  and  that  the 
quantity  of  ozone  present,  if  any,  was  too  small  to  be  de- 
tected. With  an  increase  in  the  altitude  small  quantities  of 
ozone  were  obtained  up  to  a  ^height  of  10  miles.  Concen- 
trations of  the  order  of  0'12  to  0*4  mg.  per  cubic  metre  of 
ozone,  and  smaller  quantities  of  oxides  of  nitrogen  were  ob- 
tained. 

H.  N.  Holmes  ("J.  Amer.  Chem.  Soc.,"  47,  497,  1909) 
has  shown  that  the  maximum  amount  of  ozone  is  formed  in 
moving  areas  of  air  under  a  high  barometric  pressure  when 
the  conditions  are  favourable  for  bringing  air  of  high  altitudes 
close  to  the  earth's  surface. 

It  may  be  concluded  that  ozone  is  a  normal  constituent 
of  pure  air,  and  that  the  quantity  of  ozone  in  the  air  increases 
with  the  altitude.  Seasonal  variations  in  the  ozone  content 
have  likewise  been  obtained.  Thus,  Houzeau,  as  a  result  of 
eight  years  observation  with  neutral  and  alkaline  iodide  test- 
papers  (loc.  cit.},  noted  that  the  ozone  content  of  the  atmos- 


THE   NATURAL   OCCURRENCE   OF   OZONE  21 

phere  rose  in  spring  and  summer,  but  sank  to  very  small 
proportions  in  autumn  and  winter.  Berigny  observed  a 
maximum  in  the  month  of  May,  and  a  minimum  in  Novem- 
ber, and  gives  the  following  order  for  decreasing  ozone  con- 
tent, May,  March,  April,  June,  August,  July,  September, 
January,  October,  February,  November. 

Pring  ("Proc.  Koy.  Soc.,"  96,  204,  1914)  extended  his 
investigations  to  the  air  in  the  high  Alps ;  at  2100  metres,  he 
obtained  the  value  4'7  rag.  per  cubic  metre,  and  at  3580 
metres  8*8  mg.  per  cubic  metre.  No  considerable  increase 
in  the  ozone  content  was  observed  at  altitudes  up  to  20  km. 
Oxides  of  nitrogen  and  hydrogen  peroxide  were  absent  in  the 
air  at  the  higher  altitudes. 

Usher  and  Eao  ("  J.C.S.,"  in,  779,  1917)  conducted  a 
series  of  estimations  in  India  on  the  ozone  (by  manganese 
dioxide),  hydrogen  peroxide  (by  chromic  acid),  and  oxides  of 
nitrogen  content  of  the  atmosphere.  They  could  not  detect 
the  presence  of  ozone  although  oxides  of  nitrogen  in  con- 
centrations of  from  1  to  5  parts  per  million  were  frequently 
obtained. 

The  ozone  concentration  in  the  lower  air  strata  likewise 
increases  during  periods  of  storm  or  after  heavy  rain  storms, 
and  the  south  and  south-west  winds  are  said  to  be  richer  in 
ozone  than  the  northern  ones. 

K.  Nasini  ("Atti.  d.  K.  Accad.  Lincei,"  21,  740,  1912) 
records  interesting  cases  of  naturally  occurring  ozonised 
water  ;  he  states  that  the  acid  waters  of  Bagnone,  Monte 
Annata  are  highly  ionised,  and  contain  T25  of  oxygen,  and 
0135  c.c.  of  ozone  per  litre. 


22  OZONE 

SOUECES  OF  NATUEAL  OZONE. 

Various  alternative  hypotheses  have  been  advanced  to 
explain  the  mode  of  formation  of  this  small  ozone  concentra- 
tion in  atmospheric  air.  It  is  at  once  evident  that  even  this 
minute  quantity  exceeds  the  normal  thermal  equilibrium 
amount,  and  consequently  there  must  be  a  continuous  source 
of  ozone.  We  may  classify  the  various  hypotheses  as  to  this 
source  under  three  groups  : — 

(a)  chemical ;  (b)  photo-chemical ;  (c)  electrical. 

NATUEAL  CHEMICAL  PEOCESSES. 

The  earlier  investigators  such  as  Schonbein,  Houzeau, 
Berigny,  Peyrou,  and  Marie  Davy,  were  of  the  opinion  that 
the  green  vegetation  of  plant  life  was  responsible  for  the 
production  of  ozone,  thus  accounting  for  the  observed  maxi- 
mum and  minimum  ozone  content  in  the  months  of  May  and 
November  respectively.  It  was  shown,  however,  that  coloured 
plants  yielded  no  volatile  oxidising  substances  whatever,  and 
more  recent  experiments  have  shown  that  the  oxidising  agent, 
which  can  always  be  detected  in  green  plant  growth,  is  hy- 
drogen peroxide.  According  to  Priestly  and  Usher  ("Proc. 
Phys.  Soc.,"  78,  3,  38,  1906),  the  plant  chlorophyll  serves 
merely  as  a  light  sensitiser  to  bring  about  the  reaction — 
3H20  +  C02  +  light  energy  =  HCHO  +  2H2  02, 

the  formaldehyde  thus  formed  is  subsequently  polymerised 
to  formose  (d.l.  glucose)  by  the  protoplasm  of  the  cell  chloro- 
plast. 

The  hydrogen  peroxide  is  usually  destroyed  by  one  of  the 
numerous  enzymes,  termed  oxidases,  present  liberating  mo- 
lecular oxygen — 


THE   NATUEAL   OCCUEEENCE   OF   OZONE  23 

2H202  ->  2H20  +  O2 

(see  Bach  and  Choat,  "Arch.  d.  Sci.  Phys.  et  Nat.  Geneva," 
17,  4771,  1909),  but  many  investigators  suspect  that  during 
the  decomposition  of  the  hydrogen  peroxide  small  quantities 
of  ozone  may  be  produced. 

The  production  of  ozone  by  the  atmospheric  oxidation  of 
various  gums  and  essential  oils  exuded  by  trees  and  plants, 
such  as  turpentine,  sandal-wood  oil,  or  oil  of  lavender,  has 
long  been  suspected,  and  undoubtedly  the  rapidity  with  which 
starch  iodide  slips  are  turned  blue  in  a  pine  forest  is,  in  some 
measure,  due  to  the  ozone  present  in  the  surrounding  air, 
although  the  formation  of  hydrogen  peroxide  under  these 
conditions  is  without  doubt  the  more  important  natural  pro- 
cess contributing  to  the  freshness  of  the  air. 

PHOTO-CHEMICAL  PROCESSES. 

We  shall  have  occasion  to  refer  to  the  interesting  fact  that 
oxygen  is  ozonised  by  exposure  to  ultra-violet  irradiation  of 
wave  length  \  =  120  -  180  //.^  whilst  ozonised  oxygen  is  re- 
solved into  its  original  form  by  light  of  somewhat  longer  wave 
length,  viz.  \  =  330  /JL/JL. 

Hartley  ("Trans.  Chem.  Soc.,"  39,  10,  111,  1881)  noted 
the  presence  of  Frauenhofer  lines  in  the  visible  solar  spectrum 
corresponding  to  those  which  would  be  absorbed  by  ozone. 
These  conclusions  in  the  visible  part  of  the  spectrum  were 
confirmed  by  Meyer  ("Ann.  der  Physik,"  IV,  12,  849,  1903) 
and  extended  by  C.  Fabry  and  H.  Buisson  ("  C.R.,"  156, 
782,  1913)  and  Fowler  and  Strutt  ("Proc.  Roy.  Soc.," 
93»  77,  1917)  to  the  ultra-violet  portion  of  the  spectrum. 
Furthermore,  all  experimental  evidence  indicates  that  the 


24  OZONE 

ozone  concentration  is  greatest  in  the  upper  portion  of  the 
atmosphere,  where  the  intensity  of  the  ultra-violet  radiation 
would  naturally  be  greatest  (see  also  K.  Birkeland,  "  Cairo 
Soc.,"  8,  287,  1916).  It  would  appear  that  the  presence  of 
ozone  in  atmospheric  oxygen  is  largely  due  to  the  synthetic 
operation  of  solar  radiant  energy  of  short  wave  length 
(X  =  120  -  180  ftp),  whilst  the  limitations  in  the  amount  in 
the  upper  parts  of  the  atmosphere  is  caused  by  the  destructive 
action  of  light  of  longer  wave  length  (X  =  300  /JL/J,),  a  dynamic 
equilibrium  being  finally  established  between  the  rate  of 
formation  and  the  rate  of  decay.  Near  the  earth's  surface, 
as  we  have  seen,  smaller  ozone  concentrations  are  obtained, 
partly  owing  to  the  fact  that  the  light  of  longer  wave  length 
penetrates  somewhat  further  into  a  dusty  atmosphere  than 
that  of  short  wave  length,  but  more  especially  to  the  reducing 
action  of  easily  oxidisable  substances  both  on  the  earth's 
surface,  and  carried  to  low  altitudes  by  the  wind.  Country 
air,  according  to  Houzeau,  contains  more  ozone  than  that 
around  villages,  whilst  its  presence  can  rarely  be  detected  in 
towns.  This  observer,  in  fact,  records  the  disappearance  of 
ozone  in  the  air  after  the  passage  of  a  crowd  on  a  public  fete 
day,  and  its  gradual  reappearance  when  the  normal  conditions 
had  been  re-established. 

ELECTEICAL  PEOCESSES. 

The  increasing  attention  which  during  the  last  few  years 
has  been  paid  to  a  study  of  atmospheric  ionisation  and  electri- 
fication has  not  only  clearly  demonstrated  that  the  potential 
difference  between  different  parts  of  the  atmosphere  and  be- 
tween earth  or  water  and  the  air  may  reach  extremely  high 


THE   NATURAL   OCCURRENCE   OF   OZONE  25 

values  during  periods  of  atmospheric  disturbances  such  as 
electrical  storms,  but  even  during  periods  of  fair  weather, 
local  potential  differences  of  high  magnitude  may  result. 

Evidence  for  the  ozonisation  of  oxygen  during  periods  of 
intense  electrical  discharge,  either  silent  as  in  the  aurora, 
natural  corona,  and  the  remarkable  Andes  glow  occasionally 
observed  in  S.  America  (see  "  Knoche  Meteor.  Zeit.,"  29,  329, 
1912),  or  violent  as  in  lightning  and  the  so-called  thunderbolt 
or  globular  discharge,  is  somewhat  conflicting.  Undoubtedly 
oxides  of  nitrogen  are  present,  since  these  can  always  be 
detected  during  periods  of  heavy  discharge,  and  in  many  cases 
it  appears  probable  that  ozone  is  formed  either  without  or 
more  probably  in  conjunction  with  the  oxides  of  nitrogen. 

Thornton  ("  Phil.  Mag.,"  21,  630,  1911)  has  advanced  the 
view  that  the  globular  discharges  themselves  are  purely 
gaseous  bodies  and  consist  of  ozone  in  active  combination. 

In  a  subsequent  chapter  we  shall  observe  that  the  condi- 
tions for  the  possible  ozonisation  of  oxygen  by  means  of 
ionisation  are  established  when  a  discharged  electron  or  a  gas 
ion  strikes  an  oxygen  molecule  with  sufficient  violence  so  as 
to  permit  the  temporary  distortion  of,  or  the  actual  removal 
of  one  of  the  valency  electrons  circulating  round  the  oxygen 
molecule  from  its  orbit,  and  a  rough  computation  of  the  volt- 
age of  discharge  which  is  necessary  to  give  the  emitted 
electron  this  requisite  energy  is  but  nine  volts,  a  relatively 
low  figure. 

The  potential  difference  between  strata  of  air  during 
periods  of  fine  weather  is  frequently  extremely  great  and  quite 
sufficient  to  produce  atmospheric  ionisation,  with  the  conse- 
quent possible  production  of  ozone,  thus  P,  Mercanton 


26  OZONE 

("  Terrest.  Magn.,"  22,  35, 1917)  obtained  a  P.D.  of  1200  volts 
per  metre  on  the  top  of  a  tower  at  Lausanne,  930  metres 
above  sea-level. 

C.  Chree  ("  Phil.  Trans.,"  215,  133,  1915)  gives  304  volts 
per  metre  as  the  average  potential  gradient  in  the  atmosphere 
at  Kew  for  the  last  fifteen  years. 

McLennan  ("  Nature,"  92,  424,  1913)  obtained  the  follow- 
ing values  for  the  number  of  gas  ions  formed  per  second  per 
cubic  centimetre  of  air,  nine  ions  in  the  air  over  the  land  and 
four  over  sea  water.  He  ascribed  ionisation  due  to  the  influ- 
ence of  the  ultra-violet  light  itself;  a  view  supported  by  the 
experiments  of  G.  Simpson  ("  Monthly  Weather  Keview," 
44,  115,  1916),  who  found  that  at  a  height  of  6000  metres, 
over  thirty  times  as  many  ions  were  formed  per  second  as  at 
sea-level.  W.  Swann  ("Terrest.  Magn.,"  21,  1,  1916)  like- 
wise showed  that  the  upper  air  was  a  region  of  high  electrical 
conductivity,  the  source  being  the  ultra-violet  light  of  ampli- 
tude X<135  ftp,  a  fraction  only  T61  x  10  ~5  of  the  total 
radiant  energy  derived  from  the  sun.  Production  of  ozone 
by  natural  ionisation  is  thus  chiefly  a  secondary  effect  of  ultra- 
violet irradiation,  which,  as  we  have  already  noted,  is  one  of 
the  chief  ozonising  agencies  in  the  atmosphere.  Natural  ion- 
isation and  consequent  ozonisation  is,  however,  not  entirely 
derived  from  ultra-violet  radiation,  since  ionisation  and  small 
quantities  of  both  ozone  and  hydrogen  peroxide  are  formed 
by  the  evaporation  of  water  in  air,  especially  in  the  neighbour- 
hood of  fountains  and  waterfalls,  where  conditions  of  spray 
formation  obtain. 

A.  Besson  ("  C.K.,"  153,  877,  1911)  noted  that  the  maxi- 
mum concentrations  of  ozone  and  hydrogen  peroxide  were 


THE  NATURAL  OCCURRENCE  OF  OZONE         27 

formed  in  air  during  the  fall  of  heavy  drops  of  rain  from  a 
previously  clear  sky  in  hot  summer  weather. 

C.  Oddo  ("  Gaz.  Soc.  Chim.  Ital.,"  45,  395,  1915)  ascribes 
the  formation  of  gas  ions  under  these  conditions  to  the 
spontaneous  ionisation  of  water  vapour  when  rarified.  One 
kilogram  of  moist  air  (773*4  litres  when  dry  at  N.T.P.) 
contains  89  x  10  ~ 20  hydrogen  and  hydroxyl  ions  at  15°  C. 
and  760  mm.  pressure ;  he  shows  that  a  fall  in  temperature 
naturally  diminishes  the  content  of  water  vapour  in  the  air 
but  also  increases  the  degree  of  ionisation  below  32°  C.,  where 
it  is  practically  zero.  The  optimum  temperature  range  for 
maximum  ionisation  was  found  to  be  5°  to  20°  C.,  which,  it 
may  be  noted,  is  the  optimum  for  animal  and  vegetable  life 
in  the  temperate  zones. 

Lenard,  in  a  series  of  researches  on  the  Electricity  of 
Waterfalls  («  Ann.  der  Physik,"  45,  7,  100,  1914),  showed  that 
ionisation  was  effected  not  only  by  the  impact  of  suspended 
drops  upon  obstacles  such  as  rocks  or  stones,  but  by  impact 
of  drops  against  each  other  resulting  in  the  production  of 
secondary  drops. 


CHAPTEE  III. 

CHEMICAL  PRODUCTION. 

AN  indication  of  the  production  of  ozone  can  be  observed  in 
a  great  variety  of  chemical  reactions  such  as  the  decomposi- 
tion of  certain  peroxides,  in  processes  of  autoxidation,  and  in 
many  cases  of  combustion  of  gaseous  fuels.  In  the  latter 
case  the  ozone  is  doubtless  of  a  purely  thermal  origin  and  a 
consideration  of  the  mechanism  of  production  by  this  means 
will  be  deferred  to  a  subsequent  section. 

Ozone  can  nearly  always  be  detected  in  oxygen  resulting 
from  chemical  decomposition.  The  temporary  existence  of 
atomic  oxygen  liberated  according  to  the  equation  :— 

M"02  +  H2S04  ->  MS04  +  H20  +  0 

has  not  yet  been  definitely  proved,  although  the  evidence  for 
the  formation  of  atomic  hydrogen  by  similar  processes  is  now 
extremely  strong.  In  any  case  during  the  decomposition  of 
the  peroxides,  the  atomic  oxygen  polymerises  with  great 
rapidity  to  the  molecular  form  :— 

0  +  0  -*•  02. 

C.  Brodie  ("Phil.  Trans.,"  141,  759,  1850)  first  advanced  the 
view  that  ordinary  oxygen  during  processes  of  chemical 
action  was  split  up  into  two  parts  termed  ozone  and  antozone. 

+ 
02  -»  0'  (ozone)  +  O  (antozone). 

As  we  shall  have  occasion  to  note  in  discussing  processes  of 

(28) 


CHEMICAL   PRODUCTION  29 

autoxidation,  Brodie's  hypothesis  was  strongly  supported  by 
the  experimental  work  of  Schonbein. 

K.  Clausius  ("Zeit.  Phys.  Chem.,"  103,  644,  1858)  sug- 
gested that  Brodie's  so-called  "  ozone  "  and  "  antozone  "  were 
identical  with  atomic  oxygen,  possessing  opposite  electric 
charges 

02  ->  0  +  0'. 

This  view  was  further  enlarged  upon  by  van't  Hoff 
("  Zeit.  Phys.  Chem.,'r  16,  411,  1895),  who,  as  a  result  of  his 
studies  on  the  autoxidation  of  phosphorus,  came  to  the  con- 
clusion that  there  exists  a  definite  equilibrium  in  normal 
gaseous  oxygen  between  the  molecular  and  atomic  form,  the 
atomic  being  charged  : — 

02  ^:=t  6  +  0'. 

Nernst  ("Zeit.  f.  Elektrochem.,"  9,  891,  1903)  showed,  from 
a  series  of  observations  on  the  electromotive  force  of  ozone- 
oxygen  cells,  that  if  the  three  allotropes  of  oxygen  were  as- 
sumed to  exit  in  equilibrium  with  each  other  under  normal 
conditions,  according  to  the  reversible  equations : — 

03  2  02  +  0 
02  ^  0  +  0, 

none  of  the  allotropes  possessing  an  electrical  charge,  then 
the  equilibrium  concentration  of  the  atomic  oxygen  would  be 
only  1/1023  of  the  normal  ozone  concentration,  which  we  shall 
see  is  of  the  order  10~5  per  cent.  The  normal  concentration 
of  atomic  oxygen  is,  therefore,  so  small  as  to  render  its  exist- 
ence as  a  chemical  substance,  to  which  the  ordinary  methods 
of  statistical  calculation  of  its  concentration  and  properties  in 
bulk  can  be  applied,  extremely  doubtful. 


30  OZONE 

Nevertheless,  the  formation  of  ozone  in  small  quantities 
may  be  expected  to  occur  in  the  decomposition  of  the  per- 
oxides, since,  on  the  above  assumption,  the  following  sequence 
of  chemical  reactions  may  be  assumed  to  occur  : — 
(i)  Ba02      ->  BaO  +  O. 
(ii)  O  +  O  ->  02. 
(iii)  02  +  0  ->  08. 

We  have  already  noted  that  reaction  (ii)  proceeds  with  great 
rapidity,  and  that  reaction  (iii)  is  merely  a  side  reaction, 
which  will  only  proceed  during  the  evolution  of  oxygen. 
Any  ozone  formed  may,  of  course,  be  subsequently  decom- 
posed by  catalysis  at  the  surface  of  the  decomposed  peroxide, 
or  by  the  somewhat  elevated  temperature  necessary  to  cause 
decomposition  of  the  peroxide. 

Houzeau,  in  fact,  was  able  to  obtain  concentrations  as 
high  as  28  gms.  of  ozone  per  cubic  metre  of  oxygen  evolved, 
by  gently  heating  small  quantities  of  powdered  barium  per- 
oxide in  eight  times  its  volume  of  concentrated  sulphuric 
acid.  Hydrogen  peroxide  is  likewise  formed  in  small  quan- 
tities under  these  conditions  :— 

4H2S04  +  4Ba02  ->    (i)  4BaS04  +  1H20  +  202 

(ii)  4BaS04  +  4H20  +  03  +  0 
(iii)  4BaS04  +  4H202. 

The  same  investigator  showed  that  similar  results  were 
obtained  with  other  peroxides,  notably  those  of  magnesium, 
zinc,  sodium,  and  potassium. 

Even  better  results  can  be  obtained  by  the  decomposition 
and  gentle  dehydration  of  permanganic  acid  or  potassium 
dichromate, 

Mn207  ->  2Mn02  +  03. 


CHEMICAL  PEODUCTION  31 

As  dehydrating  agent,  sulphuric  acid  is  most  conveniently 
employed  in  the  proportions  of  one  of  potassium  perman- 
ganate to  two  of  sulphuric  acid.  De  la  Coux  ("  L' Ozone,"  p. 
67)  states  that  oxalic  acid  can  be  likewise  employed  in  the 
proportion  of  10  gms.  of  permanganate  to  15  gms.  of  oxalic 
acid,  and  that  90  c.c.  of  oxygen  containing  3  mgrn.  of  ozone 
can  be  obtained  from  this  mixture. 

Satisfactory  yields  of  ozone  may  also  be  obtained  by  the 
cautious  addition  of  barium  peroxide  to  a  solution  of  potas- 
sium permanganate  in  sulphuric  acid,  of  density  1'85. 

By  the  thermal  decomposition  of  the  persulphates,  small 
quantities  of  ozone  are  likewise  disengaged,  Malaquin  ("J. 
Pharm.  Chem.,"  VII,  3,  329,  1911)  gives  the  following  details 
for  the  preparation  of  ozonised  oxygen  by  this  means. 
Twenty  gms.  of  dry  and  freshly  prepared  ammonium  persul- 
phate are  mixed  with  15  gms.  of  nitric  acid  in  a  small  flask ; 
the  air  is  subsequently  displaced  by  carbon  dioxide,  and  the 
mixture  cautiously  raised  to  65°  to  70°  C.  The  reaction, 
which  is  strongly  exothermic,  proceeds  somewhat  vigorously 
when  once  started,  and  the  resulting  oxygen,  after  removal 
of  the  carbon  dioxide,  contains  3  to  5  per  cent,  of  ozone  and 
small  quantities  of  nitrogen. 

Moissan,  in  his  researches  on  the  properties  of  fluorine, 
which  he  isolated  by  the  electrolysis  of  fused  potassium 
hydrogen  fluoride,  noted  that  appreciable  quantities  of  ozone 
were  produced  when  a  few  drops  of  water  were  introduced 
into  an  atmosphere  of  fluorine. 

The  formation  of  ozone  proceeding  according  to  the 
equation : — 

3F2  +  3H20  =  6HF  +  03, 


32  OZONE 

is  especially  marked  at  low  temperatures,  when  the  rate  of 
thermal  decomposition  of  any  ozone  formed  is  considerably 
reduced. 

An  ozone  content  of  upwards  of  14  per  cent,  in  the  oxy- 
gen disengaged  by  means  of  this  reaction  may  be  obtained, 
if  the  temperature  be  maintained  at  0°  C.  De  la  Coux 
("  L'Ozone,"  p.  70)  suggests  that  the  preparation  of  strongly 
ozonised  oxygen,  by  this  method,  offers  some  hope  of  techni- 
cal application. 

Small  quantities  of  ozone  may  likewise  be  obtained  by 
the  thermal  decomposition  of  other  oxygen-containing  salts, 
but  the  quantity  of  ozone  in  the  liberated  oxygen  is  far 
smaller  than  in  the  cases  alluded  to  above.  Thus  Rammels- 
berg  noted  that  ozone  may  be  detected  in  the  oxygen  evolved, 
on  heating  crystallised  periodic  acid  up  to  135°  C. 

Periodic  acid  is  formed  by  the  action  of  iodine  on  an 
aqueous  solution  of  perchloric  acid,  and  can  be  obtained  as 
crystals  containing  two  molecules  of  water.  When  heated 
carefully,  periodic  anhydride  is  formed. 

2(HI04 .  2H20)  ->  I207  +  5H20, 

which  on  continued  heating,  loses  oxygen  to  form  iodic  an- 
hydride : — 

I207->I205  +  02. 

The  iodic  anhydride  itself  suffers  decomposition  into  its  ele- 
ments at  300°  C.,  consequently  the  liberation  of  ozonised 
oxygen  by  decomposition  of  the  crystallised  periodic  acid 
only  takes  place  within  a  somewhat  narrow  temperature 
range.  Aqueous  solutions  of  the  acid  and  its  sodium  salt 
likewise  gradually  acquire  the  smell  of  ozone. 


CHEMICAL  PRODUCTION  33 

0.  Brunck  has  shown  that  commercial  samples  of  potas- 
sium  chlorate  liberate  ozonised  oxygen  during  thermal  de- 
composition, although  purified  samples  fail  to  yield  any  ozone. 
The  yield  of  ozone  is  sensibly  increased  by  the  addition  of 
manganese  dioxide,  thus  equal  weights  of  manganese  dioxide 
and  potassium  chlorate  liberate  0'3  per  cent,  of  the  weight 
of  chlorate  employed  in  the  form  of  ozone.     With  twenty- 
five  times  as  much  manganese  dioxide,  over  1*5  per  cent,  of 
the  weight  of  chlorate  can  be  recovered  in  this  form.     Other 
oxides,  such  as  those  of  copper,  iron  and  zinc  do  not  exhibit 
this  behaviour,  which  appears  to  be  characteristic  of  man- 
ganese dioxide,  although  slight  activity  is  noted  in  the  cases 
of  the  oxides  of  nickel  and  cobalt.     This  is  doubtless  associ- 
ated with  the  property  of  forming  unstable  peroxides,  which 
undergo  secondary  decomposition,  liberating  atomic  oxygen, 
which  can  secondarily  react  with  the  molecular  form  to  pro- 
duce ozone. 

In  the  thermal  decomposition  of  many  metallic  peroxides 
the  presence  of  ozone  may  be  detected  in  the  oxygen  evolved, 
the  yield  of  ozone  being  naturally  greater  in  the  case  of  those 
peroxides  which  undergo  thermal  decomposition  at  relatively 
low  temperatures,  such  as  silver  oxide,  yielding  oxygen 
containing  4  to  5  per  cent,  of  ozone.  Lead  peroxide  and 
mercuric  oxide  are  likewise  capable  of  yielding  small  quantities 
of  ozone. 

If  the  peroxide  of  manganese,  or  cobalt,  or  nickelic  oxide 
be  subjected  to  thermal  decomposition  in  a  current  of  oxygen, 
appreciable  quantities  of  ozone  are  stated  to  be  formed. 

All  these  oxide  decompositions,  resulting  in  the  formation 
of  small  quantities  of  ozone,  may  be  referred  to  chemical 


34  OZONE 

processes  of  activating  atmospheric  oxygen,  whilst  in  the 
case  of  the  decomposition  of  chlorates  and  iodic  anhydride 
these  salts  may  be  regarded  as  convenient  sources  of  oxygen. 
In  the  case  of  the  elements  of  the  first  group  of  the  peri- 
odic table,  namely,  copper,  silver  and  gold,  the  sub  and 
normal  oxides  of  copper,  Cu40,  Cu20,  and  CuO,  are  somewhat 
too  stable,  cupric  oxide  possessing  only  a  small  dissociation 
pressure  at  very  high  temperatures.  The  oxides  of  both  silver 
and  gold,  on  the  other  hand,  dissociate  much  more  readily, 
silver  oxide  possessing  a  dissociation  pressure  equal  to  that 
of  atmospheric  oxygen  at  250°  C.  Silver  peroxide,  Ag202, 
readily  liberates  hydrogen  peroxide  and  oxygen  containing 
ozone  when  dissolved  in  acids.  Mercuric  oxide  closely  re- 
sembles silver  oxide  in  its  chemical  properties. 

The  general  reactions  involved  may  be  expressed  by  the 
following  sequence  of  reactions  : — 

(i)  2M  +  02  =  2MO ; 
(ii)  2MO  =  2M  +  20 ; 
(iii)  20  ^02; 
(iv)  0  +  02->03; 

in  which  by  the  operation  of  the  first  two  reactions  the  oxygen 
molecule  is  temporarily  split  up  into  its  atoms,  the  necessary 
energy  to  perform  this  operation  being  supplied  by  heating  or 
cooling  the  metal  to  form  or  decompose  the  oxide.  The 
atomic  oxygen  so  formed  may  then  instantaneously  recom- 
bine  to  form  molecular  oxygen  or  combine  with  molecular 
oxygen  to  form  ozone. 

OZONE  PBODUCTION  BY  AUTOXIDATION. 
It  had  long  been  known  that  many  substances  when  ex- 
posed to  the  air  undergo  a  process  of  slow  oxidation. 


CHEMICAL   PEODUCTION  35 

Exemplifications  are  found  amongst  the  most  diverse 
types  of  substances  such  as  the  corrosion  or  rusting  of  metals, 
e.g.  zinc,  lead  and  iron,  of  certain  non-metallic  elements  such 
as  sulphur  and  more  especially  phosphorus,  and  in  many 
organic  substances,  such  as  benzaldehyde,  turpentine,  linseed 
oil  and  various  essential  oils,  such  as  oil  of  cinnamon,  lavender 
or  citronella. 

It  was  formerly  thought  that  these  reactions  were  com- 
parable to  the  ordinary  processes  of  oxidation  or  combustion 
except  in  so  far  as  the  reaction  velocity  was  exceedingly  low. 
In  1858,  however,  C.  F.  Schonbein  ("  J.  f.  Prakt.  Chemie," 
73,  99,  1858,  et  seq.,  to  1868)  opened  a  new  and  interesting 
chapter  in  the  theory  of  oxidation  by  showing  that  in  these 
cases  of  slow  oxidation,  for  every  molecule  of  oxygen  consumed 
by  the  substance  undergoing  oxidation  a  molecule  of  oxygen 
was  simultaneously  transformed  to  a  more  active  state.  This 
activated  oxygen  would  then  secondarily  react  to  form  a 
fresh  series  of  new  substances. 

Thus  in  the  presence  of  oxygen,  ozone  could  be  produced  ; 
in  the  presence  of  water  as  in  the  wet  oxidation  of  the  metals, 
an  amount  of  hydrogen  peroxide  was  produced  equivalent  to 
the  quantity  of  metal  oxidised.  In  the  presence  of  other 
oxidisable  substances  the  active  oxygen  can  oxidise  them, 
frequently  bringing  about  oxidations  which  cannot  be  accom- 
plished by  ordinary  atmospheric  oxygen  ;  thus  indigo  is 
converted  into  isatin  during  the  autoxidation  of  palladium 
hydride  or  benzaldehyde. 

The  quantitative  relationship  between  the  production  of 
active  oxygen  and  the  quantity  of  substance  undergoing  the 
process  of  slow  oxidation  was  shown  by  Schonbein  to  hold  in 


36  OZONE 

the  case  of  the  wet  oxidation  of  the  metals  by  an  estimation 
of  the  quantity  of  hydrogen  peroxide  simultaneously  produced. 
An  interesting  confirmation  of  Schonbein's  views  was 
afforded  by  A.  Genthe's  investigations  on  the  drying  of  linseed 
oils  ("Zeit.  Angew.  Chem.,"  19,  207,  1906).  It  had  been 
previously  shown  by  Lippert  ("Zeit.  Angew.  Chem./'  n, 
412,  1898)  and  Wegen  ("  Chem.  Kev.  f.  fett.  u.  Harz.," 
4,  345,  1899)  that  the  drying  of  linseed  oil  was  virtually 


Time  in  Hours 
FIG.  3. 

a  process  of  atmospheric  oxidation.  Genthe  examined  the 
reaction  velocity  of  this  process  of  oxidation  and  found  that 
the  time-increase  of  weight  curves  for  the  drying  of  a  thin 
film  of  linseed  oil  exhibited  the  sinuous  character  of  an  auto- 
catalytic  reaction. 

It  will  be  observed  that  the  initial  rate  of  dryings  increases 
somewhat  slowly  with  the  time ;  as,  however,  the  quantity  of 
autocatalyst  increases  simultaneously  the  reaction  proceeds 
at  an  ever-increasing  velocity  and  only  begins  to  sink  when 


CHEMICAL  PRODUCTION  37 

the  quantity  of  oil  remaining  to  be  oxidised  diminishes  in 
amount. 

It  had  therefore  to  be  assumed  that  in  the  process  of  dry- 
ing, a  catalyst  was  simultaneously  formed,  thus  if  a  and  b  be 
the  initial  concentrations  of  the  linseed  oil  and  catalyst,  then 
the  rate  of  oxidation  of  the  oil  after  a  time  t  will  be  given  by 
the  equation : — 

/J<r 

I?  =  K(o  -  x)(b  +  x). 

Genthe,  in  fact,  showed  by  his  experiments  on  reaction  velocity 
that  there  was  a  quantitative  relationship  between  the  quantity 
of  linseed  oil  oxidised  and  the  quantity  of  autocatalyst  simul- 
taneously produced. 

Houzeau  (1860),  Genthe  (loc.  cit.),  Hazura  ("  Zeit.  Angew. 
Chem.,"  i,  312,  1888),  Kissling  ("  Zeit  Angew.  Chem.,"  4, 
395,  1891)  and  Friend  ("  Proc.  Paint  and  Varnish  Soc.,"  1914) 
all  showed  that  the  autocatalyst  was  an  unstable  peroxide, 
since  it  liberated  iodine  from  potassium  iodide  and  showed 
the  other  reactions  of  a  peroxide  and  a  similar  catalytic 
acceleration  could  be  produced  by  the  addition  of  ozone, 
benzoyl  peroxide,  oxidised  turpentine  or  ether,  to  the  linseed 
oil.  It  is  still  a  matter  of  uncertainty  as  to  the  nature  of 
this  catalytic  peroxide.  Houzeau  was  of  the  opinion  that  it 
was  dissolved  ozone,  whilst  other  investigators  support  the 
theory  of  an  unstable  peroxide  of  linoleic  acid,  similar  in 
character  to  benzoyl  peroxide.  It  appears  probable  that  small 
quantities  of  ozone  can  be  isolated  from  turpentine,  oil  of 
cinnamon  and  other  essential  oils,  undergoing  atmospheric 
oxidation,  but  that  most  of  the  activated  oxygen  is  absorbed 
or  combines  with  part  of  the  substance  to  form  a  peroxide. 


38  OZONE 

Jorrisen  and  Keicher  ("Ber.,"  30,  1451,  1897;  "  Zeit. 
Anggw.  Chem.,"  22,  6829,  and  "  Chem.  Zeit.,"  26,  99,  1902) 
showed  that  ozone  could  be  formed  during  the  reduction  of 
certain  oxidising  acids,  such  as  chromic  acid,  attributed  to 
the  intermediary  formation  of  an  unstable  peroxide  with  its 
subsequent  decomposition  :— 

Ov          O— 0— C 
Cr03  +  (COOH)2  ->  H20  +      >Cr/  | 

or      xo— o— c 

Kelatively  large  quantities  of  ozone,  however,  are  produced 
in  the  autoxidation  of  phosphorus,  and  in  view  of  the  con- 
veniences of  this  method  of  preparation  the  following  details 
may  be  given  :  A  rapid  current  of  air  is  passed  through  a 
bottle  containing  sticks  of  yellow  phosphorus,  moistened  with 
a  dilute  sulphuric  acid  acidified  solution  of  potassium  per- 
manganate or  bichromate.  The  reaction  proceeds  but  slowly 
at  6°  C.,  whilst  the  optimum  temperature  is  stated  to  be  24°  C. 
Under  reduced  pressure  the  reaction  still  proceeds  at  0°  C. 
As  is  well  known,  pure  oxygen  reacts  but  slowly  with  phos- 
phorus except  under  reduced  pressure.  A  20  per  cent,  mix- 
ture of  oxygen  in  hydrogen  is  particularly  efficacious  for  the 
production  of  ozone,  but  the  phosphorus  is  liable  to  become 
extremely  hot,  with  the  attendant  risk  of  explosion.  Small 
quantities  of  hydrogen  peroxide  are  simultaneously  produced. 
From  time  to  time  the  stale  phosphorus  should  be  re-fused 
in  order  to  remove  the  superficial  layer  of  phosphoric  acid 
which  causes  a  diminution  in  its  activity. 

We  have  already  noted  that  the  theory  of  Brodie,  developed 
by  Clausius  and  van't  Hoff,  postulating  the  existence  of  two 
forms  of  oxygen  : — 


CHEMICAL   PKODUCTION  39 

fO  (ozone), 

o,   + 

\0  (antozone), 

was  supported  by  Schonbein  as  a  result  of  these  researches. 
According  to  this  hypothesis  all  processes  of  autoxidation  are 
dual  in  character,  since  two  substances  must  simultaneously 
undergo  oxidation.  Engler  ("  Kritische  Stiidien  liber  die 
Autoxydationsvorgange,  Braunschweig,"  1903)  has  attempted 
to  distinguish  between  these  by  terming  the  substance  under- 
going oxidation  the  autoxidiser,  and  the  substance  simultan- 
eously oxidised  the  acceptor.  Clearly,  either  the  ozonic  or 
antozonic  form  of  active  oxygen  may  react  with  the  autoxi- 
diser or  the  acceptor  to  produce  "  ozonides  "  or  "  antozon- 
ides  " ;  thus  ozone  is  an  "  ozonide,"  and  phosphoric  acid  the 
"  antozonide  "  produced  in  the  autoxidation  of  phosphorus. 
Van't  Hoff  (loc.  cit.)  noted  that  the  presence  of  excess  of 
"ozonide"  prevented  the  formation  of  the  antozonide,  and 

thus    it    necessarily   followed   that    the   primary    reaction, 

+ 

02  5  0  +  0  was  reversible  in  character.  Since  the  antozon- 
ide, viz.  phosphoric  acid,  is  not  volatile  the  escaping  ozonic 
form  of  active  oxygen  or  ozone  should  be  electrically  charged. 
A  search  for  this  electrically  charged  form  of  oxygen  in  air 
which  has  been  passed  over  phosphorus  has  yielded  conflicting 
results.  Elster  and  Geitel  ("Phys.  Zeit.,"  16,  321,  1890; 
"Wied.  Ann.,"  39,  457,  1903)  noted  that  air  thus  treated 
was  electrically  conducting  (see  also  Matteuci,  "  Enc.  Brit.," 
VIII,  622,  1855;  Naccari,  "Atti.  della  Scienze  de  Torino," 
XXV,  p.  252 ;  J.  Joubert,  "  These  sur  la  Phosphorescence 
du  Phosphore,"  1874;  T.  Evan,  "Phil.  Mag.,"  5,  38,  512, 
1897;  J.  Chappuis,  "Bull.  Soc.  Chem.,"  2,  35,  419,  1881). 


40  OZONE 

However,  Goekel  ("Phys.  Zeit.,"  IV,  1903)  showed  that  this 
conductivity  was  not  due  to  the  presence  of  ozone  which 
could  be  absorbed  without  destroying  the  conductivity. 

Barus  ("Washington,"  1901),  Harms  ("Phys.  Zeit.," 
IV,  in,  1902),  and  Bloch  ("Ann.  de  Chemie  et  de  Phys.," 
n,  25,  1905)  likewise  showed  that  the  conductivity  was  not 
due  to  the  presence  of  ozonic  oxygen  or  charged  ionic  oxygen, 
but  to  oxides  of  phosphorus  collected  round  charged  nuclei, 
forming  aggregates  of  fairly  large  dimensions  (r  =  10~6  cm.), 
while  the  actual  number  of  charged  gas  ions  observed  fell  far 
short  of  the  stoichiometric  ratio,  oxygen  absorbed — oxygen 
activated  :  1  : 1,  as  postulated  by  the  hypothesis.  A.  Blanc 
("  C.B.,"  95,  2,  1170,  1911)  showed  the  existence  of  both 
positive  and  negative  ions,  the  production  of  which  was  ac- 
companied by  the  formation  of  white  fumes.  The  production 
of  these  gas  ions  was  accelerated  by  allowing  the  process  of 
oxidation  to  take  place  in  an  electric  field. 

K.  Przibram  ("  Akad.  Wiss.  Wien.,  Ber,"  126,  247,  1912) 
showed  that  the  charge  on  each  gas  ion  was  approximately 
6  x  10~10  E.S.  units,  and  that  1'43  x  10~6  gms.  of  phosphorus 
in  the  form  of  phosphoric  acid  was  associated  with  each  E.S. 
unit,  and  1-3  x  10~15gms.  of  phosphorus  in  each  particle.  A. 
Blanc  ("C.R.,"  158,  1492,  1911)  claims  to  have  discovered 
the  existence  of  a  radiation  emitted  during  the  autoxidation 
of  phosphorus  like  7  rays,  extremely  soft  and  not  corpuscular 
in  character.  They  are  easily  absorbed  by  air.  E.  Hoppe 
Segler  ("Zeit.  Physiol.  Chem.,"  2,  23, 1878),  and  Baumann, 
adopted  the  same  hypothesis  as  Schonbein,  but  substituted 
the  somewhat  less  confusing  term  of  "  nascent "  oxygen  for 
Schonbein's  "  ozone"  and  "  antozone ".  It  is,  however, 


CHEMICAL  PRODUCTION  41 

evident  that  the  case  for  the  existence  of  charged  ions  of 
atomic  oxygen  of  opposite  electric  sign  is  not  strongly  sup- 
ported by  the  investigators  cited  above,  although,  as  we  have 
observed,  the  existence  of  uncharged  atomic  oxygen  is  a 
plausible  hypothesis. 

M.  Traube  ("  Ber,"  15,  663, 1882,  and  1471,  1843;  "  Ges- 
ammelte  Abhandlungen,"  Berlin,  1899),  A.  Bach  ("  C.K.," 
126,  2957,  1897),  and  C.  Engler  and  V.  Wild  ("Ber.,"  30, 
1667,  1897),  and  others,  on  the  other  hand,  developed  the 
theory  of  an  intermediate  compound. 

Thus,  according  to  Traube,  the  presence  of  water  is 
necessary  for  all  these  processes  of  slow  combustion,  a  point 
of  view  strongly  supported  by  the  researches  of  Mrs.  Fulhame 
("An  Essay  on  Combustion,"  London,  1794),  B.  Baker, 
H.  B.  Dixon  ("Phil.  Trans.,"  175,  630,  315,  4795,  1896),  and 
H.  E.  Armstrong  (B.A.  Eeports,  "  Proc.  Eoy.  Soc.,"  4°»  287, 
1886) ;  the  primary  reaction  taking  place  is  the  formation  of 
an  oxide  and  hydrogen  peroxide  according  to  the  equation  :— 

M  +  02  +  H20  =  MO  +  H202. 

The  formation  of  Schonbein's  ozonides  must  thus  be  con- 
sidered as  due  to  secondary  reactions  between  the  hydrogen 
peroxide  and  the  acceptor,  in  some  cases  exceedingly  improb- 
able reactions.  Thus,  it  is  difficult  to  imagine  the  formation 
of  ozone  by  the  action  of  oxygen  in  a  dilute  solution  of  hy- 
drogen peroxide  according  to  the  following  equations  : — 

P2  +  02  +  H20  =  P20  +  H202 
H202  +  02          =  H20  +  03, 

although  it  is  stated  that  by  the  distillation  of  strong  solutions 
of  hydrogen  peroxide  in  vacuo  ozone  can  be  obtained. 


42  OZONE 

Bach's  modification  of  the  hypothesis  embodied  the  con- 
ception of  the  formation  of  an  unstable  intermediary  peroxide 
prior  to  decomposition  into  an  oxide  with  simultaneous  oxi- 
dation of  the  acceptor  thus : — 

7° 
M  +  02  ->  M<;   I 


\ 


o 


O 

M/  |   +  A  ->  MO  +  AO 
\0 

Engler  and  Wild  ("Ber.,"  30,  1669,  1897),  and  Ostwald 
("Zeit.  Phys.  Chem.,"  30,  250,  1900)  applied  Bach's  concep- 
tion of  the  mechanism  of  processes  of  autoxidation  to  the 
case  under  consideration,  i.e.  the  formation  of  ozone  by  the 
autoxidation  of  phosphorus. 

Engler  and  Wild   suggested  the  following  sequence  of 
reactions  :  — 


2P  +  o2  -  p    i 
xo 


| 


02  =  P20  +  Os, 


whilst  Ostwald  suggested  that  a  still  higher  oxidation  form 
of  phosphorus  was  produced  as  an  unstable  intermediate 
product  :  — 


2P  +  200  = 


'0 


03, 


xo 

thus  giving  the  stoichiometric  ratio,  P  :  03 : :  2  : 1  which  was 
actually  obtained  by  van't  Hoff. 


CHEMICAL  PEODUCTION  43 

The  fundamental  difficulty  inherent  in  the  peroxide  theory 
was  raised  many  years  ago  in  a  remarkable  essay  by  G.  Live- 
ing  ("  Chemical  Equilibrium,  the  Eesult  of  the  Dissipation 
of  Energy,"  Cambridge,  1885).  It  is  evident  that  the  per- 
oxide formed  must  be  endowed  with  available  energy 
greater  than  that  possessed  by  atmospheric  oxygen,  and  it  is 
thus  difficult  to  explain  its  formation  as  the  result  of  an  exo- 
thermic reaction  from  phosphorus  and  air.  It  is  usually 
assumed  that  the  chemical  energy  of  one  system  is  not  avail- 
able for  another  totally  different  system,  i.e.  that  the  energy 
liberated  during  the  oxidation  of  phosphorus  is  dissipated 
through  the  system  in  the  form  of  heat.  Liveing  introduced 
the  interesting  hypothesis,  that  in  certain  cases,  the  liberated 
energy  was  not  dissipated  in  this  form,  but  stored  up  in  one, 
or  at  least  a  very  few,  neighbouring  molecules,  which  would 
thus  be  endowed  with  a  great  deal  of  energy.  Thus  we  can 
imagine  a  simple  transfer  of  energy  from  one  set  of  reacting 
molecules  to  another  set,  molecule  to  molecule,  and  thus  ex- 
plain the  simultaneous  equivalent  formation  of  an  endo- 
thermic  compound,  during  a  strongly  exothermic  reaction. 


CHAPTEK  IV. 

THERMAL  PRODUCTION. 

SINCE  the  formation  of  ozone  is  a  strongly  endothermic  re- 
action, we  would  expect,  as  pointed  out  by  Nernst  ("  Zeit. 
Elektrochem.,"  9,  891,  1903),  that  the  equilibrium:— 

302  ^  203 

would  shift  over  from  left  to  right  with  elevation  of  the  tem- 
perature. An  approximate  idea  of  the  ozone  concentration 
in  equilibrium,  with  oxygen  at  various  temperatures,  can  be 
obtained  by  two  independent  methods ;  from  a  calculation  of 
the  value  of  K,  the  equilibrium  constant  by  means  of  the 
Nernst  heat  theorem,  as  well  as  from  the  observed  measure- 
ments of  the  electromotive  force  of  the  ozone/oxygen  cell. 

According  to  the  Nernst  heat  theorem  ("  Applications  of 
Thermodynamics  to  Chemistry,"  Sillman  Lectures,  1906), 
Griineisen  ("Ann.  Phys.,"  26,  401,1912),  Pollitzer  ("Berech- 
nung  Chemischer  Amnitaten  nach  dem  Nernstchen  Warme- 
theorem.  Ahrens  Sammlung.  Encke,"  1912),  a  simple  ex- 
pression for  the  equilibrium  constant  K,  in  homogeneous  gas, 
reactions  can  be  obtained  in  terms  of  known  quantities, 
provided  two  basic  assumptions  are  made,  firstly,  that  the 
entropy  of  a  condensed  chemically  homogeneous  system 
vanishes  at  the  absolute  zero,  and  secondly,  that  the  specific 
molecular  heat  of  a  gas  can  be  approximately  evaluated 

from  a  simple  expression : — 

(44) 


THEEMAL   PEODUCTION  45 

Cp  =  3-5  +  2/3T, 

where  Cp  is  the  molecular  specific  heat  and  /3  a  constant. 
Making  these  two  assumptions  (and  much  experimental  evi- 
dence has  been  adduced  to  prove  the  validity  of  the  Nernst 
heat  theorem),  it  is  easily  shown  (loc.  cit.}  that  the  equilibrium 
constant  can  be  obtained  from  the  following  equation  :  — 


logloK  =         1T  +  1-75  Sv  log  T  -  * 

where  Q  is  the  heat  of  reaction, 

and  vaa  +  vjb  —  vcc  +  vdd 


va,  vb,  vc,  vd  being  the  number  of  molecules  of  such  species, 
a,  b,  c,  d,  reacting,  Ca,  C6,  Cc,  Cd  being  the  so-called  chemical 
constants  of  each  element  or  compound  reacting, 


applying  this  equation  to  the  case  under  consideration,  viz. : — 

203  =  302  +  68,000  calories. 
SV  =  [2]  -  [3]  =     -  1. 

Svc  =  (2  x  3)  -  (3  x  2-8)  =   -  2'4. 

Information  as  to  the  specific  heat  of  ozone  is  at  present  not 
available,  but  with  Pollitzer,  we  may  assume  that  its  value  is 
not  very  different  from  that  of  the  other  triatomic  gases,  such 
as  sulphur  dioxide,  which  has  a  molecular  specific  heat  of 
lO'o,  then 

^v/3  =  — - —  -  =  0'005. 

Hence 

68,000  0-005T 

log  KP  =   -  A^TTW  ~  l'15  log  T  +  -I^rT  ~  2'4- 


46  OZONE 

If  x  be  the  fraction  of  oxygen  converted  into  ozone  at  equi- 
librium, then  since  :  — 

ff203  _  if 
*     P30*     P 
when  x  is  small,  and  p  is  the  total  gas  pressure, 

oo  ooo 

log  z  =  -  -  0-875  log  T  +  0-0005T  -  1-2  +  i  log*, 


from  this  equation  the  values  of  x,  and  thus  the  percentage 
of  ozone  present  in  oxygen  at  equilibrium  at  various  tempera- 
tures, can  be  calculated  thus  :  — 

•p  .  -i 

T°  centigrade.  P  =  10,000  atmospheres. 


1000°  10-8  10-6 

2000°  10-5  10-3 

3000°  10-3  10-1 

It  will  be  noted  that  increase  of  pressure  greatly  favours  high 
equilibrium  amounts  of  ozone. 

Somewhat  higher  values  for  the  equilibrium  amounts  at 
various  temperatures  are  arrived  at  by  means  of  evaluating 
the  magnitude  of  the  potential  difference  between  the  ozone 
and  oxygen  electrode  (see  p.  63). 

The  potential  difference  between  two  platinum  electrodes 
immersed  in  the  same  electrolyte,  one  saturated  with  oxygen 
under  a  pressure  TT,  and  the  other  with  ozone  at  the  same 
temperature  and  pressure  irlt  is  given  by  the  equation  :  — 

TT  TT  BT    1  7T 

V-V.-aj-lpg-, 

where  V*  represents  the  value  observed  of  the  potential  differ- 
ence 02/03,  at  one  atmosphere,  under  conditions  of  reversi- 
bility. 


THERMAL   PRODUCTION  47 

There  exists  considerable  uncertainty  as  to  the  values  of 
V0,  thus  Luther  and  Inglis  ("  Zeit.  Phys.  Chem.,"  43,  203, 
1903)  obtained  the  value  V0  =  -  0'736  volts  ;  Nernst  ("  Zeit. 
Elektrochem."  9,  891,  1903),  V0  =  -  0'57;  Fischer  and 
Brauner  ("  Ber.,"  39,  3631,  1906)  -  0'64,  and  -  0'46  volts. 

Calculation  from  the  value  of  K  obtained  by  the  Nernst 
heat  theorem  as  follows,  yields  the  value  -  0'83  volts  :  — 

68,000  0-005T      0  . 

logK,=    -    -  -  1-75  log  T  +  - 


273  x  4-571  or/     34,000 


r/ 

L( 


o  , 

2+  +  °'875 


••     «"  4  x  23,046 

-  0-0005T  +  1-2)]  =  -  0-83  volts. 

It  is  evident  that  if  the  pressures  of  oxygen  and  ozone  are 
so  adjusted  that  the  cell  shall  have  zero  E.M.F.,  this  will 
represent  the  equilibrium  conditions  between  oxygen  and 
ozone. 

ET  .      TT 
V'  -  2F  IQS 


,          7T 

lo 


2FV 

where  A  is  a  constant         =          ° 


The  values  of  the  percentage  of  ozone  in  equilibrium  with 
oxygen  under  one  atmosphere  pressure  at  various  tempera- 
tures, as  calculated  from  the  above  equation  for  the  two 
extreme  values  of  V0,  i.e.  V0  =  -  0'83  volts  and  V0  =  -  0'46 
volts,  are  given  in  the  following  columns  :  — 


48 


OZONE 


fcentage  of 
Ozone. 

Equilibrium 
Temperature. 

Equilibrium 
Temperature. 

V0  =  -  0-83  volts. 

V0  =  -  0-46  volts. 

10 

7900° 

.     4400° 

1 

3950° 

2200° 

O'l 

2630° 

1460° 

o-oi 

1970° 

1100° 

o-ooi 

1580° 

880° 

It  will  be  noted  that  in  this  calculation  the  change  in 
specific  heats  of  the  gases  with  alteration  in  the  temperature 
have  been  neglected,  consequently  the  values  are  probably 
somewhat  too  high,  and  in  view  of  the  wide  discrepancies 
between  the  two  values  an  experimental  redetermination  of  the 
oxygen  ozone  electromotive  force  would  be  very  desirable. 

From  the  foregoing  considerations  we  must  conclude  that 
the  quantity  of  ozone  in  equilibrium  with  atmospheric  oxygen 
at  normal  temperature  and  pressures  is  scarcely  detectable, 
and  that  if  present  in  measurable  concentrations  under  these 
conditions  true  equilibrium  does  not  obtain.  Further  ap- 
preciable quantities  of  ozone  may  be  formed  at  high  tempera- 
tures and  should  be  capable  of  detection  and  estimation. 

The  estimation  of  ozone  in  gases  which  have  been  heated 
up  to  a  high  temperature,  is  somewhat  difficult  owing  to  the 
fact  that  ozone  rapidly  undergoes  decomposition  to  its 
equilibrium  concentration  during  the  cooling  of  the  gas  mix- 
ture. Dewar  ("  Year  Book  E.I,"  559,  1887)  inferred  that 
ozone  had  two  centres  of  stability,  one  above  the  melting-point 
of  platinum  and  the  other  at  ordinary  temperatures,  whilst 
between  these  temperatures  ozone  is  decomposed.  Chapman 
and  Jones  ("Trans.  Chem.  Soc.,"  97,  2463,  1913,  and  99, 
1811,  1911)  showed  that  at  100°  C.  nearly  75  per  cent,  of 


THERMAL   PRODUCTION  49 

the  ozone  in  excess  of  the  almost  undetectable  equilibrium 
amount  is  destroyed  in  half  an  hour,  whilst  at  300°  C.  it  is 
practically  instantaneous. 

The  explanation  of  these  observations  of  Dewar  is  that 
the  velocity  of  decomposition  of  ozone  from  high  temperatures 
down  to  100°  C.  is  extremely  rapid,  whilst  below  100°  C.  the 
velocity  of  decomposition  becomes  markedly  slower  and  the 
ozone  appears  to  be  stable  on  account  of  the  extremely  low 
velocity  of  decomposition,  the  equilibrium  being  "  frozen  ". 

The  earlier  experiments  of  Schonbein  (Engler,  "Hist. 
Kritic  Studienuber  Ozon,"  Halle,  1879),  Bottger  ("Ann.  der. 
Chem.,"  125,  34, 1861),  Pincus  ("Pogg.  Ann.,"  144,  480, 1871), 
Struve  ("  Jahresber.  f.  Chem.,"  199, 1870),  and  Traube  ("  Ber.," 
1 8,  1894,  1885),  all  indicated  that  small  quantities  of  ozone 
were  formed  during  the  combustion  of  hydrogen.  Similar 
results  were  obtained  by  the  combustion  of  coal  gas,  notably 
by  Than  ("Jour.  f.  Prakt.  Chem.,"  2,  1415,  1870),  Loew 
("Zeit.  f.  Chem.,"  65,  1870),  Ilosvay  ("  Bull.  Soc.  Chem.,'J  3, 
2,  360,  1881),  whilst  Zenghilis  demonstrated  the  presence  of 
ozone  ("  Zeit.  Phys.  Chem.,"  46,  1903)  in  the  oxygen  which 
had  been  raised  to  a  high  temperature  by  the  combustion  of 
aluminium  powder. 

Contemporary  with  these  investigations,  others  were 
carried  out  on  the  synthesis  of  ozone  by  merely  heating  air 
or  oxygen  by  means  of  an  independent  source  of  heat,  as  the 
objection  may  be  raised  to  the  former  experiments  that  the 
ozone  may  have  been  formed  by  chemical  activity  (see  Ch.  III). 

As  catalytic  agent  hot  platinum  or  silver  was  generally 
employed,  notably  by  V.  der  Willigen  ("Pogg.  Ann.,"  98, 

511,  1831),  Meissner  ("Neue   Untersuchungen  iiber  Elekt. 

4 


50  OZONE 

Sauerstoff,"  Gottingen,  1863),Leroux  ("C.R,"  50,  691, 1860), 
Troost  and  Hautefeuille  ("  C.E.,"  84,  946,  1877),  Helmholtz 
("  Wied.  Ann.,"  32,  18,  1887),  and  Elster  and  Geitel  ("  Wied. 
Ann.,"  39,  912,  1890).  Troost  and  Hautefeuille  (loc.  cit.) 
detected  the  presence  of  ozone  in  oxygen  which  had  been 
heated  up  to  only  1400°  C.  The  oxygen  was  heated  by  passage 
through  a  porcelain  tube  maintained  at  1400°  C.  In  order  to 
effect  the  rapid  cooling  of  the  gas  a  water-cooled  silver  tube 
passed  along  the  axis  of  the  porcelain  tube.  Samples  of  oxygen 
were  drawn  from  the  annular  space  between  the  porcelain 
and  the  silver  tubes  by  aspiration  through  a  small  side  tube 
which  passed  into  the  silver  tube  itself. 

J.  Clements,  at  Nernst's  instigation  in  1904  ("  Ann.  Phys.," 
14,  334,  1904),  reviewed  the  whole  subject  and  came  to  the 
conclusion  that  many  of  the  previous  observers  had  mistaken 
oxides  of  nitrogen  or  hydrogen  peroxide  for  ozone.  By  the 
use  of  Arnold  and  Mentzel's  tetramethyl  base  paper  ("  Ber.," 
35,  1324,  and  2902,  1902),  which  is  diagnostic  for  ozone, 
Clements  showed  that  ozone  could  be  detected  in  the  hot 
gases  from  a  Bunsen  burner,  but  only  in  very  small  quantities. 
(Tetramethyl  base  paper  is  stated  to  be  sensitive  to  O'OOl  per 
cent,  ozone.) 

By  the  passage  of  ozonised  air  containing  1  per  cent,  of 
ozone  over  a  glowing  Nernst  filament  maintained  at  1000°  C. 
at  various  speeds  up  to  80  cms.  per  second,  he  showed  that 
the  rate  of  decomposition  of  ozone  was  extremely  rapid,  1 
per  cent.  03  sinking  to  O'OOl  per  cent,  in  0*007  seconds. 

In  passing  air  over  a  Nernst  glower  even  when  heated  up 
to  3000° C.,  only  oxides  of  nitrogen  were  obtained,  a  result 
which  was  confirmed  by  Kossi  ("  Gaz.  Chim.  Ital.,"  35*  1>  89, 


THERMAL   PEODUCTION  51 

1905).  Clements,  however,  confirmed  the  formation  of  ozone 
by  spark  discharge,  and  Erode  ("  Zeit.  f.  Elektrochem.," 
u,  754,  1905)  observed  the  formation  of  ozone  in  the  high 
voltage  arc  at  4000°  C.  Ozone  formation  in  these  latter 
cases  may,  however,  be  attributed  to  the  action  of  ultra- 
violet light  (see  p.  79)  or  electrical  ionisation,  and  not  to  the 
result  of  the  establishment  of  a  purely  thermal  equilibrium. 

Fischer  and  his  co-workers  Brauner  ("Ber.,"  39,  940, 
1906),  Marx  ("  Ber.,"  39,  3631, 1906,  40,  443, 1907),  and  Wolf 
("  Ber.,"  44,  2956, 1911)  realised  from  Troost  and  Hautefeuille 
and  Clements'  experiments  that  rapid  cooling  was  essential  to 
preserve  any  ozone  which  might  be  formed,  from  secondary 
thermal  decomposition,  during  the  process  of  cooling  to  the 
point  where  the  reaction  of  decomposition  was  negligibly 
small.  They  showed  by  a  series  of  interesting  researches 
that  ozone  could  be  formed  by  thermal  methods  provided 
that  the  right  conditions  were  obtained. 

It  was  shown  that  ozone  could  be  produced  by  plunging 
a  jet  of  burning  hydrogen  or  acetylene  into  liquid  air,  the 
ozone  formed  by  the  local  heating  being  thus  preserved  by 
rapid  cooling. 

When  liquid  oxygen  was  substituted  for  liquid  air,  large 
quantities  of  ozone  were  formed  and  the  liquid  rapidly  be- 
came dark  blue  in  colour,  similar  to  ammoniacal  solutions  of 
copper  salts.  When  an  electrically  heated  platinised  wire 
was  immersed  in  liquid  oxygen,  practically  no  ozone  was 
formed.  This  somewhat  unexpected  result  was  shown  to  be 
due  to  the  fact  that  the  platinum  wire  in  the  liquid  oxygen 
underwent  dispersion  into  colloidal  particles  which  catalytic- 
ally  accelerated  the  decomposition  of  the  ozone.  A  bright 


52  OZONE 

platinum  wire,  protected  from  dispersion  by  a  coating  of  the 
oxides  of  zirconium  and  yttrium,  gave  a  uniform  yield  of 
ozone.  Using  a  glowing  Nernst  filament  in  liquid  air  and 
oxygen,  ozone  was  produced  and  no  oxides  of  nitrogen.  The 
yield  of  ozone  rose  steadily  with  increasing  temperature,  the 
maximum  equilibrium  amount  being  1*5  per  cent,  by  weight 
at  an  approximate  temperature  of  2200°  C.,  a  figure  which 
bears  a  strikingly  close  agreement  to  that  obtained  by  calcula- 
tion from  the  electrometric  force  of  the  oxygen  ozone  cell. 
Taking  V0  =  -  0*46  volts,  this  corresponds  to  an  equilibrium 
amount  of  1'5  per  cent,  by  weight  at  2048°  C. 

Utilising  an  arc  in  liquid  air  a  mixture  of  ozone  and  nitro- 
gen peroxide  was  obtained  which  frequently  exploded  when 
attempts  were  made  to  separate  the  residual  oxygen 
(B.P.08  -  120°C.,02  -  182-7°). 

The  maximum  yield  of  ozone  obtained  by  means  of  a 
glowing  Nernst  filament  in  liquid  oxygen  was  40  mgms.  in 
twenty-five  minutes  with  a  current  consumption  of  0'25 
amperes  at  100  volts  equal  to  a  yield  of  3*5  grams  of  ozone 
per  kilowatt  hour. 

Fischer  having  thus  demonstrated  the  thermal  production 
of  ozone  with  the  aid  of  liquid  air,  proceeded  to  extend 
Clements'  experiments  on  the  production  of  ozone  by  passing 
air  at  a  high  flow  rate  over  a  glowing  Nernst  filament.  We 
have  already  noted  that  Clements  was  not  able  to  detect  the 
thermal  synthesis  of  ozone  with  air-flows  of  linear  speeds, 
up  to  80  cms.  per  second.  Fischer  and  Marx,  using  much 
higher  velocities,  showed  that  ozone  was  formed  under  these 
conditions  and  obtained  a  series  of  interesting  results  by 
studying  the  conditions  of  oxidation. 


THEEMAL  PEODUCTION  53 

When  dry  air  is  passed  over  a  glowing  Nernst  filament 
two  endothermic  compounds  may  be  formed,  viz.  nitric  oxide 
and  ozone.  If  moist  air  be  employed  the  presence  of  hydro- 
gen peroxide  may  likewise  be  demonstrated.  The  thermal 
equilibrium  concentrations  of  nitric  oxide,  formed  according 
to  the  reaction 

N2  +  02  ^  2NO, 

have  been  obtained  by  Nernst  ("  Gottingen,  Nachricht.," 
p.  261,  1904)  and  Jellinek  and  Finck  ("  Zeit.  Anorg.  Chem.," 

49,  212,  224,  1906)  and  are  given  in  the  following  table  :— 

Temperature.  Per  Cent.  Concentration 

°C.  of  NO  in  Air. 

1811 0-35 

2033 0-67 

2580 2-02 

2675 2-35 

3200 5-0 

Jellinek  (loc.  cit.)  likewise  calculated  the  rate  of  decomposition 
of  nitric  oxide  to  its  equilibrium  value  at  various  temperatures. 
Nitric  oxide  in  this  respect  is  markedly  different  from  ozone 
since  it  is  relatively  much  more  stable  at  high  temperatures ; 
the  times  for  the  decomposition  of  half  a  given  volume  of  NO 
to  nitrogen  and  oxygen  at  atmospheric  pressure  are  as 
follows  : — 

Temperature.  Time  in  Minutes  to  Effect 

°C.  50  Per  Cent.  Decomposition. 

900 7-35  x  103 

1100 5-80  x  102 

1300 4-43  x  10 

Similar  calculations  can  be  made  for  hydrogen  peroxide.  We 
should  therefore  expect  that  with  relatively  low  velocities  of 


04  OZONE 

air-flow  over  the  glowing  filament,  only  oxides  of  nitrogen 
should  be  obtained  ;  with  higher  velocities  mixtures  of  ozone 
and  nitric  oxide,  and  with  very  high  velocities  only  ozone, 
since  the  rate  of  formation  of  nitric  oxide  as  well  as  its  rate  of 
decomposition  is  sensibly  less  than  that  of  ozone.  As  will  be 
observed  from  the  following  figures  obtained  by  Fischer  and 
Marx,  this  theoretical  deduction  is  amply  confirmed  by  ex- 
periment : — 


Flow  Rate  in 
Metres  per  Sec. 

2-8 


Reaction  to  Tetra- 
methyl  Base  Paper. 

N02 
5'2        .......          N02  +  03 

5-5        ........     O3  +  little  N0e 

6-2        ........     O3  +  trace  N02 

At  flow  rates  exceeding  30  metres  per  second  no  oxides  of 
nitrogen  could  be  detected  in  the  air  but  ozone  was  always 
present.  The  yield  of  ozone  was  influenced  both  by  the 
temperature  of  the  glowing  filament  as  well  as  by  the 
linear  velocity  of  the  gas  flow,  as  shown  in  the  appended 
tables  :  — 


Temperature  of 
Nernst  Glower 
in°C. 

Weight  Per  Cent. 
O3  in  Air. 

Weight  Per 
Cent.  03  in 
Oxygen. 

Air-flow  44  Meters 
Per  Sec.,  Oms.  O3 
PerKiv.Hr.    ' 

1479 

0-0029 

0-0126 

0-34 

1598 

0-0088 

0-0382 

0-80 

1667 

0-0118 

0-0512 

0-90 

1772 

0-0166 

00720 

1-07 

1822 

0-0218 

0-0916 

1-15 

1889 

0-0238 

0-1032 

1-19 

1930 

0-0293 

0-126 

1-30 

THERMAL   PRODUCTION 


55 


Weight  Per  Cent. 

Velocity  of  Air 

03in 

Gms.  03 
Per  Kw  Hr 

Air. 

Oxygen. 

at  1800°  C. 

30 

0-011 

0-049 

0-29 

44 

0-019 

0-082 

0-85 

57 

0021 

0-091 

1-15 

63 

0-019 

0-080 

1-28 

75 

0-012 

0-052 

1-07 

I 

In  the  presence  of  water  vapour,  the  yields  of  ozone  are 
considerably  lower,  and  hydrogen  peroxide  is  formed.  The 
influence  of  the  water  vapour  was  likewise  investigated  by 
Fischer  with  the  following  results  : — 


Water  Vapour 

Pressure  in 

mm.  Hg. 

•0   . 

0-002 

5-0   . 

10-0   . 

44-1   . 

149-2   . 


Wt.  Per  Cent. 
of03. 

0-0219 

0-0176 

0-00205 

0-00136 

0-00105 

0-00076 


Wt.  Per  Cent. 

HaO2  in 
10  Litres. 


0-011 
0-031 
0-074 


According  to  this  investigator,  the  function  of  the  water 
vapour  when  present  in  but  small  quantities  is  purely  cata- 
lytic in  depressing  the  yield  of  ozone  by  accelerating  the 
normal  decomposition  according  to  the  equation — 

203  ->  302. 

When  present  in   large  quantities   a  reversible  equilibrium 
obtains  as  follows  : — 

03  +  H202  ^  H20  +  02  +  20. 
Nernst  (loc.  cit.)  has  calculated  that  the  ratio  of  the  equili- 


56  OZONE 

brium  concentrations  of  03  and  0  formed  according  to  the 
reversible  equations 

03  £  02  +  0, 
02  ^  0  +  O, 

is  as  1023  to  1  or  atomic  oxygen  is  present  in  almost  vanish- 
ingly  small  concentrations ;  consequently,  in  the  above  reaction 
the  yield  of  ozone  and  of  hydrogen  peroxide  is  extremely 
small.  Fischer  was  thus  able  to  prepare  ozone,  nitric  oxide, 
or  hydrogen  peroxide,  all  endothermic  compounds,  from  air 
and  water  vapour  at  will  by  controlling  the  conditions  so  as 
to  take  advantage  of  the  different  rates  of  formation  and 
decomposition  of  these  substances  at  definite  temperatures. 


CHAPTEK  V. 

THE  ELECTROLYTIC  PREPARATION  OF  OZONE. 

As  early  as  1801  Cruickshank  drew  attention  to  the  fact  that 
electrolytic  oxygen,  generated  by  the  electrolysis  of  dilute 
sulphuric  acid  at  insoluble  anodes,  frequently  contained 
ozone. 

Schonbein  ("  Ann.  Phys.  Chem.,"  50,  616,  1840)  showed 
that  the  optimum  yield  of  ozone  was  obtained  when  the  sul- 
phuric acid  electrolyte  contained  23 '5  to  26*9  per  cent,  of 
sulphuric  acid  ;  solutions  of  phosphoric  acid  when  submitted 
to  electrolysis  likewise  yielded  small  quantities  of  ozone  in 
the  anodic  oxygen. 

De  Marignac  ("  C.K.,"  20,  808,  1845)  appears  to  be  the 
first  to  point  out  the  necessity  of  using  cool  electrolytes  for 
the  production  of  ozone ;  similar  observations  were  made  by 
Williamson  ("  Mem.  Chem.  Soc.,"  2,  395,  1845),  H.  Mei- 
dinger  ("Ann.,"  88,  57,  1853),  and  Baumert  ("Phil.  Mag.," 
4,  6,  51,  1853). 

The  next  advance  to  be  recorded  was  the  observation  of 
H.  Meidinger  ("  J.C.S.,"  7,  151,  1854)  that  small  anodes 
were  essential  for  the  economic  production  of  ozone.  With 
the  aid  of  an  electrode  only  20  mm.  long  by  0'5  mm.  wide,  in 
a  sulphuric  acid  electrolyte  of  density  1-9,  he  obtained  0*3 
per  cent,  ozone  in  the  anodic  oxygen. 

Soret  ("Pogg.  Ann.,"  92,  504,  1854),  showed  that   the 

(57) 


58  OZONE 

quantity  of  ozone  liberated  in  the  oxygen  was  determined  by 
various  factors.  As  electrode  material,  bright  platinum, 
gold  or  platinum  iridium  were  found  most  suitable,  since 
other  electrode  materials,  such  ;as  silver,  black  platinum, 
or  oxide  anodes,  such  as  lead,  iron  or  manganese,  exert  a  very 
considerable  activity  in  the  catalytic  decomposition  of  any 
ozone  which  might  be  formed  at  the  surface. 

The  temperature  of  the  sulphuric  acid  electrolyte  and  also 
of  the  anode  itself  plays  an  important  part  in  obtaining 
relatively  large  yields  of  ozone.  Soret  (loc.  cit.)  obtained  the 
following  ozone  concentrations  when  using  constant  currents 
and  electrolyte  composition  : — 

Temperature.  Gms.  O^per 

Cubic  Metre. 

-  21°  C 4-4 

-  13°  C 2-7 

6°C 0-9 

De  la  Coux  ("  L'Ozone,"  p.  79)  gives  the  following  values  for 
the  volume  percentage  of  ozone  liberated  in  the  oxygen  at 
different  temperatures  : — 

Electrolyte  :  H2S04 :  H20  : :  1  :  5. 

Temperature.  Volume  Per  Cent. 

of  Ozone. 

Boom 0-4 

5°-6°C 1 

Ice  and  salt  freezing  mixture     ...         .        .     2 

Andrews  ("Phil.  Trans.,"  i,  1850),  utilising  20'81  per 
cent,  sulphuric  acid  as  electrolyte  and  a  bunch  of  platinum 
wires  as  anode,  kept  cool  during  electrolysis  by  immersion  of 
the  cell  in  ice  water,  obtained  0'85  per  cent,  ozone.  Schone 
("Ber.,"  6,  1274,  1873)  claimed  the  production  of  3'29  to 


THE  ELECTROLYTIC  PREPARATION  OF  OZONE      59 

8'6  per  cent,  ozone,  and  Carius  ("  Ber.,"  174,  1,  1874),  3'44 
per  cent,  of  ozone  by  similar  means. 

Berthelot  in  1878  ("  C.R,"  86,  74,  1878)  observed  the 
formation  both  of  ozone  and  hydrogen  peroxide  in  sulphuric 
and  other  electrolytes  and  that  high  anodic  current  densities 
were  essential  for  the  production  of  ozone.  The  conception 
of  anodic  current  density  as  distinct  from  the  utilisation  of 
small  anodes  marked  a  fundamental  advance  in  the  electro- 
lytic synthesis  of  ozone. 

Persulphuric  acid  (H2S208)  is  simultaneously  produced 
when  very  concentrated  electrolytes  are  employed. 

The  investigations  of  Berthelot  were  continued  by 
Eicharz  ("  Wiedemann  Annalen.,"  24,  183,  1885  ;  31,  912, 
1887),  who  determined  the  yields  of  ozone,  persulphuric  acid 
and  hydrogen  peroxide  respectively  with  various  current 
densities  and  varying  sulphuric  acid  concentrations  at  0°  C. 
It  will  be  noted  from  the  following  tables  that  Eicharz  con- 
firmed the  previous  observer's  results  as  to  the  necessity  of 
high  anode  current  densities  and  relatively  concentrated 
electrolytes  : — 


Calculated  Volume 
of  Oz  Liberated 
during  a  Definite 
Time. 
Litres. 

Yield  of  Oxygen 

Ozone. 
Litres. 

in  the  Form  of  : 

Persulphuric  Acid. 
Litres. 

2-4      .. 

.      —     "   . 

.      0-03 

3-74    . 

— 

.       0-40 

7-47    . 

— 

.       2-32 

17-12    . 

.    0-04 

.       8*08 

30-0      . 

.    0-11       . 

.     16-25 

45-4      . 

.     0-26 

.     24-70 

65-7      . 

.    0-61 

.     39-00 

95-0     . 

,    I'X 

.    45-60 

60 


OZONE 


Per  Cent. 
Sulphuric  Acid. 

Yield  of  Ozone. 

Yield  of  Per- 
sulphuric  Acid. 

Yield  of  Hydro- 
gen Peroxide. 

In  Terms  of  Oxygen. 

Litres. 

Litres. 

Litres. 

10-1 

0-11 

0-62 

0 

19-8 

0-18 

6-79 

0 

28-3 

0-11 

16-75 

0 

39-5 

0-11 

22-01 

0 

50-7 

0-15 

18-76 

0 

60-0 

0-06 

4-85 

2-54 

69-4 

0-05 

3-49 

3-43 

77-6 

0-07 

2-55 

4-17 

89-4 

0-07 

1-21 

2-61 

McLeod  ("Trans.  Chem.  Soc.,"  44,  54,  1886)  conducted  a 
very  thorough  investigation  into  the  electrolytic  preparation 
of  ozone,  he  showed  the  importance  of  the  various  factors, 
viz.  acid  density,  temperature  of  the  solution  and  current 
density  to  which  attention  had  been  drawn  by  previous 
investigators. 

From  the  following  figures  the  extraordinary  good  yields 
obtained  by  McLeod  are  evident : — 


1 

Electrode  Material. 

Density  of 
Acid. 

Per  Cent. 
Ozone  by 
Volume. 

Current  Density 
Amps,  per  Sq.  Cm. 

6  platinum  wires 
1  mm.  long  by  0-045  mm. 
diameter 

f  1-086 

\  1-075 
U-25 

13-96 
16-7 
9-5 

30-76 
50-6 
50 

6  platinum  wires 
6  mm.  long  by  0'045  mm. 
diameter 

IW 

U-6 

16-7 
0-6 

50 
50 

The  influence  of  the  acid  density  on  the  production  of 
available  oxygen  in  the  electrolyte  in  the  form  of  persulphuric 
acid  and  hydrogen  peroxide  was  likewise  investigated,  the 
optimum  production  occurring  with  an  acid  density  of 
specific  gravity  T20  with  a  current  density  of  50  amps,  per 


THE  ELECTROLYTIC  PEEPAEATION  OF  OZONE      61 

sq.  cm.,  as  is  evident  from  the  following  figures  taken  from 
McLeod's  data : — 

Current  Density  Mols.  active  02 

Amps.ISq.Cm.  Acid  Density.  Per  100  Mols.  H.2 

evolved. 

51  .  .  .  .  1-05  ....  11-08. 

53  .  .  .  .  1-10  ....  20-80 

54  .  .  .  .  1-15  ....  25-8 
53  .  .  .  .  1-20  ....  34 
50  .  .  .  .  1-25  ....  29-9 

With  the  introduction  of  the  ionic  theory  by  Arrhenius 
and  van't  Hoff  in  1887  a  more  systematic  investigation  of  the 
anodic  reactions  taking  place  during  the  electrolysis  of  dilute 
sulphuric  acid  was  commenced. 

It  was  shown  that  if  the  potential  difference  between  two 
platinum  electrodes  in  dilute  sulphuric  acid  be  gradually 
raised  and  the  current  intensity  be  plotted  against  the  ap- 
plied electromotive  force  a  series  of  breaks  occurs,  which 
breaks,  on  the  ionic  theory,  correspond  to  different  anodic 
ionic  discharges,  the  discharge  of  hydrogen  being  the  only 
cathodic  reaction.  Careful  investigation  has  shown  that  the 
ionic  discharges  associated  with  each  break  in  sulphuric  acid 
and  electrolyte  are  as  follows  : — 

P.  D.  Anodic  Discharge. 

1-08 0"->02 

1-67 20H'  ->  02  +  H20 

1-95 S04  "  -*  H2S04  +  0, 

2-60 HS04'  ->  (HS04)2 

2-83 30"  -^Og 

In  1889  Nernst,  by  the  introduction  of  the  conception  of 
electrode  solution  pressure,  pointed  out  the  method  of  deter- 
mining the  influence  of  the  anode  potential  on  the  discharge 


62 


OZONE 


of  anions  without  having  to  take  into  account  any  cathodic 
reactions. 

If  a  platinum  electrode  be  saturated  with  oxygen  under  a 
definite  pressure  at  a  temperature  of  T°,  and  immersed  in  a 
sulphuric  acid  electrolyte,  normal  in  respect  to  its  hydrion 
concentration,  electrical  equilibrium  will  finally  be  arrived  at 
between  the  oxygen  molecules,  atoms  and  ions  in  the  elec- 
trode and  electrolyte,  the  electrode  becoming  positively 
charged  relatively  to  the  solution  by  the  discharge  of  nega- 
tively charged  oxygen  ions, 

+  ve 


and  a  condition  of  equilibrium  will  obtain  when  the  potential 
difference  between  solution  and  electrode  becomes  sufficiently 
great  to  prevent  the  discharge  of  any  more  negative  ions. 

If  V  be  the  electrode-electrolyte  potential  difference, 
fjb02  and  yu,O"  the  molecular  chemical  potentials  of  the  oxygen 
gas  in  the  electrode  and  of  oxygen  ions  in  the  solution,  then 
if  we  imagine  the  transfer  of  a  quantity  of  electricity  Se  from 
electrode  to  solution,  the  electrical  work  will  be  equal  to 
-  VSe,  the  change  in  molecular  chemical  potential  per  mol. 
will  be  fj,02  -  2/u,0",  therefore,  the  work  done  on  the  transfer 
of  this  quantity  of  electricity  is  equal  to 

-  yuO"  , 
~ 


where  e  is  the  charge  associated  with  one  gram  ion  of  a 
monovalent  element. 


THE  ELECTEOLYTIC  PKEPAKATION  OF  OZONE      63 

If  the  conditions  of  reversible  equilibrium  obtain,  then 


Now  //X)2  =  T(<£02  +  R  log  7r02)  for  a  dilute  solution,  where 
<f>  is  independent  of  the  concentrations,  it  being  merely  a 
function  of  the  temperature, 
similarly  yuO"  =  T(c/>0"  +  E  log  CO"). 


ET  .   CO" 

+  -t  -  log 


Further,  since  in  aqueous  solutions 


^7logi 

which  gives  an  expression  for  the  variation  of  the  oxygen 
electrode  potential,  with  alteration  in  the  hydrion  concentra- 
tion of  the  solution  and  the  pressure  of  the  oxygen  gas. 

The  value  of  V0  is  approximately  -  1*35  vollfe,  whence 
the  value  of  the  cathode  potential  for  a  hydrogen  electrode  in 
normal  hydrion  solution  under  a  pressure  of  one  atmosphere 
is  +  1-08  -  1-35  =  -  0-27  volts. 

If  an  oxygen  electrode  be  set  up  against  an  ozone  elec- 
trode, the  difference  in  potential  between  the  two  electrodes 
can  be  calculated  in  a  similar  manner  and  found  equal  to  : — 

V  '  +  ??  lo    ^ 

Luther  and  Inglis  ("  Zeit.  f.  Physik.  Chem.,"  43,  203,  1903) 
first  attempted  to  obtain  an  approximate  value  for  V</  by 


64  OZONE 

measurement  of  the  potential  difference  between  an  oxygen 
and  an  ozone  charged  platinum  electrode  immersed  in  dilute 
sulphuric  or  nitric  acid.  They  obtained  the  value : — 

V;  =  -  0-736  volts. 

We  have  noted  that  approximately  the  same  value,  viz. 
-  0'83  volts,  can  be  obtained  by  calculation  from  the  Nernst 
heat  theorem.  Subsequent  investigators  have  found  con- 
siderably lower  values:  Nernst  ("Zeit.  Elektrochem.,"  9,  89, 
1903)  obtained  the  value  V0  =  -  0'57,  and  Fischer  and 
Brauner("Ber.,"  34,  3631, 1906)  the  values  -  0'64  to  -  0'46 
volts.  It  would  appear  from  the  experiments  of  these  latter 
observers  on  the  thermal  equilibrium,  that  the  lower  value, 
viz.  -  0*50  volts  is  probably  more  correct.  It  is  possible 
that  the  higher  values  obtained  by  the  earlier  experimenters 
were  occasioned  by  the  presence  of  oxozone  04  in  the  ozone 
round  the  electrode,  and  a  reinvestigation  of  this  electrodic 
reaction  is  clearly  eminently  desirable. 

From  the  above  calculation  it  is  evident  that  an  extremely 
high  anode  potential  is  required  to  remove  the  formation  of 
ozone,  whilst,  in  order  to  ensure  its  stability  when  produced, 
both  electrode  and  electrolyte  must  be  kept  cold.  Fischer, 
Massenez,  and  Bendixsohn  ("  Zeit.  Anorg.  Chem.,"  52,  202, 
1907;  11,229,  1907;  61,  13,153,  1909),  realising  these  im- 
portant factors,  improved  upon  McLeod's  results  by  adopting 
the  artifice  of  water-cooled  electrodes,  in  addition  to  the  sup- 
plementary cooling  of  the  electrolyte. 

In  their  earlier  experiments,  a  small  platinum  tube  6  mm. 
long  was  sealed  to  two  terminal  glass  tubes  and  served  as 
anode ;  the  tube  itself  was  covered  with  glass  with  only  a 
thin  strip,  0'4  mm.  wide,  exposed  to  the  electrolyte. 


THE  ELECTEOLYTIC  PREPAKATION  OF  OZONE      65 

Cold  water  was  circulated  through  this  anode,  and  the 
electrolyte  was  kept  cool  by  immersion  in  ice  water. 

Utilising  an  anodic  current  density  of  58  amperes  per  sq. 
cm.,  and  a  sulphuric  acid  concentration  of  density  1*075  to 
I'lO,  a  yield  of  17  per  cent,  ozone  by  weight  (11*3  per  cent, 
by  volume)  was  obtained  in  the  anodic  oxygen. 

A  glass-covered,  rhomboidal,  platinum  tube  was  then 
substituted  for  the  cylindrical  one,  and  one  edge,  01  mm.  in 
width,  was  exposed  by  grinding  away  the  glass.  The  length 
of  the  tube  was  11'5  mm.,  and  it  was  maintained  at  -  14°  C. 
by  circulation  of  a  solution  of  cold  calcium  chloride.  A  yield 
of  28  per  cent,  ozone  by  weight  (19  per  cent,  by  volume)  was 
thus  obtained.  By  embedding  platinum  foil  in  glass,  and 
exposing  one  edge  only,  O'Ol  mm.  wide,  to  the  electrolyte, 
slightly  lower  yields  were  obtained,  viz.  23  per  cent,  of  ozone 
by  weight. 

In  confirmation  of  McLeod's  results,  the  optimum 
concentration  of  sulphuric  acid  lay  between  =  1'075  and 
110. 

They  noted  that  the  quantity  of  ozone  produced  per  kw. 
hr.  rose  steadily  with  continued  use  of  the  platinum,  which 
became  quite  bright  and  burnished  by  the  gas  evolution  in 
course  of  time. 

A  yield  of  71  gms.  per  kw.  hr.  was  obtained  at  a  potential 
difference  of  7*5  volts,  and  an  anodic  current  density  of  80 
amperes  per  sq.  cm.  If  we  calculated  the  theoretical  pro- 
duction of  ozone  per  kw.  hr.  from  its  heat  of  formation,  i.e. 
34,000  calories,  the  yield  of  71  gms.  per  kw.  hr.  indicates  an 

7*1 

electrical  efficiency  of  only  -^T^  =  0*6  per  cent. 


66  OZONE 

P.  Fischer  assumed  that  the  primary  discharge  of  ozone 
occurs  according  to  the  equation— 

30"  ->  03  +  60,  % 

which  ozone  is  partly  decomposed  by  the  catalytic  action  of 
the  anode  surface. 

Other,  but  less  efficacious  methods  have  been  suggested 
from  time  to  time  for  raising  the  anode  discharge  potential, 
and  thus  increasing  the  yield  of  ozone.  Donovan  and  Gard- 
ner utilised  a  saturated  solution  of  potassium  permanganate 
in  from  5  to  10  per  cent,  of  sulphuric  acid,  and  obtained 
relatively  high  concentrations  of  ozone ;  chromic  acid  can 
likewise  be  substituted  for  the  permanganate. 

St.  Edme  adopted  the  somewhat  ingenious  method  of 
obtaining  a  high  anode  current  density,  by  employing 
moistened  crystals  of  phosphoric  acid,  or  caustic  potash  or 
soda,  as  the  electrolyte. 

In  this  way,  the  electrolyte  was  given  sufficient  conduc- 
tivity for  passage  of  the  current,  yet  at  the  same  time  only 
point  contact  between  the  moistened  crystals  and  the  anode 
was  ensured. 

Archibald  and  von  Wartenberg  ("  Zeit.  Elektrochem.,"  17, 
812,  1911)  pointed  out  that  the  low  yields  of  ozone  accom- 
panying the  electrolytic  decomposition  of  dilute  sulphuric 
acid  were  probably  occasioned  by  the  high  degree  of  anodic 
polarisation  that  was  produced  when  operating  at  high  cur- 
rent densities.  In  agreement  with  Fischer  they  considered 
that  the  primary  formation  of  ozone  takes  place  according 
to  the  equation — 

30"  ->  03  +  60, 


THE  ELECTROLYTIC  PREPARATION  OF  OZONE      67 

but  that  the  subsequent  catalytic  decomposition  of  ozone  at 
the  electrode  surface 

203  ->  302 

was  not  the  most  important  factor.  It  was  suggested  that 
the  ozone  thus  formed  is  further  oxidised  at  the  anode — 

O3  +  0"  ->  202  +  20, 

consequently,  if  the  anodic  polarisation  could  be  diminished 
without  alteration  of  the  anodic  current  density,  increased 
yields  of  ozone  could  be  obtained,  since  the  secondary  oxida- 
tion would  be  diminished. 

A  series  of  experiments  were  carried  out  in  which  an 
alternating  current  was  super-imposed  on  the  direct  current 
flowing  through  the  cell ;  this  method  of  reducing  the  elec- 
trode polarisation  having  been  utilised  in  the  Wohwill  pro- 
cess for  the  electrolytic  parting  of  gold  and  silver,  and  in  the 
electrolytic  preparation  of  hydrogen  peroxide.  As  electrodes, 
short  platinum  wires  or  platinum  capillaries  cooled  with 
water  were  utilised,  as  electrolyte  sulphuric  acid  of  varying 
density,  whilst  a  direct  and  alternating  current  of  variable 
periodicity  was  applied  simultaneously  to  the  cell. 

It  was  established  that  the  optimum  acid  density  varies 
with  the  area  of  the  electrode  and  not  only  with  the  current 
density,  more  concentrated  electrolytes  being  desirable  for 
big  electrodes  as  indicated  by  the  following  figures  : — 

Area  of  Electrode  Optimum  Acid 

in  S%.  Cm.  Density. 

0-041  1-34 

0-333  1-478 

The  yield  of  ozone  was  also  affected  by  the  periodicity  of  the 


68 


OZONE 


alternating  current,  especially  with  currents  of  low  frequency  ; 
above  20  periods  per  second  the  effect  was  not  so  marked. 


Direct 
Current. 

Alternating 
Current. 

Periods  per 
Second. 

Per  Cent.  O3. 

Amperes. 

0-61 

0-50 

11 

3-04 

0-61 

0-50 

17 

4-10 

0-61 

0-50 

25 

4-77 

It  was  noted  that  the  applied  potential  difference  necessary 
for  the  passage  of  the  current  rose  until  the  ratio 

alternating  current  exceefled  3  md  then        idl    sunk  until 
direct  current 

this  ratio  became  equal  to  6,  which  was  found  to  be  an 
optimum.  The  potential  difference  was  found  to  sink  with 
increasing  current  density. 

Their  optimum  yield  was  obtained  under  the  following 
conditions : — 

Electrode  area :    -  0*333  sq.  cm.  periodicity  18  v.  per  sec. 

Temperature :  10°  C. 

Acid  density :  1*478. 


Direct 
Current. 

Alternating 
Current. 

A.C. 

Anode 
Current 
Density. 

Per  Cent.  03 
by  Volume 
Calculated  on  the 
Direct  Current. 

D.C. 

Amperes. 

0-25 

1-50 

6 

0-75 

37 

The  most  important  result  from  a  technical  point  of  view 
was  the  effect   of  the   alternating  current  on  the  potential 


THE  ELECTKOLYTIC  PREPARATION  OF  OZONE      69 

difference  necessary  to  effect  the  passage  of  this  current ;  in 
the  above  case  only  2' 75  volts  being  necessary  with  an  anode 
potential  of  0'71  volt  as  opposed  to  7'5  volts  required  by  F. 
Fischer  for  direct  current.  We  can  calculate  from  the  above 
data  the  production  of  ozone  per  kw.  hr.  as  follows; 
96,540  coulombs  or  26*8  ampere  hrs.  liberate  1  gm.  equiv- 
alent, or  11 '2  litres  of  oxygen  gas.  Under  the  conditions  of 
operation,  however,  the  liberated  gas  contains  37  per  cent, 
of  ozone  which  would  result  from  the  condensation  of  55 '5 
per  cent.  (37  per  cent.  +  \  37  per  cent.)  of  the  oxygen,  which 
weighs  4'40  gms.  Hence  26'8  ampere  hrs.  liberated  4'40 
gms.  of  ozone.  The  potential  difference  which  has  to  be  ap- 
plied to  the  cell  to  effect  this  liberation  is  2' 75  volts,  thus 
4*4  gms.  of  ozone  are  produced  by  the  expenditure  of  energy 
equal  to  26*8  x  2*75  or  73'7  watt-hrs.,  representing  an  output 
of  59  gms.  per  kw.  hr.,  or  over  eight  times  the  yield.  This 
yield  approximates  to  those  obtained  by  the  method  of  the 
silent  discharge,  and  it  would  appear  possible,  if  larger 
electrodes,  and  a  cooled  electrolyte  were  employed,  to  develop 
this  method  of  producing  ozonised  oxygen  both  for  strong 
and  weak  gas  concentrations  for  the  purposes  of  technical 
production. 


CHAPTER  VI. 

PRODUCTION  BY  ULTRA- VIOLET  RADIATION. 

IN  1900,  Ph.  Lenard  ("Ann.  der  Physik,"  i,  480, 1900),  utilis- 
ing a  quartz  mercury  vapour  lamp  as  a  source  of  energy, 
showed  that  ultra-violet  light  of  extremely  short  wave  length 
was  an  effective  agent  for  ozonising  oxygen.  Both  Lenard 
and  E.  Goldstein  ("Ber.,"  36,  3042,  19fe)  showed  that 
ultra-violet  light  in  the  Schumann  portion  of  the  spectrum 
within  the  spectral  region  X  =  120  ^  to  X  =  180  pp  exerted 
the  maximum  activity  in  this  respect;  Goldstein  (loc.  cit.) 
actually  obtaining  pure  liquid  ozone  by  means  of  a  quartz 
vacuum  tube.  Regener  ("  Ann.  der  Physik,"  20,  1033, 1906), 
who  reinvestigated  the  matter,  noticed  the  interesting  fact 
that  although  light  of  wave  length  X  =  120  //,/-&  to  180  fi/j,  was 
a  powerful  ozonising  agent,  yet  light  still  in  the  ultra-violet 
portion  of  the  spectrum  of  wave  length  A,  230  //./-i  to  A  =  290  ^ 
(especially  \  =  257  fiji)  exerted  an  equally  effective  catalytic 
decomposing  effect.  Ozone  is  thus  formed  by  light  of  short 
wave  length  and  decomposed  again  by  light  of  slightly  longer 
wave  length.  According  to  E.  'Warburg  the  ozonisation 
effected  by  ultra-violet  light  likewise  increases  steadily  with 
the  pressure  of  the  gas  ("  Deut.  Phys.  Ges.  Vehr.,  17,  10, 

184,  1915). 

(70) 


PRODUCTION  BY  ULTRA-VIOLET  RADIATION  71 

EFFECT  OF  KADIATION  OF  SHORT  WAVE  LENGTH. 

Since  marked  concentrations  of  ozone  result  when  oxygen 
is  subjected  to  irradiation  in  light  of  this  wave  length,  it 
necessarily  follows  that  the  energy  necessary  for  the  forma- 
tion of  ozone  from  oxygen  is  derived  from  the  light,  the 
process  of  ozone  formation  being  a  typical  photo-chemical 
synthesis. 

According  to  Planck's  quantum  theory  ("  Vorlesungen 
iiber  die  Warmestrahlung,"  M.  Planck,  Leipzig,  1906,  pp.  100, 
et  seq.),  radiant  energy  is  discrete,  and  can  only  be  emitted 
by  an  oscillator  or  absorbed  by  a  resonator  in  definite  quanta.1 
The  magnitude  of  the  quantum  bears  a  definite  relationship 
to  the  frequency,  of  the  light  e  =  hv,  where  e  is  the  magnitude 
of  the  quantum,  v  the  light  frequency,  and  h  Planck's  constant 
equal  to  6*85  x  10~27erg  seconds. 

Quanta,  or  the  units  of  energy,  may  be  emitted  or  absorbed 
in  single  units  or  in  even  multiples  of  that  unit  at  a  time. 
Two  hypotheses  have  been  advanced  to  explain  the  directional 
motion  of  the  quanta  since  it  is  evidently  rectilinear  in 
motion.  A.  Einstein  ("  Ann.  der  Physik,"  17,  133, 1905)  pos- 
tulates an  entity  for  the  quantum  in  the  form  of  a  light  cell 
which  moves  uniformly  in  the  direction  in  which  its  centres 
of  gravity  is  projected.  Sir  J.  J.  Thomson  ("  Proc.  Phys.  Soc.," 
14,  540,  1908;  "Phil.  Mag.,"  792,  1913)  has  advanced  the 
ingenious  hypothesis  which  assumes  that  the  light  travels  in 
the  wave  form  as  postulated  on  the  old  hypothesis,  but  that 
these  waves  are  confined  to  certain  directions,  being  virtually 

xlt  is,  of  course,  possible,  and  indeed  more  probable,  to  assume  that  radiant 
energy  appears  discrete  because  matter  is  discrete,  and  that  the  radiation 
itself  is  continuous. 


72  OZONE 

kinks  in  Faraday  tubes  which  project  from  the  point  source. 
A  beam  of  light  is  thus  compared  to  a  bundle  of  a  number  of 
Faraday  tubes,  and  light  transmission  is  effected  by  trans- 
mission of  pulses  naturally  of  definite  magnitudes,  and  there- 
fore in  quanta  along  these  tubes. 

The  elements  when  raised  to  a  high  temperature  emit 
light  in  the  form  of  spectral  series.  In  all  elements  two 
distinct  types  of  light  emission  can  usually  be  observed, 
namely,  band  spectra  and  line  spectra.  Various  investigators, 
notably  H.  Delandres  ("  C.E  ,"  100,  1256,  1885,  et  seq.)  have 
shown  that  the  elementary  band  spectra  can  be  divided  into 
groups  related  by  the  expression  v  =  Bn2  +  /3,  where  /3  and  B 
are  constants,  and  n  a  series  of  integers,  whilst  in  each  group 
the  frequency  of  the  bands  v0  are  also  related  by  the  ex- 
pression : — 

i/0  =  A  (m  +  a)2  -f  d, 

where  A,  a,  d  are  constants,  and  m  a  series  of  integers. 
Again,  in  the  line  spectra,  J.  J.  Balmer  ("  Verb.  d.  Natur.  ges. 
Basel,"  2,  648,  750,  1885;  "  Wied.  Ann./'  25,  40,  1885), 
C.  Eunge  ("B.A.  Eeports,"  576,  1888),  F.  Paschen  ("Ann. 
der  Physik,"  27,  537, 1908,  35,  860,  1911),  and  J.  E.  Eydberg 
("K.  Svenska,  Vet.  Akad.  Handl.,"  23,  155,  1890)  have 
shown  similar  relationships. 

From  these  and  other  considerations  (see  J.  J.  Thomson, 
"Proc.  Eoy.  Soc.,"  14,  540,  1908;  "Phil.  Mag,"  19,  331, 
1910) ;  J.  Stark  ("  Prinzipien  der  Atorndynamik,"  Leipzig, 
1911)  we  deduce  that  a  chemical  element  is  not  composed  of 
homogeneous  atoms  or  molecules,  but  that  each  atom  or 
molecule  is  composed  of  at  least  two  parts,  one  which  gives 
rise  to  a  line  spectrum,  and  the  other  to  a  band  spectrum 


PRODUCTION   BY  ULTRA-VIOLET  RADIATION  73 

when  excited.  The  light  thus  emitted  is  periodic  in  character, 
being  produced  by  some  form  of  oscillation  or  oscillators, 
each  periodic  movement  corresponding  to  one  series  of  bands, 
or  lines  in  the  spectrum. 

From  other  considerations,  such  as  the  electrical  pro- 
perties and  radioactivity  of  certain  elements,  the  composite 
nature  of  the  atom  receives  confirmation. 

Sir  J.  J.  Thomson,  who  first  suggested  this  actual  structure 
for  the  atoms,  although  speculations  on  the  electrical  nature 
of  matter  had  long  been  made  for  the  purpose  of  calculation, 
assumed  the  existence  of  a  relatively  large  positive  nucleus 
with  the  negative  electrons  (or  corpuscules)  distributed  in  it. 
A  small  positive  nucleus  with  the  electrons  rotating  round  it, 
in  fact  a  small  planetary  system,  is  now  a  common  hypoth- 
esis. It  is  at  present  uncertain  whether  the  inverse  square 
law  or  some  higher  power  such  as  the  inverse  fifth  power 
conditions  the  rotation  of  the  electrons.  Information  is  also 
lacking  whether  the  electrons  rotate  in  big  or  small  circles, 
i.e.  whether  the  plane  of  their  rotation  passes  through  the 
centre  of  gravity  of  the  atom  or  not,  and  it  is  also  a  matter 
of  speculation  whether  the  electrons  are  point  charges,  or 
consist  of  rings  such  as  are  found  in  the  satellites  of  Saturn. 

It  can  easily  be  shown  (see  F.  A.  Lindemann,  "  Verh.  d. 
Phys.  Ges.,"  13,  482,  1911)  that  the  amplitude  of  the  vibrat- 
ing particle  in  the  oscillator  emitting  light  radiation  is  of  the 
order  10~9  to  10~10  cms.,  or  from  10  to  100  times  smaller 
than  the  actual  diameter  of  an  atom  ;  we  are  therefore  forced 
to  the  conclusion  that  the  oscillators,  both  for  the  band  and 
line  spectra  emissions,  are  to  be  found  in  the  atom  itself. 
Much  evidence  has  been  adduced  to  show  that  the  source  of 


74  OZONE 

infra-red  radiation  is  the  atom,  of  the  visible  light  the  charged 
atom,  and  of  the  ultra-violet  light  the  electron,  the  band 
spectra  owing  their  origin  to  the  oscillations  caused  by  the 
swing  of  a  valency  electron  about  the  positive  nucleus. 

If  we  imagine  a  valency  electron  circulating  in  its  orbit 
with  a  definite  and  constant  momentum,  a  definite  amount 
of  energy  E  must  be  supplied  to  remove  the  electron  from 
the  system.  If  the  electron  be  nearly  but  not  quite  removed 
from  the  sphere  of  action  of  the  atom  it  will  oscillate  about 
its  mean  position  of  rotation  and  emit  light.  The  energy  of 
oscillation  must,  according  to  Planck's  hypothesis  be  a  mul- 
tiple of  quanta,  or  :— 
nhv  where  n  is  a  whole  number,  h  Planck's  constant,  and  v 

the  light  frequency. 

hence  E  must  be  >  nhv  to  cause  deformation  and  light 
emission ;  the  smallest  value  of  n  is  unity,  so  to  cause  light 
emission  by  deformation  of  the  orbit  of  a  valency  electron — 

E  =  kv, 

TT 

or  v  must  be  less  than  _.     It  necessarily  follows  that  as  E, 

n 

the  energy  of  deformation,  is  decreased,  the  wave  length  of 
the  light  will  increase  or  be  shifted  towards  the  infra-red 
portion  of  the  spectrum.  The  band  spectrum  of  an  element 
thus,  according  to  this  view,  optically  expresses  the  configur- 
ation of  the  valency  electrons  in  the  molecular  system. 

The  relationship  between  position  of  the  band  spectrum 
and  complexity  of  the  molecule  in  which  the  valency  electron 
is  oscillating,  can  be  clearly  shown  in  the  case  of  oxygen. 

Monatomic  oxygen,  viz.  0,  has  a  band  spectrum  in  the 
region  X  =  245  up  to  \  =  333  p^  (W.  Steubing,  "  Ann.  der 


PEODUCTION  BY  ULTRA-VIOLET  RADIATION  75 


Physik,"  33,  £53,  1913).  In  diatomic  or  molecular  oxygen, 
viz.  02,  the  band  spectrum  is  shifted  towards  the  ultra-violet, 
viz.  X  =  120  /jL/jb  to  X  =  190  ftp,  since  the  energy  E,  required 
to  remove  a  valency  electron  from  two  positive  nuclei,  is 
much  greater  than  is  required  to  remove  one  of  the  two 
valency  electrons  from  the  single  positive  nucleus  of  the 
atomic  oxygen.  In  ozone,  on  the  other  hand,  not  only  is  it 
evident  from  its  endothermic  character,  but  also  from  a  visual 
representation  of  three  positive  nuclei  coupled  by  valency 
electrons,  that  the  energy  required  to  remove  a  valency 
electron  from  the  ozone  molecule  will  be  less  than  from  the 
molecular  form,  i.e.  the  band  spectrum  will  be  between  the 
two  former.  In  fact,  a  strong  absorption  is  noted  at  X  = 
258  fifi  (W.  N.  Hartley,  "  Chem.  News,"  42,  268,  1888). 

The  oscillator  of  the  series  spectrum,  on  the  other  hand 
(J.  Stark,  "  Die  Elektrizitat  in  Gasen,"  Leipzig,  p.  447,  1902), 
is  to  be  found  in  the  positive  ion  resulting  from  the  complete 
removal  of  a  valency  electron  from  an  atom  or  a  molecule. 
The  notable  experiments  of  Sir  J.  J.  Thomson  ("  Phil.  Mag.," 
13,  561,  1907,  21,  275,  1911,  et  seq.}  on  cathode  ray  analysis 
have  indicated  that  such  ions  which  have  lost  or  gained  a 
valency  electron  possesses  actual  entities,  and  can  be  distin- 
guished one  from  another  by  their  difference  in  electrical 
charge  ;  thus  in  the  case  of  oxygen  there  have  been  isolated 
the  charged  gas  ions  :  — 

++    +    -   — 

o,  o,  o,  o, 

as  well  as  oxygen  molecules  of  various  charges. 

R  Horton  ("Phil.  Mag.,"  22,  214,  1914)  has  identified  as 
carriers  of  positive  electricity,  giving  positive  band  spectra  in 


76  OZONE 

oxygen,  the  following  polymers  of  electric  atomic  weights 
8,  16,  32,  48,  96. 

N.  Bohr  ("Phil.  Mag.,"  26,  476,  1913)  attributes  to  the 
oxygen  atom  a  nucleus  carrying  eight  unit  position  charges 
with  eight  electrons,  of  which  only  two  appear  removable  by 
methods  at  present  available. 

The  Mechanism  of  Ozone  Formation. 
We  have  already  noted  that  the  oxygen  molecule  when 
subjected  to  ultra-violet  light  radiation  of  the  correct  fre- 
quency for  resonance  may  absorb  quanta  of  energy.  Similar 
conditions  obtain  for  the  iodine  molecule  in  the  infra-red 
spectral  range  and  we  may  regard  the  primary  cleavage  to 
occur  in  a  similar  manner,  viz.  : — 

I2  JT I  +  I 
02  ^  0  +  O. 

Warburg  ("  Preuss.  Akad.  Wiss.,"  Berlin,  872,  1914)  has 
adopted  this  hypothesis  to  explain  the  mechanism  of  ozone 
formation.  He  assumes  that  the  atomic  oxygen  resulting 
from  the  cleavage  of  the  molecule  secondarily  reacts  either 
with  atomic  oxygen  to  reform  molecular  oxygen  as  indicated 
by  the  reversibility  of  the  above  equation,  a  point  clearly 
emphasised  by  Nernst  from  thermal  considerations  (see  p.  29), 
or  it  may  react  with  molecular  oxygen  to  form  ozone — 

02  +  0  =  03. 

Warburg's  experiments,  conducted  under  pressures  of  from 
30  to  400  kgm.  per  sq.  cm.,  yielded  a  photo-chemical  efficiency 
of  55  per  cent,  at  120  kgm.  cm.2  and  29  per  cent,  at  300  kgm. 
cm.2,  indicating  the  plausibility  of  the  above  hypothesis. 
Weigert  ("  Zeit.  Wiss.  Photochem.,"  n,  381,  1912)  obtained 


PEODUCTION  BY  ULTRA-VIOLET   EADIATION 


77 


a  photo-chemical  efficiency  of  46  '0  per  cent,  and  a  thermo- 
dynamic  efficiency  of  27'7  per  cent. 

The  reaction  is  primarily  a  molecular  one  and  the  energy 
of  formation  of  a  gram.  mol.  of  03  (34,000  cal.)  should  have 
its  corresponding  photo-chemical  equivalent  equal,  as  cal- 
culated by  Warburg,  to  ~Nhv  where  N  is  the  number  of 
molecules  per  gram.  mol.  From  the  relationship 


(cal.)          ncm. 

with  a  value  of  34,000  calories  for  the  heat  of  formation  of 
ozone  we  obtain  X  =  800  ^/JL,  as  the  critical  ozonising  wave 
length,  or  we  must  assume  that  one  quantum  of  X  =  200 
will  form  four  ozone  molecules. 

Ozone  formation  may  therefore  occur  without  gas  ionisa- 
tion,  a  fact  which  was  first  demonstrated  by  Lenard  and  con- 
firmed by  Ludlam  ("  Phil.  Mag./' 
23,  757,  1912). 

We  can  easily  deduce  from  our 
previous  considerations  on  the 
mechanism  of  photo-chemical 
processes  in  the  light  of  the 
quantum  theory  that  ionisation 
of  oxygen  will  be  brought  about 
by  light  of  shorter  wave  length 
than  that  required  to  produce 
atomic  oxygen  and  hence  ozone. 

If  A,   B,   represent   the  two 
positive  nuclei  of  oxygen  atoms 
in  a  neutral  molecule  and  c,  c, 
one  valency  electron  of  each  atom  which  has  come  within  the 
attraction  (partially  saturated)  of  the  positive  charge  of  the 


78  OZONE 

neighbouring  atom ;  the  simplest  line  of  cleavage  is  along  a, 
a',  resulting  in  the  formation  of  an  oxygen  molecule  with  one 
bond  as  link  0 — 0,  which  can  then  react  to  form  ozone — 
30—0  ->  203. 

This  requires  the  smallest  amount  of  energy,  and  hence  is 
effected  by  light  of  the  longest  wave  length  (>  X  =  200  /A/A). 
The  simplest  cleavage  into  neutral  atoms  takes  place  along 
the  line  b,  b\  requiring  more  energy  than  is  necessary  to 
effect  partial  unsaturation,  and  hence  shorter  wave  length 
light  X  =  200  /A/A.  Cleavage  along  the  line  d,  df  necessitates 
the  removal  of  one  electron  from  an  oxygen  atom  and  conse- 
quent increase  of  energy  or  light  of  a  still  shorter  wave  length, 
ca.  X  =  180  /A/A.  The  energy  required  to  move  a  valency 
electron  which  is  partially  attached  to  two  atoms  on  to  one 
atom  is  of  the  order  1  x  10~12  ergs,  to  completely  remove 
the  electron  requires  a  considerably  greater  expenditure 
of  energy.  A  quantum  of  light  energy  in  the  visible  or 
ultra-violet  portion  of  the  spectrum  is  of  the  order  of 
>  3  x  10~12  ergs ;  thus  with  very  short  wave  length  light 
electron  removal  can  easily  be  effected. 

Under  these   conditions  we   obtain  monatomic   oxygen 
ions — 

02  ;±  6  +  6. 

Ozone  formation  may  result  from  this  ionisation  according  to 
the  following  reaction  : — 

202  +  O  +  6  =  203. 

Light  of  still  shorter  wave  length  will  actually  remove  elec- 
trons from  the  monatomic  oxygen  ion  (Ca.  X  =  130  /A/A) 

+       ++ 

0  ->  0  +  0, 

which  electron  may  attach  itself  to  the  charged  -  ve  residue 


PBODUCTION  BY  ULTKA-VIOLET  KADIATION  79 

provided  that  it  be  projected  from  the  original  ion  with  suf- 
ficient kinetic  energy — 

6  +  0  ->  0. 

Or  again,  it  may  attach  itself  more  easily  than  as  above  to  a 
neutral  molecule — 

02  +  ©  -»  62. 

In  this  way  we  can  imagine  the  formation  of  the  various 
charged  ions  actually  observed  during  irradiation  of  oxygen 
by  ultra-violet  light  of  short  wave  lengths  within  the  range 
X  =  130  to  200  w. 

Construction  of  Apparatus. 

(a)  Source  of  Ultra-violet  Light. — We  have  already  in- 
dicated that  for  the  production  of  ozone  a  source  of  ultra- 
violet light  rich  in  lines  of  the  Schumann  region  (below 
A,  =  200)  and  if  possible  free  from  light  of  longer  wave  length, 
especially  in  the  region  X  =  230  to  290  /-t^,  which  exerts  a 
strong  catalytic  activity  in  deozonisation. 

A  glance  at  the  curves  representing  the  distribution  of 
energy  over  the  spectrum  radiated  from  a  black  body  at 
various  temperatures  will  suffice  to  indicate  that  "black 
body  "  radiation  is  unsuitable  as  an  efficient  source  of  Schu- 
mann light.  In  agreement  with  the  theoretical  calculations 
of  Wien  and  Planck  the  experimental  observations  of  Lummer 
and  Pringsheiin  ("  Ver.  d.  Deut.  Phys.  Gesell.,"  i,  23,  1899  ; 
2,  163,  1900)  have  indicated  that,  with  elevation  of  the 
temperature  of  the  radiator,  the  maximum  energy  emission 
E™  shifts  from  the  longer  to  the  shorter  wave  length  portion 
of  the  spectrum.  Even,  however,  at  sun  temperature, 
ca.  5500°  C.,  which  temperature  can  only  be  approached  with 
difficulty  by  the  utilisation  of  carbon  arcs  under  high  gas 


80  OZONE 

pressures,  a  black  body  radiator  will  have  its  E  position  at 
about  X  =  500  /JL/J,,  or  in  the  green  of  the  visible  spectrum. 
The  fraction  of  the  total  energy  emitted  which  will  lie  in  the 
Schumann  region  of  the  spectrum  XI  to  200  //,/*  will  be  re- 
markably small. 

We  must,  therefore,  reject  black  body  radiators  and  fall 
back  on  methods  of  obtaining  selective  emission  and  as  such 
light  sources  we  may  utilise  the  arc,  spark,  or  vacuum  tube 
illumination  of  various  elements. 

Arc  and  Spark  Light  Sources. 

Most  metals  exhibit  strong  Schumann  and  ultra-violet 
light  radiation  when  the  arc  or  spark  electric  discharge  is 
made  to  pass  between  metallic  electrodes. 

We  may  argue  from  the  electronic  structure  of  the  atom 
that  since  the  removal  of  a  second  electron  from  an  atom 
which  has  already  lost  one,  necessitates  the  supply  of  a  still 
greater  quantity  of  energy  for  its  removal  than  the  first,  and 
as  this  energy  is  supplied  in  quanta,  the  value  of  hv  must  rise 
with  each  subsequent  removal.  Atoms  which  can  loose  many 
electrons  without  loss  of  atomic  identity  will  therefore  radiate 
light  corresponding  to  high  values  of  hv,  i.e.  of  extremely 
short  wave  length. 

The  elements  which  have  been  most  widely  used  as 
sources  of  ultra-violet  light  are  those  of  aluminium,  iron  and 
especially  mercury,  which  can  loose  as  many  as  eight  electrons. 

The  aluminium  ultra-violet  spectrum  has  been  investigated, 
more  particularly  by  Lenard  ("  Sitz.  Heidelberg,  Akad.  Wiss. 
Abh.,"  31,  1910)  and  Morris  Airy  ("  Man.  Lit.  Phil.  Soc.," 
XLIX,  1,  1905),  and  iron  by  Lyman  ("  Astrophys.  Jour.," 
38,  282, 1913). 


PEODUCTION   BY   ULTRA-VIOLET   RADIATION  81 

Much  work  has  been  accomplished  on  the  mercury  arc, 
which  is  especially  rich,  both  in  green,  violet,  and  ultra-violet 
radiation,  most  conspicuous  where  mercury  vapour  lamps  are 
used  as  light  sources. 

The  investigations  of  Tian  ("  C.R,"  155,  141,  1912)  and 
Lyman  ("  Astrophys.  Jour.,"  38,  282,  1913)  have  shown  that 
"the  spectrum  is  dominated  by  the  broad  unsymmetrical 
line  X  =  184'96  /z/z,  ".  The  spark  spectrum  of  mercury  is 
rich  in  lines,  whilst  the  arc  spectrum  contains  only  a  few. 
Other  lines  in  the  same  series  predicted  by  Paschen  have 
likewise  been  observed  at  X  =  140*2  ji/j,  and  A,  =  126'9  /u/4. 
When  viewed  through  a  short  column  of  air  the  line  X  = 
184'96  pi*  is  replaced  by  three  groups  of  faint  lines  observed 
by  Steubing  ("  Ann.  der  Physik,"  33,  573,  1910).  lonisation 
of  the  mercury  atom  by  collision  commences  at  X  =  253 '6  /Ayit, 
equivalent  to  a  fall  of  potential  of  the  colliding  electron  of 
4'8  volts— A.  Lande  ("Phys.  Zeit.,"  15,  793,  1914),  J.  Franck 
and  G.  Hertz  ("Deut.  Phys.  Ges.,"  16,  407,  1914). 

Vacuum  Tube  Discharge. 

Of  the  elements  investigated  by  means  of  the  vacuum 
tube  discharge,  the  remarkable  activity  of  mercury  vapour  in 
the  emission  of  light  of  short  wave  length  has  already  been 
discussed,  two  other  substances  also  exhibit  a  marked  selec- 
tive emission  in  the  ultra-violet  region,  namely  hydrogen 
and  carbon,  the  latter  usually  introduced  into  the  vacuum 
tube  in  the  form  of  one  of  its  oxides,  carbon  monoxide  or 
dioxide.  Lyman  ("  The  Spectroscopy  of  the  Extreme  Vio- 
let," Longmans,  1914)  states  that  hydrogen  surpasses  all 
other  gases  in  the  wealth  and  strength  of  lines  in  the  Schu- 
mann region.  They  extend  at  pressures  of  1  to  5  mm.  from 

6 


82  OZONE 

X  =  90  ftp  to  X  =  167*5  pp,  and  this  light  forms  one  of  the 
most  important  of  all  three  distinct  spectra  which  the 
element  possesses.  St.  John  ("Astrophys.  Jour./'  XXV, 
p.  45,  1907)  found  hydrogen  to  emit  250  times  as  much 
energy  of  short  wave  length  as  a  mercury  vapour  lamp. 
There  appears  to  be  a  distinct  gap  in  light  emission  within 
the  spectral  region  X  =  167*5  pp  to  X  =  243*3  pp. 

Delandres  ("  C.K.,"  106,  842,  1888)  noted  a  great  number 
of  bands  in  the  ultra-violet  spectrum  with  the  rarified  oxides 
of  carbon  in  the  vacuum  tube  within  the  range  X  =  130  /*//, 
and  X  =  210  /^. 

When  consideration  is  taken  of  the  difficulties  in  the 
operation  of  an  arc  lamp  with  iron  or  aluminium  electrodes, 
such  as  the  automatic  adjustment  of  the  arc  gap,  the  removal 
of  the  oxides  produced  during  combustion,  if  the  arc  be  open, 
or  the  volatilisation  of  the  metals  on  to  the  walls,  if  the  arc 
be  of  the  enclosed  type,  as  well  as  the  great  thermal  effects 
produced  by  an  arc  lamp  in  continuous  operation,  which,  as 
we  have  seen,  militates  against  a  high  yield  of  ozone,  it  will 
be  clear  that  the  mercury  vapour  lamp  operating  at  low  volt- 
ages and  relatively  high  internal  mercury  vapour  pressures, 
or  working  at  high  voltages  with  only  a  few  millimetres 
pressure  of  vapour,  is  the  most  suitable  source  of  ultra-violet 
light  which,  up  to  the  present  time,  has  received  systematic 
investigation. 

The  utilisation  of  hydrogen  vacuum  discharge  tubes, 
however,  may  possibly  receive  more  attention  in  the  future, 
since  discoloration  of  the  tube  walls,  so  frequently  noticed  in 
mercury  lamps,  would  be  greatly  minimised  (see  also  Lyman, 
"Astrophys.  Jour.,"  27,  87,  1908). 


PRODUCTION   BY   ULTRA-VIOLET   RADIATION  83 

(b)  Material  for  Lamp  Construction. 
The  walls  of  the  mercury  vapour  lamp  must  be  trans- 
parent to  radiation  of  this  extremely  short  wave  length,  in 
order  that  ozonisation  of  the  surrounding  oxygen  may  be  ef- 
fected. Ordinary  glasses  are  singularly  opaque,  thus  boro- 
silicate  crown-glass,  which  is  the  most  transparent  of  the 
ordinary  glasses,  passes  only  8  per  cent,  of  light  of  wave 
length  X  =  309  pp  (Kriiss,  "  Zeit.  f.  Instrumentkunde,"  23, 
197,  1903),  and  is  opaque  to  light  below  X  =  297  ^/*.  Schott 
of  Jena's  uviol  glass  was  specially  prepared  by  Zschimmer 
("  Zeit.  f.  Instrumentkunde,"  23,  360,  1903)  for  ultra-violet 
transparency.  With  a  thickness  of  1  mm.  fifty  per  cent, 
transmission  of  light  at  X  =  280  w  is  effected,  whilst  a  uviol 
microscope  cover-slip  is  still  transparent  to  X  =  248  pp. 
Zschimmer  further  indicated  ("Phys.  Zeit.,"  8,  611,  1907) 
that  pure  boric  anhydride  and  silica  are  very  transparent 
even  below  X  =  200  ^  (and  even  below  X  =  185  /z/t), 
but  that  the  addition  of  certain  salts  lessens  their  transpa- 
rency. Boric  anhydride  is  slightly  inferior  to  silica,  its  lower 
limit  of  transparency,  according  to  Lyman,  being  X  =  170 
IJLfi,.  Fritsch  ("Phys.  Zeit.,"  8,  518,  1907)  gives  the  follow- 
ing composition  of  a  glass  extremely  transparent  down  to 

X  =  185  fifi  :— 

CaF2          6  parts. 

B203        14     „ 

M.  Luckiesh  ("  J.  Franklin  Inst.,"  186,  111,  1918)  claims 
that  a  special  cobalt-blue  glass  is  more  transparent  than 
ordinary  glass  to  ultra-violet.  With  the  exception  of  Fritsch's 
borate  glass,  which  does  not  appear  to  have  received  any 
technical  application  as  yet,  extremely  pure  fused  silica  is  the 
most  suitable  material  for  lamp  construction. 


84  OZONE 

Hughes  ("Photo  Electricity,"  1913,  p.  137)  has  shown 
that  fused  quartz  is  still  transparent  down  to  X  =  145*0  /JL/JL  ; 
a  thickness  of  0'3  mm.  will  transmit  24  per  cent,  of  X  =  184'9 
fji/ju,  36  per  cent,  of  X  =  197  /*/*,  and  40  per  cent,  of  X  = 
200'2  pp. 

Mention  may  be  made  of  the  naturally  occurring  sub- 
stances, which  are  even  more  transparent  to  ultra-violet 
light  than  fused  silica,  viz.  quartz,  fluorite,  and  rock  salt. 
Quartz  in  very  thin  laminae  is  transparent  down  to  X  =  145 
/jifi,  rock  salt  to  X  =  175  /up,  and  fluorite  to  X  =  123  /i/x,. 

In  the  Quain  apparatus,  which  is  the  only  form  of  quartz 
mercury  vapour  lamp  ozoniser  in  technical  use,  the  lamp 
which  is  of  the  vacuum  type  and  operated  by  a  coil  or  mag- 
neto, at  a  terminal  potential  difference  of  circa  7000  volts, 
is  inserted  in  a  hollow  aluminium  tube  through  which  the 
air  or  oxygen,  undergoing  ozonisation  by  irradiation  from 
the  lamp,  is  passed  at  a  relatively  low  velocity.  No  litera- 
ture has  been  published  dealing  with  the  problem  of  the  in- 
fluence of  gas  velocity  on  ozone  concentration  and  ozone 
production  per  minute,  but  the  following  considerations  will 
indicate  that  the  optimum  conditions  will  very  likely  be 
formed  when  only  a  thin  film  of  air  passes  over  the  lamp  at 
high  velocity.  Dry  and  dust-free  air  is  relatively  transparent 
to  light  above  X  =  186  /-tyu,  but  nearly  opaque  to  light  below 
X  =  178  /iyLt.  Lyman  ("  Astrophys.  Jour.,"  27,  87,  1908) 
states  that  1  mm.  of  air  will  cut  off  most  of  the  light  below 
X  =  185  fjifi,  which,  as  we  have  seen,  is  the  active  light  for  the 
production  of  ozone.  Kreusler  ("Ann.  der  Physik,"  6,  418, 
1901)  gives  the  following  figures  for  the  absorption  produced 
by  20 '45  cms.  of  oxygen  : — 


PRODUCTION  BY  ULTRA-VIOLET  RADIATION  85 

Light  of  Per  Cent. 

Wave  Length.  Absorption. 

186  32-5 

193  6-2 

200  negligible 

whilst  Schumann  observed  an  air  film  only  4  mm.  thick 
(equal  to  '8  mm.  of  oxygen  approximately)  was  sufficient  to 
render  all  lines  below  X  =  178  p/j,  extremely  faint.  With  0'5 
mm.  of  air,  light  down  to  X  =  168  /J,/JL  would  be  transmitted, 
and  below  0*05  mm.  in  air  thickness,  the  spectrum  stretched 
considerably  below  X  =  160  //,/z.  It  will  thus  be  observed 
that  the  ozonising  action  of  ultra-violet  light,  in  so  far  as  it 
is  caused  mainly  by  light  of  wave  length  smaller  than  X  = 
200  /tyi  is  confined  to  but  a  millimetre  thickness  or  so  of  air. 

Lamp  Efficiency. 

Figures  are  not  available  as  to  the  optimum  conditions 
for  the  production  of  ultra-violet  light  from  mercury  vapour 
lamps.  As  is  to  be  expected  the  ultra-violet  light  fraction 
increases  with  increasing  voltage  (see  A.  Tian.,  "  C.R.,"  155, 
141,  1912). 

J.  N.  Pring  ("  Proc.  Eoy.  Soc.,"  96,  204,  1914)  showed 
that  no  oxides  of  nitrogen  or  hydrogen  peroxide  were  formed 
during  operation  and  that  the  average  ozone  content  of  the 
air  in  the  neighbourhood  of  the  lamp  was  O'Ol  per  cent,  at 
760  mm.  and  0'0014  per  cent,  at  30  mm.  air  pressure.  W. 
Chlopin  ("  Zeit.  Anorg.  Chem.,"  71,  2198,  1911),  on  the  other 
hand,  detected  the  presence  of  both  hydrogen  peroxide,  ozone 
and  nitrous  anhydride  by  exposure  for  a  few  minutes  of 
ordinary  moist  air  to  the  rays  of  a  Westinghouse  quartz 
mercury  vapour  lamp. 


86  OZONE 

The  ultra-violet  efficiency  of  the  various  types  of  mercury 
vapour  lamps  on  the  market  was  examined  by  C.  Fabry  and 
Buisson  in  1911  ("C.R.,"  153,  93,  1911),  who  obtained  the 
following  results  :  — 

Percentage  of  Power  Sup- 

Lamp.  plied,  Radiated  in   Wave 

Lengths  below  320. 

Westinghouse     .......         6 

A.E.G  .........       4-7 

,  .........     0-85 

Westinghouse  (ii)        .        .        .     .  •        .        .0*13 

PKODUCTION  BY  IONIC  COLLISION. 

In  the  previous  discussion  we  have  noted  that  molecular 
cleavage  of  oxygen  into  neutral  atoms  with  or  without  sub- 
sequent ionisation  may  be  brought  about  by  absorption  of 
light  energy,  provided  that  this  latter  is  of  the  correct  fre- 
quency for  absorption. 

The  production  of  ozone  depends  primarily  on  the  simplest 

cleavage,  viz  :  — 

O2  ->  0  +  O, 

with  subsequent  synthesis  of  ozone,  whilst  secondary  ozone 
formation  probably  results  from  the  combination  of  charged 
ions,  e.g.  :  — 

2 
(see  W.  W.  Strong,  "  J.  Amer.  Chem.  Soc.,"  50,  104,  1913) 

The  cleavage  and  ionisation  of  the  oxygen  molecule  may 
also  be  brought  about  by  other  means  than  by  the  absorption 
of  light  quanta,  such  as  by  direct  impact  by  a  particles  or 
electrons. 

Madame  Curie  noticed  that  radium  salts  were  effective  in 
ozonising  oxygen  ("  C.B,"  183),  a  point  at  first  disputed  by 


6   +  6  =  0 


PRODUCTION   BY   ULTRA-VIOLET   RADIATION  87 

Ramsay  and  Soddy,  but  finally  confirmed  by  Giesel  and 
Nasini  and  Levi  ("  Atti.  R  Accad.  Lincei,"  17,  46, 1908).  S.  C. 
Lind  ("  J.  Amer.  Chem.  Soc.,"  47,  397, 1912)  and  0.  Schoner 
("C.R.,"  159,  423, 1914)  showed  that  the  a  particles  projected 
from  radium  ozonised  oxygen ;  Lind  showed,  inter  alia,  that 
the  number  of  ozone  molecules  formed  were  equal  to  the 
number  of  ions  made  by  the  a  particles — 

02  ->  O  +  O 
202  +  2O  =  203. 

In  many  of  his  experiments  a  slight  deficiency  in  ozone 
formation  was  observed  from  that  calculated,  but  under  no 
circumstances  was  more  ozone  than  the  theoretical  obtained. 
(See  also  W.  Duane,  "  C.R.,"  153,  336,  1911.)  It  may  be 
noted  in  passing  that  similiar  results  were  obtained  for 
hydrogen  by  W.  Duane  and  Wendt  ("  Phys.  Rev.,"  10,  110, 
1917),  the  presence  of  H3  being  clearly  demonstrated.  F. 
Kriiger  ("  Phys.  Zeit.,"  13,  1040,  1912)  obtained  ozone  by  the 
ionising  action  of  Lenard  rays  obtained  by  the  projection  of 
cathode  rays  through  an  aluminium  window  and  showed,  as 
indicated  in  the  following  tables,  that  more  ozone  was  formed 
per  second  than  ions  in  oxygen,  the  number  of  molecules  of 
ozone  formed  approximately  more  closely  to  the  ionisation 
of  nitrogen  under  similar  conditions  : — 

No.  Ions  Produced  per  Sec.  in  No.  Mols.  O3  Produced  per  Sec. 

O2  x  1014.  N2  x  1014.  4        x  1014. 

0-70  6-0  7-0 

0-56  1-2  1-1 

0'41  1-4  1-4 

0-21  0-5  0*33 

In  the  case  of  radium,  practically  all  the  ozone  produced  is 
formed  through  the  agency  of  the  a  particles,  the  j3  and  7 


88  OZONE 

radiation  producing  but  minor  and  secondary  effects.  The 
energy  associated  with  each  group  of  rays  is  clearly  demon- 
strated from  the  following  figures  ("Phil.  Mag.,"  22,  567, 
1907) :- 

Heating  effect  of  1  gm.  radium  =110  gms.  cal.  per  hr. 
a  rays  =  103*5 
0    „     =      2-0 
7     „     =      4-5 

With  Kontgen  or  X-rays,  on  the  other  hand,  ionisation  is  not 
so  marked,  since  only  about  1  atom  in  1212  is  ionised  by  the 
penetration  of  the  rays  into  a  substance. 

For  ionisation  to  be  effected  by  collision,  the  molecule  or 
atom  must  be  struck  by  the  a  particle  or  electron  with  suffi- 
cient energy  to  discharge  a  valency  from  its  normal  orbit  in 
the  atomic  sphere.  It  will  thus  leave  the  atom  with  a  certain 
critical  velocity  which  it  would  also  acquire  if  it  had  been 
acted  on  by  a  definite  potential  difference.  We  may  there- 
fore equate  the  loss  in  kinetic  energy  sustained  by  the  im- 
pinging a  particle  or  electron  as  a  result  of  collision  with  the 
molecule  and  the  energy  of  discharge  of  the  electron. 

If  m  be  the  mass  of  the  impinging  electron,  VQ  its  incident 
and  v1  final  velocity,  its  loss  in  kinetic  energy  will  be  : 
l/2m(v02  -  v-f)  whilst  the  discharged  electron  of  charge  e  will 
possess  an  energy  Ve. 

Hence  l/2ra«  -  v^)  =  Ne. 

A  discharged  electron  or  a  particle  will  thus  continue  its 
passage  through  the  gas,  causing  ionisation  by  collision  on  its 
way  until  its  velocity  sinks  to  the  value  vs  where 

l/2m^32  =  Ve, 


PRODUCTION  BY   ULTRA-VIOLET   RADIATION  89 

the  minimum  velocity  necessary  to  cause  ionisation  by  colli- 
sion. Below  this  velocity  the  electron  will  merely  adhere  to 
a  neutral  molecule  to  form  a  negatively  charged  ion,  the  a 
particle  will  loose  its  charge  to  a  neutral  molecule  to  form  an 
atom  of  helium  and  a  negatively  charged  gas  ion,  provided  that 
they  have  not  come  in  contact  with  the  walls  of  the  con- 
taining vessel  before  their  journey  is  completed. 
In  the  case  of  ionisation  by  electrons  the  value  of 

—  =  1-77  x  107, 

m 

.'.  V  =  2-82  x  10  - 1G  vz 

or  vcc 

In  the  following  table  are  given  the  electron  velocities  in 
cms.  per  second  and  the  potential  difference  in  volts  required 
to  bring  them  to  rest : — 

V.  v. 

1  5-9  x  107 

10  1-88  x  108 

100  0-595  x  109 

1000  1»88  x  109 

10,000  5-95  x  10* 

100,000  18-8  x  109 

200,000  27  x  109 

For  the  minimum  velocity  required  for  an  electron  to 
cause  ionisation  of  an  oxygen  molecule  by  collision  Franck 
and  Hertz  ("Verb.  d.  Deut.  Phys.  Ges.,"  XV,  34,  1913) 
found  the  value  1'80  x  108  cms.  per  second  corresponding  to 
a  fall  of  9-0  volts. 

This  value  is  in  extremely  good  agreement  with  that  cal- 
culated from  the  critical  wave  length  requisite  for  ionisation 
by  absorption  of  ultra-violet  light  quanta.  It  is  evident  that 


90  OZONE 

the  requisite  energy  equal  to  ~Ve  can  be  supplied  by  the  kinetic 
energy  lost  by  an  impinging  electron,  i.e.  l/2?/w2,  or  by  the 
absorption  of  a  light  quantum  hv  thus — 

Ne  =  I/ton .  v2  =  hv. 

Taking  X  =  135  fifi  we  obtain  the  value  9*20  volts  for  the 
value  of  V  determined  in  this  manner.  A  value  of  8 '6  volts 
being  obtained  by  Compton  ("  Phys.  Kev.,"  8,  412,  1916),  by 
calculation  of  the  work  necessary  to  remove  a  valency  electron 
from  an  atom  possessing  Bohr's  hypothetical  structure. 

Quantitative  agreement  between  the  yield  of  ozone  calcu- 
lated and  that  actually  obtained  has,  as  has  already  been 
mentioned,  been  shown  to  hold  for  the  case  of  ozonisation  by 
a  particle  discharge  by  Lind.  Cases  of  ionisation  and  ozon- 
isation by  electron  emission  have  given  more  variable 
results.  This  is  in  part  due  to  the  great  velocity  and  rela- 
tively small  size  of  the  electrons  which  can  pass  through  a 
vessel  containing  gas  and  come  to  rest  on  the  walls  without 
having  made  a  great  number  of  collisions,  thus  the  major 
part  of  its  kinetic  energy  is  still  retained  when  it  emerges 
from  the  gas  and  strikes  the  walls.  Again,  it  appears  that 
every  collision  which  an  electron  makes  with  a  molecule  of 
oxygen,  with  sufficient  energy  to  dissociate  the  molecule,  is 
not  always  effective  in  doing  so.  According  to  P.  Kirkby 
("  Proc.  Eoy.  Soc.,"  85,  151,  1911),  only  50  per  cent,  of  such 
collisions  are  effective.  The  yield  of  ozone  by  electron  col- 
lision in  oxygen,  therefore,  usually  falls  far  short  of  the 
theoretical  quantity. 


CHAPTEK  VII. 

PKODUCTION  BY  MEANS  OF  THE  SILENT  ELECTRIC  DISCHARGE. 

THE  formation  of  ozone  by  the  action  of  the  silent  discharge 
on  air  is  the  only  process  of  ozone  production  which  has 
received  considerable  technical  development  and  a  great 
number  of  ozonisers  of  various  types  and  designs  have  been 
incorporated  in  installations  for  the  economic  manufacture 
of  ozone  and  ozonised  air. 

It  may  be  stated  at  the  outset  that  we  do  not  possess 
sufficient  information  about  the  mechanism  of  the  silent 
discharge  to  put  forward  a  satisfactory  explanation  as  to  the 
modus  operandi  of  a  "  Siemens  tube,"  nor  can  it  be  said  that 
the  design  and  construction  of  ozonisers  is  on  a  scientific 
basis,  since,  with  the  exception  of  a  few  generalisations  based 
on  experiment  and  a  few  suggestions  based  upon  somewhat 
unsatisfactory  and  frequently  incomparable  theories,  ozonisers 
have  been  built  on  the  rule  of  thumb  and  hit-or-miss 
principle. 

The  following  considerations,  however,  will  indicate  in 
some  measure  the  intricacy  of  the  problem  : — 

If  the  potential  difference  between  a  point  and  a  plate 
separated  by  a  few  millimetres  of  air  space  from  it  be 
gradually  raised,  the  current  potential  difference  curves  be 
plotted,  and  they  will  be  found  to  possess  certain  character- 
istic features  for  both  direct  and  alternating  currents.  The 
V,  i  characteristic  curves  for  point  plate  and  plate  discharges 

have  been  obtained  with  great  accuracy  by  Toepler  ("  Drud. 

(91) 


92 


OZONE 


Ann.,"  7,  477,  1902)  and  Brion  ("  Zeit.  Elektrochem.,"  14,  245, 
1906)  for  direct  currents,  and  by  Cramp  and  Hoyle  ("  Electro- 
chem.  Ind.,"  7,  74,  1909)  for  alternating  currents. 

The  following  indicate  the  series  of  changes  in  the  char- 
acter of  the  discharge  obtained  by  Toepler  :— 

~  ve  point  to 

"*•  t/e  point 


Low  Tens/on 
Arc. 


FIG.  5. 

+  ye  point  to 
—  ue  point 


PEODUCTION   BY   SILENT   ELECTEIC   DISCHAEGE  93 

If  a  very  small  potential  difference  be  applied  between  a 
negative  point  and  positive  plate,  a  small  amount  will  flow, 
the  current  being  carried  entirely  by  the  negative  ions 
present  in  the  gas ;  the  i,  V  curve  will  then  follow  Ohm's 
law  until  the  rate  of  removal  of  gas  ions  by  the  electric  cur- 
rent becomes  equal  to  the  rate  of  supply,  when  the  so-called 
saturation  current  is  arrived  at,  which  is  independent  of  the 
applied  potential  difference. 

As  the  P.D.  is  gradually  raised,  negative  electrons  are 
discharged  from  the  point  and  participate  in  carrying  off  the 
current.  The  area  around  the  point  now  becomes  luminous, 
which  luminosity  extends  towards  the  plate  with  increasing 
P,D.,  the  discharge  becoming  a  typical  brush  accompanied 
by  a  slight  crackling  noise.  At  this  point  positive  gas  ions, 
produced  at  the  anode  by  detachment  of  an  electron  from  a 
gas  molecule  through  collision  with  an  electron  travelling  at 
high  speed,  will  also  augment  the  current  capacity  of  the 
system. 

The  resistance  of  the  air  circuit  now  falls  quite  rapidly 
owing  to  increase  in  conductivity  by  collision  between 
electrons  and  the  gas  molecules  and  the  brush  discharge  is 
converted  into  a  high  tension  arc  discharge.  During  the 
high  tension  arc  discharge,  the  cathode  gets  extremely 
hot  owing  to  bombardment  by  positive  ions  and  the 
thermionic  emission  of  electrons  as  well  as  particles  of 
vaporised  electrode  material  charged  positively  commences, 
resulting  in  a  still  greater  increase  in  conductivity  ;  the  high 
tension  arc  is  therefore  not  stable  but  is  transformed  into  the 
more  usual  low  tension  arc.  For  the  production  of  ozone 
the  electrically  stable  part  of  the  discharge  only,  viz.  the 


94  OZONE 

non-luminous,  the  glow,  and  the  brush  discharge,  come  under 
consideration,  since,  as  we  have  already  had  occasion  to 
observe,  the  high  thermal  effects  associated  with  both  the 
high  and  low  tension  arc  discharges  are  more  than  sufficient 
to  mask  any  electronic  formation  of  ozone. 

We  have  noted  that  the  transformation  of  the  silent  dis- 
charge into  the  high  tension  arc  discharge  occurs  after  the 
whole  inter-electrode  space  has  been  filled  with  the  discharge 
glow,  which  makes  its  first  appearance  in  the  so-called 
corona  light.  The  nature  of  this  luminescence  is  not  clearly 
understood  ;  that  it  is  a  function  of  the  composition  of  the  gas 
is  shown  by  the  experiments  of  E.  Eiesenfeld  ("  Zeit.  Elektro- 
chem.,"  17,  725,  1911),  who  noted  that  the  discharge  is  pink 
in  nitrogen,  blue  in  hydrogen,  white  in  chlorine,  "  like  the 
combustion  of  iron  wire  in  oxygen,"  and  greenish-blue  in 
oxygen. 

Sir  J.  J.  Thomson  ("  Conduction  of  Electricity  through 
Gases,"  1906,  pp.  478-512)  has  shown  that  at  the  moment 
when  both  anode  and  cathode  glow  make  their  appearance 
there  is  a  very  great  increase  in  conductivity  of  the  gas 
space,  and  advanced  the  hypothesis  that  just  prior  to  the 
appearance  of  the  glow  discharge  the  atoms  have  acquired 
internal  energy  by  collision  with  electrons  and  by  absorption 
of  soft  Eontgen  rays,  generated  by  collisions  of  electrons  with 
other  atoms,  until  it  has  nearly  approached  the  critical  value 
at  which  the  atom  becomes  unstable  and  luminous ;  that 
ionisation  precedes  the  luminous  discharge  was  clearly 
indicated  by  D.  Mackenzie  ("  Phys.  Kev.,"  5,  294,  1915). 

According  to  K.  Nesturch  ("  Phil.  Mag.,"  30,  244,  1915) 
there  always  exists  a  definite  ratio  between  the  amount  of 


PEODUCTION  BY   SILENT  ELECTEIC  DISCHAEGE  95 

radiation   and    the   number  of    gas  ions   formed    by   such 
collision. 

Sir  J.  J.  Thomson  and  E.  Threlfall  ("Proc.  Eoy.  Soc.," 
40,  340,  1886)  clearly  showed  that  ozone  formation  in  the 
silent  discharge  tube  was  associated  with  the  production  of  a 
luminous  glow,  whilst  a  similar  conclusion  was  arrived  at  by 
E.  Warburg  ("Ann.  der  Physik,"  17,  1,  1905),  who  advanced 
the  hypothesis  that  ozone  is  only  produced  by  electrons  with 
sufficient  kinetic  energy  to  cause  themselves  to  become 
luminous. 

The  view  that  the  corona  and  brush  discharges  are  at 
least  in  part  due  to  ionisation  by  collision  is  supported  by  a 
series  of  experiments  which  have  been  made  on  the  corona 
"pressure"  phenomenon  by  S.  P.  Farnweld,  J.  Kunz,  and 
especially  Townsend  and  E.  Warner  ("  Phys.  Eev.,"  8,  285, 
1916).  It  is  evident  that  if  the  molecules  break  up  into  ions 
as  a  result  of  ionic  collision  an  increase  of  pressure  should 
result.  In  an  enclosed  gas  space  subjected  to  the  brush  dis- 
charge, this  pressure  increase  has  actually  been  noted,  and 
when  corrected  for  the  unavoidable  temperature  increase  the 
following  relationship  was  found  to  hold  good  : — 

Vi  =  v0Bp, 

where  V  is  the  applied  voltage,  i  the  corona  current,  v0  the 
volume  of  the  gas  subjected  to  the  silent  discharge. 

In  oxygen,  however,  there  is  scarcely  any  increase  in 
pressure,  due  to  the  formation  of  ozone ;  in  other  words,  the 
ozone  formation  is  strictly  proportional  to  the  ionisation. 

It  would  therefore  appear  that  only  a  relatively  small 
portion  of  the  discharge  is  effective  in  the  production  of 
ozone  and  that  the  optimum  results  are  to  be  obtained  when 


96  OZONE 

the  ozoniser  is  so  operated  that  the  luminosity  of  the  silent 
discharge  glow  is  at  a  maximum. 

Many  conflicting  statements  have  been  published  relative 
to  the  yield  of  ozone  per  kilowatt  hour  obtainable  in  ozonisers ; 
on  analysis  these  are  found  to  be  due  to  the  fact  that  many 
investigators  have  ignored  the  primary  consideration  affecting 
the  production,  of  ozone  by  this  means,  viz.  the  relationship 
between  ozone  production  per  kilowatt  hour  and  the  concen- 
tration of  the  ozone.  It  is  evident  that  if  a  definite  volume 
of  air  be  subjected  to  the  silent  electric  discharge,  the  ozone 
concentration  in  that  air  will  rise  to  a  certain  definite  value, 
Co,  the  "  limiting  "  concentration.  When  this  concentration 

is  reached,  the  rate  of  formation  of  ozone  ~~  will  be  equal  to 

its  rate  of  destruction  by  thermal,  catalytic  and  other  effects. 
Thus,  in  the  enclosed  volume  of  air,  the  apparent  ozone  pro- 
duction per  kilowatt  hour  will  be  zero  whilst  the  actual  pro- 
duction will  be  -~.  As  a  first  approximation  it  may  be 
taken  that  the  rate  of  catalytic  deozonisation  is  proportional 
to  the  concentration  of  ozone  or  -—  =  KC0  5  thus  the  energy 

required  to  produce  strong  concentrations  of  ozone  in  a 
stream  of  gas  will  be  a  great  deal  more  than  is  necessary  to 
produce  the  same  amount  of  ozone  in  a  very  dilute  state. 

As  clearly  pointed  out  by  Allmand  ("  The  Principles  of 
Applied  Electro-Chemistry  "),  the  duty  of  an  ozoniser  cannot 
be  obtained  without  a  knowledge  of  the  following  data  :— 

(1)  The  limiting  yield,  or  the  yield  per  ampere  hour  at 
zero  concentration. 

(2)  The  maximum  concentration  of  ozone  obtainable. 


PEODUCTION  BY   SILENT  ELECTRIC   DISCHARGE  97 

(3)  The  rate  of  variation  of  the  yield  with  the  concentra- 
tion. 

INFLUENCE  OF  CURRENT  ON  YIELD  OF  OZONE. 

Owing  to  the  difficulties  inherent  in  the  construction  of 
high-tension  (4000,  10,000  volts)  generators  the  current  for 
ozone  installations  is  usually  derived  from  static  transformers. 

The  static  transformer  may  operate  either  on  alternating 
current  or  on  direct  current  with  a  suitable  interrupter  in  the 
primary  circuit. 

(1)  Static  Transformer  with  Alternating  Current  Generator. 

If  a  conductor  such  as  a  piece  of  wire  describe  simple 
harmonic  motion  in  front  of  the  pole  of  a  magnet  as  is  ob- 
tained in  the  rotation  of  an  armature  between  the  poles  of  a 
magnet,  a  current  varying  in  intensity  from  moment  to 
moment  will  be  induced  in  the  conductor. 

Both  E.M.F.  and  current  time  curves  will  follow  those  of 
the  sine  or  cosine  curve. 


Time 


FIG.  7. 
The  E.M.F.  at  any  time  t  being  given  by  the  relationship 

E  =  Kcos2^ 

the  current  in  a  similar  manner  by  i  =  ig  x  sin 

7 


98  OZONE 

If  the  circuit  were  entirely  non-inductive,  the  volt-ampere 
curves  would  naturally  be  superimposed,  since  at  any  time 
the  current  flowing,  would,  in  accordance  with  Ohm's  law, 
be  strictly  proportional  to  the  applied  E.M.R  In  actual 
practice  self-induction  is  always  present,  being  defined  as  the 
value  of  the  integral — 

"cos  e  dl  dl' 


-if 


r 

where  e'&dl  dl'  in  the  current  circuit. 

The  product  L*  may  be  termed  the  electrical  momentum 
acquired  by  the  current  in  the  circuit,  and  Ohm's  law  has  to  be 
modified  to  include  the  rate  of  change  of  electrical  momentum 
as  well  as  the  instantaneous  current — 

<*©  -  E  cos 

or 

<ft      E.  =  E 

The  solution  of  this  equation  is  given  by — 

E  cos  (-J-  -  a 


,  i 

where  a  =  tan" 


-^^. 
xil 

There  is  therefore  a  lag  between  the  current  and  E.M.F. 
curves,  and  the  maximum  value  of  the  current  never  exceeds 

TC 

where  the  expression  under  the  square  root 


the  "  impedance  "  takes  the  place  of  K. 


PRODUCTION  BY   SILENT  ELECTRIC   DISCHARGE  99 

Several  generalisations  which  have  an  important  bearing 
on  ozoniser  designs  follow  from  these  considerations. 

Firstly,  large  currents  cannot  be  obtained  in  systems  of 
high  inductances,  and  with  increasing  values  of  the  periodicity 

( PpJ  the  inductance  term  becomes  the  only  one  of  significance 
in  the  resistance  of  circuit. 

For  high  frequencies  „  will  be  large,  consequently 
augmenting  tan  a,  making  a  the  angle  of  lag  approximately 

7T 

2 

E0  sin 


equal  to  _-. 


or  when  cos  -~-  =  0,  sin  -^-  =  1,  the  E.M.F.  will  therefore 

be  at  a  maximum  when  the  current  is  zero  and  vice  versa. 
The  Wattage  consumption  ~Ei0i0  will  be  equal  to 


,-,„  27T£  f^TTt  \ 

Baa  cos  -jjp  cos  (-y-  -  a) 


7T 


-  =  o  when  a  is  ^, 
\* 


or 

— for  small  values  of  a 


For  R  =  o  or  a  there  is  therefore  no  energy  consumption, 
whilst  for  some  intermediate  value  there  is  a  maximum  energy 
consumption. 


100 


OZONE 


E 


to  be  a  minimum 


For 


.  /27r\2-r  ,  27rL 

E2  must  equal  \jp]  L2  or  E  =  -=-. 

If  a  condenser  of  capacity  C  be  placed  in  circuit  with  the 
secondary  system  we  can  in  a  similar  manner  obtain  the  ex- 
pression for  the  relationship  between  the  varying  potential 
difference  and  the  charge  on  the  plates. 

T?     ( 

E  ( 


cos  --  - 


27T 


Q 


By  suitable   adjustment   of   the   condenser,  i.e.  making 

T 
C  =  o~Y>  we  can  Se*  larger  amounts  through  the  circuit  for 

a  given  applied  potential  difference  than  if  the  circuit  were 
closed  by  a  wire. 

According  to  Kabakjian  ("  Phys.  Rev.,"  31,  117,  1910)  the 
limiting  yield  of  ozone  increases  with  decreasing  capacity, 


whilst  the  efficiency  of  ozone  production  at  a  definite  con- 
centration increases  with  decreasing  capacity. 

So  far  we  have  assumed  that  the  resistance  E  of  the  cir- 


PKODUCTION  BY   SILENT   ELECTRIC   DISCHARGE          101 

cuit  is  non-variant,  but  as  we  have  had  occasion  to  observe 
the  conductivity  of  the  air  gap  in  the  discharge  apparatus 
varies  with  the  current.  Large  currents  cause  the  gap  to 
become  more  conducting,  permitting  under  a  constant  applied 
E.M.F.  still  higher  currents  to  pass,  ending  finally  in  spark 
and  arc  discharges.  The  sinuous  character  of  the  curve  will 
thus  be  altered,  the  maxima  a,  a  being  increased  for  this 
reason  to  higher  values  b,  b. 

(2)  Direct  Current  with  Interrupter. 
Small  coils  with  magnetic  or  larger  induction  apparatus, 
with  mercury  or  Wehnelt  type  of  make  and  break  on  the 
primary,  also  yield  a  periodic  current  which,  however,  no 
longer  possesses  the  sinuous  character  of  the  alternating 
current  machine,  but  consists  of  a  number  of  periodic  current 
makes  and  breaks  as  is  depicted  in  the  following  curves  : — 


FIG.  9. 

It  will  be  noted  that  under  both  conditions  of  operation 
there  exists  a  great  danger  of  the  spark   discharge   taking 


102 


OZONE 


place  at  the  point  of  optimum  current  flow,  which  practically 
coincides  with  the  period  of  maximum  conductivity  of  the 
gas.  The  spark  discharge  itself  is  oscillatory  in  character 
having  a  period  T  =  2ir^/~LC  and  will  possess  a  curve  of  the 
following  form  : — 


FIG.  10. 

According  to  the  investigations  of  Kabakjian  ("  Phys. 
Rev.,"  31,  117,  1910)  the  brush  discharge  itself  may  under 
conditions  of  high  rates  of  discharge  assume  the  oscillatory 
character  of  the  spark  discharge. 

(a)  Influence  on  Voltage  on  Ozone  Yield. — Chassy  ("  Etude 
sur  TOzone,  C.R.V.,"  135,  1902)  claimed  that  there  was  for  a 
fixed  air  gap  in  a  given  ozoniser  a  practically  linear  relation- 
ship between  the  ozone  yield  per  ampere  hour  and  the 
potential  difference  between  the  electrodes.  Later  experi- 
ments have  shown  that  Chassy's  conclusions  were  not  entirely 
correct.  E.  Warburg  ("Ann.  der  Physik,"  13,  464,  1904) 
showed  that  provided  that  the  potential  difference  applied 
was  sufficient  to  maintain  a  uniform  glow  at  the  point  of  the 
air  gap,  the  yield  of  ozone  was  practically  independent  of  the 
voltage  as  is  shown  by  the  following  figures  :— 


PRODUCTION  BY   SILENT  ELECTRIC  DISCHARGE 


103 


Current 
I  x  10- 6. 

57 


57-5 
57-2 


Voltage  of 
Point. 

.       4,200 

.       9,880 

11,700 


Gms.  Ozone 
per  Coulomb. 

•0375 
•0386 
•0387 


A.  W.  Gray  ("  Ann.  der  Physik,"  13,  477,  1904),  utilising  a 
standard  Siemens  ozoniser,  likewise  found  that  the  yield  per 
coulomb  was  constant  and  independent  of  the  voltage  pro- 
vided that  uniform  illumination  was  maintained  in  the  dis- 
charge space. 

Kabakjian  ("  Phys.  Rev.,"  V,  31.  17, 1910)  found  that  the 
ozone  output  per  coulomb  rapidly  rose  witti  the  voltage  until 
a  potential  difference  of  2,700  volts  with  a  1  mm.  air  gap  and 
3,200  volts  with  a  2  mm.  air  gap  was  reached  after  which  no 
further1  increase  was  noted.  At  these  voltages  presumably 
"saturation"  of  the  field  with  the  brush  discharge  was  just 
effected. 

Influence  of  Current  Density. 

With  a  constant  regime  established  in  the  working  of  an 
ozoniser  the  quantity  of  ozone  produced  per  coulomb  is 
practically  constant,  for  a  point  discharge  on  the  other  hand, 
the  yield  per  coulomb  varies  with  the  current  flowing,  as  is 
shown  from  the  following  figures  obtained  by  Warburg:— 


Positive  Point. 

Negative  Point  and 
Positive  Plate. 

Negative  Point  and 
Positive  Cylinder. 

Voltage. 

Current 
1  x  10-B. 

Grammes 
03per 
Coulomb. 

Current 
I  x  10-  6. 

Grammes 
03per 

Coulomb. 

Current 
1  x  10  -6. 

Grammes 
O3  per 
Coulomb. 

8,420 
10,400 
12,000 

28-8 
57-2 
94-2 

0-0172 
0-0600 
0-0630 

17-4 
25-1 
57'2 

0-0489 
0-0459 
0-0375 

29-1 
94-2 

0-0431 
0-0386 
0-0370 

104  OZONE 

The  variation  in  these  figures  is  attributed  by  Warburg 
to  the  alteration  in  the  volume  of  the  corona  glow  in  the  dis- 
charge space  under  the  varying  voltages.  In  a  Siemens  type 
of  ozoniser  operating  under  the  optimum  conditions,  he  ob- 
tained a  value  of  0-260  gms.  per  coulomb,  a  figure  confirmed 
by  A.  W.  Gray  ("  Ann.  der  Physik,"  13,  477,  1904). 

These  figures  show  some  light  on  the  mechanism  of  ozone 
formation.  Since,  as  can  be  shown  by  electrolytic  decom- 
position or  by  measurement  of  the  charge  of  the  electron, 
together  with  the  number  of  molecules  in  a  gram  molecule, 
96,540  coulombs  are  associated  with  one  equivalent  of  a 
substance,  it  necessarily  follows  that  if  the  ozone  were  pro- 
duced by  some  form  of  electrolytic  action  in  the  gas  space,  a 
limit  is  set  to  the  quantity  of  ozone  obtained  per  coulomb. 

We  may,  of  course,  make  various  assumptions  as  to  the 
magnitude  of  the  electronic  transfer  associated  with  the  for- 
mation of  a  molecule  of  ozone,  but  it  will  be  evident  that  the 
maximum  yield  for  the  minimum  transfer  will  be  obtained 
on  the  assumption  of  the  following  possible  sequence  of 
reactions : — 

02  +  2®  ->  6  +  6 

O2  +  6    -»  03 

i.e.  one  molecule  of  ozone  would  be  formed  for  each  electron 
transferred.  Forty-eight  gms.  of  ozone  should  therefore  .be 
formed  for  a  current  consumption  of  96,540  coulombs,  or 
0'0005  gms.  per  coulomb.  It  is  evident  that  the  quantities 
of  ozone  actually  produced  per  coulomb  exceed  the  amount 
some  520  times,  even  though  these  conditions  represent  those 
most  favourable  to  ozone  formation  by  electronic  transfer. 


PRODUCTION  ,BY   SILENT   ELECTRIC   niSCHARGE  105 

We  are  forced  to  the  conclusion,  assuming  the  accuracy  of 
Warburg  and  Gray's  experimental  data,  that  most  of  the 
ozone  is  of  secondary  origin  and  is  produced  by  collision 
between  electrons  both  primary  and  secondary  and  gas 
molecules. 

Kriiger  and  Moeller  ("  Nernst  Festschrift,"  240,  1912) 
have  suggested  that  one  electron  may  liberate,  in  the  case  of 
the  positive  point  discharge,  seventeen  secondary  electrons, 
and  for  a  silent  discharge  in  metallic  tubes  287  secondary  elec- 
trons, which  would  necessitate  velocities  produced  by  applied 
voltages  of  approximately  100  and  50,000  volts  respectively. 

Influence  of  Wave  Form. 

The  hypothesis  advanced  from  the  previous  considerations 
that  ozone  formation  is  produced  by  inter-molecular  and 
electronic  collision,  and  is  not  a  phase  of  gaseous  electrolysis 
between  the  electrodes,  is  supported  by  a  consideration  of  the 
effect  of  the  wave  form  on  the  ozone  yield.  A.  Vosmaer 
("Ozone,"  Constable,  1916,  p.  70)  states,  "a  very  peaked 
wave  form  would  cause  a  greater  distance  between  regular 
working  tension  and  ordinary  maximum  tension  and  thus 
facilitate  the  brush  discharge.  On  the  other  hand  a  flattened 
curve  would  give  more  available  energy  in  the  domain  of 
working  and  would  give  a  better  output  of  ozone  .  .  .  there 
is  not  so  much  difference  in  wave  form  to  be  of  any  import- 
ance." It  is  clear  that  this  investigator  does  not  consider 
wave  form  of  great  importance  and  his  views  have  been  sup- 
ported by  many  other  observers.  Those,  however,  who  have 
had  occasion  to  make  use  of  the  oscillograph  and  thus  have 
been  able  to  plot  the  wave  form  with  accuracy  have  noticed 


106  OZONE 

that  the  wave  form  does  have  an  important  bearing  on  the 
yield  of  ozone.  Amongst  the  more  important  investigations 
published,  may  be  mentioned  those  of  0.  Frohlich  ("  Elek- 
trotech.  Zeitschrift,"  12,  340,  1901).  E.  Warburg  and  Leit- 
hauser  ("  Ann.  der  Physik,"  4,  28, 17,  1909),  and  G.  Lechner 
("Zeit.  Elektrochem.,"  17,  414,  1911;  21,  309,  1915). 

The  following  is  a  brief  summary  of  the  more  important 
conclusions : — 

The  ozone  yield  per  coulomb  rises  at  first  rapidly  with  the 
periodicity  of  the  alternating  current  and  thereafter  more 
slowly.  Up  to  500  periods  have  actually  been  employed  in 
technical  installations.  It  is  evident  from  a  consideration  of 
the  form  of  the  sine  curve  that  an  increasing  periodicity  in- 
creases the  steepness  of  the  curve,  i.e.  high  periodicity  ensures 

a  large  value  for  the  rate  of  alteration  of  the  current  flow  ^. 

(it 

A  high  periodicity  likewise  lowers  the  minimum  potential 
difference  to  produce  a  silent  discharge  across  a  fixed  inter- 
polar  space  (E.  Eiesenfeld,  "  Nernst  Festschrift,"  374,  1912). 
More  ozone  is  produced  per  coulomb  with  a  periodically 
broken  direct  current  than  with  an  alternating  current  of  the 
same  current  strength  and  periodicity.  A  glance  at  the 
typical  wave  forms  for  these  two  types  of  current  flow  will 

suffice  to  indicate  that  in  the  former  case  the  -r,  values  are 

dt 

much  larger  than  in  the  symmetric  alternating  currents.  A 
further  advantage  of  the  direct  current  is  that  for  the  same 
effective  potential  difference  a  larger  current  can  be  passed 
through  the  silent  discharge  tube  with  a  consequent  increase 
in  ozone  production. 


PRODUCTION  BY  SILENT  ELECTBIC  DISCHARGE          107 

Puschin  and  Kauchtschev  ("  J.  Euss.  Phys.  Chem.  Soc.," 
46,  576,  1914;  have  likewise  shown  that  the  yield  of  ozone 
increases  with  the  frequency,  but  that  the  optimum  frequency 
was  dependent  on  the  applied  voltage  as  indicated  by  the 
following  figures : — 

Periodicity.  Applied  Voltage. 

1240 6500 

950 7000 

660 8000 

For  a  constant  air-flow  an  increase  in  periodicity  above 
these  limits  decreases  the  output  of  ozone,  whilst  an  increas- 
ing air-flow  displaces  the  maximum  towards  increasing 
frequencies. 

In  a  general  way  it  is  not  difficult  to  offer  an  explanation 
of  the  increase  in  yield  of  ozone  per  coulomb  with  rapid 
alteration  of  the  current  flow  or  increasing  tension  of  a 
current  impulse,  if  we  regard  the  formation  of  ozone  due  to 
electronic  and  molecular  collisions. 

At  any  given  instant  we  may  regard  the  current  flow  as 
constant ;  then  proceeding  from  the  negative  to  the  positive 
electrode  there  will  be  a  stream  of  electrons  which,  in  their 
passage  through  the  gas  space  will  ionise  part  of  the  gas 
therein.  The  effect  of  the  electron  stream  on  the  oxygen 
fraction  of  the  gas  is  three-fold  : — 

(a)  A  splitting  of  the  molecule  into  two  neutral  atoms  by 
direct  impact — 

02  =  0  +  0. 

As  we  have  seen  it  is  by  this  disruption  that  ozone  is  chiefly 
formed  in  ultra-violet  light. 

(6)  An  ionisation  of  the  molecule  or  atom  by  impact — 


108  OZONE 

02  -*  62  4-  0 

o  -»6  +  0 

the  positive  ions  so  formed  which  may  be  atomic  or  consist 
of  molecule  clusters  (we  have  already  indicated  that  evidence 
for  clusters  up  to  06  in  complexity  is  at  hand,  this  probably 
represents  the  extreme  upper  limit,  as  large  clusters  easily 
break  down  again).  The  atoms  or  molecules  with  one  or  two 
positive  charges  naturally  proceed  in  the  reverse  direction  to 
the  stream  of  electrons  and  by  impact  and  combination  with 
them  neutralisation  to  atoms  and  molecular  groups  is  once 
more  effected — 

62  +  20  =  02. 

(c)  An  ionisation  of  the  molecule  or  atom  by  impact  and 
adherence  of  the  electron. 

The  electron  having  spent  most  of  its  kinetic  energy  with 
which  it  left  the  electrode  or  dielectric,  may,  on  contact  with 
a  neutral  atom  or  molecule,  not  possess  sufficient  energy  to 
detach  a  valency  electron  from  its  orbit  of  rotation  and  may 
actually  adhere  to  the  system  it  strikes  forming  a  negatively 
charged  atom  or  molecule — 

02  +  0  =  0'2 
0  +  0  =  0'. 

Oppositely  charged  atoms  and  molecules  may  then  react  to 
form  ozone — 

62  +  0'  =  08. 

If  the  current  flux  be  subject  to  violent  changes  then  the 
stream  density  of  both  electrons  and  gas  molecules  will  not 
be  constant,  but  will  proceed  by  a  series  of  irregular  spasmodic 
bursts  of  varying  velocities,  thus  greatly  enhancing  the  pos- 
sibilities of  collision. 


PRODUCTION  BY  SILENT  ELECTRIC  DISCHARGE 


109 


Influence  of  Gas  Flow  and  Composition. 

We  have  already  referred  to  the  fact  that  the  output  of  an 
ozoniser  is  governed  by  the  concentration  of  the  ozone  formed 
during  the  discharge,  since  for  high  concentrations  the  rate 
of  deozonisation  is  increased  and  the  apparent  yield  of  ozone 
per  kw.  hr.  decreased.  In  the  curves  on  next  page  are 
shown  the  relationships  obtained  between  yield  and  concen- 
tration by  utilising  a  standard  Siemens  and  Halske  industrial 
ozoniser  (Erlwin,  "  Zeit.  f.  Sauerstoff  and  Stickstoff,"  e,  143, 
1911). 

Warburg  and  Leithauser  ("Drud.  Ann.,"  28,  1,  1908) 
made  a  very  extensive  series  of  experiments  on  both  glass 
and  metallic  ozonisers  to  determine  the  influence  of  the  ozone 
concentration  derived  upon  the  yield.  Their  results  are 
tabulated  in  the  following  columns : — 


Gms.  per 

Limiting 

Ampere 

Hour  for 

Distance 

Concentra- 

Type of 
Ozoniser. 

between 
Electrodes 

Voltage. 

Period- 
icity. 

Amperes. 

Cos  e. 

tions  of 
Gms.  per 
Cubic 

Yield 

Concen- 
tration 

in  Cms. 

Metre. 

Gms.  1  Amp. 

in  Gms. 

Hr. 

per  Cubic 

Metro 

10 

4 

Mil.  &  Vf&  . 

Glass 

0-51 

8,050 

50 

0-182 

0-185 

38-3 

41-9 

45-5 

3-5 

> 

1-40 

10,080 

50 

0-102 

0-314 







, 

1-40 

16,900 

50 

0-193 

0-243 

52-3 

55-1 

56-8 

59-2 

i 

3-72 

17,500 

50 

0-160 

0-415 

51-1 

56-1 

60-1 

31-2 

M  tal 

2-26 

10,800 

50 

0-182 

0-431 

72-2 

78-4 

82-6 

40-5 

4-66 

13,900 

50 

0-169 

0-450 

53-3 

62-4 

68-4 

20-2 

2-26 

9,480 

100 

0-308 

0-451 

75-7 

81-4 

88-2 

51-6 

4-66 

12,300 

100 

0-280 

0-447 

54-0 

63-0 

69-0 

16-8 

2-26 

9,340 

510 

1-58 

0-537 

57-1 

66-0 

71-9 

18-3 

4-66 

12,100 

510 

1-19 

0-704 

33-0 

58-0 

74-7 

11-4 

110 


OZONE 


10 


IS          ^0          25          30 
Cubic  Metres  per  Hour 


10  15  20      ~~E  T~ 

Cubic  Metres  per  Hour 

FIG.  11. 


Mr  Flow  Rate 


PBODUCTION  BY  SILENT   ELECTRIC  DISCHARGE          111 

It  will  be  noticed  that  the  conditions  most  favourable  for 
the  economic  production  of  ozone  obtain  with  low  concentra- 
tions of  ozone  or  relatively  high  flow  rate  of  air.  High  air 
flow  rates  likewise  serve  to  keep  the  ozoniser  cool,  an  import- 
tant  consideration  since  the  catalytic  decomposition  of  ozone 
is  considerably  accelerated  by  high  temperatures. 

For  technical  operations  the  air-flow  rates  are  accordingly 
adjusted  as  to  give  the  minimum  concentration  of  ozone 
which  will  prove  effective  for  the  process  under  consideration ; 
for  under  these  conditions,  although  more  energy  must  be 
expended  for  pumping  air,  yet  a  very  considerable  economy 
is  effected  in  the  ozone  production. 

The  concentrations  of  ozone  and  the  yields  obtainable  per 
kw.  hr.  are  higher  in  oxygen  than  in  air,  but  the  employment 
of  oxygen  instead  of  air  does  not  prove  to  be  economical  in 
practice,  although  concentrations  up  to  150  gms.  per  cubic 
metre,  or  nearly  three  times  the  maximum  concentration 
attainable  with  air,  can  be  carried  out. 

The  yield,  however,  does  not  increase  indefinitely  with 
the  oxygen  pressure,  thus,  H.  von  Wartenberg  and  L.  Max 
("  Zeit.  f.  Elektrochem.,"  14,  879,  1913),  operating  with  an 
ozoniser  constructed  to  withstand  high  pressures  with  an 
interpolar  space  of  from  2  to  6  mm.  and  a  current  of  1  milli- 
ampere  at  23,000  volts  and  50  periods,  obtained  the  maximum 
ozone  concentration  and  ozone  yield  per  watt-second  at  0'5  to 
1  atmosphere.  Pressures  up  to  5  atmospheres  were  reached 
during  the  course  of  their  experiments. 

It  will  be  noted  that  60  gms.  of  03  in  air,  and  180  gms. 
of  ozone  in  oxygen  per  kw.  hr.  represent  the  best  results 
yet  obtained  with  ozonisers  operating  under  the  optimum 


112  OZONE 

conditions.  Taking  34,000  calories  as  the  heat  of  formation 
of  ozone,  this  represents  a  theoretical  yield  of  1'2  kgms.  per 
kw.  hr.,  or  industrial  ozonisers  have  an  efficiency  of  only 
5  per  cent,  in  air  or  15  per  cent,  in  oxygen. 

Air  suitable  for  ozonisation  should  be  free  from  dust, 
which  favours  the  passage  of  sparks,  and  from  certain  gaseous 
impurities  such  as  oxides  of  nitrogen,  chlorine,  and  more 
especially  water  vapour.  All  three  gases  appear  to  exert  a 
distinct  inhibiting  effect  on  the  formation  of  ozone,  in  addi- 
tion to  a  deozonising  action,  which  is  especially  marked  in 
the  case  of  chlorine  and  nitrogen  dioxide.  The  function  of 
these  gases  as  catalytic  deozonisers  will  be  referred  to  later. 
The  inhibiting  action,  which  is  most  marked  in  the  case  of 
water  vapour,  has  been  attributed  to  the  formation  of  mist, 
which  serve  as  nuclei  for  the  condensation  of  the  gas  ions. 
Under  these  conditions  of  condensation,  the  velocity  of  the 
ions  is  naturally  reduced  and  their  power  of  ionising  or 
breaking  down  a  molecule  into  atoms  is  correspondingly 
lowered.  Undried  air  at  ordinary  temperatures  having  a 
water  vapour  pressure  of  ca.  7mm.  H20  has  a  limiting  yield 
of  ozone  which  is  only  60  to  70  per  cent,  of  the  air  when 
dry  ;  in  the  presence  of  moisture  likewise  the  formation  of 
oxides  of  nitrogen,  due  to  the  thermal  effects  of  sparking  as 
well  as  the  possible  interaction  of  ozone  with  some  form  of 
active  nitrogen  produced  in  the  spark  discharge,  is  usually 
occasioned.  T.  Lowry  ("  J.C.S.,"  101,  1152,  1912),  in  an  in- 
teresting research  on  the  effect  of  the  silent  and  spark  dis- 
charges on  air,  showed  that  in  dry  air  oxides  of  nitrogen  were 
not  formed  under  the  experimental  conditions  by  passage 
through  the  ozoniser  or  the  spark  discharge  gaps.  When 


PRODUCTION  BY   SILENT  ELECTRIC  DISCHARGE          113 

passed,  however,  through  both  in  series,  or  when  the  air 
currents  subjected  to  each  discharge  were  mixed,  oxides  of 
nitrogen  were  produced. 

Lowry  came  to  the  conclusion  that  in  the  spark  discharge 
an  active  variety  of  nitrogen  was  formed  which  was  easily 
oxidised  by  ozone. 

DIELECTRIC  MATERIAL. 

We  have  already  noted  that  the  yield  of  ozone  per  kw. 
hr.  at  a  definite  concentration  increases  with  the  increase  in 
capacity  of  the  ozoniser,  but  that  the  limiting  yield  decreases. 
In  addition  it  must  be  remembered  that  extreme  variation  in 
the  size  of  the  air  gap  or  interpolar  free  space  is  not  permis- 
sible, since  too  small  a  gap  will  permit  the  passage  of  sparks 
and  possible  arcing  with  minute  variations  in  the  applied 
voltage,  whilst  with  a  large  interpolar  free  space  the  luminous 
discharge,  on  which  the  formation  of  ozone  appears  to  be 
largely  dependent,  will  not  fill  or  "  saturate  "  the  field.  It  is 
evident  that  the  use  of  dielectric  material  other  than  air,  by 
which  alterations  in  the  capacity  and  interpolar  distances 
can  readily  be  made,  offers  the  designers  of  ozonisers  a  very 
considerable  latitude  in  these  factors. 

For  the  purposes  of  calculation  we  may  take  a  simple 
plate  form  of  ozoniser  and  consider  the  effect  of  inserting  a 
plate  of  dielectric  material  in  the  air  space  between  the  two 
metallic  electrodes. 

T* 

i 

FIG.  12. 
8 


114  OZONE 

If  the  two  plates  are  charged  with  a  quantity  of  electricity 
of  surface  density  <r,  the  attraction,  at  a  point  P  situate  in  one 
plate,  by  the  other  plate  which  is  separated  from  it  by  the 
interpolar  distance  (a)  is — 

r^irrdra  cos  0  _  fS^rEUan  0crQ  sec2  0d0  cos  6 
"  }       a2  sec2  6        "  }  ~ 


F 


a2  sec2 


-f 


2-7r<7  sin  6  =  27r<r. 


At  a  point  between  the  two  plates  the  attraction  due  to  each 
plate  is  27r«r,  thus,  with  a  positive  charge  on  one  plate  and  a 
negative  charge  on  the  other,  F  =  47r<r. 

If  the  potential  difference  be  V,  and  the  total  charge  Q, 

then  —  =  47ra-  =  -5$,  A  being  the  area  of  each  plate,  or  the 

Cb  A. 

capacity  = 


if  we  neglect  the  irregular  distribution  of  the  stream  lines 
near  the  edges  of  the  plates. 

On  the  insertion  of  a  piece  of  plate  glass  of  thickness  b 

between  the  plates,  the  equivalent  air  thickness  is  ^,  where 

K  is  the  specific  inductive  capacity  of  the  glass,  hence  the 

O  A 

new  capacity  will  be  augmented  to        =  --  -  --  -  ,  and 


(a  - 


the  interpolar  distance  of  air  space  reduced  to  a  -  b.  By 
this  means  we  have  augmented  the  capacity  and  decreased 
the  interpolar  distance  of  the  ozoniser,  and  thus  increased  its 
efficiency  ;  at  the  same  time  the  tendency  to  sparking  and 
rupture  of  the  gap  between  the  electrodes  is  diminished,  since 
the  mechanical  force  per  unit  area  is  likewise  reduced,  and 


PRODUCTION  BY  SILENT  ELECTEIC  DISCHARGE          115 

the  possibility  of  the  flow  of  currents  of  high  densities  natur- 
ally excluded. 

The  following  are  the  approximate  values  of  the  specific 
inductive  capacities  of  the  more  common  dielectric  materials, 
dry  air  being  taken  as  unity  : — 

Material  K. 

Paraffin  wax 2-3 

Eosin 2-6 

Ebonite 8-2 

Sulphur 3-8 

Glass 6  to  7 

The  choice  of  dielectric  material  is  naturally  limited,  since 
it  has  to  withstand  both  high  temperatures  and  the  destruc- 
tive oxidising  action  of  the  ozone.  Amongst  those  which 
have  been  suggested  may  be  mentioned :  shellac,  mica, 
quartz,  glass,  and  artificial  insulators  formed  by  condensation 
of  phenol  and  formaldehyde,  the  so-called  Baekelites ;  glass, 
however,  is  the  only  material  which  has  received  extended 
technical  application. 

The  effect  of  insertion  of  a  solid  dielectric  in  the  inter- 
polar  space  is,  however,  not  so  simple  as  indicated  by  the 
above  considerations,  since  like  other  materials,  not  only  are 
they  imperfect  insulators,  but  many  of  them  possess  the  in- 
teresting property  of  acquiring  residual  charges.  We  may 
regard  the  dielectric  medium  to  consist  of  a  number  of  con- 
ducting particles  embedded  in  an  insulating  material,  the 
fraction  being  smaller  in  the  case  of  the  more  perfect  insula- 
tors. If  this  fraction  be  denoted  by  u,  then  the  specific  in- 
ductive capacity  can  be  calculated  approximately  from  the 
relationship : — 


116  OZONE 


1  -  u 

u  being  determined  from  the  molecular  specific  volume  -,. 

Cu 

When  a  strip  of  dielectric  material  is  charged  up  to  a 
high  potential,  after  discharge  it  will  be  found  to  acquire  a 
small  charge  on  standing,  which  is  often  sufficiently  great  to 
raise  the  potential  of  the  dielectric  up  to  300  volts.  This 
property  of  acquiring  a  residual  charge  is  only  possessed  by 
those  bodies  which  possess  the  property  of  exhibiting  elastic 
after-effects,  it  is  never  shown  by  simple  substances,  but  only 
by  mixtures  such  as  the  glasses  ;  thus  xylene  and  paraffin 
oil  alone  do  not  show  this  effect,  but  on  mixing  the  two,  the 
residual  charge  is  apparent.  One  of  the  constituents  must 
also  possess  a  certain  amount  of  electrical  conductivity. 

It  is  interesting  to  note  that  Eiesenfeld  ("  Zeit.  Elektro- 
chem.,"  725,  1911)  failed  to  obtain  a  brush  discharge  with 
pure  quartz  glass,  although  such  discharges  are  readily  ob- 
tained with  all  forms  of  glass  which  may  contain  quite  large 
percentages  of  silica,  attributable  to  the  slight  electrical  con- 
ductivity of  the  glass,  and  its  possession  of  a  residual  charge. 

Two  other  important  properties  of  dielectric  materials 
must  be  briefly  alluded  to,  namely,  the  alteration  in  conduct- 
ivity with  elevation  of  the  temperature,  and  the  modification 
which  the  dielectric  undergoes  when  subjected  to  electrical 
stress. 

It  is  well  known  that  the  conductivity  of  glass  at  even 
slightly  elevated  temperatures  rapidly  increases.  Mond  and 
Langer  and  Huber  have  actually  used  solid  glasses  as 
electrolytes  at  temperatures  between  200°  and  500°  C.  Local 


PEODUCTION   BY   SILENT   ELECTKIC   DISCHAEGE          117 

overheating  at  one  point  in  the  dielectric,  due  to  a  slight  ir- 
regularity in  the  current  flow,  will  thus  cause  an  increase  of 
conductivity  at  this  point,  with  a  corresponding  augmentation 
of  the  current.  Fusion  and  finally  perforation  of  the  glass 
results.  It  is  for  this  reason  that  porcelains,  which  possess 
temperature  coefficients  even  higher  than  those  of  the  glasses, 
are  unsuitable  for  dielectric  material  in  ozonisers. 

Kerr  noted  that  the  optical  properties  of  dielectrics  were 
considerably  modified  by  the  application  of  electric  stresses. 
These  modifications  are  influenced  by  the  period  of  time  for 
which  the  stress  has  been  applied,  thus  Fleming  (see  "  Amer. 
Suppl.,"  45,  1912)  showed  that  the  conductivity  of  a  dielec- 
tric varied  with  the  frequency  of  the  applied  alternating 
current,  and  Lipp  ("  Hochspanning  Technik  ")  found  that 
the  applied  voltages  necessary  for  perforation  of  thin  sheets 
of  dielectric  material  varied  with  the  period  to  which  the 
dielectric  material  had  been  subjected  to  the  electrical  stress. 
The  perforating  potentials  for  the  usual  dielectric  materials 
are  approximately  as  follows  : — 

Material  Perforating  Potentials 

lal-  in  Kilovolts  per  Cm. 

Mica 600  to  750 

Micanite 400  „  500 

Porcelain 100 

Glass 75  „  300 

Lead  glass 1000 

Air  ("  Amaduzzi  N.  Cimenta,"  3,  51,  1912) — 

7000  volts  per    1-5  cm. 
97,000     „       „    13-5   „ 
100,000     „       „    14       „ 

The  potential  difference  necessary  for  discharge  between 
two  conductors  in  air  varies  with  the  size  and  shape  of  the 


118  OZONE 

conductor,  point  discharge  taking  place  much  more  readily 
than  discharge  across  plane  surfaces.  The  following  figures 
(Abraham  and  Villard,  "Physical  Constants,"  1910)  indicate 
the  potential  difference  required  to  cause  a  30  mm.  spark  to 
strike  between  two  spherical  electrodes  of  varying  radius  :— 

Radius  in  Mm.  Potential  Difference. 

a  (plane)  82,700 

300  85,100 

100  84,400 

50  90,000 

0  (point)  30,500 

The  relationship  between  the  potential  difference  and  the 
striking  distance,  is  also  not  a  simple  one,  as  indicated  from 
the  experimentally  derived  figures  for  spherical  electrodes 
1  cm.  in  radius  — 

Distance  in  Cms.  Kilovolts. 
0-06  27 

0-10  41 

0-40  13-0 

0-50  15-6 

For  very  small  interpolar  distances,  say  1  to  50  /A/A,  the  volt- 
age necessary  is  independent  of  the  distance  and  equal  to 
about  350  volts  (E.  Williams,  "Phys.  Chem.,"  31,  216,  1910). 
0.  Hoveda  ("Phys.  Eev.,"  34,  25,  1912)  gives  the  follow- 
ing relationship  for  point  to  plane  discharges  :— 


c  + 

where  a,  b,  c,  are  constants,  and  D  is  the  interpolar  distance. 
The  use  of  minute  points  corrugated  or  roughened  metal 
electrodes  in  industrial  ozonisers  is  very  frequent,  since,  as 
we  have  seen,  the  presence  of  points  facilitates  the  electrical 
discharge, 


PKODUCTION  BY   SILENT  ELECTRIC   DISCHARGE          119 

L.  Decombe  ("Jour,  de  Physique,"  2,  181,  1912)  has  at- 
tempted to  calculate  the  energy  dissipated  in  a  condenser 
when  connected  to  an  alternating  current  source  ;  he  shows 
that  the  energy  absorbed,  i.e.  V&q  (where  V  is  the  applied 
E.M.F.  and  q  the  charge  on  the  condenser),  can  be  expressed 
in  the  form  :  — 


where  in  is  the  polarisation  and  E0  a  constant,  this  is  equi- 
valent to  the  dissipated  energy  :  — 


or  the  dissipated  energy  is  proportional  to  the  square  of  the 
polarisation  current  and  independent  of  the  periodicity. 
V.  Ehrlich  and  F.  Euss  ("  Zeit.  Elektrochem.,"  19,  330, 1913), 
as  a  result  of  an  investigation  on  the  measurement  of  the 
electrical  quantities  in  a  Siemens  ozone  tube,  showed  that 
the  ionisation  or  polarisation  current  and  applied  potential 
were  always  in  phase,  and  that  the  potential  difference 
across  the  gas  gap  was  a  direct  measure  of  the  energy. 

Chassy  ("Jour,  de  Physique,"  2,  876,  1912),  on  the  other 
hand,  showed  that  the  energy  absorbed  per  second  by  a  gas 
under  alternating  fields,  varied  as  the  charge  Q  and  not  as 
the  square  of  the  charge,  as  in  metallic  conductors. 

An  increase  in  conductivity  of  the  solid  dielectric  is  also 
to  be  expected  from  its  exposure  to  the  ultra-violet  light 
generated  by  the  brush  discharge  in  the  interpolar  air  gap. 
A.  Goldmann  ("Ann.  der  Physik,"  36,  3584,  1911)  has 
shown  that  solid  dielectrics  exhibit  both  an  increase  in  con- 
ductivity and  a  negative  discharge  of  electrons  similar  to  the 
Hallwachs  effect  in  metals  when  subjected  to  ultra-violet 


120  OZONE 

irradiation  ;  the  conductivity  of  sulphur  is  said  to  increase 
1500  times  when  thus  illuminated. 

Types  of  Industrial  Ozonisers. 

Industrial  ozonisers  may  be  grouped  into  two  distinct 
types :  those  in  which  the  silent  discharge  passes  across  the 
air  gap  without  the  interposition  of  any  solid  dielectric,  and 
those  in  which  one  or  both  of  the  electrodes  are  protected  by 
some  suitable  dielectric  material,  usually  glass. 

Non-dielectric  Ozonisers. 

Several  attempts  have  been  made  to  produce  an  ozoniser 
without  any  dielectric  and  although  large  units  on  various 
systems  have  been  constructed  from  time  to  time,  their 
efficiency  has  usually  been  extremely  low  and  at  the  present 
time  all  industrial  ozonisers  contain  dielectric  material. 

Schneller,  Wisse  and  Sleen,  in  1894,  were  the  first  to 
construct  large  ozonisers  without  a  dielectric.  One  dis- 
charging surface  consisted  of  a  sheet  of  platinum  gauze  to 
provide  a  great  number  of  small  points  some  30  mm.  from 
the  other  surface  formed  of  perforated  metal  sheet,  the  air 
current  being  forced  through  the  perforations  in  the  sheet. 
The  two  electrodes  were  cylindrical  in  shape  and  were 
mounted  in  glass  tubes.  The  operating  voltage  was  at  first 
15,000,  which  was  subsequently  raised  to  50,000.  To  avoid 
the  formation  of  an  arc  discharge,  which  in  the  absence  of 
any  dielectric  between  the  electrodes  would  be  attended  with 
disastrous  effects,  a  high  resistance  was  inserted  in  series 
with  the  ozoniser. 

These  investigators  found  much  difficulty  in  the  con- 
struction of  a  resistance  suitable  for  the  purpose.  It  was 


PEODUCTION  BY  SILENT  ELECTEIC  DISCHARGE          121 

necessary  to  obtain  a  suitable  resistance  of  1*5  megohms, 
capable  of  carrying  O01  ampere  ;  moist  unglazed  porcelain 
and  glass  tubes  containing  80  per  cent,  glycerine  were  found 
most  suitable.  Vosmaer  ("  Ozone,"  p.  94)  showed  that 
sparking  and  arcing  could  not  be  avoided  even  with  this 
artifice,  and  that  the  external  resistance  served  to  increase 
the  capacity  of  the  circuit  rather  than  the  resistance. 

Slate  was  found  to  be  more  suitable  than  either  glycerine 
or  moist  unglazed  porcelain,  which  rapidly  lost  its  humidity 
and  suffered  an  increase  in  resistance. 

Patin's  ozoniser  followed  a  similar  construction ;  re- 
frigeration of  the  air  prior  to  ozonisation  was  employed  to 
increase  the  yield,  and  a  number  of  small  metallic  perforated 
prisms  enclosed  in  a  single  unit  comprised  the  electrodes  in 
lieu  of  the  perforated  plates  and  gauzes  in  Schneller's  ap- 
paratus. 

Various  improvements  in  design  of  this  type  of  ozoniser 
were  introduced  by  Tindal  in  1894  and  more  especially  by 
De  Frise  in  1904.  De  Frise's  plant  was  actually  employed 
for  a  short  period  in  the  sterilisation  of  water  on  a  large 
scale  at  the  Saint-Maur  Water  Works  at  Paris,  but  the 
dielectric  Siemens-Halske  ozoniser  was  subsequently  in- 
stalled and  adopted  as  proving  itself  more  economical  in 
operation. 

Tindal  employed  Schneller's  method  of  augmenting  the 
capacity  of  the  system  by  the  insertion  of  liquid  resistances 
in  series  with  the  ozoniser.  The  ozoniser  consisted  essenti- 
ally of  a  system  of  compartments  separated  by  perforated 
metal  plates,  containing  alternately  a  set  of  electrodes  and  a 
water-cooling  device, 


122 


OZONE 


The  perforated  metal  plates  attached  to  the  cooling  tubes 
served  as  one  set  of  electrodes  of  the  ozoniser  and  fine  gauze 
as  the  other  set. 


FIG.  13. 

The  water  was  maintained  in  active  circulation  to  aug- 
ment the  cooling. 

De  Frise  likewise  adopted  Tindal's  arrangement  of  alter- 
nating ozonisation  and  refrigeration,  but  adopted  a  different 
arrangement  for  the  distribution  of  the  electrodes. 

These  consisted  essentially  of  a  series  of  crescent-shaped 
discs  mounted  in  parallel  each  with  its  own  liquid  inductive 
capacity  in  a  water-cooled  metallic  trough  which  served  as 
the  other  electrode. 


FIG.  14, 


PRODUCTION  BY   SILENT  ELECTRIC   DISCHARGE          123 

Each  disc  was  furnished  with  a  number  of  minute  points  to 
facilitate  the  discharge. 

These  ozonisers  were  in  successful  operation  with  voltages 
up  to  10,000,  although  an  applied  voltage  of  from  15,000  to 
20,000  was  usually  employed. 

It  possessed  distinct  advantages  over  Tindal's  apparatus 
in  that  the  distance  between  the  electrodes  could  be  reduced 
very  considerably  without  risk  of  arcing,  thus  increasing  the 
efficiency  of  the  apparatus. 

Various  types  of  mechanical  ozonisers  operating  without 
the  interposition  of  any  dielectric  have  been  constructed  from 
time  to  time,  the  most  successful  being  that  of  Otto. 


FIG.  15. 

The  frequent  occurrence  of  arcing  in  non-dielectric 
ozonisers  as  well  as  the  necessity  for  obtaining  a  very  small 
polar  distance  between  the  electrodes  for  efficient  working, 
led  Otto  to  construct  an  ozoniser  in  which  by  the  rotation  of 
one  electrode  the  polar  distance  was  always  varying,  so  that 
if  for  any  accidental  cause  an  arc  should  be  formed  at  one 


124  OZONE 

point,  it  would  speedily  be  broken  again  by  the  subsequent 
increase  in  the  arc  gap.  A  diagram  of  an  improved  form  of 
Otto  ozoniser  is  shown  above.  One  electrode,  which  is  fixed, 
consists  essentially  of  an  aluminium  disc,  studded  with  a 
great  number  of  small  points,  the  rotor  being  a  metallic  disc 
segmented  with  insulating  material.  Up  to  80,000  volts  have 
been  employed  on  these  machines. 

None  of  these  ozonisers  have  proved  sufficiently  economi- 
cal or  reliable  for  industrial  operations  in  which,  at  the 
present  time,  ozonisers  containing  one  or  more  dielectrics 
are  practically  universally  employed. 

Ozonisers  Containing  a  Dielectric. 

Tubular  Ozonisers. — The  present  construction  of  service- 
able apparatus  for  the  production  of  ozone  by  means  of  the 
silent  discharge  has  developed  from  the  simplest  forms  of 
ozone  tubes  constructed  by  von  Siemens,  in  1857,  in  Germany, 
by  Brodie  in  England,  and  Berthelot  in  France. 

Siemens'  first  ozoniser  consisted  essentially  of  two  coaxial 
glass  tubes,  the  outer  coated  externally  and  the  inner  intern- 
ally with  tin-foil,  the  air  being  passed  through  the  annular 
space.  Brodie  substituted  water  as  electrode  material  in  the 
place  of  tin-foil,  and  Berthelot  used  sulphuric  acid. 

These  ozonisers  thus  contained  two  dielectric  plates  cover- 
ing each  electrode,  and  in  practice  gave  serviceable  and 
uniform  results. 

Brodie's  and  Berthelot's  system  gives  somewhat  better 
results  than  that  of  Siemens',  since  it  is  a  simple  matter  in 
these  forms  to  arrange  for  efficient  cooling  of  the  electrodes 
and  interpolar  space.  When  oxygen  is  employed  instead  of 


PEODUCTION   BY   SILENT   ELECTEIC  DISCHAEGE          125 

air,  a  10  per  cent,  ozone  concentration  can  easily  be  obtained 
at  room  temperature  and  over  20  per  cent,  at -25°  C. 

Yet  another  type  of  apparatus  was  introduced  by  Dr. 
Oudin  and  Andreoli  in  1893.  As  one  electrode  a  simple 
form  of  glass  vacuum  tube  was  used,  electrical  connection 
being  furnished  by  means  of  a  sealed-in  platinum  wire, 
which  in  Andreoli's  apparatus  ran  through  the  whole  length 
of  the  tube.  The  second  electrode  consisted  of  a  series  of 
equally  spaced  indented  steel  rings,  surrounding  the  vacuum 
tube,  or  a  copper  spiral  coiled  in  the  form  of  a  helix  round 
the  tube;  the  whole  unit  being  inserted  in  a  glass  tube 
through  which  the  current  of  air  was  passed.  Gaiffe  and 
E.  Chatelain  at  a  later  date  modified  Oudin  and  Andreoli's 
apparatus  by  substituting  a  second  annular  vacuum  tube  in 
place  of  the  steel  or  copper  electrode  by  the  former  investi- 
gators. 

Small  types  of  Oudin  and  Andreoli's  machines  were  at 
one  time  popular  for  medical  work  but  have  not  been 
developed  for  industrial  purposes. 

The  Siemens  type  of  ozoniser  was  developed  by  Froh- 
lich  and  Erlwein,  of  Siemens  and  Halske,  to  its  present  form, 
which  has  proved  to  be  eminently  suitable  for  industrial 
purposes. 

The  tubular  form  of  the  earlier  form  of  apparatus  was 
preserved,  but  various  modifications  were  made  in  the  dis- 
position of  the  electrodes. 

For  the  internal  tin-foil  coated  glass  tube,  a  cylindrical 
aluminium  tube  was  substituted,  maintained  in  position  in 
the  outer  tube  (of  glass),  and  separated  from  it  by  1'5  mm. 
by  means  of  three  ebonite  spring  plungers.  As  the  other 


126 


OZONE 


electrode,  Brodie's  idea  of  using  water  was  adopted  and  pro- 
vision was  made  for  circulation  to  ensure  cooling,  the  cast- 
iron  frame  containing  the  ozoniser  and  water  cooling  being 
carefully  earthed.  The  smallest  technical  unit  contains  two 
ozone  tubes,  the  largest  eight  in  the  same  water-cooling 
frame,  which  is  provided  with  a  glass  inspection  plate. 
The  operating  voltage  varies  between  4000  and  10,000 
volts. 


Ozone 


\luminium  Cylinder 
G/ass  Tube 


FIG.  16. 

We  have  already  discussed  the  results  obtained  by 
Erlwein  with  this  type  of  apparatus  and  will  refer  to  the 
technical  applications  in  a  subsequent  section. 

A  similar  type  of  apparatus  has  been  developed  by  the 
General  Electric  Company,  but  the  somewhat  cheaper  and 
equally  efficient  enamelled  iron  has  been  substituted  for  the 
aluminium. 

The  Westinghouse  Company  in  the  Gerard  ozoniser  sub- 


PRODUCTION  BY  SILENT  ELECTRIC  DISCHARGE          127 

stitute  oil  for  water  as  the  external  electrode,  and  utilise,  as 
in  the  early  Brodie  tube,  a  double  dielectric  system. 

According  to  the  investigations  of  Vosmaer,  the  economi- 
cal production  of  high  concentrations  of  ozone  is  realisable 
in  this  type  of  apparatus. 

Various  other  forms  of  tubular  ozonisers  have  been  the 
subject  of  patent  literature,  a  few  of  which,  such  as  the 


H 

Air 

i 

j. 

on/s< 

q/vi 

s 

?d 

afi 

Oil 

k 

%*~ 

/V- 

/ 

-~J 

Oil 

G/ass 


Metal 


FIG.  17. 

Elworthy,  Yarnold  and  Gaiffe,  have  been  sporadically  de- 
veloped for  a  short  time  by  small  companies,  only  to  sink 
again  into  oblivion.  In  Europe  the  only  representation  of 
this  type  of  ozoniser  which  may  be  said  to  have  established 
its  footing  industrially  is  that  of  Siemens  and  Halske. 

Plate  Type  Ozonisers. — In  the  tubular  type  of  ozoniser, 
owing  to  the  disposition  of  the  electrodes,  it  would  appear 


128  OZONE 

that  the  use  of  cooling  water  is  essential  in  order  to  keep 
the  interpolar  space  and  the  inner  electrode  relatively  cool, 
which,  as  we  have  seen,  is  one  of  the  most  important  factors 
in  the  economic  production  of  ozone.  Since  the  thermal 
radiation  obtainable  from  two  parallel  plates  is  much  greater 
than  in  the  tubular  form,  where  the  radiation  is  practically 
confined  to  the  interpolar  area  and  to  the  external  surface  of 
one  electrode,  the  provision  of  cooling  water  is  not  essential 
for  efficient  operation.  Although  better  results  are  obtain- 
able in  well-designed  ozonisers,  where  water  cooling  is  used 
in  addition  to  the  cooling  effected  by  the  air  passage,  yet 
with  air-cooled  plate  ozonisers  of  the  proper  design  and  with 
the  enhanced  capital  and  running  costs  entailed  in  water- 
cooling  devices,  the  cost  of  ozone  production  by  either 
method  is  approximately  equal.  Nevertheless,  in  those  cases, 
usually  exceptional,  where  high  ozone  concentrations  or  low 
air  current  velocities  are  required,  provision  for  water  would 
appear  desirable.  Modern  plate  form  ozonisers  have  con- 
sequently developed  on  these  two  distinct  lines,  those  in 
which  air  cooling  only  is  utilised  and  those  in  which  supple- 
mentary water  cooling  is  made  use  of. 

Air-cooled  Plate  Ozonisers.  —  The  earlier  ozonisers  of 
this  type,  such  as  those  of  Villon  and  Genin  of  Prepoignot, 
and  an  experimental  one  of  Otto's,  were  not  a  success,  since  it 
was  found  impossible  to  prevent  a  very  considerable  rise  in 
temperature  during  continuous  operation,  resulting  in  a 
serious  loss  of  efficiency. 

The  first  technical  ozoniser  which  showed  promise  was 
that  of  Andreoli,  and  possessed  the  great  merits  of  simplicity 
of  construction  and  uniformity  in  operation. 


PRODUCTION   BY  SILENT  ELECTRIC   DISCHARGE          129 

Andreoli's  ozoniser  consisted  of  a  series  of  serrated  alu- 
minium plates  separated  from  each  other  by  a  sheet  of  glass, 
ca.  2  mm.  thick,  and  an  air  gap.  Each  plate  had  an  area  of 
30'5  x  30*5  cms.,  and  possessed  17,760  points  formed  by  in- 
dentation of  the  serrations.  These  units  were  mounted  in  a 
wooden  box  ;  provision  was  made  for  possible  expansion  and 
contraction,  and  various  artifices  were  devised  to  ensure  the 
plates  being  inserted  quite  parallel  with  each  other. 

With  an  eight-plate  ozoniser,  operating  with  a  voltage  as 
low  as  3,300  volts,  an  energy  consumption  of  550  watts  could 
easily  be  maintained,  producing  ca.  60  to  100  gms.  of  ozone 
per  kw.  hr.,  naturally  at  very  low  concentrations.  In  his 
later  models  Andreoli  likewise  introduced  water  cooling,  and 
could  consequently  elevate  to  applied  voltage  from  3,300  to 
10,000,  without  any  undue  rise  in  temperature. 

Experimental  ozonisers  of  similar  construction  have  been 
designed  by  Vohr  and  Vosmaer  but  do  not  appear  to  have 
been  developed  for  industrial  purposes.  The  only  air-cooled 
type  plate  ozoniser  which  appears  to  have  outgrown  the  ex- 
perimental state,  and  to  be  actually  employed  in  the  various 
industries,  is  that  of  the  Ozonair  Company. 

This  ozoniser  is  extremely  simple  in  construction  and 
efficient  in  operation.  The  electrodic  discharge  through  the 
glass  dielectric  plates  and  air  gap  is  facilitated  by  employing 
flat  sheets  of  metallic  aluminium  alloy  gauze  as  electrode  ma- 
terial ;  this  possesses  the  dual  advantage  of  an  even  distribution 
of  points  over  the  whole  electrode  area,  and  of  being  practi- 
cally resistant  to  corrosion  or  tarnishing.  Cooling  is  effected 
by  the  air  current  and  concentrations  up  to  3,  and,  for 

short  periods,  even  4  gms.  of  ozone  per  cubic  metre  may  be 

9 


130 


OZONE 


obtained  under  conditions  of  actual  operation  without  undue 
elevation  of  the  temperature.  The  operating  voltage  is  usu- 
ally 5000  at  periodicities  varying  from  25  to  100.  Tests  have 
shown  that  at  a  concentration  of  2  gms.  per  cubic  metre  the 
output  may  exceed  100  grns.  per  kw.  hr. 

Aluminium 

Glass, 


FIG.  18. 

Water-Cooled  Plate  Ozonisers. — This  type  of  ozoniser 
was  developed  chiefly  by  the  early  investigations  of  Otto, 
and  subsequently  by  those  of  Marmier  and  Abraham ;  as  a 
result  the  Otto  Marmier  Abraham  Ozoniser  was  constructed, 
which  has  found  a  by  no  means  insignificant  number  of  in- 
dustrial applications,  especially  in  France. 

The  unit  consists  of  a  pair  of  hollow  disc  electrodes  with 
perfectly  plain  faces  which  are  protected  by  plates  of  the 
dielectric,  in  this  case  glass,  some  2  mm.  thick.  The  inter- 
polar  air  space  through  which  the  air  flows  in  a  radial 
direction  varies  from  1'3  to  1'8  mm.  in  width.  Each  electrode 


PBODUCTION  BY  SILENT  ELECTEIC   DISCHAEGE 


131 


is  cooled  by  running  water,  a  broken  fall  providing  against 
short  circuiting  through  the  earth. 


Air 


FIG.  19. 

The  outer  casing  is  made  of  earthenware,  the  air  being 
forced  in  at  one  end  and  emerging  after  ozonisation  at  the 
other. 


The  ozoniser  operates  successfully  at  a  voltage  of  30,000 
to  40,000  volts,  although  voltages  much  lower  than  this  can 
naturally  be  employed,  12,000  being  quite  normal. 


132  OZONE 

Otto  introduced  for  these  machines  a  simple  form  of 
electric  safety  valve  consisting  of  two  horn-shaped  electrodes, 
separated  by  a  variable  air  gap  and  placed  in  series  with  the 
ozoniser. 

An  accidental  rise  in  the  operating  voltage  would  cause  a 
discharge  to  take  place  across  the  air  gap  when  suitably  ad- 
justed, and  thus  obviate  any  break-down  in  the  dielectric 
plates  of  the  ozoniser. 

Labille  suggested  the  use  of  mica  as  a  dielectric  in  place 
of  glass  plate  in  ozonisers  of  this  type,  but  in  practice,  owing 
to  the  disintegration  of  this  laminated  material,  unfavourable 
results  were  obtained. 


CHAPTEK  VIII. 

THE  CATALYTIC  DECOMPOSITION  OF  OZONE. 

SINCE  the  quantity  of  ozone  in  equilibrium  with  atmospheric 
oxygen  at  ordinary  temperatures  is,  as  we  have  seen,  almost 
vanishingly  small,  it  follows  that  in  ozonised  oxygen  or 
ozonised  air  conditions  of  unstable  equilibrium  obtain,  and 
the  apparent  stability  of  the  ozone  is  due  to  the  fact  that  the 
equilibrium  is  "  frozen,"  or  the  rate  of  decomposition  of  ozone 
at  these  temperatures  is  almost  negligible.  The  rate  of  de- 
composition of  the  ozone  in  excess  of  the  minute  amount 
present  at  the  true  equilibrium  can  be  accelerated  in  a  variety 
of  ways,  such  as  by  the  addition  of  catalytic  materials,  either 
solid,  liquid,  or  gaseous,  by  photo-chemical  action,  or  by 
purely  thermal  methods,  by  slightly  elevating  the  temperature. 

The  rate  of  decomposition  of  ozone  has  been  the  subject 
of  many  investigations.  Shenstone  in  1897  considered  dry 
ozone  to  be  extremely  unstable,  and  to  undergo  decomposi- 
tion with  extreme  rapidity.  At  a  later  period,  H.  E.  Arm- 
strong showed  that,  in  the  absence  of  oxides  of  nitrogen,  the 
rate  of  decomposition  was  sensibly  lessened. 

Jahn  ("Zeit.  Anorg.  Chem.,"  48,  260,  1906),  at  Nernst's 
suggestion,  conducted  a  series  of  experiments  on  the  rate 
of  decomposition  with  a  view  to  elucidating  the  mechan- 
ism of  decomposition.  The  ozone  molecule  may  undergo 

(133) 


134  OZONE 

decomposition  in  a  variety  of  ways,  as  indicated  in  the  fol- 
lowing equations  :  — 

(i)          208  =  302 

(ii)  03  =  02  +  0 

(iii)  O3  +  O  =  202. 

Jahn's  experimental  results  appeared  to  indicate  that  the  rate 
of  decomposition  could  be  formulated  in  the  expression  — 

<fo(0,)       KC(03)2 
dt  C(02)' 

He  consequently  argued  that  the  decomposition  followed  the 
course  indicated  by  equations  (ii)  and  (iii)  on  the  assumption 
that  the  first  reaction  was  rapid  and  reversible  — 

03  ^  02  +  0, 

and  the  second  slow  and  irreversible— 
03  +  0  ->  202. 

Perman  and  Greaves  ("Proc.  Eoy.  Soc.,"  4,  807,  353,  1908) 
likewise  stated  that  the  rate  of  decomposition  was  inversely 
proportional  to  the  concentration  of  the  oxygen  — 

K 


dt      ~  0(02)' 

Chapman  and  Clark  ("  Trans.  Chem.  Soc.,"  93,  1638,  1908) 
and  Chapman  and  Jones  ("  Trans.,"  254,  2463,  1910)  subjected 
the  whole  matter  to  an  exhaustive  examination  ;  they  showed 
that  Jahn's  and  Perman's  interpretation  was  not  correct  and 
that  very  serious  errors  due  to  the  catalytic  effect  of  the  sur- 
face of  the  glass  vessel  vitiated  the  accuracy  of  their  results. 
These  investigators  found  that  the  rate  of  decomposition  was 
proportional  to  the  square  of  the  concentration  of  the  ozone 
and  independent  of  the  oxygen  pressure  ; 


THE   CATALYTIC  DECOMPOSITION   OF   OZONE  135 

^  =  KC(0,)«. 

E.  Weigert  ("  Zeit.  Phys.  Chem.,"  80,  78,  1912),  however, 
obtained  values  corresponding  to  an  order  of  decomposition 
somewhat  exceeding  two  in  the  dark. 

The  rate  of  decomposition  is  greatly  accelerated  by  rise 
in  temperature,  being  almost  instantaneous  at  270°  C.  and 
quite  rapid  at  100°  C.  This  fact,  namely,  that  the  equilibrium 
is  not  to  be  regarded  as  "  frozen  "  until  room  temperatures 
are  arrived  at  is,  as  we  have  seen,  the  factor  militating  against 
the  successful  thermal  production  of  ozone  in  contradistinc- 
tion to  nitric  oxide  where  the  equilibrium  is  practically 
"  frozen  "  at  600°  C. 

The  decomposition  may  likewise  be  accelerated  by  solid 
catalytic  agents  such  as  the  following : — 

Ag,  Cu,  Co,  Ni,  Cr203,  Pb304,  V205,  Mn02,  Ti02,  Th02, 

Ce02,  U308,  W203,  BaO,  CaO,  Hg,  Ni,  Pt,  Pd,  V, 
and  powdered  glass. 

The  catalytic  activity  of  platinum  black  is  extremely 
great.  Mulder  and  v.  d.  Meulen  ("  Eec.  Trans.  Chem.  Pays.- 
Bas.,"  i,  167)  and  Warburg  ("  Berl.  Akad.,Ber.,"  i,  1900, 1176, 
1901)  noted  the  rapidity  of  decomposition  of  ozone  rich  gas 
when  passed  over  cold  or  warm  platinum  black,  whilst  Elster 
and  Geitel  ("  Ann.  der  Physik,"  2,  39,  321, 1890)  and  Well  and 
Kopp  ("Jahresber.  Chem.,"  270,  1889,322,  1890)  observed 
the  formation  of  ozone  by  passing  oxygen  over  hot  platinum, 
indicating  the  reversibility  of  the  catalytic  activity  of  the 
platinum. 

In  many  cases  the  activity  of   these  catalytic  materials 


136  OZONE 

can  be  attributed  to  the  formation  and  subsequent  decom- 
position of  an  unstable  oxide  or  peroxide,  e.g. 

2Ag  +  03  ->  Ag20  +  02  ->  Ag. 

Manchot  ("Ber.,"  39,  1510,  1906;  40,  2891,  1907;  and  42, 
3948,  1908),  as  a  result  of  a  series  of  experiments,  obtained 
some  interesting  results  with  silver  as  a  catalytic  agent.  He 
noted  that  pure  silver  was  relatively  stable  in  the  presence 
of  ozone,  it  being  necessary  to  warm  the  metal  up  to  24°  C. 
before  decomposition  of  the  ozone  and  formation  of  silver 
oxide  occurred. 

A  trace  of  iron,  usually  obtained  from  the  emery  powder 
employed  for  cleaning  the  silver,  serves  as  an  excellent 
catalyst  promoter,  silver  containing  but  a  minute  trace  of 
iron  reacts  already  at  normal  temperatures,  and  less  than 
O'Ol  per  cent,  of  ozone  can  be  detected  by  this  means.  Man- 
chot states  that  silver  thus  prepared  is  even  more  sensitive 
than  alcoholic  tetramethyl  base  paper  and  that  the  ozone 
present  in  hot  flames  can  easily  be  detected. 

J.  Strutt  ("Proc.  Koy.  Soc.,"  87,  302,  1912)  examined 
Chapman  and  Jones'  results  from  a  statistical  point  of  view. 
He  showed  that  in  the  case  of  catalytic  decomposition  at  a 
gas-solid  surface,  the  rate  of  decomposition  depends  on  the 
number  of  collisions  of  a  gas  molecule  with  the  surface  neces- 
sary to  effect  the  rupture  of  the  molecule  or  reaction  with 
the  surface. 

Calculating  the  number  of  collisions  necessary  to  effect 
the  decomposition  of  ozone  in  the  presence  of  metallic  silver 
from  the  minimum  area  of  silver  necessary  to  effect  such 
change,  he  showed  that  only  1'6  collisions  of  an  ozone  mole- 
cule with  the  silver  was  necessary,  or  practically  every  col- 


THE  CATALYTIC  DECOMPOSITION  OF  OZONE  137 

lision  was  effective.  In  the  absence  of  any  surface,  i.e.  in  free 
space,  Strutt  concluded  that  at  100°  C.  two  molecules  of  ozone 

must  collide  6  x  1011  times  before  a  favourable  collision  re- 
sulted. 

The  effect  of  various  gases  on  the  rate  of  decomposition 
of  ozone  was  likewise  investigated  by  Chapman  and  Jones, 
who  showed  that  oxygen,  nitrogen  and  carbon  dioxide  had 
no  effect,  whilst  nitrogen  dioxide  and  chlorine  accelerated 
the  rate  of  decomposition.  The  influence  of  water  vapour 
was  not  very  marked.  Shenstone  ("  Trans.  Chem.  Soc.,"  71, 
47,  1897)  stated  that  water  vapour  did  not  retard  the  forma- 
tion of  ozone ;  but,  as  pointed  out  by  Armstrong,  Shenstone 
probably  included  any  nitrogen  dioxide  formed  at  the  same 
time  in  his  ozone  estimations.  Warburg  and  Leithauser 
("Ann.  der  Physik,"  IV,  20,  757,  1906)  found  that  the  for- 
mation of  ozone  was  retarded  by  water  vapour  whilst  the  rate 
of  decomposition  was  not  affected.  The  specific  influence  of 
water  vapour  in  accelerating  the  decomposition  of  ozone  was, 
however,  noted  by  Warburg  ("  Sitzungs  K.  Akad.  Wiss.," 
Berlin,  644,  1913)  in  the  course  of  his  investigations  on  the 
photo-catalytic  decomposition  of  ozone. 

It  may  be  concluded  from  these  experiments,  as  well  as 
from  the  somewhat  scanty  data  collected  on  the  rate  of 
decomposition  of  ozone  in  solutions,  that  water  vapour  exerts 
a  slight  yet  distinct  catalytic  action.  This  is  only  to  be  ex- 
pected if  it  can  be  assumed  that  ozone  is  slightly  acidic,  since 
when  passed  into  strong  alkalis  it  forms  the  somewhat  un- 
stable coloured  ozonates  MH04.  In  this  case  (see  Chapman 
and  Jones,  "Trans.  Chem.  Soc.,"  208,  1811,  1911)  an  equili- 
brium is  probably  set  up  represented  by  the  equation— 


138  OZONE 

20H'  +  203  ^  2H04' 
2H04'  ->  H20  +  02  +  20. 

If  the  rate  of  decomposition  in  the  presence  of  moisture  is 
thus  accelerated  by  the  HO/  ion  then  the  observed  rate  of 
decomposition  will  be  — 

^g^  =  K(03)2  +  K'C(03)2(OH)2, 

where  C(OH)  is  very  small  and  K'  relatively  small  compared 
toK. 

Eoth  ("  Monatsheft,"  34,  665,  1913)  investigated  the  de- 
composition of  ozone  in  acid  solutions  ;  he  showed  that  the 
rate  of  decomposition  in  strong  acids  was  nearly  bimolecular 
and  in  very  weak  acids  practically  monornolecular,  the  rate 
for  any  acid  strength  being  determined  by  the  equation  — 

=  KC(03)2  +  K'C(03), 


by  suitable  choice  of  the  values  from  K  and  K',  the  velocity 
coefficients. 

The  catalytic  activity  of  light  in  decomposing  ozone  was 
first  studied  by  Eegener  ("  Ann.  der  Physik,"  20,  1033,  1906), 
who  showed  that  light  of  a  certain  wave  length  in  the  ultra- 
violet portion  of  the  spectrum,  viz.  in  the  region  230  to  290  /JL^L, 
exerted  a  very  marked  deozonising  action,  and  we  have 
already  discussed  the  interesting  fact  that  light  of  shorter 
wave  length  exerts  an  ozonising  action. 

The  reaction  of  kinetics  of  this  reaction  was  investigated 
by  von  Bahr  ("  Ann.  der  Physik,"  4,  33,  598,  1910)  and  especi- 
ally by  Weigert  ("  Zeit.  Phys.  Chem.,"  80,  78,  1912),  who 
showed  that  with  complete  absorption  of  light  the  reaction 
velocity  of  decomposition  was  proportional  to  the  ozone  con- 


THE    CATALYTIC  DECOMPOSITION   OF   OZONE  139 

centrations.  On  the  other  hand,  under  conditions  of  homo- 
geneous illumination,  when  the  emergent  and  entrant  beams 
are  equally  intense,  conditions  obtaining  approximately  in 
very  thin  gas  films,  the  reaction  velocity  was  found  to  be  pro- 
portional to  the  square  of  the  ozone  concentration.  From 
Weigert's  data  it  may  be  calculated  that  in  his  experiments, 
approximately  100  molecules  of  ozone  were  decomposed  per 
quantum  of  light  energy  absorbed.  We  have  already  noted 
that  the  magnitude  of  the  quantum  hv  necessary  to  effect 
any  given  photo-chemical  action  increases  as  the  required 
energy  increases,  and  consequently  the  photo-chemical  activity 
of  light  is  greatest  as  we  approach  the  extreme  ultra-violet. 
In  the  case  of  the  formation  of  ozone  from  oxygen  we  have 
already  discussed  the  effect  of  quanta  of  various  magnitudes 
on  both  the  oxygen  atom  and  molecule ;  we  would  expect 
that  the  magnitude  of  the  quantum  necessary  to  detach  an 
oxygen  atom  from  an  ozone  molecule  would  be  very  small,  on 
account  of  its  instability  ;  and  again  that  the  period  of  natural 
vibration  of  an  ozone  molecule,  which  determines  the  absorb- 
tive  power  for  light  of  a  definite  wave  length,  would  be  larger 
than  that  for  the  smaller  oxygen  molecule  or  atom.  Both 
these  expectations  are  fulfilled  in  the  experimental  results 
since  deozonising  light  has  a  longer  wave  length  than  that 
effective  in  ozonisation.  Photo-chemical  equivalence,  how- 
ever, is  not  obtained  as  in  the  case  of  ozonisation.  The 
absorption  of  a  quantum  of  light  energy  by  the  already  ex- 
tremely unstable  ozone  molecule  causes  it  to  explode  with 
considerable  violence,  and,  as  we  have  seen,  the  energy  liber- 
ated during  the  explosion  is  able  to  cause  the  primary  and 
secondary  decomposition  of  over  100  other  molecules  before 


140  OZONE 

the  energy  is  dissipated  into  the  surrounding  medium.  M. 
Saltmarsh  ("  Proc.  Phys.  Soc.,"  27,  357,  1915)  regards  the 
ultra-violet  light  in  deozonisation  as  the  source  of  nuclei  in 
the  ozonised  oxygen  which  serve  as  centres  of  decomposition. 
Similar  results  were  obtained  by  E.  Warburg  ("  Berlin  Akad. 
Sitzungsber.,"  2,  644,  1913).  He  obtained  values  for  the 
specific  photo-chemical  activity  by  filling  a  little  quartz  cell 
with  ozonised  oxygen  and  exposing  it  to  radiation  for  a 
definite  length  of  time. 

The  specific  photo-chemical  activity  </>  was  obtained  from 

the  ratio — 

,  __  m0  -  ms 

where  E  is  the  energy  absorbed  from  the  light  m0,  and  ms  the 
ozone  concentration  before  and  after  irradiation. 

The  rate  of  deozonisation  was  calculated  from  the  relation- 
ship— 

,, 3   =  <4AJ  -  gm. 
ctt 

J  =  Light  energy  in  gm.  calories  per  second. 
A  =  Fraction  of  light  energy  absorbed  =  am. 
m  =  Concentration  of  ozone  in  cell. 

g  =  A  cell  constant  correcting  for  the  spontaneous  decomposi- 
tion of  the  ozone  in  the  cell. 

in 

Hence  log  —  =  t  log  (6AJ  +  g\ 

ms 

and  in  the  absence  of  radiation — 

log  ^V  =  if  log  g- 

He  obtained  the  following  values  for  the  specific  photo- 
chemical activity  with  light  of  wave  length  X  =  253  /A/A  for — 


THE    CATALYTIC   DECOMPOSITION   OF   OZONE  141 

Ozone  in  $  x  105. 

Oxygen 0-253 

Nitrogen 0-975 

Helium 1*520 

Hautefeuille  and  Chappuis  ("  C.E.,"  91,  762,  1880)  were  the 
first  to  notice  the  inhibiting  effect  of  chlorine  on  the  forma- 
tion of  ozone,  which  result  was  confirmed  by  Shenstone  and 
Evans  ("  J.C.S.,"  73,  246,  1898).  The  work  of  Bodenstein 
and  others  on  the  hydrogen  chlorine  combination  leads  one  to 
conclude  that  chlorine  is  an  optical  sensitiser  for  the  decom- 
position of  ozone.  F.  Weigert  ("  Zeit.  Elektrochem.,"  14, 
591,  1908)  clearly  showed  that  ozone  containing  small 
quantities  of  chlorine  was  rapidly  decomposed  by  blue  and 
violet  light,  whilst  pure  ozone  is,  as  we  have  seen,  only  sensi- 
tive to  ultra-violet  radiation.  The  rate  of  decomposition  was 
found  to  be  proportional  to  the  intensity  of  the  light  and 
independent  of  the  ozone  concentration.  It  would  appear 
that  the  molecules  of  chlorine  absorb  the  smaller  quanta  of 
longer  wave  length  and  are  consequently  endowed  with  an 
excess  of  kinetic  energy  equal  in  magnitude  to  the  quantum 
absorbed.  By  subsequent  collision  with  an  ozone  molecule 
this  increment  of  energy  is  transferred,  causing  a  rupture  of 
the  ozone  molecule  into  oxygen.  The  chlorine  molecule  thus 
serves  as  a  conveyor  of  energy  and  makes  the  medium  sensi- 
tive to  this  particular  radiation  frequency. 


CHAPTEE  IX. 

INDUSTRIAL  USES  OF  OZONE. 

APPLICATION  TO  HYGIENIC  PURPOSES. 

The  Sterilisation  of  Water  by  Ozone. — The  earliest  experi- 
ments on  the  use  of  ozone  as  a  germicide  for  the  sterilisation 
of  water  were  made  by  De  Meritens  in  France  (1886). 

He  showed  that  even  dilute  ozonised  air  would  effect  the 
sterilisation  of  polluted  water,  provided  that  intimate  contact 
between  gas  and  liquid  was  effected.  A  few  years  later  the 
subject  was  reinvestigated  by  Frohlich("  Elektrochem.  Zeit," 
344, 1891),  of  the  firm  of  Siemens  and  Halske,  who  erected 
a  semi-technical  experimental  plant  at  Martinikenfeld. 
Ohmiiller  and  Prall  ("Arbeit.  Kais.  Gesund,"  229,  1892),  at 
the  request  of  the  German  Government,  investigated  the 
process  in  great  detail  and  as  a  result  showed  that  ozone 
energetically  attacked  bacteria  in  water  from  which  any 
excess  of  inert  organic  matter  had  been  previously  removed. 
Sufficient  evidence  as  to  its  practical  utility  was  thus  at 
hand  to  warrant  a  closer  examination  as  to  its  suitability  for 
municipal  work. 

Chemically,  ozone  is  the  ideal  agent  for  purification,  since 
it  leaves  behind  it  nothing  foreign  in  the  treated  water,  with 
the  exception  of  oxygen,  which  assists  in  the  normal  aeration 

and  greatly  augments  the  palatability  of  the  water. 

(142) 


INDUSTEIAL  USES   OF   OZONE  143 

For  technical  purposes,  however,  the  important  factors  of 
reliability  and  cost  are  all  important. 

As  a  result  of  Frohlich's  experiments  and  the  satisfactory 
report  of  Ohmuller  and  Prall,  the  firm  of  Siemens  and  Halske 
developed  their  process  of  sterilisation  with  ozone,  and  large 
plants  were  installed  at  Wiesbaden  and  Paderborn  and  at  a 
later  date  at  St.  Maur,  Paris,  and  Petrograd. 

Contemporary  with  these  developments,  Tindal,  Schneller 
and  Van  Sleen  installed  an  ozone  sterilisation  plant  at  Oud- 
shoorn  in  Holland  which  was  subjected  to  a  detailed  in- 
vestigation by  Van  Ermengem  on  behalf  of  the  Belgian 
Board  of  Agriculture,  and  by  Drs.  Ogier,  Koux  and  Eepin 
("  Eev.  Gen.  des  Sciences,"  596,  1896)  for  the  municipality 
of  Paris,  with  the  result  that  an  ozone  installation  on  Tindal 
and  de  Frise's  system  was  installed  at  St.  Maur,  Paris.  At 
a  later  date  a  combination  of  the  Tindal-de  Frise  and 
Siemens-Halske  systems  was  utilised  by  the  Parisian  muni- 
cipal authorities. 

In  1898,  Abraham  and  Marmier  erected  an  installation  at 
Lille,  and  in  1904  an  ozone  plant  on  Otto's  system  was  sup- 
plied to  the  municipality  of  Nice. 

Small  installations  were  likewise  erected  on  Vosmaer's 
system  in  Holland  and  at  Philadelphia,  U.S.A. 

As  a  result  of  these  developments  at  the  present  time 
there  are  in  operation  over  fifty  plants  for  the  industrial 
sterilisation  of  water  by  this  means. 

We  have  already  given  a  brief  description  of  the  various 
types  of  ozonisers  developed  for  the  purpose  of  the  economical 
production  of  ozonised  air  ;  the  various  installations  naturally 
differed  in  their  methods  of  ensuring  contact  of  the  water 


144 


OZONE 


with  the  ozonised  air  and  in  the  preliminary  treatment  of 
the  water. 

Systems  of  Ensuring  Intimate  Contact  Between  the  Ozone 
and  Water. — In  the  earlier  Siemens  and  Halske  installations 
at  Wiesbaden  and  Paderborn,  ozonised  air  containing  from 
2-j-  to  3  grammes  of  ozone  per  cubic  metre  passed  upwards 


'Qzomzed 


Air 


Water 


FIG.  21. 


through  a  tower  filled  with  broken  flint  and  met  a  descend- 
ing current  of  water  which  had  passed  through  roughing 
filters. 

Fairly  satisfactory  bacterial  reduction  was  effected  by 
this  method,  the  count  ranging  from  9  to  2  organisms  per 
c.c.  after  ozonisation,  and  these  organisms  were  of  the 
resistant  and  harmless  B.  subtilis  types.  Economically, 


INDTJSTBIAL  USES   OP  OZONE  145 

however,  these  contact  towers  left  much  to  be  desired,  the 
cost  being  about  2'75d.  per  1000  gallons  of  water  treated. 

The  fundamental  point  to  be  considered  in  the  design  of 
a  contact  tower,  viz.  the  intimate  contact  of  every  drop  of 
water  with  the  ozonised  air,  was  clearly  not  realised  in  this 
elementary  type  of  tower.  It  is  evident  that  every  flint  is 
covered  with  a  thin  film  of  water  into  which  the  ozone  can 
only  penetrate  by  diffusion.  Now  the  rate  of  diffusion  of  the 
ozone  into  the  liquid  film  is  proportional  to  its  partial  pres- 
sure in  the  gas  above  the  film  and  approximately  inversely 
proportional  to  the  square  root  of  its  density.  Thus,  although 
it  is  not  a  difficult  matter  to  oxygenate  water  in  this  type  of 
tower  by  means  of  air,  since  the  partial  pressure  of  oxygen 
in  air  is  about  20  per  cent,  of  one  atmosphere,  yet  in  the 
case  of  ozonised  air  containing  2*4  gms.  per  cubic  metre 
the  partial  pressure  of  ozone  is  only  1/1000  of  an  atmosphere 
and  its  relative  rate  of  diffusion  only  J% f ,  or  three-quarters 
that  of  oxygen.  As  a  consequence,  to  ensure  sterility,  high 
concentrations  of  ozone  had  to  be  employed  and  much  ozone 
was  lost  at  the  top  of  the  tower. 

Trouble  was  also  caused,  especially  at  Wiesbaden,  by 
clogging  of  the  contact  tower  with  precipitated  ferric 
hydroxide.  The  water  contained  small  quantities  of  ferrous 
salts  up  to  0*5  parts  per  million,  which  were  precipitated  on 
oxidation  as  ferric  hydroxide  in  the  interstices  of  the  quartz 
packing. 

Vosmaer  states  that  by  proper  design  of  this  type  of 
tower,  relatively  long  and  slender  in  proportion,  sterilisation 
can  be  accomplished  by  uzing  an  ozone  concentration  of 

only  1  gramme  per  cubic  metre,  and  that  complete  removal 

10 


146 


OZONE 


of  the  ozone  is  effected  by  such  a  tower  when  gas  and  liquid 
flow  rates  are  properly  adjusted ;  thus  a  tower  1  foot  in 
diameter  and  33  feet  high  was  shown  capable  of  treating 
10,000  gallons  an  hour,  one  3  feet  diameter  at  Philadelphia 
dealt  with  50,000  gallons  per  hour. 

In  the  first  plants  of  Tindal  and  de  Frise  both  ozonised 
air  and  raw  water  entered  by  separate  pipes  at  the  base  and 
flowed  in  the  same  direction  through  the  flint  packed  tower, 
which  was  divided  up  into  segments  by  means  of  perforated 
metal  or  celluloid  plates. 


=—  Water 


FIG.  22. 

A  considerable  advance  on  this  practice  was  made  by 
Otto  by  the  introduction  of  an  emulsifier  for  ensuring  inti- 
mate mixture  of  ozonised  air  and  water  at  the  base  of  the 
tower.  The  emulsifier  is  constructed  on  the  lines  of  a  Korting 
injector  or  simple  water  vacuum  pump,  the  water  supplied 
under  pressure  drawing  in  the  ozonised  air  under  vacuum. 
Not  only  was  a  very  intimate  mixture  of  the  ozone  and  the 
water  effected  by  this  means  but  by  utilising  the  vacuum 
produced  by  the  water  stream  the  necessity  for  pumping 
ozonised  air,  which  requires  pumps  of  special  construction,  is 
avoided.  Otto's  installation  at  Nice  included  a  sectional 
contact  tower  of  cement  using  shingle  as  packing,  and 


INDUSTRIAL  USES  OF  OZONE 


147 


operated  by  means  of  these  emulsifiers.  The  plant  is 
capable  of  dealing  with  5,000,000  gallons  per  day. 

In  the  Ozonair  Sy 'stem,  which  has  been  successfully 
developed  for  installations  of  capacity  from  1000  to  2000 
gallons  per  hour,  the  ozonised  air  is  drawn  into  an  injector 
by  the  pressure  of  the  water  which  is  of  the  order  of  about 
two  atmospheres.  It  is  thoroughly  emulsified  by  the  injector 
and  passes  into  a  sterilising  tank  which  is  so  proportioned  as 
to  give  a  contact  of  about  80  minutes.  The  concentration  of 
ozone  used  is  from  2  to  3  gms.  per  cubic  metre  produced  in 
the  standard  type  of  ozonisers  (see  p.  129). 

The  greatest  economy  in  the  utilisation  of  ozone  was 
effected  by  de  Frise,  who  introduced  a  cyclic  system  at  the 
St.  Maur  waterworks,  where  it  operated  with  great  success. 

A  diagrammatic  sketch  of  De  Frise's  system  is  shown 
below. 


FIG.  23. 


The  ozone  compressor  draws  the  ozonised  air  from  the 
ozonisers  and  forces  it  into  the  plate  sterilising  tower ;  from 
the  top  of  the  steriliser  the  air  returns  to  the  ozonisers, 


148  OZONE 

passing  on  its  way  through  a  separator  and  a  dryer  in  which 
it  comes  in  contact  with  calcium  chloride,  which  is  cooled  in 
hot  weather.  A  small  suction  valve  on  the  inlet  of  the 
separator  admits  fresh  air  to  make  up  for  the  loss  through 
absorption  by  the  sterilised  water.  The  sterilising  tower  is 
constructed  on  the  sectional  system,  being  built  up  of 
enamelled  iron  sections  20  inches  long  and  3  feet  in  diameter, 
each  section  (there  being  30  sections  in  the  tower)  being 
separated  from  the  others  by  perforated  plates. 

With  a  ratio  of  ozonised  air  to  water  of  two  to  five, 
88  per  cent,  removal  of  the  ozone  is  effected  in  the  tower 
system,  the  residue  being  returned  after  drying  to  the  ozon- 
isers. 

Pre-treatment  of  the  Water. — Since  inert  organic  matter 
is  readily  oxidised  by  ozone  to  carbon  dioxide  and  water,  in 
order  to  exercise  every  possible  economy  in  the  utilisation  of 
ozone,  it  is  necessary  to  subject  polluted  waters  to  some  form 
of  preliminary  purification. 

The  earlier  installations  at  Wiesbaden  and  Paderborn 
were  equipped  with  simple  roughing  filters  with  small  pebbles 
as  filtration  medium,  through  which  the  water  passed  at 
relatively  high  speeds.  In  the  later  plants,  where  it  was 
realised  that  the  cost  of  removing  organic  matter  by  oxida- 
tion with  ozone  was  higher  than  by  the  usual  methods  of 
sedimentation  and  filtration,  more  attention  was  paid  to  the 
pre-treatment  of  the  water,  and  the  ozone  was  utilised  merely 
to  remove  the  last  traces  of  organic  matter  and  ensure  steri- 
lising of  the  water,  conditions  unobtainable  by  any  method 
of  sedimentation  and  filtration  except  at  prohibitive  costs. 

In  the  Otto  installation  at  Nice  sand  filtration  alone  is 


INDUSTRIAL  USES   OF   OZONE  149 

employed  as  pre- treatment.  At  Ginnekin,  in  Holland,  the 
Mark  water  is  subjected  to  gravel  filtration,  followed  by  a 
sand  filter.  At  Oudshoorn  the  Khine  water  is  passed  through 
sedimentation  basins  and  sand  filters  prior  to  sterilisation. 
At  St.  Maur,  Paris,  sedimentation  basins  in  conjunction  with 
gravel  and  sand  filters  are  utilised  for  the  Marne  water, 
whilst  at  Petrograd  the  Neva  water,  which  is  subjected  to 
very  serious  pollution  and  liable  to  contain  pathogenic  or- 
ganisms, is  passed  through  a  battery  of  mechanical  filters 
using  alumina  as  coagulant  prior  to  the  ozone  treatment. 

For  the  sterilisation  of  upland  waters  which  are  subjected 
to  sporadic  or  seasonal  pollution  such  pre-treatment  is  gener- 
ally unnecessary,  especially  in  those  cases  where  adequate 
storage  is  provided  by  means  of  impounding  reservoirs  which 
act  as  sedimentation  basins  of  large  capacity.  Exceptions, 
however,  are  to  be  found  when  the  upland  water  is  derived 
from  .a  peaty  or  recently  flooded  watershed,  where,  during 
the  summer  months,  the  quantity  of  oxidisable  organic  matter 
in  the  water  suffers  a  considerable  augmentation  and  neces- 
sitates a  correspondingly  increased  dosage  of  ozone. 

For  the  treatment  of  river  supplies  which  are  already 
polluted,  and  with  the  growth  of  the  riparian  population  are 
becoming  increasingly  contaminated  in  organic  matter,  some 
form  of  pre-treatment  is  necessary,  and  it  must  be  emphasised 
that  the  utility  of  ozone  in  the  present  stages  of  its  industrial 
manufacture  is  most  marked  when  it  is  employed  as  a  finisher 
or  sterilising  agent  for  waters  which  are  already  good  enough 
but  not  safe  enough  for  public  supplies,  rather  than  when  at- 
tempts are  made  to  purify  highly  contaminated  streams, 


150 


OZONE 


EESULTS  EFFECTED  BY  OZONE  TREATMENT. 

The  change  in  appearance  of  many  waters  after  ozonisa- 
tion  is  frequently  very  striking,  attributable  to  the  removal 
of  traces  of  coloured  organic  compounds  and  to  the  aeration 
and  oxygenation  effected  during  the  process.  Marshy  or 
polluted  river  waters,  which,  although  clear  after  effective 
sand  filtration,  are  brown  or  greenish-brown  in  colour,  and 
generally  flat  and  insipid,  after  ozonisation  they  become 
bright  and  sparkling,  generally  colourless  or  acquiring  a  faint 
bluish  tint. 

The  physical  changes  are  accompanied  by  chemical 
changes  no  less  marked  and  characteristic  of  the  process, 
thus  the  oxygen  consumed  figure  determined  by  the  potas- 
sium permanganate  under  standard  conditions  (usually  at 
60°  C.  in  acid  solution)  is  usually  reduced  from  30  to  70  per 
cent.,  as  is  indicated  by  the  following  figures : — 


Installation. 

Oxygen  Consumed  Figure. 
Parts  per  100,000. 

Observers. 

Before 
Treatment. 

After 
Treatment. 

Nice     .... 
Seine  .... 
St.  Maur      . 

Philadelphia 

1-4 
0-43 
0'124 

0-3 
0-21 
0-060 
(mean  of  14) 
40  per  cent, 
reduction 

Buisine  &  Bouriez. 
Van  der  Sleen. 
Rideal. 

Vosmaer. 

The  quantity  of  organic  matter  oxidised  by  the  ozone  as  in- 
dicated by  the  difference  in  the  oxygen  consumed  figures 
naturally  varies  with  the  three  factors,  the  nature  of  the 
organic  matter,  the  concentration  of  the  ozone,  and  the  time 


INDUSTEIAL  USES  OF  OZONE  151 

of  contact  between  the  ozone  and  the  water.  Thus  S.  Kideal 
("J.  Eoy.  S.  Inst.,"  30,  32,  1909)  found  at  St.  Maur  the 
following  distribution  of  the  ozone  at  two  different  gas  con- 
centrations : — 

Ozone  in  Gins.  Per  Cubic  Metre,  in 

Entering  air 1'68          2-65 

Entering  water 0-679  1-071 

Escaping  into  the  air 0-137  0*291 

Escaping  in  solution  in  the  water        .         .         .  0'048  0'162 

Used  in  oxidising  organic  matter         .         .         .  0-494  0-618 

Gms.  Per  Cub.  Metre. 
Oxygen    consumed    for    the    water    untreated 

(from  acid  KMn04  at  60°  C.)  1-73          1-12 

Oxygen     consumed    for     the     water     treated 

(from  acid  KMn04  at  60°  C.)  1-25          0-61 

Decrease      0-48          0'51 

=  28  %      =  45  % 

During  these  experiments  it  was  noticed  that  the  dissolved 
ozone  left  in  the  water  after  it  left  the  ozonising  column  dis- 
appeared after  a  few  hours,  and  it  seemed  probable  that 
oxidation  of  the  more  resistant  organic  matter  was  still  pro- 
ceeding. This  was  confirmed  by  the  determination  of  the 
oxygen  consumed  figures  from  time  to  time. 

Two  typical  tests  giving  the  following  results  : — 

Oxygen  Consumed  Gins. 

Per  Cubic  Metre. 
Filtered  water 1-73        1-12 

After  ozonisation 1-25        0'61 

One  hour  later 1-15        0'58 

Two  hours  later       .        .        .        .         .         .  0-90        0'51 

A  corresponding   decrease   in   the   free   and   albuminoid 
ammonia  takes  place  simultaneously  with  the  reduction  in 


152  OZONE 

the  oxygen  consumed  figures,  as  is  shown  from  the  investiga- 
tions of  Van  der  Sleen  on  the  Seine  water  : — 

Before  Treatment.        After. 
Free  ammonia         ....  0-271  0-136 

Albuminoid  ammonia     .        .        .  0-536  0-189 

As  in  the  case  of  the  organic  matter  in  the  water,  a 
marked  reduction  in  the  number  of  bacteria  takes  place.  In 
common  with  other  germicides  ozone  is  selective  in  its  action 
on  micro-organisms,  certain  organisms  being  more  resistant 
than  others  to  its  action.  It  so  happens  that  pathogenic 
organisms  which  are  already  enfeebled  by  their  unsuitable 
environment  are  easily  destroyed,  whilst  the  non-pathogenic 
sporing  organisms,  such  as  B.  subtilis,  are  frequently  found  in 
ozonised  waters.  This  point  was  fully  investigated  by  Otto 
in  his  experiments  on  the  treatment  of  the  Seine  and  Vannel 
waters. 

He  showed  that  all  pathogenic  organisms  and  the  follow- 
ing common  bacteria  which  he  isolated  were  rapidly  removed  : 
these  included  B.  fluorescens  liquefaciens,  B.  coli,  B.  termo, 
B.  proteus  vulgaris,  B.  prodigiosus  and  Aspergillus  niger. 
More  resistant  were  B.  subtilis,  B.  luteus  and  Penicillium 
glaucum,  which  only  succumbed  to  prolonged  treatment  with 
ozone. 

Camlette  showed  that  Lille  installation  effected  an  average 
reduction  in  the  bacterial  pollution  of  the  water  from  28,000 
per  c.c.  in  the  untreated  water  to  10  c.c.  in  the  treated  water  ; 
these  latter  were  found  to  consist  entirely  of  the  B.  subtilis 
type.  These  conclusions  were  confirmed  by  S.  Rideal  in  his 
investigations  on  the  Siemens-de  Frise  plant  at  St.  Maur. 
In  the  filtered  water,  prior  to  ozonisation,  B,  coli  was  usually 


INDUSTEIAL  USES  OF  OZONE  153 

present  in  40  c.c.  of  water  and  occasionally  in  20  c.c.,  whilst 
it  was  never  found  in  the  treated  water.  The  20  c.c.  count 
was  reduced  from  an  average  of  74  to  11  per  c.c.,  which  were 
found  to  be  spore-bearing  organisms  of  B.  subtilis  type. 
Whilst  purification  can  be  effected  by  the  consumption  of 
0'53  gms.  of  ozone  per  cubic  metre  of  water,  the  ozone 
consumption  per  cubic  metre  of  water  naturally  varies  with 
the  quality  of  the  water  to  be  treated,  the  economic  limit 
appears  to  be  in  the  neighbourhood  of  2  gms.  ozone  per  cubic 
metre  of  water.  If,  by  preliminary  testing  with  permanganate, 
it  is  found  that  more  than  this  quantity  would  be  required 
then  some  form  of  preliminary  treatment  is  essential  for 
economic  operation. 

The  Purification  of  Air. 

Several  types  of  apparatus  have  been  designed  for  the 
the  purification  of  air  in  confined  spaces,  such  as  theatres, 
lecture  halls,  slaughter  houses,  tanneries  and  breweries,  whilst 
extended  use  of  ozonised  air  has  been  made  upon  the  Central 
London  Railway  systems  and  public  lavatories. 

Although  the  immunity  of  the  motor  drivers  on  the 
London  Tubes  during  the  recent  epidemics  of  influenza  has 
been  somewhat  remarkable  it  is  more  probable  that  this  must 
be  attributed  to  the  uniformity  of  temperature  and  to  the 
circulation  of  the  air  rather  than  to  any  specific  action  of  the 
ozone  in  the  air. 

The  evidence  for  the  specific  utility  of  ozone  as  a  means 
of  purifying  air  is  somewhat  conflicting. 

Dewar  and  McKendrick  ("Pogg.  Ann.,"  152,  329,  1874) 
showed  that  by  the  inhalation  of  strongly  ozonised  air  the 
frequency  of  pulsation  of  the  heart  is  lowered  very  consider- 


154  OZONE 

ably,  the  blood  temperature  sinks  from  3°  to  5°,  and  post 
mortem  examination  showed  that  the  blood  had  become 
venous  in  appearance.  Thenard  ("  O.K.,"  82,  157, 1876)  and 
Biny  ("Med.  C.  Bl.,"  20,  721,  1882)  confirmed  these  observa- 
tions of  Dewar's.  Schultz  ("  Arch.  f.  Exper.  Path.  u.  Plan.," 
29,  365,  1892)  records  several  cases  of  chronic  poisoning  by 
ozone.  Jordan  and  Carlson l  ("  J.  Amer.  Med.  Assoc.,"  61, 1007, 
1913)  confirmed  the  deodorant  action  of  ozone  on  air  but 
showed  that  long  before  the  concentrations  reached  those 
necessary  for  germicidal  action  injury  was  caused  to  the  res- 
piratory tract. 

The  lowest  concentrations  of  ozone  in  air  which  can 
exert  a  definite  disinfecting  action  (Schultz,  "  Zeit.  f.  Hyg.," 
75,  1890  ;  and  de  Christmas,  "  Ann.  de  1'inst.  Pasteur,"  7,  689, 
1893)  appears  to  be  in  the  neighbourhood  of  13'53  mgm.  per 
litre.  With  such  concentrations  sterilisation  can  usually  be 
effected  in  air,  but  the  presence  of  large  quantities  of  moisture 
lowers  its  germicidal  activity.  According  to  Labbe  and 
Oudin  ("C.K.,"  113,  141,  1891)  the  highest  concentration 
which  may  be  inhaled  without  deleterious  effects  is  approxi- 
mately O'll  to  0'12  mgm.  per  litre. 

They  state  that  beneficial  results  obtain  by  the  inhalation 
of  ozonised  air  of  this  concentration,  a  marked  increase  in  the 
oxyhsemoglobin  contact  of  the  blood  taking  place  after  an 
interval  of  from  ten  to  fifteen  minutes. 

It  is  therefore  evident  that  there  is  no  question  of 
germicidal  activity  in  ozonised  air  of  concentrations  suitable 
for  respiration.  As  a  powerful  oxidant  it  doubtless  removes 

1  Leonard  Hill  and  Flack  ("  Proc.  Eoy.  Soc.,"  B  84,  405, 1911)  state  that  a 
concentration  of  1  in  106  of  ozone  produces  irritation  of  the  respiratory  tract. 


INDUSTRIAL  USES  OF  OZONE  155 

small  traces  of  hydrogen  sulphide  and  other  impurities  in  air, 
whilst  the  unpleasant  smells  associated  with  crowded  places 
are  amenable  to  treatment  with  oxidising  agents  such  as 
ozone.  It  is  somewhat  remarkable  that  most  odoriferous  sub- 
stances contain  unsaturated  valencies  and  as  such  would 
naturally  be  attacked  by  means  of  ozone.  The  odour  of  ozone 
in  itself  when  very  dilute  is  by  no  means  unpleasant  and  thus 
provides  a  counter  irritant  to  the  olfactory  organs,  a  matter 
of  psychological  importance  if  of  no  physiological  significance. 

The  use  of  ozone  even  in  dilutions  of  O'll  to  0'12  mgm. 
per  litre  has  frequently  been  condemned  on  account  of  its 
supposed  physiological  activity,  in  many  cases  erroneously, 
since,  unless  the  usual  precautions  such  as  drying  the  air  and 
the  avoidance  of  sparking  are  taken  in  the  preparation  of  the 
ozonised  air,  oxides  of  nitrogen  are  liable  to  be  formed  which 
have  an  extremely  high  physiological  activity  even  in  extreme 
dilutions.  Schwarg  and  Munchmeyer  ("Zeit.  f.  Infekt. 
Krankh,"  75,  81,  1913)  investigated  in  great  detail  the  de- 
odorising action  of  small  concentrations  of  ozone  in  air  ;  they 
observed  that  hydrogen  sulphide  was  rapidly  oxidised,  sulphur 
dioxide  slowly  converted  to  sulphuric  anhydride,  whilst  the 
mercaptans,  skatol,  and  indol  were  oxidised  to  somewhat 
pleasant  smelling  substances. 

Carbon  monoxide  was  but  slowly  oxidised  whilst  ammonia 
was  not  affected.  Franklin  ("  IV.  Int.  Congress  on  School 
Hygiene,"  1918)  and  Biesenfeld  and  Egidius  ("  Zeit.  Anorg. 
Chem.,"  85,  217,  1914)  confirmed  these  observers'  results  on 
the  action  of  ozone  on  hydrogen  sulphide.  This  reaction 
was  studied  in  detail  by  Eiesenfeld,  and  it  was  found  that 
oxidation  takes  place  through  a  series  of  intermediate  com- 
pounds according  to  the  following  scheme  ; — - 


156  OZONE 

hyposulphate,  sulphite 
Sulphide  ->  thiosulphate  ' 

^polythionates. 

->  dithionates  ->  sulphates. 

Although  the  germicidal  activity  of  ozone  in  concentrations 
which  exert  no  injurious  action  on  the  respiratory  organs  is 
practically  negligible,  yet  as  has  been  shown  by  Dibden  some 
sterilisation  is  effected  by  ozonisation  of  air,  since  a  marked  re- 
duction is  obtained  in  the  bacterial  count  of  the  air  which  has 
actually  passed  through  the  ozoniser.  This  air,  which  has  been 
relatively  strongly  ozonised  and  subjected  to  the  ultra-violet 
radiation  in  the  ozoniser,  is  practically  sterile,  and  a  consequent 
improvement  in  the  bacterial  pollution  of  air  which  has  been 
admixed  with  this  purer  air  was  naturally  expected  and  in 
fact  obtained.  The  largest  system  of  air  purification  by 
means  of  ozone  is  that  of  the  Central  London  Eailway  which 
is  equipped  with  air  screens,  washers,  and  ozonisers  of  the 
.Ozonair  type.  The  total  air  supply  treated  for  this  system 
being  of  the  order  of  eighty  million  cubic  feet  per  day.  The 
average  concentration  of  ozone  in  the  Tube  air  is  of  the  order 
of  one  part  in  two  millions  (1*2  mgm.  per  cubic  metre)  which, 
under  special  circumstances,  it  is  stated  that  it  is  increased  to 
five  parts  per  million  (12  mgm.  per  cubic  metre). 

The  installation  on  the  Central  London  Kailway  was  so 
successful  in  operation  that  the  system  of  ventilation  has  been 
extended  to  practically  every  other  London  Tube. 

Surgical  and  Therapeutic  Uses  of  Ozone. 

During  the  period  of  the  war  small  portable  ozonisers 
have  been  in  use  in  many  of  the  military  hospitals  of  the 
nations  for  the  treatment  of  wounds.  Major  Stokes,  of  the 


INDUSTEIAL   USES   OF   OZONE  157 

Queen  Alexandra  Military  Hospital  ("Lancet,"  1918),  de- 
scribes the  following  method  of  wound  sterilisation  which 
appears  to  have  given  excellent  results.  Wounds  and  sinuses 
are  washed  twice  daily  with  boiled  water  and  a  dressing  of 
oxy  gauze  is  applied.  The  ozone  (ozonised  air)  is  applied  on 
the  wounded  surface  or  on  to  the  cavities  and  sinuses  twice 
daily  for  a  maximum  time  of  fifteen  minutes  or  until  the  sur- 
face becomes  glazed.  At  first  ozone  causes  an  increase  of  the 
discharge  of  pus,  which  is  gradually  replaced  by  clear  serum. 
The  serum  at  a  still  later  stage  becomes  coloured  reddish  or 
purplish.  It  was  found  that  ozonised  air  applied  in  this  way'j 
was  a  strong  stimulant,  and  increased  the  flow  of  blood  to  the  f 
affected  part,  that  it  was  a  germicide  and  exerted  great  powers  I 
in  the  formation  of  oxyhsemoglobin.  A  record  of  seventy- 
nine  cases  so  treated  is  given,  in  which  the  period  of  treat- 
ment varied  from  a  few  days  to  three  months  and  was  in 
practically  all  cases  completely  successful. 

Curie  ("  Practitioner,"  864, 1912)  describes  the  application 
of  ozonised  air  to  the  liberation  of  iodine  in  the  lungs  for  the 
treatment  of  phthisis.  Potassium  iodide  is  introduced  into 
the  lungs  by  ionisation,  and  the  iodine  is  subsequently  liber- 
ated by  the  inhalation  of  ozonised  air.  For  disinfection  of 
the  intestinal  canal  in  cases  of  enteritis  and  dysenteries, 
Lessing  ("  Lancet,"  Nov.,  1913)  records  the  improvement 
obtained  by  washing  out  the  intestinal  canal  with  ozonised 
water.  The  treatment  of  ulcers  and  pyorrhoea  of  the  teeth  has 
been  successfully  accomplished  with  ozonised  air.  Ozonised 
medicaments  and  ointments,  such  as  vaseline,  have  been  stated 
to  possess  a  superior  curative  value  to  those  not  so  treated. 
Ozone  appears  to  be  slightly  soluble  in  these  semi-fluid 


158  OZONE 

hydrocarbons,  and  would  naturally  give  the  medicaments  a 
germicidal  activity.  Information  is  not  available  as  to  the 
extent  of  the  solubility  of  ozone  in  vaseline,  fats,  and  lards, 
or  how  long  the  substance  retains  any  germicidal  activity  due 
to  dissolved  ozone  or  to  the  presence  of  unstable  ozonides. 
As  has  already  been  mentioned,  the  action  of  dilute  ozonised 
air  in  stimulating  the  production  of  oxyhsemoglobin  has  been 
successfully  utilised  for  the  treatment  of  cases  of  anaemia, 
whilst  its  application  in  cases  where  there  is  a  shortage  of 
oxygen  absorbed  in  the  system,  such  as  in  asthma  or  heart 
weakness,  is  not  without  benefit. 

Applications  for  Bleaching  Purposes. 
Houzeau  ("  O.K.,"  75,  349, 1872)  and  Boillot  ("  C.R,"  80, 
1187,  1875)  showed  that  dilute  ozonised  air  possessed,  in 
common  with  chlorine  and  bromine,  the  property  of  selectively 
oxidising  the  colouring  matters  present  in  various  natural 
substances.  Ozone  appears  to  be  even  more  efficacious  than 
either  chlorine  or  bromine,  since  not  only  is  the  danger  of 
forming  coloured  substitution  chloro-  or  bromo-derivatives 
avoided,  but  the  oxidising  power  of  ozone  greatly  exceeds  that 
of  chlorine  or  bromine.  For  this  reason  only  very  dilute 
concentrations  of  ozone  may  be  utilised  for  bleaching  purposes, 
and  many  unsuccessful  results  are  directly  attributable  to  the 
employment  of  air  relatively  highly  ozonised. 

Fibres. 

Linen  and  cotton  goods  are  slowly  attacked  by  ozone 
("  Kolb.  Bull.  Soc.  Ind.  Mulhouse,"  38,  94, 1868),  and  accord- 
ing to  Witz("Bull.  Soc.  Eouen,"  u,  198,  1883),  in  the 
presence  of  moisture,  oxycellulose  is  formed.  The  subject 


INDUSTRIAL  USES  OF  OZONE  159 

was  reinvestigated  by  Cunningham  and  Doree  ("  J.C.S.," 
103,  1347,  1912)  employing  high  concentrations  of  ozonised 
oxygen  (20  to  25  gms.  per  cubic  metre).  They  showed  that 
an  oxycellulose  and  cellulose  peroxide  accompanied  by  a  de- 
struction of  the  fibre  were  produced  with  the  liberation  of 
carbon  dioxide. 

Unbleached  samples  of  cotton  became  white  in  from  one  to 
two  hours,  but  when  dried  the  fibre  was  found  tendered  and 
dusty.  Jute  fibres  were  likewise  rapidly  bleached,  but  became 
acid  and  tender  when  subjected  to  a  similar  treatment  for  a 
few  hours.  The  cellulose  peroxide  affected  a  photographic 
plate,  and  possessed  oxidising  properties  such  as  the  liberation 
of  iodine  from  potassium  iodide.  In  the  presence  of  water 
hydrogen  peroxide  was  formed.  In  general  the  action  of 
ozone  on  cellulose  closely  resembles  that  of  ammonium  per- 
sulphate ("Ditz.  Chem.  Zeit,"  31,  833, 1907 ;  "  J.  Prakt.  Chem.," 
78,  343, 1908),  and  with  more  dilute  concentrations  of  ozonised 
air  bleaching  without  subsequent  tendering  might  be  ob- 
tained. 

The  application  of  ozonised  air  to  the  conditioning  of  tex- 
tile materials  is  stated  to  be  entirely  successful ;  exceedingly 
dilute  concentrations  of  ozonised  air  are  employed,  and  under 
suitable  conditions  of  humidity  the  period  of  conditioning 
can  be  reduced  from  several  months  to  a  few  days. 

Oils,  Fats,  and  Waxes. 

The  applications  of  the  use  of  ozone  in  the  oil,  fat,  and 
wax  industries  are  very  considerable,  the  reagent  being  useful 
for  a  great  diversity  of  purposes.  Amongst  the  more  im- 
portant of  these  may  be  mentioned  : — 


160 


OZONE 


1.  Eemoval  of  odour,  flavour,  and  colour  from  oils  and 
fats  intended  for  edible  purposes. 

2.  Bleaching  and  refining  of  oils  and  fats  for  use  in  the 
soap  industry. 


A.  Air  Cleaner. 

B.  Electrically  driven  Blower. 

C.  Air  Delivery  Pipe. 

D.  Air  Cooling  Machine. 
K.  Electric  Motor. 

E.  Cold  Air  Delivery  Pipe. 


F.  Ozone  Generators. 

G.  Transformer. 
H.  Valve. 

I.  Ozone  Pipe. 

J.  Ozone  Injectors. 

L.  Switchboard. 


FIG.   24. 


3.  Bleaching  of  oils  for  paint  and  varnish-making. 

4.  Bleaching  and  refining  of  waxes  for  use  in  the  manu- 
facture of  candles,  polishes,  and  ointments. 

In  the  above   sketch  is  given  a  diagrammatic  arrange- 


INDUSTEIAL  USES  OF  OZONE  161 

ment  of  a  bleaching  plant  on  the  Ozonair  system.  The 
temperatures  at  which  selective  oxidation  of  the  colouring 
matter  or  objectionable  odoriferous  substances  in  the  oil,  fat, 
or  wax  commences,  the  concentration  of  ozone,  and  the  period 
of  action  naturally  vary  with  the  nature  of  the  substance 
treated  and  the  degree  of  refining  required.  Generally  a  fairly 
dilute  concentration  of  ozone  at  not  too  elevated  temperatures 
passed  through  the  material  for  a  relatively  long  time  gives 
the  best  results.  In  certain  cases  it  is  found  advantageous 
to  add  small  quantities  of  catalytic  materials,  such  as  salts 
of  manganese,  vanadium,  or  cerium,  to  accelerate  the  process 
of  oxidation. 

The  application  of  ozonised  air  to  the  bleaching  and 
deodorisation  of  oils,  fats,  and  waxes  is  well  exemplified  by 
the  following  list  (see  pp.  162,  163),  in  which  a  summary  is 
given  of  the  effect  of  dilute  ozonised  air  on  various  com- 
mercial products. 

Application  to  the  Paint  and  Varnish  Industry. 

As  has  already  been  observed  all  the  industrial  oils  and 
waxes  readily  undergo  partial  or  complete  deodorisation  by 
fractional  oxidation  of  the  coloured  chlorophyllic  constituents 
utilising  ozonised  air  as  oxidising  agent.  For  the  production 
of  clear  and  transparent  varnishes  this  is  a  matter  of  some 
technical  importance,  and  the  use  of  ozone  for  this  purpose  has 
been  frequently  suggested. 

It  would  appear  that  the  use  of  ozonised  air  as  a  substitute 
for  siccatives  in  the  preparation  of  drying  oils  is  already 
out  of  the  experimental  state.  It  has  long  been  known  that 
the  drying  of  oils  is  a  process  of  slow  oxidation  and  poly- 
merisation (see  Lippert,  "  Zeit.  Angew.  Chem.,"  11,  412,  1895; 


162 


OZONE 


Oil,  Fat, 
or  Wax. 

Bleaches. 

Deodorises. 

Remarks. 

Arachide  or 
ground  nut 

Yes 

Yes 

Bleaches  easily,  but  requires  subsequent 
treatment  for  deodorisation  and  im- 
provement of  flavour. 

Bone  fat 

Yes 

Yes 

Bleaches  easily,  at  the  same  time  the 
odour  is  much  improved,  in  fact  rank 
or  ill-smelling  fats  are  readily  con- 
verted into  perfectly  sweet  substances. 

Borneo 
Tallow 

Yes 

— 

bleaches  readily,  but  requires  subse- 
quent treatment  for  deodorisation 
and  improvement  of  flavour. 

Coconut  oil 

Yes 

No 

1  Bleaches  readily,  but  requires  subse- 
quent treatment  for  deodorisation 
and  improvement  of  flavour. 

Coconut  acid 
oil 

Yes 

Yes 

Yields  readily  to  treatment  for  bleach- 
ing and  deodorisation. 

Cod  oil 

Yes 

Yes 

Cotton  seed 

Yes 

No 

bleaches  fairly  easily  according  to 
grade  or  quality,  but  requires  subse- 
quent treatment  for  deodorisation 
and  improvement  of  flavour. 

Japan  wax 
tallow 

Yes 

Yes 

Bleaches  well  according  to  grade  or 
quality. 

Lard  oil 

Yes 

No 

Bleaches  very  easily. 

Mowrah 

Yes 

— 

Bleaches  readily,  but  requires  subse- 
quent treatment  for  deodorisation 
and  improvement  of  flavour. 

Neatsfoot  oil 

Yes 

Yes 

Bleaches  readily. 

Oleine 

Yes 

Yes 

Bleaches  easily.  At  the  same  time  the 
odour  is  much  improved,  but  the 
characteristic  odour  still  remains. 

Olive 

Yes 

Yes 

Bleaches  very  easily,  the  darkest 
varieties,  such  as  that  known  as 
"sulphur,"  are  bleached  to  the  or- 
dinary olive  colour. 

Palm 

Yes 

No 

Bleaches  very  easily  in  most  cases. 
Even  "  unbleachable  "  varieties  such 
as  Congo  and  saltpond  are  much  im- 
proved in  colour. 

Sesame 

Yes 

No 

Bleaches  fairly  easily  according  to 
grade  or  quality,  but  requires  subse- 
quent treatment  for  deodorisation 
and  improvement  of  flavour. 

INDUSTEIAL   USES   OF  OZONE 


163 


Oil,  Fat, 
or  Wax. 

Bleaches. 

Deodorises. 

Remarks.' 

Soya 

Yes 

Yes 

Bleaches    easily.     At   the   same   time 

the  odour  is  much  improved,  but  the 

characteristic  odour  still  remains. 

Stearine 

Yes 

Yes 

Bleaches  easily.     At  the   same   time 

the  odour  is  much  improved,  but  the 

characteristic  odour  still  remains. 

Tallow 

Yes 

Yes 

Bleaches  easily.    Odour  is  much  im- 

proved. 

Turnip  seed 

Yes 

Yes 

bleaches    fairly   easily  according  to 

grade  or  quality,  but  requires  subse- 

quent   treatment    for    deodorisation 

and  improvement  of  flavour. 

Beeswax 

Yes 



There  are  at  least  twenty  well-known 

commercial    varieties    all    of  which 

possess  different  characteristics,  and 

respond  to  the  treatment  in  varying 

degree.     Some  are  bleached  entirely 

by   ozone,   others  only  partially,   so 

requiring  subsequent  sun  treatment 

in   the  ordinary  way.      Contrary  to 

what  might   be  presumed,  the  dark 

varieties,  such  as  "  Cuba"  or  "West 

Indian,"  give  the  best,  whereas  the 

lighter  varieties  may  give  the  least 

satisfactory  results. 

Cardililla 

No 

— 

— 

Carnauba 

No 

— 

— 

Lanoline 

Yes 

_ 

The  bleaching  is  effected  very  readily 

after  removal  of   all   the  free  fatty 

acids.    The  degree  of  colour  or  bleach 

obtained  depends  very  much  on  the 

quality  of  the  original  wax  and  the 

care  with  which  it  has  been  collected. 

Montana 

No 

— 

— 

aln  all  these  cases,  if  deodorisation  only  is  desired,  this  can  be  obtained 
in  certain  cases  by  using  a  low  concentration  of  ozone  and  without  any  sub- 
sequent treatment,  but  when  bleaching  and  deodorisation  are  both  required, 
subsequent  treatment  is  necessary. 

It  has  been  stated  that  the  actual  cost  of  bleaching  and  deodorising  oils 
and  fats  for  edible  purposes  or  for  soap-making,  by  means  of  ozonised  air, 
does  not  exceed  five  shillings  per  ton,  a  figure  which  compares  quite  favour- 
ably with  the  alternative  methods  of  treatment  with  superheated  steam,  fuller's 
earth  or  charcoal  filtration. 


164  OZONE 

Weger,  "  Chem.  Eev.  Fett.  Harz.  Ind.,"  4,  301,  1899).  A. 
Genthe  ("Zeit.  Angew.  Chem.,"  19,  2087, 1906),  who  investi- 
gated the  process  in  detail,  showed  that  the  action  was  auto- 
catalytic  in  character,  or  that  the  rate  of  oxidation  of  the  oil 
after  a  time  when  a  quantity  x  had  already  been  oxidised  was 

given  by  the  equation  -^-  —  K(a  -  x)(b  +  x\  where  a  and  b 

were  the  initial  concentrations  of  linseed  oil  and  catalyst 
originally  present.  Further  experiments  (see  also  Engler 
and  Weiszberg,  "  Chem.  Zeit.,"  27,  1196,  1903)  showed  that 
the  catalytic  material  naturally  formed  was  some  form  of 
unstable  peroxide  which  accelerated  the  oxidation  of  the  oil 
by  air. 

This  unstable  peroxide  could  be  supplemented  or  replaced 
by  other  peroxides  of  a  similar  character,  such  as  those  ob- 
tained on  the  exposure  of  turpentine  to  the  air  or  even  by 
the  agitation  of  an  ether  air  mixture.  These  peroxides  are 
destroyed  by  boiling  the  oil  but  can  be  regenerated  by  aera- 
tion. (For  a  consideration  of  the  composition  of  these 
peroxides  formed  in  the  drying  of  linseed  oil  see  Orloff  ("  J. 
Buss.  Phys.  Chem.  Soc.,"  42,  658,  1910);  Fahrion  ("Zeit. 
Angew.  Chem.,"  23,  723,  1910);  Salway  and  Kipping 
("J.C.S.,"  95,  166,  1909),  and  others.) 

The  addition  of  siccatives,  such  as  salts  of  lead,  manganese 
zinc,  frequently  with  the  addition  of  certain  promotors  as 
cobalt,  vanadium,  cerium,  and  uranium  is  now  common 
practice.  The  salts  are  either  those  of  weak  acids  such  as 
the  borates,  or  of  soluble  organic  acids  and  oleates,  linoleates, 
or  resinates  (see  Ingle,  "J.C.S.  Ind.,"  454,  1917).  The 
siccatives  are  pseudo-catalytic  in  behaviour,  and  serve  to 


INDUSTKIAL  USES  OF  OZONE  165 

stabilise  or  assist  in  the  formation  of  the  auto-catalytic  per- 
oxide. 

Identical  results  are  obtained  with  the  use  of  ozonised 
air,  the  oils  can  be  easily  thickened  and  the  process  conducted 
at  much  lower  temperatures.  Linseed,  Chinese  wood,  poppy 
seed,  rape,  and  similar  oils,  rapidly  thicken  at  comparatively 
low  temperatures  (upwards  of  35°  C.),  and  at  the  same  time 
their  colour  is  much  improved  by  selective  oxidation  of  the 
coloured  constituents.  Bleaching  usually  proceeds  anterior 
to  the  thickening  process,  consequently  an  improvment  in 
colour  may  be  obtained  without  drying  the  oil. 

Linseed  oil  may  readily  be  thickened  to  a  syrup  or  to  a 
jelly  for  the  manufacture  of  linoleum ;  Chinese  wood  oil 
likewise  rapidly  undergoes  oxidation,  whilst  poppy-seed  and 
rape  oils  thicken  less  readily  than  linseed  oil.  The  utilisation 
of  ozonised  air  in  the  oxidation  of  linseed  oils  has  naturally 
been  extended  from  the  simple  preparation  of  drying  oil  to 
the  drying  of  the  oil  in  its  various  technical  applications, 
such  as  linoleum  manufacture,  the  preparation  of  waterproof 
materials,  fish  netting,  and  other  similar  manufactures. 

Ozone  in  the  Fine  Chemical  Industries. 

The  oxidation  of  organic  substances  by  means  of  ozone  has 
been  the  subject  of  numerous  investigators.  Carbon  monoxide 
is  readily  oxidised  to  carbon  dioxide  in  the  presence  of  moisture 
(Clausman,  "  C.B.,"  150,  1332,  1910),  but  only  slowly  when 
the  gases  are  dry  (Remsen,  "  Ber.,"  8,  1414,  1875).  Alde- 
hydes are  readily  oxidised  to  alcohols  ;  iodobenzene  to  iodoso 
benzene  whilst  the  saturated  hydrocarbons  themselves  are 
readily  attacked  at  low  temperatures.  Thus,  methane  is 


166  020Nfi 

converted  below  100°  C.  into  a  mixture  of  methyl  alcohol 
formaldehyde  and  formic  acid.  Drugrnan  ("  J.C.S.,"  89,  1614, 
1906)  has  shown  that  gradual  hydroxylation  of  one  carbon 
atom  takes  place,  the  corresponding  alcohol  is  first  formed 
which  is  then  oxidised  rapidly  to  the  more  stable  aldehyde 
acid.  Subsequently  slow  oxidation  to  the  acid  proceeds  : — 

/OH 

C  .  CH3  ->  C .  CH2OH  ->  G .  CH(    ->  C  .  HOH 

XOH 

->C  -  OH 

^OH  -»  COOH 
\OH 

Harries  ("Ann.,"  374,  288,  1910)  suggests  that  oxidation 
proceeds  through  the  intermediary  formation  of  unstable 
peroxides,  e.g.,  C  .  CH2OH  ->  C .  CH20  =  0,  and  not  through 
a  series  of  unstable  hydroxylations  as  postulated  by  Drugrnan. 

Ozone  is  finding  many  applications  as  an  oxidising  agent 
in  the  fine  chemical  industries.  It  has  been  successfully 
employed  for  the  preparation  of  several  synthetic  perfumes, 
such  as  the  methyl  ether  of  pyrocatechaldehyde  (vanillin), 
piperonal  (heliotrope),  and  anisic  aldehyde  ;  the  manufacture 
of  vanillin  being  accomplished  on  a  very  large  scale  in  France, 
America,  and  in  this  country. 

Vanillin  is  prepared  from  eugenol  according  to  the  follow- 
ing scheme : — 

C3H5  CH  =  CH .  CH3        CHO 


+  CHaCHO 
COCH, 


Eugenol. 


INDUSTRIAL  tJSES  OF  OZONE  167 

the  isoeugenol  being  oxidised  by  ozone  to  vanillin  and 
acetaldehyde  by  rupture  of  the  double  bond. 

According  to  Trillat  the  process  is  conducted  as  follows : 
Eugenol  is  converted  into  isoeugenol  by  treatment  with 
caustic  potash  and  amyl  alcohol,  from  which  solution  it  is 
liberated  by  sulphuric  acid. 

About  25  litres  of  isoeugenol  are  dissolved  in  acetic  acid 
and  subjected  to  a  current  of  ozonised  air  (2  to  2'5  gms. 
per  cubic  metre)  for  a  period  of  six  hours  at  a  low  temperature 
(ca.  2°  C.)  in  an  enamel-lined  vessel  fitted  with  a  tall  recti- 
fication column.  When  the  oxidation  to  the  aldehyde  is 
completed  the  acetic  acid  is  removed  by  distillation,  ether  and 
sodium  bisulphate  are  added  and  the  solution  warmed  to  30°  C. 
The  bisulphate  aldehyde  compound  is  washed  with  ether 
decomposed  with  sulphuric  acid,  and  the  vanillin  finally  ex- 
tracted with  ether. 

In  a  similar  manner  heliotropin  is  prepared  from  safrol 
according  to  the  equations — 

CH,  -  CH  =  CH2    CH  =  CH.CH3 

CHO 


Safrol.  Isosafrol.  Piperonal  (heliotropin). 

The  safrol  is  converted  into  isosafrol  by  heating  with  an 
alcoholic  solution  of  caustic  potash,  from  which  it  is  subse- 
quently extracted  by  means  of  ether.  For  treatment  with 
ozone  (as  in  the  case  of  isoeugenol)  it  is  dissolved  in  acetic 
acid,  from  which  the  heliotropin  is  recovered  in  like  manner. 


168  OZONE 

Anisaldehyde  can  be  prepared  in  a  similar  manner  by  the 
oxidation  of  anethol — 

CH  =  CH .  CH,  CEU .  CH  =  CH9  CHO 


OCH3  OCH3 

Attempts  have  also  been  made  to  utilise  ozone  for  the 
oxidation  of  aniline  to  aniline  black  and  the  leuco  bases  of 
various  dyes,  such  as  indigo,  to  the  coloured  dye-stuffs,  but  do 
not  appear  to  have  received  any  extensive  technical  applica- 
tion. 

For  analytical  purposes  in  organic  chemistry  ozone  merits 
some  attention,  since  by  the  preparation  of  ozonides  and  their 
subsequent  decomposition  the  structure  of  various  complex 
compounds  containing  ethylene  linkages  has  been  elucidated. 

The  investigations  of  Harries  ("  Ber.,"  37,  839,  842,  2708, 
3431,  1904,  et  seq.)  and  his  co-workers,  have  been  the  most 
remarkable  in  this  direction.  Unsaturated  compounds  are 
ruptured  at  the  double  bond  and  converted  into  aldehydes 
and  ketones. 

In  the  absence  of  water,  however,  a  direct  addition  of 
ozone  to  the  double  bond  occurs  with  the  formation  of 
ozonides — 

>C:C<-»>C  -.    C< 

I          I 

o      o 

V 

which  on  the  subsequent  addition  of  water  undergo  decom- 
position to  ketones  and  hydrogen  peroxide — 


INDUSTKIAL  USES   OF  OZONE  169 

>C    .     C< 


+  H20  ->>C  =  0  +  O  =  C<+  H202. 
0       0 

V 

The  ozonides  are  colourless,  viscid,  oily  substances,  highly 
explosive  and  possessing  a  penetrating  odour. 

In  common  with  ozone  they  affect  a  photographic  plate, 
attributable  to  the  chemioluminescence  produced  on  oxida- 
tion of  organic  matter  by  means  of  ozone  (see  pp.  8,  159). 

The  ozonides  behave  like  powerful  oxidising  agents  them- 
selves, akin  to  the  peroxides  in  chemical  behaviour  in  that 
they  bleach  indigo,  liberate  iodine  from  potassium  iodide  and 
react  with  potassium  permanganate. 

Benzene  triozonide  or  ozobenzene,  isolated  by  Kenard,  is 
a  relatively  stable  substance,  easily  produced  by  passing 
ozonised  air  into  dry  benzene,  from  which  it  is  precipitated 
as  a  gelatinous  amorphous  product.  It  explodes  somewhat 
violently  on  the  addition  of  warm  water. 

Oleic  acid,  either  when  dissolved  in  acetic  acid  (Harries 
and  Thieme,  "Ber.,"  39,  28,  44,  1906),  or  when  treated  with 
ozonised  air  without  a  solvent  (Molinari  and  Sonicini,  "  Ber.," 
39,  27,  34,  1906),  forms  a  normal  ozonide.  In  chloroform 
four  atoms  of  oxygen  are  taken  up  to  form  an  ozonide  per- 
oxide. 

By  analysis  of  the  products  of  decomposition  the  position 
of  the  unsaturated  linkage  in  oleic  acid  was  established  be- 
tween the  atoms  C9  and  C10,  thus  giving  oleic  acid  the  structure 
CH3(CH2)7CH  =  CH(CH2)7COOH. 

If  strong  concentrations  of  ozone  be  employed  for  the 
preparation  of  ozonides,  oxozonides  are  said  to  be  formed  at 


170  OZONE 

the  same  time  (Harries,  "  Ber.,"  43,  936,  1912,  et  seq.\  thus 
s.  butylene  yields  the  following  substances  on  ozonisation  : — 


1 


(1)  GH3 .  CH  .  CH  .  CH3  (2)  fCH3 .  CH  .  CH  .  CH, 

v>  {       \/ 

o3  I  o3 

Normal  ozonide.  The  dimeric  ozonide. 

(3)  CH3 .  CH  .  CH  .  CH3 
and  \  / 

04 

Oxozonide. 

and  a  dimeric  oxozonide — 

(4)  f  CH8 .  CH  .  CH  .  CH31 

I     Y      1 

cyclo  pentene  exhibits  a  similar  behaviour  in  that  two  ozonides 
and  two  oxozonides  are  formed  on  oxidation  (Harries,  "  Ann.," 
4,  4101,  1915). 

Harries  (loc.  cit.)  attributed  the  formation  of  oxozonides 
to  the  presence  of  oxozone  in  the  ozonised  oxygen  which  he 
postulated  to  be  present  by  analysis,  employing  the  iodide 
method  of  estimation.  The  existence  of  oxozone  in  ozonised 
air  or  oxygen  has,  however,  not  been  confirmed  (see  p.  184) 
and  some  other  structural  formula  must,  consequently,  be 
adopted  for  the  oxozonides. 

The  ozonisation  of  rubber  was  first  attempted  unsuccess- 
fully by  Wright  in  1897  ("Bull.  Soc.  Chem.,"  18,  438,  1897), 
and  was  subsequently  investigated  in  great  detail  by  Harries 
and  his  co-workers,  Langheld  and  Haeffner,  in  the  hope  of 
elucidating  the  complex  structure  of  the  isoprene  polymer. 

Harries  ("  Ber.,"  37,  2708,  1904)  ozonises  rubber  by  the 
following  procedure  :  Ozonised  air,  washed  with  caustic  soda 


INDUSTRIAL  USES  OF  OZONE 


171 


and  sulphuric  acid  to  remove  the  oxozone,  containing  from 
6  to  12  per  cent,  of  ozone,  is  passed  for  ten  hours  into  a  1  per 
cent,  solution  of  purified  rubber  in  chloroform.  The  end  of 
the  reaction  is  ascertained  by  the  decolorisation  of  bromine. 

The  ozonide  is  obtained  by  evaporation  at  20°  C.  in  vacuo, 
subsequently  reprecipitating  from  ethyl  acetate  by  petroleum 
ether  in  the  form  of  a  thick  oil  solidifying  to  a  vitreous  mass. 

Rubber  ozonide  is  soluble  in  ethyl  acetate,  benzene  and 
alcohol  is  explosive  and  like  ozone  it  acts  on  a  photographic 
plate. 

Analysis  gives  the  average  composition  C  49  per  cent., 
H2  6'9  per  cent.,  and  the  molecular  weight  about  526,  corre- 
sponding to  the  compound — 
(C10H16O6)2  (C  51-72  per  cent.,  H2  6'70  per  cent.,  m.w.  =  464). 

Adopting  Harries'  structural  formula  for  isoprene  rubber — 

fCH3C— CH2— CH2— CH   ] 

\      ?  #         f 

[HC— CH2— CH2    C.CH3L 

the  rubber  ozonide  would  possess  the  following  structure  : — 
CH3  H 

\j — C» — OH2 — OH2 — C\^^ 


0— C— CH2— CH2— C 

I  -  / 

H  CH, 


:0 


Rubber  ozonide  suffers  decomposition  on  boiling  with 
water,  forming  levulinic  aldehyde  and  levulinic  aldehyde 
peroxide,  as  indicated  by  the  following  equation  : — 


172  OZONE 

CH3 

O— C— CH2— CH2— CH— Ov 
0( | > 

X0— C—CH2— CH2— C-  -0 

H  CH3 

CH3 

0= C—CH2— CH2— CH-  0 
->   I!  II 

O =0 

+ 
CH3.CO.CH2    CH2.CHO 

On  continued  boiling  of  the  aqueous  solution  the  levulinic 
aldehyde  peroxide  undergoes  autoxidation  to  levulinic  acid, 
and  the  a  and  /3  lactones  of  this  acid — 

CH3 

0= C— CH2— CH2— CH=  O 

II  [I   ->  CH3 .  CO  .  CH2CH2COOH 

0  .Q  Levulinic  acid. 

OH3 — 0  *  OH — OH2 

I         i 

O CO 

Levulinic  lactone. 

Gottlob  ("  Zeit.  f.  Anal.  Chem.,"  20,  2213, 1907)  investigated 
the  action  of  ozone  on  many  varieties  of  African  rubbers, 
especially  those  from  Uganda  and  the  Upper  and  Lower 
Congo  areas ;  he  obtained  the  following  mean  values  for  the 
decomposition  products  : — <* 

Per  Cent. 

Yield. 
Levulinic  acid 49-8 

aldehyde         . 25'8 

„  „        peroxide 3-8 

Resin  5-0 


INDUSTKIAL  USES  OF  OZONE  173 

Paulsen  ("Le  Caoutchouc  et  la  Guttapercha,"  7,  4177,  1913) 
has  shown  that  the  ozonides  of  various  resins,  such  as  sandarac 
and  dammar,  are  precipitated  by  carbon  tetrachloride,  a 
property  which  Dubosc  and  Luttringer  ("  Kubber,  Its  Pro- 
duction, Chemistry  and  Synthesis,"  Griffin,  1918)  has  applied 
to  the  estimation  of  rubber  resins.  Molecular  weight  deter- 
mination has  shown  that  the  natural  rubber  molecule  is 
exceedingly  complex,  whilst  the  ozonide  consists  of  but  two 
molecules  of  ozonised  dimethyl  cyclo  octadiene.  It  there- 
fore follows  that  ozone  exerts  a  depolymerising  action  on  the 
rubber  molecule. 

Attempts  to  prepare  dimethyl  octadiene  itself  by  reduction 
of  the  ozonide  have,  however,  proved  fruitless. 

Cyclo  octadiene,  the  simplest  of  the  cyclo  octane  deriva- 
tions, has,  however,  been  isolated  by  Willstatter,  and  on 
ozonisation  and  hydrolysis  this  yields  succinic  dialdehyde, 
whilst  on  polymerisation  it  yields  a  product  very  similar  to 
natural  rubber  — 

CH—  CHo—  CH—  CH  2  .  CHO  .  CH2  .  CH2CHO 


CH—  CH2—  CH2—  CH,, 

On  this  evidence,  supported  by  his  previous  work  on  the 
nitrosites  and  tetrabromide  of  rubber,  Harries  adopted  the 
somewhat  unusual  eight-ring  structure  as  the  unit  in  the 
polymer  of  natural  rubber.  Although  Harries'  views  have 
received  wide  acceptation,  yet  this  theory  is  contested  by 
several  workers  in  the  field;  notably  by  Pickles,  who  ad- 
vances various  arguments  why  rubber  should  be  represented 
as  an  open-chain  polymer  :  — 


174  OZONE 

f    CH,  I 

1  =  C  .  CH2 .  CH2CH  =  J , 

the  ozonide  of  which  would  naturally  possess  the  following 
structure:  — 

CH3 

°\  I  /°\ 

|  XC— CH2— CH2—CH/    X0 

°\  ' 

A  few  applications  of  the  use  of  ozonised  air  have  been 
made  in  preparative  inorganic  chemistry ;  thus  the  oxidation 
of  manganates  to  permanganates,  chlorates  to  perchlorates, 
ferrous  chloride  to  ferric  chloride  are  reactions  which  pro- 
ceed smoothly  and  rapidly  with  the  aid  of  ozone.  The 
preparation  of  permanganates  by  means  of  ozone  is  said  to 
possess  advantages  over  the  usual  chemical  methods  of 
manufacture. 

APPLICATION  TO  BREWING  AND  FOOD  PRESERVATION. 

Ozonised  air  has  found  increasing  application  in  the 
brewery,  not  only  to  prevent  the  ingress  of  adventitious 
micro-organisms  during  the  process  of  fermentation  and 
cooling  of  the  wort,  but  also  the  refrigerating  and  bottling 
the  beer.  By  enclosure  of  the  fermenting  tuns  and  the 
cascade  coolers  in  a  suitable  air  shaft  high  concentrations  of 
ozone  may  be  used,  which  ensures  the  sterility  of  the  air  in 
contact  with  the  liquid.  Minor  applications  are  found  in  the 
treatment  of  filtering  material,  the  cleansing  of  clarifying 
chips  and  the  sterilisation  of  bottles  and  casks. 

According  to  V.  Vetter  ("  Zeit.  f.  Brauerie,"  Feb.,  1911) 
all  utensils,  with  the  exception  of  rubber  goods,  which  are 


INDUSTRIAL  USES  OF  OZONE  175 

rapidly  attacked  by  ozone  can  be  cleaned  after  washing  with 
water  by  subjecting  these  to  an  air  current  containing  0*5  gm. 
ozone  per  cubic  metre  for  half  an  hour. 

Will  and  Wiensiger  and  V.  Vetter  (loc.  cit.)  have  shown 
that  yeast  has  a  higher  power  of  resisting  ozone  than  other 
organisms  met  with  in  brewing.  On  this  observation  pro- 
cesses of  selective  sterilisation  of  the  fermenting  liquid  have 
been  devised  in  which  by  the  aeration  with  ozonised  air  for  a 
suitable  period  of  time  all  organisms  such  as  sarcina,  with 
the  exception  of  the  yeast  cells,  are  destroyed.  It  was  found 
that  aeration  with  ozonised  air  containing  3  gms.  per  cubic 
metre  at  the  rate  of  12  cubic  metres  per  hour  per  kilogram  of 
pressed  yeast,  for  a  period  of  from  fifteen  to  twenty  minutes, 
ensured  the  production  of  a  normal  and  energetic  fermenta- 
tion. It  was  further  claimed  that  the  flavour  of  the  beer 
was  unchanged,  its  keeping  qualities  improved,  together  with 
its  power  of  resisting  infection  on  storage.  In  forcing  tray 
experiments  beer  from  untreated  yeast  turned  at  the  end  of 
thirty-two  days,  whereas  that  from  ozonised  yeast  remained 
good  for  eighty-six  days,  showing  only  a  slight  haze  at  the 
end  of  this  period.  Similar  applications  of  ozone  in  the 
other  fermentative  industries,  such  as  the  manufacture  of 
wines,  cider,  perry,  alcohol  and  vinegar,  have  been  frequently 
proposed.  From  time  to  time  proposals  have  been  advanced 
to  accelerate  the  normal  ageing  of  wines  and  especially  spirits 
by  treatment  with  ozonised  air.  According  to  De  la  Coux 
("  L'Ozone,"  p.  378)  the  process  of  ageing  is  virtually  one  of 
slow  oxidation  by  means  of  atmospheric  oxygen.  Not  only 
is  a  small  fraction  of  the  alcohol  oxidised  direct  to  acetic  acid, 
as  indicated  by  an  increase  in  the  quantity  of  ethyl  acetate 


176  OZONE 

in  the  spirit,  but  the  bouquet  is  in  part  due  to  the  formation 
of  acetal  produced  by  interaction  of  aldehyde  and  alcohol  in 
the  presence  of  ozonised  oxygen.  Eesinous  matter,  which  in 
the  normal  process  of  ageing  is  precipitated  from  the  wine  or 
spirit,  is,  it  is  said,  also  removed  by  treatment  with  ozone 
(together  with  fusel  oils  from  whisky).  De  la  Coux  likewise 
records  an  improvement  in  colour.  Such  a  process  of 
artificial  ageing,  first  proposed  by  Pasteur,  has  been  at- 
tempted on  a  small  industrial  scale  by  numerous  investi- 
gators, notably  Villon,  Broyer,  and  Petit,  and  others.  The 
type  of  plant  employed  follows  closely  on  those  adopted  for 
the  sterilisation  of  water,  either  plate  towers  or  spray 
systems  being  utilised  to  ensure  intimate  contact  between 
the  ozonised  air  and  the  liquid.  For  the  artificial  ageing  of 
wines  it  is  said  that  20  to  40  litres  of  oxygen  should  be 
utilised  per  hectolitre  of  liquid  by  the  continued  passage  of 
cooled  ozonised  air  for  a  suitable  length  of  time.  The  wine 
so  treated,  it  is  claimed,  will  become  fully  mature  in  from 
two  to  three  months. 

For  spirits  some  50  litres  should  be  utilised  per  hectolitre. 
Three  days  after  treatment  the  spirit  is  clarified  by  precipita- 
tion with  magnesia  or  filtration  through  suitable  clarifying 
agents  and  reozonised,  until  another  50  litres  of  oxygen  are 
absorbed.  The  process  is  repeated  three  or  four  times  and 
the  spirit  finally  stood  for  a  few  months.  Villon  claims  that 
twenty  year  old  cognac  may  thus  be  prepared  in  less  than 
six  months. 

Although  these  claims  are  distinctly  interesting,  and  any 
method  of  rapidly  maturing  wines  and  spirits  would  possess 
great  economic  advantages,  yet  it  must  be  confessed  that 


INDUSTKIAL   USES   OF   OZONE  177 

apart  from  the  fact  that  no  process  of  industrial  ageing  on 
these  lines  appears  to  be  in  actual  operation,  ageing  is  prob- 
ably not  entirely  a  process  of  oxidation  but  results  from  a 
great  number  of  chemical  reactions  produced  from  enzyme 
activity  taking  place  but  slowly  in  the  wine.  Probably  not 
the  least  important  are  the  proteoclastic  ferments  effecting 
the  gradual  hydrolysis  of  the  small  quantities  of  protein  sub- 
stances present  in  the  liquid. 

A  useful  field  for  the  application  of  ozonised  air  is  to  be 
found  in  the  preservation  of  food,  especially  in  connection 
with  refrigeration. 

For  the  prolonged  storage  of  fresh  meat  it  is  necessary  to 
maintain  it  at  a  low  temperature,  in  order  to  lower  the  rate 
of  hydrolysis  both  proteoclastic  and  lipoclastic  produced  in 
the  meat  by  the  naturally  occurring  enzyme.  At  normal 
temperatures  meat  can  only  be  preserved  for  a  few  days 
without  its  quality  being  seriously  affected  by  such  changes 
occurring.  During  storage  and  transit  not  only  are  the  sub- 
stances subjected  to  internal  attack  by  the  natural  enzymes 
but  frequently  external  sources  of  contamination  are  to  be 
found,  especially  flies  and  air-borne  micro-organisms. 

Thus  maintaining  perishable  foodstuffs  at  a  low  tempera- 
ture and  in  a  sterile  atmosphere  ideal  conditions  for  preserva- 
tion obtain.  Several  large  refrigerating  warehouses  and  ship 
holds  have  been  equipped  with  ozonisers  on  the  air-circulating 
systems. 

We  have  already  alluded  to  the  enhanced  efficiency  of 
ozonisers  at  low  temperatures,  hence  the  conjunction  of  an 
ozoniser  in  a  refrigerating  system  is  a  particularly  economical 

installation. 

12 


178  OZONE 

Useful  applications  for  ozone  are  likewise  to  be  found  in 
the  drying  of  copra,  which,  when  subjected  to  the  ordinary 
sun-drying  process,  is  liable  to  acquire  an  exceedingly  offensive 
smell.  The  conditioning  of  air  in  flour  mills  by  means  of 
ozonised  air  is  said  to  be  attended  with  a  possible  increase 
in  the  mill  capacity  of  30  per  cent,  and  the  practical  elimina- 
tion of  the  flour  moth. 


CHAPTEE  X. 

METHODS  OF  DETECTION  AND  ANALYSIS. 

THE  presence  of  one  part  of  ozone  in  a  million  of  air  can  be 
detected  by  means  of  its  characteristic  odour,  which,  however, 
is  liable  to  be  confused  with  that  of  dilute  chlorine  or  nitrogen 
peroxide. 

In  common  with  other  powerful  oxidising  agents,  it  will 
readily  liberate  iodine  from  the  usual  starch  iodide  papers, 
colouring  them  a  brilliant  blue,  a  method  employed  by  Schon- 
bein,  Wolffhiigel  and  Van  Bastelaer. 

Houzeau  indicated  that  by  a  simple  modification  of  the 
test  paper  ozone  could  be  distinguished  from  acid  oxidants, 
such  as  chlorine  or  nitrogen  peroxide.  A  strip  of  filter  paper 
is  impregnated  with  a  solution  of  neutral  potassium  iodide 
and  one  half  is  then  treated  with  starch  and  the  other  with 
an  alkali  indicator  such  as  phenolphtalein  or  rosolic  acid. 
Ozone  is  sharply  distinguished  from  chlorine  and  nitrogen 
peroxide  by  liberating  both  iodine  and  alkali  from  neutral 
potassium  iodide — 

03  +  2KI  +  H20  =  02  +  2KOH  +  I2. 

The  paper  is  not  entirely  diagnostic  for  ozone,  since  carbon 
dioxide  will  slowly  liberate  iodine  from  neutral  potassium 
iodide  solutions  and  will  not  turn  the  paper  distinctly  acid. 
Hydrogen  peroxide  will  give  the  same  indications  as  ozone 

itself.     Iodine  is  also  set  free  by  photolysis  (Loew,  "  Zeit.  f. 

(179) 


180  OZONE 

Chem.,"  5,  625, 1869),  and  the  papers  should  be  guarded  from 
direct  sunlight.  Guiacum  test  papers  are  turned  blue  by 
ozone  in  common  with  other  oxidants. 

Cazeneuve  has  shown  that  m.-phenylene  diamine  test 
papers  are  sensitive  to  oxidants  and  that  ozone  can  be  dis- 
tinguished from  hydrogen  peroxide,  since  the  former  gives  a 
brown  coloration  and  the  latter  an  intense  blue. 

Arnold  and  Mentzel  ("  Ber.,"  35,  1324,  2902,  1902),  as  a 
result  of  a  series  of  experiments,  showed  that  benzidine  and 
dimethyl  p.-phenylene  diamine,  or  better  the  tetramethyl 
derivative  (the  sensitivity  to  oxidising  agents  increases  with 
the  number  of  methyl  groups  inserted),  were  extremely 
sensitive  and  at  the  same  time  gave  a  ready  means  of  dis- 
tinguishing between  the  different  oxidising  substances,  as 
indicated  by  the  following  table  : — 


Colour  shown  by  : — 

Oxidant.  Benzidine.  Tetramethyl 

Base. 


Ozone 

.     Brown. 

Violet. 

Nitrous  acid    . 

.     Blue. 

Straw  yellow. 

Halogens 

.     Blue  then  red. 

Deep  blue. 

Hydrogen  peroxide 

.     nil. 

nil. 

Mention  may  be  made  of  the  following  test  papers  which 
indicate  the  presence  of  ozone  :  Silver  foil  is  coloured  black 
by  the  formation  of  the  somewhat  unstable  silver  oxide  in 
the  presence  of  ozone ;  hydrogen  sulphide  will,  of  course, 
form  a  sulphide  coloration  somewhat  similar  to  that  of 
oxide ;  lead  acetate  paper  is  sufficiently  diagnostic  of  hydro- 
gen sulphide  ;  whilst  lead  sulphide  paper  is  bleached  by  ozone 
and  hydrogen  peroxide  owing  to  conversion  into  lead  sulphate. 


METHODS  OF  DETECTION  AND  ANALYSIS       181 

Manchot  ("  Ber.,"  39,  3570,  40,  289, 1907,  42,  3948,  1908) 
notes  that  silver  is  extremely  sensitive  to  the  presence  of  a 
little  metallic  iron  as  catalytic  agent,  a  coloration  is  easily 
produced  by  O'Ol  per  cent,  ozone,  and  he  claims  this  to  be 
more  sensitive  than  the  tetramethyl  base  paper. 

Manganous  sulphate  impregnated  filter  papers  turn  brown 
in  the  presence  of  ozone,  due  to  the  formation  of  Mn203. 
Manganous  oxide  may,  of  course,  be  formed  if  any  alkali  be 
present  in  the  gas,  e.g.  ammonia,  and  this  in  turn  will  undergo 
atmospheric  oxidation  to  the  coloured  manganese  oxide, 
especially  in  the  presence  of  light  (Danhary,  "  J.C.S.,"  5, 
1,  1867).  Thallous  oxide  is  converted  into  the  brown  thallic 
oxide  T1203  by  the  action  of  ozone.  Nitrous  acid  is  without 
effect,  since  the  nitrite  and  nitrate  of  thallium  are  not  coloured. 
Halogens  and  hydrogen  sulphide,  however,  produce  a  brown 
coloration,  the  former  due  to  oxidation  and  the  latter  due  to 
conversion  into  a  coloured  sulphide.  Carbonic  acid  present 
in  the  gas  to  be  detected  causes  a  considerable  decrease  in 
sensitivity  of  thallous  oxide  paper  owing  to  conversion  to  the 
somewhat  insoluble  carbonate. 

Various  investigators,  notably  Poe'y  and  Berigny,  have 
used  these  test  papers  in  the  form  of  long  strips  in  a  suitable 
recorder  mechanism  for  the  continuous  detection  of  ozone  in 
gases.  By  means  of  a  simple  clockwork  escapement  a  small 
piece  of  a  ribbon  of  impregnated  paper  is  exposed  to  the  gas 
stream  for  a  short  period,  and  by  noting  the  time  and  coloration 
of  the  paper  the  presence  of  ozone  in  the  gas  at  any  time 
during  the  period  of  operation  can  easily  be  detected. 

Some  attempts  have  been  made  to  convert  these  so-called 
cronozoscopes  into  cronozometers  for  giving  some  idea  as  to 


182  OZONE 

the  quantity  of  ozone  in  the  gas  at  different  intervals  of  time. 
These  experiments  have  usually  been  directed  along  one  of 
the  following  lines :  either  the  time  of  exposure  is  increased 
until  the  test  slip  becomes  sufficiently  coloured  to  be  indis- 
tinguishable from  a  standard  colour,  when  the  amount  of 
ozone  present  is  naturally  inversely  proportional  to  the  time ; 
or  a  series  of  standard  colours  are  made  up  and  each  test  slip 
is  exposed  for  a  definite  and  constant  time  interval.  It  would 
appear  that  the  former  method  gave  more  accurate  results. 
It  is  evident  that  human  control  is  necessary  for  this  type  of 
cronozometer,  but  a  simple  mechanical  mechanism  could 
doubtless  be  fitted  to  make  the  machine  automatic  and  not 
merely  semi-automatic  in  action.  Thus,  for  example,  the 
difference  in  reflecting  powers  of  various  shades  of  thallium 
oxide  T1203  could  be  made  to  actuate  a  system  of  balanced 
thermocouples. 

METHODS  OF  ESTIMATION. 

(a)  Iodide  Method. — Bunsen's  method  of  estimating  ozone 
by  the  liberation  of  an  equivalent  of  iodide  from  a  neutral 
solution  of  potassium  iodide  according  to  the  equation — 

(1)  2KI  +  03  +  H20  =  2KOH  +  I2  +  02, 
followed  by  back  titration  after  acidification  with  sodium 
thiosulphate  or  sodium  hydrogen  sulphite,  using  starch  as 
indicator,  is  liable  to  give  unsatisfactory  results  owing  to  the 
further  oxidation  of  the  liberated  iodine  into  iodite,  iodate, 
and  periodate  (Garzarolli,  "  Thurnlackh.  Monatsh.,"  22, 
455,  1901).  With  a  large  excess  of  potassium  iodide  and  in 
a  slightly  acid  solution,  however,  the  sensible  error  due  to  the 
formation  of  iodate  and  periodate  js  not  large,  According  to 


METHODS  OF  DETECTION  AND  ANALYSIS       183 

Ladenburg,  hydrogen  peroxide  is  formed  under  these  condi- 
tions ("  Ber.,"  V.,  34,  1187,  1901)- 

(2)  403  +  10HI  =  5L  +  4H20  +  302  +  H202, 

and  although  a  slight  loss  may  result,  due  to  interaction  of 
the  ozone  and  the  hydrogen  peroxide  thus  formed — 

H202  +  03  =  H20  +  202, 

the  results  are  usually  somewhat  higher  than  those  determined 
by  physical  methods,  which  we  will  shortly  refer  to.  Laden- 
burg  ("  Ber.,"  36,  115,  1903)  obtained  excellent  results  by 
performing  the  estimation  in  the  reverse  manner,  viz.  passing 
ozone  through  standardised  sodium  hydrogen  sulphite  and 
back  titrating  with  standard  iodine  solution. 

Ingles  ("J.C.S.,"  98,  1010,  1903)  showed  that  the  acid 
iodide  method  invariably  gave  high  results,  neutral  solutions 
yielding  more  accurate  determinations.  He  found  that 
neutral  potassium  bromide  solutions  gave  discordant  results. 

Houzeau's  modification  of  the  iodine  method  is  extremely 
accurate.  A  consideration  of  the  equation  (1)  will  indicate 
that  for  every  equivalent  of  ozone  two  equivalents  of  alkali 
are  liberated,  and  consequently  the  increase  in  alkalinity  of 
the  solution  gives  a  measure  of  the  amount  of  ozone.  A  very 
dilute  sulphuric  acid  solution  of  potassium  iodide  is  usually 
employed,  followed  after  absorption  of  the  ozone  by  back 
titration  with  standard  alkali,  using  litmus  or  congo-red  as 
an  indicator.  The  liberated  iodine  can  be  determined  in  the 
neutral  solution  by  means  of  sodium  thiosulphate,  thus  giving 
a  check  on  the  former  figure. 

E.  H.  Eiesenfeld  and  F.  Bencher  ("  Zeit.  Anorg.  Chem.," 
98,  167,  1916)  investigated  the  effect  of  the  addition  of 


184  OZONE 

to  neutral  potassium  iodide  solutions  for  the  estimation  of 
ozone  in  great  detail.  They  showed  that  in  all  cases,  although 
the  main  reaction  proceeded  according  to  the  equation— 

03  +  KI  ->  I  +  02, 
a  side  reaction  took  place  simultaneously — 

03  +  3X1  ->  31. 

The  side  reaction  was  found  to  be  uninfluenced  by  the  ozone 
concentration  in  the  gas,  but  greatly  favoured  by  low  tempera- 
tures and  relatively  strong  acid  solutions.  Any  values  be- 
tween 1  and  3  atoms  of  iodine  per  molecule  of  ozone  could 
be  obtained  by  altering  these  conditions,  a  value  of  2  '1  being 
readily  obtainable. 

They  suggest  that  the  intermediary  ions  10',  I03',  IO/  play 
a  part  in  the  reaction,  and  that  in  all  solutions  containing 
ozone  and  potassium  iodide  an  equilibrium  between  the  fol- 
lowing ions  is  invariably  obtained,  K,  OH',  I',  10',  I03'  and 
I04'.  The  production  of  oxozone  (04),  in  the  silent  electric 
discharge  was  suspected  by  Harries  ("  Zeit.  Elektrochem.,"  17, 
629,  1911)  as  a  result  of  analyses  of  the  ozonised  air  by  the 
iodide  method.  It  appears  more  than  probable  that  the 
above  side  reaction  fully  accounts  for  Harries'  results. 

N 

Vosmaer  ("  Ozone,"  Constables,  1916)  employs  ^j  sul- 
phuric acid,  and  finds  that  no  appreciable  loss  of  accuracy 
results  by  using  acid  of  this 'strength. 

(b)  Arsenious  Oxide  Method. — Thenard's  method  of  esti- 
mating ozone,  similar  to  that  employed  for  evaluating  bleach- 
ing powder  or  permanganate  solutions,  is  based  upon  the 
oxidation  of  arsenious  acid  to  arsenic  acid  by  these  oxidising 
agents,  according  to  the  equation — » 


METHODS  OF  DETECTION  AND  ANALYSIS       185 

3As(OH)3  +  03  +  3H20  =  3As(OH)5. 

A  dilute  solution  of  potassium  arsenite  is  prepared  by  the 
solution  of  arsenious  oxide  in  potassium  bicarbonate,  and  after 

N 

filtration  is  standardised  by  -./w     iodine  solution,  using  starch 


as  indicator. 

The  ozonised  air  is  metered  after  absorption  of  the  ozone 
by  the  potassium  iodide  or  arsenious  acid  solutions  (1  wash 
bottle  is  sufficient  for  gas  flow  rates  up  to  10  litres  per  hour); 
for  higher  flow  rates  a  greater  number  must  be  employed. 
De  la  Coux  ("  L'Ozone,"  p.  530)  states  that  five  1-litre  wash 
bottles  are  ample  up  to  500  litres  per  hour. 

Ozone  concentrations  are  usually  expressed  in  grammes 
per  cubic  metre  of  air.  It  will  be  noted  that  in  the  case  of 
arsenious  acid  absorption  the  equivalent  volume  of  oxygen 
is  not  returned  to  the  gas,  as  is  the  case  in  absorption  by 
means  of  potassium  iodide,  and  consequently  in  this  case  the 
gas  suffers  a  diminution  in  volume  when  passing  through  the 
absorbers;  this  correction,  however,  is  but  a  small  one.  (4*8 
gms.  03  per  cubic  metre  would  give  a  diminution  of  but 
0-224  per  cent.) 

(c)  David  ("  C.E.,"  164,  430,  1917)  suggests  the  use  of 

N 
=-      ferrous  ammonium  sulphate  solution,  slightly  acidified 


with  sulphuric  acid,  as  absorbent  ;  back  titration  is  accom- 
plished with  potassium  permanganate.  It  is  stated  that  the 
solution  is  unaffected  by  air  at  this  concentration. 

(d)  Physical  Methods.  —  Otto  has  made  use  of  a  barograph 
for  obtaining  a  continuous  record  of  the  density  of  a  stream 
of  ozonised  air  ;  it  is  claimed  that  the  apparatus  is  extremely 


186  OZONE 

sensitive  owing  to  the  great  difference  in  densities  between 
ozone  and  oxygen.  The  same  investigator  also  devised  a 
dilatometer  for  the  estimation  of  ozone  based  upon  the  in- 
crease of  volume  which  ozone  undergoes  when  subject  to 
thermal  decomposition.  A  500  c.c.  flask,  terminating  in  a 
graduated  neck,  is  filled  at  atmospheric  pressure  and  at  a  given 
temperature  with  the  ozonised  oxygen.  The  flask  is  inverted 
and  the  graduated  neck  is  immersed  in  a  mercury  bath,  and 
the  ozone  is  then  decomposed  by  heat.  Boiling  amyl  benzoate 
(b.p.  261°  C.)  has  been  found  to  be  a  suitable  substance  for 
this  purpose.  The  increase  in  volume  after  cooling  to  the 
original  temperature  and  readjusting  to  the  original  pressure 
is  noted  and  thence  the  ozone  content  of  the  gas  can  be  cal- 
culated from  the  equation — 

203  =  302. 

Otto  further  devised  an  optical  method  based  on  the  principle 
of  the  tintometer,  more  recently  applied  by  Lovibond  to  similar 
purposes.  A  series  of  coloured  cobalt-blue  glasses  serve  as 
standards  of  comparison  with  a  tube  of  definite  length  of 
ozonised  air  or  oxygen  under  standard  conditions  of  tempera- 
ture and  pressure. 

F.  Kriiger  and  M.  Moeller  ("  Physik.  Zeit.,"  13,  779, 1912) 
have  suggested  the  measurement  of  ozone  concentrations  by 
the  absorption  of  ultra-violet  light.  The  maximum  absorp- 
tion of  ultra-violet  light  by  ozone  is  found  in  the  region  X  = 
200  to  300  P/JL,  especially  at  X  =  254  /JLJJL.  According  to  Beer's 
law  the  absorption  coefficient  may  be  expressed  in  the  form 
I  =  I0e"ked  or  log  I  =  log  10-kea,  where  I0  is  the  initial  inten- 
sity of  the  ultra-violet  light,  I  the  intensity  after  absorp- 
tion by  a  layer  of  ozonised  oxygen  d  cms.  thick  containing 


METHODS  OF  DETECTION  AND  ANALYSIS       187 

e  gms.  per  cubic  metre,  and  K  the  absorption  coefficient  for 
ozone ;  thus,  under  constant  illumination  in  a  tube  of  con- 
stant length,  the  ozone  concentration  is  proportional  to  the 
logarithm  of  the  intensity  of  the  emergent  ultra-violet  light, 
determined  by  means  of  a  potassium  photo-electric  cell. 

None  of  these  methods  of  physical  analysis  have,  however, 
received  technical  application,  reliance  having  usually  been 
placed  on  some  modification  of  the  iodide  volumetric  method. 

The  analysis  of  gases  containing  ozone  and  other  oxidising 
agents  has  been  the  subject  of  investigation  by  Tommasi,  as 
early  as  1879  ("  Chem.  News.,"  29,  289,  1874). 

The  gases  containing  ozone  and  chlorine  or  nitrous  acid 
are  passed  into  a  normal  solution  of  potassium  ferrocyanide, 
and  the  total  oxidising  power  determined  by  the  conversion 
to  potassium  ferricyanide  effected.  Another  portion  of  the 
gas  is  then  passed  over  hot  platinum  black,  or  through  a  hot 
tube  containing  manganese  dioxide,  when  the  ozone  is  de- 
stroyed. The  potassium  ferrocyanide  conversion  is  then  de- 
termined, and  from  the  difference  in  the  two  estimates  the 
ozone  content  of  the  gas  can  be  determined. 

Analysis  of  mixtures  containing  ozone  and  the  oxides  of 
nitrogen  may  also  be  accomplished  by  passing  the  gases 
into  liquid  air,  when  the  oxides  of  nitrogen  are  solidified  and 
may  be  separately  determined. 

Hauser  and  Herzfeld  ("Ber.,"  45,  3575,  1912)  cite  an  in- 
teresting method  for  the  analysis  of  small  quantities  of 
methane,  which,  it  would  appear,  would  also  be  applicable  to 
the  estimation  of  ozone.  They  note  that  methane  is  quanti- 
tatively oxidised  at  ordinary  temperatures  to  formaldehyde 
by  ozone  according  to  the  equation— 


188  OZONE 

CH4  +  203  =  HCHO  +  H20  +  02. 

It  has  been  suggested  that  this  method  of  ozone  estimation 
might  be  applied  to  the  detection  of  electrical  leaks  and  corona 
discharge  in  tunnels  through  which  insulated  high-tension 
electric  cables  are  led  (e.g.  the  Lotschberg  Simplon  Tunnel), 
as  the  amount  of  ozone  resulting  from  the  ionisation  of  the 
air  in  the  tunnel  would  give  an  approximate  idea  as  to  the 
magnitude  of  the  electrical  leak. 

An  instrument  for  the  detection  and  estimation  of  dissolved 
ozone  in  water  was  devised  some  years  ago  by  U.  Evans 
and  the  author  ("  An  Electro-chemical  Indicator  for  Oxidising 
Agents,"  "  The  Analyst,"  August,  1913).  This  consists  essen- 
tially of  a  small  cell  formed  by  a  platinum  rod  surrounded 
by  a  copper  tube.  The  water  containing  the  ozone  flows 
through  the  annular  space  between  the  platinum  and  the 
copper  at  a  good  rate,  and  forms  the  electrolyte  of  the  cell, 
whilst  the  platinum  rod  and  the  copper  tube  forming  the 
electrodes  of  the  cell  are  connected  to  a  microammeter  or 
thread  recorder.  In  the  absence  of  any  oxidising  agents  in 
the  water,  the  small  cell  rapidly  becomes  polarised,  the  current 
flowing  through  the  microammeter  sinks  to  zero,  and  the 
platinum  becomes  charged  with  hydrogen  corresponding  to 
the  electrolytic  solution  pressure  of  the  copper  in  the  water. 
Most  potable  waters  contain  a  quantity  of  dissolved  salts  to 
make  the  internal  resistance  of  the  cell  sufficiently  low  for 
practical  operation.  On  the  addition  of  any  oxidising  agent 
to  the  water,  the  cell  is  partly  depolarised  by  the  removal  of 
hydrogen  from  the  platinum  electrode,  and  the  rate  of  re- 
moval of  hydrogen  by  the  ozone  is  measured  on  the  micro- 
ammeter, 


METHODS  OF  DETECTION  AND  ANALYSIS       189 

Since  96,540  coulombs  are  associated  with  1  gm.  equivalent 
or  8  gms.  of  ozone,  assuming  the  electrode  reaction — 

60  ©  03  =  30", 

this  quantity  passing  through  the  cell  per  second  would 
generate  a  maximum  current  of  96,540  amperes.  If  the 
liquid  flow  rate  were  1  c.c.  per  second,  a  fairly  normal  rate  for 
the  instrument,  one  part  of  ozone  in  10,000,000  of  water  would 
correspond  to  a  passage  of  10~4  mgm.  of  ozone  through  the 
cell  per  second,  equivalent  to  a  possible  current  of  12  x  10~4 
amperes.  It  is,  however,  evident  that  all  the  ozone  cannot 
act  as  a  depolariser,  since  half  of  it  at  least  is  wasted  at  the 
other  electrode,  and  for  convenience  of  operation  the  cell  and 
flow  rates  are  not  so  proportioned  as  to  effect  complete  re- 
duction of  the  ozone  in  the  water  flowing  through  the  cell. 
In  actual  operation  the  recorded  current  is  about  25  per  cent, 
of  the  theoretical  maximum.  The  instrument  is  remarkably 
sensitive,  easily  estimating  or  recording  one  part  of  ozone  in 
10,000,000  of  water  or  '00001  per  cent. 


NAME  INDEX. 


ABRAHAM,  130,  143. 
Allmand,  96. 
Andreoli,  125,  128,  129. 
Andrews,  2,  58. 
Archibald,  66. 
Armstrong,  41,  132,  137. 
Arnold,  50,  180. 
Arny,  19. 
Arrhenius,  61. 

BACH,  42. 

Baeyer,  14. 

Balmer,  72. 

Barus,  40. 

Baumann,  40. 

Baumert,  2,  57. 

Becquerel,  2,  11. 

Beger,  9. 

Bencher,  183. 

Bendixsohn,  64. 

Berigny,  21,  22,  181. 

Berthelot,  7,  59,  124. 

Besson,  26. 

Bineau,  16. 

Binz,  154. 

Birkeland,  24. 

Blanc,  40. 

Bloch,  11,  40. 

Bodenstein,  141. 

Bohr,  76,  90. 

Boillot,  158. 

Bonyssy,  20. 

Bottger,  49. 

Brauner,  47,  51,  64. 

Brion,  92. 

Erode,  51. 

Brodie,  28,  38,  124,  126,  127. 

Brooksbank,  10. 

Broyer,  176. 

Brunck,  33. 

Buisson,  17,  18,  23,  86. 

Bunsen,  182. 

CAMLETTE,  152. 
Carius,  7,  59. 
Carlson,  154. 
Cazeneuve,  180. 
Chapman,  48,  134,  137. 


Chappuis,  6,  9,  141. 
Chassy,  102,  119. 
Chatelain,  125. 
Chlopin,  85. 
Chree,  26. 
Clark,  134. 
Clausius,  29,  38. 
Clements,  50,  51,  52. 
Cloez,  16. 
Compton,  90. 
Cramp,  92. 
Croze,  10. 

Cruickshank,  1,  57. 
Cunningham,  159. 
Curie,  86. 
Curie,  157. 

DAVID,  185. 

De  Christmas,  154. 

Decombe,  119. 

De  Frise,  121,  122,  143,  146,  147. 

De  la  Coux,  6,  31,  32,  58,  175,  176, 

185. 

Delandres,  72,  82. 
De  Marignac,  57. 
De  Meritens,  142. 
De  Varigny,  19. 
Dewar,  6,  8,  48,  49,  153,  154. 
Dibden,  156. 
Dixon,  41. 
Donovan,  66. 
Doree,  159. 
Drugman,  166. 
Duane,  87. 
Dubosc,  173. 

EGIDIUS,  155. 
Ehrlich,  119. 
Einstein,  71. 
Elster,  39,  135. 
Elworthy,  127. 
Engler,  19,  39,  41,  42. 
Erlwein,  109,  125,  126. 
Evans,  141,  188. 

FABRY,  17,  18,  23,  86. 
Farnweld,  95. 
Finck,  53. 

(191) 


192 


OZONE 


Fischer,  8,  47,  51,  52,  54,  55,  56,  64 

66,  69. 

Fleming,  117. 
Fowler,  10,  17,  23. 
Franck,  81,  89. 
Franklin,  155. 
Freny,  2. 
Friend,  37. 
Fritsch,  83. 

Frohlich,  13,  106,  125,  142. 
Fulhame,  41. 

GAIFFE,  127. 
Gardner,  66. 
Geitel,  39,  135. 
Genin,  128. 
Genthe,  36,  37,  164. 
Gerard,  126. 
Giesel,  87. 
Goekel,  40. 
Goldmann,  119. 
Goldstein,  6,  10,  70. 
Gottlob,  172. 
Graham,  5. 
Gray,  103,  104,  105. 
Greaves,  134. 

HABER,  116. 

Haeffner,  170. 

Hall\vachs,  119. 

Harries,  15, 166, 168, 170, 171, 173, 184. 

Hartley,  16,  18,  23. 

Hatcher,  19. 

Hauser,  187. 

Hautefeuille,  6,  50,  51,  141. 

Hayhurst,  20. 

Hazura,  37. 

Henriet,  20. 

Hertz,  81,  89. 

Herzfeld,  187. 

Hollman,  7. 

Holmes,  20. 

Hoppe,  40. 

Horton,  11,  75. 

Houzeau,  11,  20,  22,  23,  30,  37,  158, 

179,  183. 
Hoveda,  118. 
Hoyle,  92. 
Hughes,  84. 

ILOSVAY,  49. 
Ingles,  183. 
Inglis,  47,  63. 

JAHN,  7,  132,  134. 
Jellinek,  53. 
Jones,  48,  134,  137. 
Jordan,  154. 
Jorrisen,  38. 


KABAKJIAN,  100,  102,  103. 

Kauchtschev,  107. 

Kerr,  117. 

Kirkby,  90. 

Kissling,  37. 

Kopp,  135. 

Kreusler,  84. 

Kron,  17. 

Kriiger,  105,  186. 

Kunz,  95. 

LABBE,  154. 

Labille,  132. 

Ladenburg,  5,  6,  7,  9,  10,  18,  183. 

Lande,  81. 

Langer,  126. 

Langheld,  170. 

Lechner,  106. 

Lehmann,  9,  10,  18. 

Leithauser,  106,  109,  137. 

Lenard,  10,  27,  70,  80. 

Leroux,  50. 

Lessing,  157. 

Levi,  87. 

Lind,  87. 

Lipp,  117. 

Lippert,  36. 

Liveing,  43. 

Loew,  49. 

Lovibond,  186. 

Lowry,  112,  113. 

Luckiesh,  83. 

Ludlam,  77. 

Lummer,  79. 

Luther,  47,  63. 

Luttringer,  173. 

Lyman,  80,  81,  83,  84. 

MACKENZIE,  94. 
McKendrick,  153. 
McLellan,  26. 
McLeod,  7,  60,  61,  65. 
Mailfert,  7. 
Malaquin,  31. 
Manchot,  14,  136,  181. 
Marie"  Davy,  22. 
Marmier,  130,  143. 
Marx,  51,  54. 
Massenez,  64. 
Meidinger,  57. 
Meissner,  49. 
Mentzel,  50,  180. 
Mercanton,  25. 
Meyer,  17,  23. 
Moeller,  105,  186. 
Moissan,  81. 
Mond,  116. 
Vlorris  Airy,  80. 
Moufgang,  8. 


NAME  INDEX 


193 


Mulder,  135. 
Miinchmeyer,  155. 

NASINI,  21,  87. 

Nernst,  29,  44,  47,  50,  53,  55,  61 

76,  133. 
Nesturch,  94. 

ODDO,  27. 

Ogier,  143. 

Ohmiiller,  142,  143. 

Olzweski,  6. 

Ostwald,  42. 

Otto,  4,  8,  123,  124,  128,  130,  132, 

146-152,  185,  186. 
Oudin,  125,  154. 
Ozonair,  129,  147,  161. 

PASCHEN,  72,  81. 
Patin,  121. 
Paulsen,  173. 
Perman,  134. 
Petit,  176. 
Peyron,  22. 
Pickles,  173. 
Pincus,  49. 
Planck,  71,  79. 
Poey,  181. 
Pollitzer,  45. 
Prall,  142,  143. 
Pre"poignot,  128. 
Priestly,  22. 
Pring,  20,  21,  85. 
Pringsheim,  79. 
Przibram,  40. 
Puschin,  107. 

QUAIN,  84. 


,  64 


143 


,  32. 

Ramsay,  87. 

Rao,  21. 

Regener,  10,  70,  138. 

Reicher,  38. 

Remsen,  7. 

Renard,  169. 

Repin,  143. 

Richarz,  59. 

Rideal,  151. 

Riesenfeld,  12,  94,  116,  155,  183. 

Rossi,  50. 

Roth,  138. 

Rothmund,  8,  12. 

Roux,  143. 

Runge,  72. 

Russ,  119. 

Rydberg,  72. 


SALTMARSH,  140. 

St.  Edme,  66. 

St.  John,  82. 

Schneller,  120,  121,  143. 

Schonbein,  1, 16,  22,  29,  35,  39,  49,  57, 

179. 

Schone,  7,  9,  19,  58. 
Schoner,  87. 
Schott,  83. 
Schultz,  154. 
Schumann,  85. 
Schuster,  10. 
Schwarg,  155. 
Segler,  40. 

Shenstone,  133,  137,  141. 
Siemens,  124,  125. 
Simpson,  26. 
Sleen,  120. 
Soddy,  87. 
Soret,  2,  3,  4,  5,  56. 
Stark,  9,  10. 
Steubing,  10,  11,  81. 
Stokes,  157. 
Strong,  86. 

Strutt,  9,  17,  18,  23,  136,  137. 
Struve,  49. 
Swann,  26. 

TAIT,   2. 

Than,  49. 

Thenard,  154,  184. 

Thierry,  19. 

Thomson,  11,  71,  73,  75,  94,  95. 

Thornton,  25. 

Threlfall,  95. 

Tian,  81. 

Toepler,  91,  92. 

Tommasi,  187. 

Townsend,  95. 

Traube,  41. 

Trillat,  167. 

Troost,  7,  50,  51. 

Tropsch,  8. 

USHER,  21,  22. 

VAN  BASTELAEB,  177. 

Van  de  Meulen,  7,  135. 

Van  der  Willigen,  49. 

Van  Ermengen,  143. 

Van  Marum,  1. 

Van  Sleen,  145,  152. 

Van't  Hofi,  29,  38,  39,  42,  61. 

Vetter,  174,  175. 

Villiger,  11. 

Villon,  128,  176. 

Vohr,  129. 

Von  Bahr,  138. 


13 


194 


OZONE 


Von  Wartenberg,  66,  111. 
Vosmaer,  105,  121,  127,  129,  143,  145, 
184. 

WARBURG,  70,   76,  77,  95,  102,  103, 

104,  105,  106,  109,  135,  137,  140. 
Warner,  45. 
Weger,  36. 

Weigert,  76,  135,  138,  139,  141. 
Well,  135. 
Wendt,  87. 
Wien,  79. 
Wiensiger,  175. 
Wild,  19,  41,  42. 


Will,  175. 
Williamson,  2,  56. 
Willstatter,  173. 
Wisse,  120. 
Witz,  158. 
Wohlwill,  67. 
Wolff,  51. 
Wolffhiigel,  179. 
Wright,  170. 

YARNOLD,  127. 

ZENGHILIB,  49. 
Zschimmer,  83. 


SUBJECT  INDEX. 


ACCEPTORS,  37,  42. 

Active  nitrogen,  113. 

—  oxygen,  35,  37. 

Air  conditioning,  179. 

Air-cooled  ozonisers,  128. 

Air  purification,  13,  153-156. 

Allotropes,  2,  10,  29. 

Alternating  currents,  67-69. 

Altitude,  effect  of,  on  03  content,  17, 


19,  20,  26. 
Aluminium  arc,  82. 

—  oxidation,  49. 
Ammonia,  151. 
Ammonium  ozonate,  14. 
Analysis  of,  180-187. 

—  uses  in,  168. 
Anethol,  168. 
Aniline  black,  168. 
Anisaldehyde,  166,  168. 
Anode  material,  58. 
Anodic  polarisation,  67. 
Antozone,  28,  29,  39. 
Antozonides,  2,  28,  39. 

a  particles,  87,  88,  90. 
Arc  discharge,  51,  80,  82,  93,  94,  120. 
Arsenious  oxide,  184. 
Atmospheric  ionisation,  24-27. 

—  ozone,  17-27. 
Atomic  diameters,  73. 

—  hydrogen,  28,  87. 

—  oxygen,  29,  56. 
—  structure,  72-76. 

Aurora,  25. 
Autocatalysis,  36,  37,  164. 


Autoxidation,  1,  23,  29,  34,  35,  37,  38, 


39. 

BACTERIA,  142. 

Baekelite,  115. 

Band  spectra,  9,  72,  73,  90. 

Barium  peroxide,  30,  31. 

Barograph,  185. 

Benzaldehyde,  35. 

Benzene  ozonide,  169. 

Benzidine,  180. 

—  derivatives,  180. 

Black  body  radiators,  79,  80. 

Boiling-point,  7. 


Boric  acid,  83. 
Boro-silicate  glass,  83. 
Brewing,  175. 
Brush  discharge,  93,  95. 

CALCIUM  peroxide,  14. 
Capacity  influence,  100,  113. 
Carbon  arc,  79,  81. 

—  monoxide  oxidation,  165. 
Catalytic  autoxidation,  161,  164. 

—  decomposition,  12,  30,  58,  67,  96, 

134,  135,  136,  137,  181. 

—  preparation,  49. 
Cathode  rays,  75. 
Cellulose,  action  on,  158. 
Chemical  properties,  11. 
Chemioluminescence,  169. 
Chinese  wood  oil,  165. 
Chlorates  decomposition,  33. 
Chlorine,  action  of,  112,  137,  141, 179, 

180. 

—  discharge,  94. 
Chlorophyll,  22,  161. 
Chromic  acid,  38. 
Cinnamon  oil,  37. 
Citronella,  35. 
Cobalt-blue  glass,  83. 
Cognac  ageing,  175. 
Cold  electrolysis,  57-69. 

—  storage,  13,  177. 
Colloidal  platinum,  51. 
Colour,  6,  8,  17,  51,  150. 
Combustion,  formation  in,  35,  49,  51. 
Condensers,  100. 

Conditioning  of  textiles,  159. 
Contact  towers,  144. 
Copra  drying,  178. 
Corona,  25,  95,  188. 

—  pressure,  95. 
Corrosion,  35,  129. 
Cronozometers,  181. 
Cronozoscopes,  181. 

Current  density  influence,  59,  103. 


DAMMAR,  173. 
Decomposition,  134-139. 
Density,  4,  5. 
Deodorant  action,  13,  154. 

(195) 


196 


OZONE 


Depolymerisation,  173. 

Detection,  180-187. 

Dielectric  ozonisers,  124. 

Dielectrics,  113-125,  132. 

Diffusion,  5,  145. 

Drying  oils,  13,  36,  37. 

Dust,  influence  of,  17,  24,  84,  112. 

ELECTRIC  safety  valve,  132. 
Electrode  materials,  58. 

—  potentials,  47,  61,  63,  64. 
Electrolysis,  31. 
Electrolytic  preparation,  1,  57. 
Electron  velocities,  25,  89,  105. 
Emulsifiers,  146. 

Endothermic  actions,  7,  43,  44,  75. 
Enzymes,  177. 

Equilibrium,  29,  46,  48,  49. 
Essential  oils,  23. 
Estimation,  chemical,  48,  179. 

—  electrical,  188. 

—  photo-chemical,  187. 
Ethylene  linkages,  13,  15,  168. 
Explosive  properties,  6,  52,  169. 

FAT  bleaching,  13,  162,  163. 

—  deodorising,  13,  162,  163. 
Ferric  chloride  preparation,  174. 
Fibres,  action  on,  158. 

Field  saturation  in  corona,  103. 

Filtration,  144,  148. 

Flour  mills,  178. 

Fluorine,  31. 

Fluorite,  84. 

Food  preservation,  177. 

Formaldehyde,  22,  166. 

Formic  acid,  166. 

Frauenhofer  lines,  17,  23. 

Frozen  equilibria,  49,  133,  135. 

7  BAYS,  40. 
Gas  velocities,  109. 
Germicidal  action,  13,  142. 
Glass,  83,  115,  117. 
Guiacum  test,  180. 

HEAT  of  formation,  7,  77. 

—  theorem,  45. 
Heliotropin,  14,  167. 
Hide  preservation,  13. 
Historical,  1. 

Hydrogen  peroxide,  2,  12,  22,  38,  41, 
50-56,  59,  84,  180. 

—  sulphide,  155. 

—  vacuum,  82. 

IMPEDANCE,  98. 
Indigo,  35,  168. 
Indol,  155. 


Induction,  98. 

Infra-red  radiation,  9,  10,  74,  76. 

Intermediate  compounds,  41. 

Interpolar  distances,  114,  115. 

Iodide  estimation,  19-21,  182,  184. 

Ionic  theory,  61. 

lonisation,  24,  25,  26,  77,  86,  88,  90. 

Ionising  potentials,  25,  81,  89. 

Iron  arc,  80. 

Isatin,  35. 

Isoeugenol,  167. 

Isosafrol,  167. 

JUTE,  action  on,  159. 
KEER  effect,  117. 

LAVENDER,  23. 
Leucobase  oxidation,  163. 
Levulinic  acid,  171. 
—  aldehyde,  172. 
Limiting  yield,  96. 
Line  spectra,  9,  72,  73-90. 
Linoleum,  13,  165. 
Linseed  oil,  165. 
Lithium  ozonate,  14. 
Liquefaction,  6. 

MAGNETIC  susceptibility,  11. 
Manganese  dioxide,  33,  58. 
Manganous  salts,  action  on,  12, 19, 181. 
Matter,  structure  of,  73. 
Mechanism  of  formation,  76. 
Medicinal  uses,  156. 
Mercaptans,  155. 
Mercury  arc,  6,  70,  81. 
M.-phenylene  diamine,  180. 
Methane  oxidation,  165,  187. 
Methyl  alcohol,  166. 
Mica,  115,  132. 
Monatomic  oxygen,  29,  56,  74,  78. 

NASCENT  state,  40. 

Natural  sources,  16. 

Nitrogen  peroxide,  action  of,  112. 

detection  of,  180. 

formation  of,  52-56. 

Nitrosites,  173. 
Non-dielectric  ozonisers,  120. 

ODOUR,  5,  6. 

Oleic  acid,  169. 

Optical  sensitation,  141. 

Organic  combustion,  12,  13,  142,  150, 

152. 

Oscillatory  sparks,  102. 
Oscillators,  molecular,  73,  75. 
Oscillograph,  105. 


SUBJECT   INDEX 


197 


Oxides  of  metals,  33,  34. 

nitrogen,  14,  16,  21,  25,  50,  54 

85,  112,  133. 

Oxidising  powers,  11,  169. 
Oxozone,  15,  173,  174,  184. 
Oxozonides,  15,  170. 
Oxycellulose,  158. 
Oxyhsemoglobin,  154,  158. 
Ozobenzene,  169. 
Ozonates,  14,  137. 
Ozonic  acid,  14. 
Ozonide  peroxides,  169. 
Ozonides,  2,  14,  15,  41,  168-171. 
Ozonised  water,  21. 

—  waxes,  157. 
Ozonisers,  120-132. 

PAINTS,  161. 

Palladium,  35. 

Pathogenic  organisms,  149,  152. 

Perchlorates,  13,  174. 

Periodic  acid,  32. 

Periodicity,  68,  99,  106,  130. 

Permanganates,   13,   30,  32,  66,  150, 

153,  169,  174. 

Peroxides,  30,  31,  33,  37,  42,  136,  164. 
Persulphates,  31,  59. 
Phosphorescence,  8,  9. 
Phosphoric  acid  electrolytes,  57,  66. 
Phosphorus,  28,  38,  39,  40,  42. 
Photo-chemical  activity,  140. 
-  decomposition,  23,  70,  138. 

—  efficiency,  76-77. 

—  production,  23,  70,  77,  138-141. 
Photo-electric  cells,  187. 

—  effect,  119. 

Photographic  action,  159,  169,  171. 

Photolysis,  180. 

Physiological  activity,  154. 

Piperonal,  166. 

Plate  ozonisers,  127. 

Platinum  electrodes,  58-67,  120. 

Point  discharge,  118,  123. 

Polymerisation,  28,  76,  161. 

Poppy-seed  oil,  165. 

Positive  carriers,  11,  40,  93. 

Potassium  chlorate,  33. 

—  iodide,  2,  4,  11,  20,  157. 
Potential  electrode,  47,  58,  63. 
Pressure,   effect  of,   46,   70,  76,    111, 

Pre-treatment  of  waters,  148. 
Promoters,  164. 
Pseudocatalysis,  164. 
Pyrocatechaldehyde,  166. 

QUANTA,  71,  72,  77,  89,  139,  141. 
Quartz,  70,  84,  115,  116,  140. 


RADIUM,  86,  88. 

Rape  oil,  165. 

Bate  of  decomposition,  24,  49,  53, 133, 

138,  137,  177. 
Reaction  rates,  12,  30,  53. 
Refrigeration,  122,  177. 
Residual  charge,  115. 
Rock  salt,  84. 
Rontgen  rays,  88,  94. 
Rotating  electrode,  124. 
Roughing  filters,  144,  148. 
Rubber  analysis,  15,  173,  175. 
Rusting  of  metals,  35. 

SAFROL,  167. 

Sandal-wood  oil,  23. 

Sandarac,  173. 

Saturation  current,  93,  113. 

Schumann  rays,  80. 

Seasonal  variations,  20,  21. 

Secondary  production,  26,  86,  105. 

Selective  oxidation,  13,  161,  165. 

Self-induction,  98. 

Shellac,  115. 

Siccatives,  161,  164. 

Silent  discharge,  25,  91,  95. 

Silica,  83. 

Silver,  action  of,  3,  33,  49,  136,  181. 

Skatol,  155. 

Solubility,  7,  8. 

Spark  discharge,  1,  51,  102,  118. 

Specific  heat,  45. 

—  inductive  capacity,  114,  115. 
Spectra,  9,  10,  72,  73-90. 
Spirit  ageing,  179. 
Spontaneous  ionisation,  27. 
Stability,  48. 

Starch  iodide  tests,  16,  23,  180. 
Static  transformers,  97. 
Storms,  21. 

Sulphates,  action  on,  12. 
Sulphuric  acid^electrolytes,  57. 

TANNERIES,  153. 

Temperature,  influence  of,  46-52,  58. 

Tetrabase  papers,  136,  180,  181. 

Tetrabromides,  173. 

Textile  conditioning,  159. 

Thallous  salts,  action  on,  181. 

Therapeutic  uses,  156. 

Thermal  decomposition,  32,  111,  134. 

—  equilibrium,  22,  45. 

—  production,  44,  50,  51-56. 
Thermionic  emission,  93. 
Thiosulphates,  action  on,  12. 
Thunderbolts,  25. 
Tintometer,  186. 

Tubular  ozonisers,  124. 
Turpentine,  3,  25,  37. 


198 


OZONE 


ULTRA-VIOLET  lamps,  70,  85. 

—  light,  17,  23,  26,  70,  76,  79,  86, 107. 

estimation  by,  186. 

Uviol  glass,  83. 


VANILLIN,  14,  166,  167. 
Varnishes,  161. 
Vaseline,  ozonised,  157. 
Velocity  of  decomposition,  49. 
Voltage,  influence  of,  102. 
—  perforating,  117,  118. 


WATER-COOLED  ozonisers,  130. 
Waterfalls,  ionisation  in,  27. 
Water,  influence  of,  58,  112,  137. 

—  sterilisation,  142-153. 
Wave  form,  influence  of,  105. 
Waxes,  bleaching  of,  13,  162,  163. 
Wine  ageing,  175. 

Wound  treatment,  13,  157. 

YIELDS,  chemical,  30,  31,  33. 

—  electrolytic,  65,  69. 
—  silent  discharge,  111. 


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