Studies on the phenoldisulphonic acid method for determining nitrates in soils

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i IN
AGRICULTURAL SCIENCES
Vol. 1, No. 2, pp. 21-37 October 15, 1912

STUDIES ON THE PHENOLDISULPHONIC
ACID METHOD FOR DETERMINING
NITRATES IN SOILS

BY

C. B. LIPMAN anp L. T. SHARP

Despite the fact that some careful research has been earried
out on the colorimetric method for determining nitrates, many
factors concerned with it have not been studied, and the some-
what uncertain nature of the method makes it imperative to con-
trol, so far as possible, every factor which may interfere with
the accurate analysis of nitrate-containing material. These
statements apply particularly to the analysis of soils for nitrates
and the authors therefore deem ‘the subjoined data, derived from
a thorough investigation, deserving of the attention of every soil
chemist.

Among the interfering factors in the phenoldisulphonic acid
method which have been either studied inadequately or not at
all, are the effects of salts, the effects of agents employed to pre-
cipitate the clay and organic matter, and the effects of decolor-
izing agents. Cognizance must be taken of all of these factors
by the chemist in the determination of nitrates and in the ar-
rangement and interpretation of results. The importance of salt
effects and their significance in this connection are emphasized
by the fact that many soils, and particularly those of arid and
semiarid regions, may frequently be found to a greater or less
degree impregnated with one or more of the so-called ‘‘alkali
salts,’? together with which, it often happens indeed, consider-

bo
bo

University of California Publications in Agricultural Sciences [Vol. 1

able quantities of nitrates are to be found. So far as the clay-
coagulating substances are concerned, it has always been a com-
mon practice in soil work to employ varying amounts of a satur-
ated solution of alum to obtain a clear soil solution, and more
recently it has been proposed by investigators who have studied
the method under discussion to use aluminum cream for the pur-
pose in place of alum. For decolorizing solutions both aluminum
cream and bone black have been used. The methods for both
clay coagulation and decolorization are obviously essential in most
soil work, since ordinary filtration, without the use of such
agents, can rarely be depended on to yield a clear, colorless soil
solution, even if time be no object. The employment of the
Pasteur Chamberland filter to remove clay has been found by
direct investigation to involve well-defined losses of nitrates.

It is not our purpose here to enter into a lengthy review of
other investigations bearing on the subject in hand, but into a
brief discussion of the more important ones which show the ques-
tions still remaining unsolved or bring out certain results with
which ours do not agree. .

In 1894 Gill’ carried out a series of painstaking investiga-
tions which, briefly, indicate (1) that for purposes of accuracy
the phenoldisulphoniec acid employed in the nitrate determination
must be carefully prepared to insure a uniform compound for
use as a standard; (2) that chlorine induces losses of nitrie acid
both when the solution containing nitrate is evaporated on the
water bath and when the residue is treated with the reagent;
(3) that Na,CO, added to the nitrate-containing solution to pre-
vent escape of nitric acid during evaporation induces losses of
nitrates varying in quantity from four to six per cent; (4) that
alumina may be used to precipitate colloidal material for obtain-
ing a clear solution; (5) that silver sulphate, if free from nitrate,
may be employed to precipitate chlorine, thus removing an im-
portant interfering agent.

More recently Chamot and his coworkers? have prosecuted an
even more thoroughgoing investigation than the preceding, in
which the most emphasis has been placed, however, on the mode
of preparation of the tripotassium salt of nitrophenoldisulphonie

1 Jour. Am. Chem. Soc. vol. 16, p. 122. 1894.

1912] Lipman—-Sharp: Phenoldisulphonic Acid Method 23

acid used as the reagent. Their results indicate (1) that in
order to obtain the phenoldisulphonie acid free from the mono
and tri-phenolsulphonie acid a careful digestion of the phenol
and sulphuric acid under certain constant conditions must be
assured; (2) that the mono and tri-phenolsulphonie acids intro-
duce other colors which interfere with the readings in the colori-
meter; (3) that the tri-potassium salt of nitrophenoldisulphonie
acid gives the characteristic color employed in the determination
and should always be used as a standard; (4) that heating the
dry residue of nitrates even for several hours on the water bath
occasions no losses; (5) that aluminum cream is the best pre-
cipitating agent for organic matter of several used and occasions
no losses of nitrates; (6) that 2 ¢.c. of the phenoldisulphonie acid
should be used in uniform amounts in all determinations; (7)
that KOH was to be preferred to NaOH and NH,OH, as the
alkali employed; (8) that chlorides induced losses of nitrates;
(9) that carbonates and organie matter did lhkewise; (10) that
temperature, concentration, and length of exposure to reagent
greatly affect results; and (11) that there have been other minor
effects of iron, magnesium, and nitrites.

Reference must also be made here to the brief investigation of
Stewart and Greaves® pertaining to the effect of chlorine in de-
termining nitrates in soils, both because the work is recent and
because it is the only one published which is derived from re-
searches on soils. This investigation and those above reviewed
cover most completely the questions involved and reference will
be made in the discussion of our experimental work below to
those questionable points which were considered settled but which
our work shows were far from being so.

THE INTERFERENCE OF SALTS WITH THE NITRATE DETERMINATION

As has been above indicated the salt accumulations which
occur in the soils of California, Nevada, Utah, and other arid
or semi-arid regions frequently contain considerable quantities
of nitrates and the determination of the latter in the presence
of the ‘‘alkali salts’’ is, as has been found, frequently attended

2 Ibid., vols. 21, p. 922; 32, p. 630; 33, p. 366
3 Ibid., vol. 32, p. 756.

24 University of California Publications in Agricultural Sciences [ Vol. 1

with losses of nitrie acid. While all the investigations above re-
viewed have pointed out the interference of chlorine and chlorides
with the nitrate determination, and while some of them have
also considered the losses occurring through the use of Na,CO,,
no mention is made of the effects of the most common and widely
spread of the alkali salts, Na,SO,, or Glauber salt. It seems fur-
ther to have been taken for granted that Na,CO, and Na,SO,
should, for obvious reasons, have the same effects on the nitrate
determination by the phenoldisulphonic acid method. Our re-
sults do not, however, bear out this opinion. Under this head
were also studied the effects of the kation as well as the anion
of salts on the same determination.

Varying quantities of the salts tested were here added to the
same amounts of nitrates in solution, and uniform quantities of
salts were also tested as to their effects on varying quantities
of nitrates. Everyone of the following tables gives the effects
of one of the salts tested in accordance with the scheme above
indicated and in some eases also shows how the nitrate deter-
mination is affected by varying the quantities of both the nitrates
and other salts. The residue containing the salts and the nitrates
was treated with 2 ¢.e of phenoldisulphonic acid thoroughly
stirred for about two or three minutes, 25 ¢.e. of nitrate-free dis-
tilled water was added, and then strong ammonia drop by drop
until the odor of ammonia persisted and the color was per-
manent. The solution was then diluted as necessary and com-
pared in the Sargent-Kennicott colorimeter with a standard
solution similarly and always freshly prepared, whose strength
was in every case carefully tested. The results of these experi-
ments are given in the following tables.

1912] Lipman-Sharp: Phenoldisulphonic Acid Method 2

TABLE I

EFFECTS OF NaCl

NaCl added N. added as N. found as

mgs. nitrate mgs. nitrate mgs.
Uniform quantities .25 -050 .045
KNO,—varying 50 .050 041
amounts NaCl 1.00 .050 .035
2.50 .050 .026
Varying quantities 1.00 050 038
KNO, -uniform 1.00 100 O70
amounts NaCl 1.00 250 215
1.00 500 460
1.00 1.000 .940
1.00 2.500 2.300
Varying quantities of 25 050 -046
both KNO, and NaCl 50 100 O78
1.00 .250 .230
1.50 500 .460
2.00 1.000 -900
2.50 2.500 2.300
Uniform amounts KNO.- aOuit .050 .051
small amounts NaCl i .05 -050 051
10 .050 .049
Color blanks .00 .050 .050
on both salts 2.50 -000 -000

TABLE II
Errects or Na,SO,

Na,SO, added N. added as N. found as

mgs. nitrate mgs. nitrate mgs.
Amounts of nitrate 1.000 -0500 .0480
uniform and sulfate 5.000 -0500 -0420
varying 10.000 .0500 0400
20.000 -0500 .0280
30.000 -0500 .0270
Amounts of sulfate 15.000 .1500 .1420
uniform and nitrate 15.000 .5000 -4950
varying 15.000 1.0000 -9000
15.000 2.0000 1.9500
15.000 3.0000 2.8500
Amounts of both salts 1.000 .1500 .1420
varying 5.000 .5000 .4800
10.000 1.0000 -9200

20.000 2.0000 1.9300

O1

26 University of California Publications in Agricultural Sciences [Vol.1

TABLE III
Errects or Na,CO,

Na,CO, added N. added as N. found as

mgs. nitrate mgs. nitrate mgs.
Amounts of nitrate 1.00 .1000 0995
uniform and carbonate 2.50 .1000 .1040
varying 5.00 .1000 - 1010
10.00 .1000 .1010
20.00 .1000 .1020
30.00 .1000 1020
Amounts of carbonate 10.00 * .1000 .1000
uniform and nitrate 10.00 -2000 -2100
varying 10.00 .5000 5000
10.00 1.5000 1.4900
Amounts of both salts 1.00 .1000 .1010
varying 5.00 .2000 1970
10.00 .5000 -5050
20.00 .5000 5100
30.00 1.5000 1.4900

The results set forth in tables I, II, and III leave no room
for doubt as to the effects of ‘‘alkah’’ salts on the nitrate deter-
mination by the colorimetric method. Both NaCl and Na,SO,
induce large losses of nitrate, and especially is this true of NaCl,
which may be responsible for losses equivalent to forty-five per
cent and more of the total nitrate present as indicated in Table
I. While Na,SO, induces smaller absolute losses than NaCl, they
are none the less marked, and where large amounts of the sulfate
are present very considerable losses of nitrate occur.

Perhaps the most striking feature of the foregoing results is
what appeals to one at first sight as the singular difference in
the behavior of Na,SO, and Na,CO,. Whereas the former is
always responsible for losses in the determination of nitrates,
the latter is the only one of the salts tested which has no effect
and the presence of which in a long series of tests has never,
except in one case, decreased the amount of nitrate present as
shown by the colorimeter readings. It was naturally assumed
that Na,CO,, after the addition of the phenoldisulphonie acid,
would be converted in the presence of an excess of sulphuric
acid into Na,SO, and should therefore show the same decreases
in the nitrate content as the latter salt. To clear up these rather
puzzling facts, as above given, we decided to run a special series
of experiments based on a suspicion which we had as to the

1912] Lipman-Sharp: Phenoldisulphonic Acid Method 27

nature of the action of the salts in question. The results of
these experiments, which will be given below, make entirely clear
what seemed at first quite puzzlng.

In further general discussion of the tables above given, it
must be added that the decreases in the nitrate content of the
solutions tested as induced by the presence of salts never oc-
eurred in accordance with any definite law, the losses at times
being greater with smaller amounts of salts than with larger
amounts, the amounts of nitrates being constant. On the other
hand, with a given amount of nitrates not exceeding one-tenth
of a milligram the salts seemed always to induce larger per-
centage losses than they did in the case of the larger amounts of
nitrates. Our results not only give good opportunity for a com-
parison of the effects of varying quantities of salts on the same
nitrate content, but point out all the relationships between the
salts and nitrates where first the former, then the latter, and
finally both, are varied. There are two other points, also, which
they would not seem to confirm ; indeed they give entirely different
evidence on these than was obtained by other investigators. The
first is that small amounts of NaCl do not induce losses of
nitrates, as claimed by Stewart and Greaves, and Table I indi-
cates that amounts of NaCl below .1 milligram do not occasion
any losses. The other point of difference between our results and
those of the others mentioned is that Na,CO, does not decrease the
amounts of nitrates, no matter to what extent it is used, as shown
in Table III. This is in entire disagreement with the results of
Gill and Chamot and his coworkers, who claimed that Na,CO,
and other carbonates induced losses of nitrates, in the determina-
tion outlined. It must also be added here that the effects of
Na,SO, as given in Table II constitute the first published results,
so far as we are aware, on the effects of Glauber salt on the
nitrate determination, and they have indeed been indirectly re-
sponsible for the discovery of one or two other points of interest
which will be discussed below.

The results above given indicate the effects of each of the
salts taken singly on the nitrate determination. To make the
data more complete it was thought desirable to test various mix-
tures of the same salts and note their effects. Table IV gives
the results obtained.

28 University of California Publications in Agricultural Sciences [Vol.1

TABLE IV

EFFECTS OF MIxED ALKALI SALTS

Na,CO, Na,SO, NaCl N. added as N. found as

mgs. mgs. mgs. nitrate mgs. nitrate mgs.
1 1 1 .1000 .055
5 5) 5 .1000 .026
10 10 10 .1000 021
20 20 20 .1000 O17
1 et .1000 .082
5 5 apie .1000 081
10 10 oe .1000 .075
20 20 eile .1000 O77
i a 1 .1000 061
5) ahs 5) .1000 -060
10 ius 10 .1000 .055
20 we 20 .1000 .028
il .1000 -050
5 5 .1000 .042
10 10 .1000 .033
ae. 20 20 .1000 -030
10 10 10 .2000 .086
10 10 10 -5000 125
10 10 10 1.000 .360
10 10 10 2.000 1.140

The same marked losses in nitrates occur here as where the
salts are employed singly. NaCl seems to be responsible again
for the greatest losses, Na,SO, is next in order, and Na,CO,
seems to have little or no effect. Since these salts occur together
in alkali soils, however, the results in Table IV possess consider-
able significance and interest, especially since they point out what
enormous losses of nitrates occur where such large amounts as
ten milligrams of each of the salts are added to the nitrate-con-
taining solution.

Tur INTERFERENCE OF PRECIPITANTS OF CLAY AND ORGANIC

MATTER ON THE NITRATE DETERMINATION

It is very singular that analytical chemists have for so long
a time been employing such materials as saturated alum solu-
tions, aluminum cream, and bone black for precipitating clay
and organie matter in obtaining the soil solution to be used for
nitrate determinations without ever having attempted to ascer-

1912] Lipman—Sharp: Phenoldisulphonic Acid Method 29

tain if such materials in any way affect the accuracy of the
determination. Indeed Chamot and his coworkers have recom-
mended the use of aluminum cream for removing suspended
material from the solution, and claim to have had very satis-
factory results in the use of that material. Our experiments
in this series were intended to clear up this question and the
following results show very strikingly that none of the materials
mentioned may be employed in the nitrate determinations with-
out incurring very serious losses. Table V gives results obtained
in the use of potash alum, and Table VI gives results obtained
in the use of bone black and aluminum cream.

TABLE V

Errects or K,A1,(SO,),

K,Al,(S0,), N. added as N. found as
added nitrate nitrate
mgs. mgs. mgs.

Amounts of nitrate 5.00 .050 .040
uniform and alum 12.50 .050 .036
varying 25.00. 050 033
50.00 .050 031
100.00 .050 .034
150.00 .050 .040
Amounts of alum uniform 45.00 .050 .035
and nitrate varying 45.00 -100 -075
45.00 .250 168
45.00 .500 2345
45.00 1.000 .675
45.00 2.500 1.800
Amounts of both 5.00 .050 .040
salts varying 12.50 .100 O74
25.00 .250 75
50.00 500 1300
100.00 1.000 .690
150.00 2.500 1.850
Color blanks on .00 .050 .049
both salts .00 .500 .480

TOO: OON TR Me 089 gists 22 bi Gil a Fees

30 University of California Publications in Agricultural Sciences | Vol. 1

TABLE VI
EFFECTS OF ALUMINUM CREAM AND BONE BLACK
N. added as N. found as

nitrate nitrate

mgs. mgs.

Sufficient aluminum cream to 5000 254
clear solution. Five minutes 1.000 .648
exposure 2.000 1.460
Twice the amount of aluminum 5000 .100
cream used above. Exposed one 1.0000 300
and one-half hours 2.0000 1.180
Sufficient bone black to clear 1.0000 plied
and decolorize solution 2.5000 .650
5.0000 2.200

The data in Tables V and VI are clearly very striking. The
enormous losses of nitrates sustained through the use of a satur-
ated solution of alum, varying quantities of aluminum cream and
bone black, make these substances entirely unfit for use as
precipitants for clay, or organic matter, or both, when nitrates
are to be determined. While bone black occasions the largest
losses, and potash alum the smallest, of any of the substances
above described, the losses of nitrates brought about through the
use of all the precipitants are too great to permit of their con-
tinuance in a method for nitrate determinations which is none
too accurate under the best of conditions. It is therefore evident
that nitrates are lost not merely through the loss of nitrie acid,
as is the case where salts are used, but that there is a loss of
nitrates mechanically through adsorption on the part of the
colloidal material of the precipitant, as must be the case where
such substances as aluminum cream and bone black are used.
The large amounts of colloids possessed by these substances, with
the accompanying large surface areas, evidently prevent some of
the nitrate in solution from going through the filter.

On casting about for a method to precipitate clay or organic
matter, we first tried the Briggs filter pump, but found that open
to two objections. First, the losses of nitrates through what we
look upon as adsorption on the part of the clay filter, though not
very large, were nearly equal to those induced by small amounts
of sulfates. Second, while the filter pump yields a clear solu-
tion, it does not serve to decolorize solutions. After several fur-

1912] Lipman-—Sharp: Phenoldisulphonic Acid Method 31

ther attempts to find a coagulating and decolorizing agent which
might promise well for this method, it struck us that quicklime,
being the best coagulating material for clay, might perhaps also
serve to remove organic matter and yet might not decrease
seriously the amount of nitrates in the solution to be tested.
Accordingly, tests were carried out by adding lime to solutions
containing known amounts of nitrates, to soils containing known
amounts of nitrates and to soils with unknown amounts of
nitrates, in which latter a comparison was also especially made
between lime and aluminum cream. We found in these experi-
ments that the losses of nitrate through the use of lime were not
only very small or negligible, but that the action of lime in
precipitating both clay and organic matter was equal to or better
than that of the best of the coagulating and decolorizing agents.
Its coagulating action on clay has of course always been recog-
nized in soil physics. The results of the experiments are given
in Table VII.
TABLE VIL
EFFECTS OF LIME

A—Solutions of known nitrate content

CaO present N. added as nitrate N. found as nitrate
grms. mgs. mgs.
1 1.0000 1.0150
3 1.00006 -9800
5 1.0000 -9550
3 5.0000 4.6500

B—Soils of known nitrate content

CaO present N. present as nitrate N. found as nitrate

grms, mgs. mgs.
Lime ground with
soil and water 2 3.280 3.150
Lime added to muddy
suspension 2 3.280 3.200

C—Comparison of lime and aluminum cream on soil of unknown nitrate

content
CaO present N. found as nitrate
grms. mgs.

Lime ground with

soil and water 2 1.210
Lime added to
muddy suspension
Sufficient aluminum
cream added to
clear solution fe .800

bo
ie
bo
i)
Or

32 University of California Publications in Agricultural Sciences | Vol. 1

It would seem from these results therefore that lme can
yield a clear, colorless solution without decreasing the quantity
of nitrates present in the solution appreciably, and that it is
therefore the only one of the coagulating agents above tested
which can be safely used in the work. We commend it to soil
chemists and others making nitrogen determination under similar
conditions. Only where very large quantities of lime are em-
ployed, and they are not necessary, have we found definite losses
of nitrates. We find that 2 grams of CaO is sufficient to
coagulate the clay in 100 grams of loam soil and to remove
whatever color may be present at the same time.

While lime has been used by some chemists in accordance
with the method above outlined, its use has by no means been
general and no data prior to this existed with reference to its
effects on the nitrate determination. J. G. Lipman and P. E.
Brown give directions in their laboratory manual on Soil Bae-
teriology for the use of 2 grams of lime to precipitate the clay
in the 100 gram samples of soil used in nitrification experiments,
but we have never seen any published statements beyond that
as to the advisability or feasibility of employing lime. It is cer-
tainly surprising that those who have tested the method for
nitrate determination should not have tried and urged the use
of lime as a substitute for alum or aluminum cream.

OTHER EXPERIMENTS ON SALT EFFECTS

It appeared interesting, when the results in Tables I, II, and
III were obtained, to ascertain if the kation as well as the anion
of salts was responsible for losses of nitrates. Accordingly a
series of experiments was instituted in which the effects of
NaCl, KCl, and MgCl, could be compared. The following re-
sults were obtained.

1912] Lipman—Sharp: Phenoldisulphonic Acid Method 33

TABLE VIII
Errects or IONS
N. added as N. found as
KCl MgCl, NaCl nitrate nitrate
mgs. mgs. mgs. mgs. mgs.
1 35e5 = -1000 .070
5 cs a .1000 .063
10 = fee .1000 .055
20 eas nee .1600 .050
1 ae .1000 .057
5 pos .1000 .028
10 ses .1000 .016
20 an .1000 O11
il .1000 .065
5 .1000 .043
10 -1000 .035
20 .1000 .038

It is evident from Table VIII that the chlorine and not the
base is the interfering element, and while the amounts of chlorine
were not so proportioned as to be equivalent in the case of the
two monovalent bases, the effect is clearly seen of the smallest
and the largest amounts of chlorine present in the salts, which
can be ecaleulated from the molecular weights. The negative
ion therefore seems to be the active agent in setting free nitric
acid, but the decreases, depending as they do on other conditions
such as evaporation on the water bath and length of exposure, do
not take place in accordance with any definite law.

The last phase of the salt effects studied was that above re-
ferred to in the discussion of Tables I, II, and III, namely, the
reason for differences in the action of Na,CO, and Na,SO, on
nitrate-containing material. Since it was evident that Na,CO,
should react similarly to Na,SO, when the phenoldisulphonic
acid was added to the dried residue to be analyzed, we suspected
that the losses occurring when Na,SO, was employed came about
on the water bath in evaporating the solution, under which con-
ditions only, according to our work, could there have been a
difference in the action of the two salts.

The results given in the following table prove that our sus-
picions were well founded. In this series the dry salts were
thoroughly mixed with the nitrate-containing residue obtained by
evaporating standard nitrate solutions, and then the phenoldi-
sulphonic acid reagent was added. NaCl was similarly tested.

34 University of California Publications in Agricultural Sciences [Vol.1

TABLE IX
Errects oF Dry MIXING oF NITRATES AND SALTS
N. added as N. found as
Na,CO, Na,SO, NaCl nitrate nitrate
mgs. mgs. mgs. mgs. mgs.
50 ea a .1000 097
100 ae phe 1000 102
50 = .1000 103
100 oe .1000 102
50 .1000 .080
100 .1000 .062

The data in Table IX make it quite clear that the losses due
to Na,SO, occur only when the latter salt is present in solution
with nitrates and the solution is evaporated on the steam bath.
When, however, the salt is mixed dry with the dry nitrate no
losses of the latter occur any more than they do when Na,CO, is
added. The same is not true, however, of NaCl, as is shown in
the last table. That salt causes losses of nitrates during both
the evaporation on the steam bath and the reaction setting
chlorine free in the treatment of the dry residue with phenoldi-
sulphonic acid. This latter fact is a confirmation of work done
by Gill and reviewed above. We have thus shown the individual
reaction of each of the salts as related to the nitrate determina-
tion and the causes which are responsible for the difference.
Nitrie acid is evidently set free from nitrates through the com-
bined action of heat and the SO, radicle on the steam bath and
in the evolution of chlorine when the phenoldisulphonie acid is
added to nitrate and chloride-containing material. Na,CO,,
however, possessing only a weak and unstable acid radicle is
powerless to set free nitric acid either through the help of heat
on the steam bath or by its reaction with the phenoldisulphonie
acid.

GENERAL REMARKS

So many factors may interfere with the determination of
nitrates by the phenoldisulphonie acid method that it would ap-
pear to be almost worthless, and yet it would seem to us that
since there is no other good method to take its place which is
nearly as simple and capable of use in very numerous deter-
minations, it is worth while taking certain precautions to avoid

1912] Lipman-Sharp: Phenoldisulphonic Acid Method 35

error, and to establish the method on a firmer basis. Our results
as above outlined show that losses of nitrates are induced by
the presence of NaCl and Na,SO,, and such losses are indeed
hard to avoid when working with ‘‘alkali soils.’’ Even the
suggestion of Chamot that AgSO, might be used to precipitate
chlorides would seem, from our results, not to be useful, since
the addition of sulfate to the solution would accomplish very con-
siderable losses itself, even if the silver sulfate can be obtained
nitrate-free, which Chamot claims is seldom the case. So that while
we deem it unsafe in the presence of considerable quantities of salts
containing chlorides and sulfates to determine nitrates by the
phenoldisulphonie acid method and would therefore recommend
the Street modification of the Ulsch method in such eases, it is
likewise clear that many of the nitrate determinations made
in soil laboratories, as is especially the case in soil bacteriological
work, would not be interfered with by salts. In such cases the
method can be safely depended on if potash alum, aluminum
eream, and bone black are not used to coagulate clay and or-
ganic matter, since they have been found in the researches above
described to be productive of very serious errors. We recom-
mend as a substitute for these coagulating agents the oxide of
lime in its chemically pure state, to be employed in accordance
with the method above given. The losses of nitrates sustained
through its use have been shown to be very small in the work
above reported, and it may be employed by grinding the soil
with water or by direct addition to the muddy suspension pre-
pared from the soil.

Other sources of loss such as those brought about through
the sterilization of controls in the autoclave are unavoidable.
They have been found at times to be distinctly appreciable, and
especially in the presence of considerable quantities of organic
matter. It is further of the greatest interest to learn, from the
experiments above described, of the action of the anion of the
salts employed in our studies and the losses of nitrates occurring
on the water bath from solutions being evaporated there when
either NaCl or Na,SO, is present.

We should also make mention here of our attitude toward
the use of NH,OH instead cf KOH, which was found superior

36 University of California Publications in Agricultural Sciences [Vol.1

to the former in the investigations above reviewed. While higher
absolute results may no doubt be obtained from the use of KOH
than from NH,OH, and while in addition ammonia _ possesses
other objectionable features, we were not aware of the first of
these objections when these investigations were begun and did
not deem the others serious enough to warrant a change in the
established method. Moreover, the same relative values would
exist for the data above given if obtained with one or the other
of the hydrates, and therefore our results, having been obtained
throughout by the use of ammonia, do not in any way lose their
value. We do intend, however, in the future to employ KOH
exclusively in nitrate determinations made in this laboratory.
Finally we desire to call the attention of soil chemists to the
fact that losses of nitrates by the agencies above described never
seem to occur in accordance with any definite law, with the
exception of the case in which the various alkali chlorides are
compared. In these it would appear, from calculations which
we have made, that the losses of nitrates are proportional to
the amounts of chlorine present. While no law can be formu-
lated, however, in accordance with which nitrates are lost in
the presence of salts, it may be possible to work out tables
for the losses of nitrates incurred in the presence of varying
quantities of chlorides and sulphates, and to make corrections,
therefore, in samples whose composition is unknown after alkali
determinations are made. It is true, however, that calculation
has shown on the basis of data in Table VIII that the losses
of nitrates induced by chlorides alone are proportional to the
amount of chlorine present.

CONCLUSIONS

1. The “‘alkali’’ salts NaCl and Na,SO, induce losses of
nitrates when the latter are determined by the phenoldisulphonic
acid method. Na,CO, has no such effect. NaCl induces much
greater losses than Na,SOQ,.

2. Among the substances used to coagulate clay and organic
matter from solutions in which nitrates are to be determined,
potash alum, aluminum cream, and bone black have been found
decidedly unreliable. They all induce large losses of nitrates.

1912] Lipman-Sharp: Phenoldisulphonic Acid Method o7

3. Lime has been found to be much more reliable for the pur-
pose named than any of the other substances, the losses incurred
through its use being very small.

4. The reason for the difference between the action of Na,SO,
and Na,CO, so far as the nitrate losses are concerned is to be
found in the fact that Na,SO, induces the loss of nitrie acid
from the solution while the latter is being evaporated, while
Na,CO, containing only a weak acid radicle has no power to
set nitric acid free. Neither Na,SO, nor Na,CO, has the power
to set nitric acid free from nitrates when the dry residues of the
two are mixed prior to treatment with phenoldisulphonie acid.

9. Losses of nitrates from solutions as induced by chlorides
alone seem to be proportional to the amount of chlorine present.

6. The work of Gill which showed that chlorine induces losses
both on the water bath and in mixing the dry residue with
phenoldisulphonie acid is confirmed.

( oinoattas! of. Southern. “cafornia, by: acre: Moore. ‘Hall, Pp. 4
8025. “plates 1-3, with a map. - December, 1907 .

pasoeee er nt Seen e anaes pile

3 -amplexicaulis Nutt. by He .D: ‘Densmore. . _»Pp._ 303-330; plates 4-8. ;

‘Protective: Action, — On: the “Effects of: Certain Poisonous Gases on
‘Plants, “By.

‘Plate Ou" ‘December, gfe Riva Sele saan

~eeous Corallines.— “Ti, by Maurice ‘Barstow bal igs RPE 349-370;

1 dies os 5 Graceatat Trees art shrubs, by) Harvey Monroe Hail, “Pp.
a 1-74; p plates 1-11; 15 text-figures. March, 19100-0202.
2. Gracilariophila, | a New Parasite on Gracilaria sont er volltes: by Harriet

8. Plantae Mexicanae Purpusianae, sa by TT: ‘8. Brandegee. Pp. 85-95.

4, Leuvenia, a New Genus-of Flagellates, by N.L. Gardner. Pp, 97-106;

, The Genus Sphaerosoma, by. William Albert Setchell. Pp. 107-120;
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Pp, 143-150; alate: 19. ‘February, ‘1911-. SRA ESE ait os Serge eee Sct nt CO 4A FS So

Reet en delesserioides J. Ag. by Wilfred. Charles ‘Twiss. Pp.
159-1765 plates iis Match, 1911 nimi enn

Pp. 195-208, - A March, PN one SR cacti asec thceee ttc cet cones eea denen atdoecnnsnoane =
7 OOF e. March, Ch b Reena irene ete Ee pistian ct su onkavep adnan Vatie panceres
229-268; plates: “QB-BL, May, VOVQ oo saitceeeceoenssentcnssesecenscnsennecedtcnne bape

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e Origin, “Structure, and. Function of the Polar Caps in Smilacina

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‘By W. J. V. Osterliout.. Pp. 331-340, June, 1908 2.002.000.
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Contributions- +o-the “Knowledge of the ‘California Species of Crusta- i

aces 10-13. ; Ape 1909: “Sesbis oes Secs Pn cbecwaanene mate belnain wach ta taabgnbatersckauomaa egos ae

“L. Wilson, Pp. 75-84; plates 12-13. May, 1910 0. eee aac

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Ay May, LQLO. 2a eechscetccsectnnamae escnenebenjeshnanbnnnnstiedensaratcennsenensrennette)

6. Variations. in Nuclear “Extrusion Among. ‘the Fucaceae, by Nathaniel oe
Lyon Gardner. Pp. 121-136; plates 16-17. August, 1910 ian -we

7. The Nature of the ‘Carpostonies in the Cystocarp of Ahnfeldtia gigarti-

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:) On & Colacodasya from Southern. California, by. Mabel Effie McPadden.

9 Fructification of Macrocystis, by ‘Edna Juanita ‘Hoffman, Pp. 151- 158;
Uplate 28. -Eebrumry, 1902 2 0 ha ca ati ieee sa

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