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The Assay of Tin 
and Antimony. 



Assayer and Consulting Metallurgist. 






~r/V 3 

%i*v. ; 



regard to the high price of the metal, it is a most 
FJJ remarkable fact that the methods usually described in text 
books, taught in school and classes, and in common use, for 
the separation and analytical determination of tin, are as a rule 
both unpractical and unreliable. 

In commercial work it is, in general, of far greater importance 
to employ methods which can be relied upon to yield results accurate 
to within a quarter or a half per cent, in every case, and in a few 
hours at most, or in which the possibilities of error are definitely 
limited to a known minimum, rather than to spend three or four 
days in finicking with methods which are, properly speaking, only 
suitable for atomic weight determinations or other academic research, 
in order to obtain results which may possibly be exact to two or three 
decimal places and possibly inexact to the extent of 10 per cent. 
Gravimetric methods possess the single advantage over volumetric 
that one can weigh with greater precision than one can measure ; 
the difference is absolutely immaterial in most cases, as far as 
buying and selling are concerned, and in every other respect volumetric 
methods are far more reliable and are much more rapid than gravi- 
metric, involving fewer and less complicated separations ; speaking 
generally, and having regard to differences in the kind of error in- 
volved in gravimetric as against volumetric assays, it is not going too 
far to say that in commercial work gravimetric assays should be 
avoided as far as possible they leave too much to chance unless used 
with great discrimination. In an atomic weight determination one 
starts with pure chemicals, in assaying there are too many unknowns, 
involving too many assumptions, for rapid and accurate gravimetric 
work. It may as well be pointed out here that very few combining 
weights are known with certainty beyond the first decimal place, or 
nearer than i part in 1,000. In the majority of analytical methods 
greater accuracy than i part in 1,000 is a physical and chemical 
impossibility ; in very many the limit may be put at i part in 500 
indeed, a method of analysis which is properly systematised and 
capable of this degree of accuracy is a most excellent method, and, 
what is more, a rara avis. For instance, a really good method may 
enable us to report 50-1 per cent., or even 50-15 in very rare cases, 
in preference to 50-0 per cent, or 50-2 per cent. ; but such a result 
as 50-12 per cent, is a scientific absurdity in the second decimal place. 

A 2 

I{ Hrjyon^doubtS tMs4et him study care fully Ostwald's "Foundations 
k of Analytical Chemistry." 

As regards tin, the commonest and most glaring source of error 
is undoubtedly the indiscriminate and unreasoning abuse of the estima- 
tion by weighing as Stannic Oxide (SnO 2 ). It is, or should be, 
perfectly well known to chemists that the usual methods employed 
in order to obtain tin as SnO 2 involve at the same time the con- 
tamination of the stannic oxide with SbO 2 , As 2 O 5 , P 2 O 5 , Fe 2 O 3 , 
PbO, and even with biO 2 and WO 3 , and occasionally CuO, unless 
special methods are adopted which ensure the quantitative separation 
of these elements. The separation of tin for volumetric estimation 
is easy and certain ; on the other hand, its separation for gravimetric 
estimation as SnO 2 is complicated, tedious, and unreliable. The 
result is that the presence of impurities in stannic oxide is very 
commonly ignored unless they are in very obvious quantities. Thus, 
it is usual to assume that solder is composed of tin and lead, whilst 
as a matter of fact nearly all the tinman's and plumber's solder which 
is made to-day contains from i to 3 per cent, antimony; in con- 
sequence, determinations of tin in solder are very generally too high, 
either because the antimony is simply allowed to score fully as tin, 
or because an imperfect separation of tin and antimony is employed ; 
moreover, such determinations are usually too high on account of the 
presence of PbO in the SnO 2 , to the extent of about i per cent., 
as it is a common error to assume that treatment of solder with 
HNO 3 is a quantitative separation of tin from lead, or of tin and 
antimony from lead. When worked without a due appreciation of 
its limitations, there is probably not a more unreliable analytical 
method in common use than the gravimetric estimation of tin as 
SnO 2 ; yet in certain special cases it is undoubtedly useful. No wet 
method of assaying tin can be considered justifiable which does not 
exclude the interference of Sb, As, Fe, Pb, Cu, W, and bi, to mention 
the most usual sources of error, and weighing as SnO 2 does not permit 
of this with certainty. It is probably in consequence of the general 
use of this method that errors of 5, 10, and even 20 per cent, in tin 
and antimony assays (reported to two decimal places) are of common 
every-day occurrence. 

Tin ores are very frequently sold on a dry assay basis. Any 
dry assay of tin is, however, rather misleading as a valuation basis 
unless it is combined with a wet assay for Sw, of the button of 
metal obtained in the crucible. Mere oxidation with HNO 3 and 
weighing the residue as SnO 2 will not do common sense should 
teach that ; it is necessary to use a method which will show the 
percentage of Su and exclude the possibility of any possible impurity 
scoring as Sn. If this is done, the result obtained (as in any 
crucible assay) must of necessity be lower than the actual tin contents 
of the ore. If it is not done if the metal button is simply weighed 
as tin, or if the percentage of Sn is determined negatively by assaying 
the button for one or two impurities, or positively by oxidation with 
HNO 3 and weighing the residue as SnO 2 the result will in many cases 

be too high. It is a common practice to report the percentage of 
tin in tin ore as so many per cent, "fine tin." The use of this 
term appears to be sanctioned by buyers and sellers alike, though 
it is scientifically an indefensible expression with a far too indefinite 
meaning. One man's idea of "fine tin" may include tin of 5 per 
cent, of impurities, while another man may think of " fine tin " as 
something containing ggf per cent, or over of the definite entity 5w, 
and altogether it is scarcely advisable to expect concordance of results 
between two assayers, one of whom reports the percentage of " fine 
tin," and the other the percentage of Sn. As regards tin ores, the only 
method of assay which would, in the opinion of the writer, receive 
the mutual assent of buyers and sellers, if they could be brought 
to see the technical points involved, would be reduction to metal in 
hydrogen combined with a volumetric or electrolytic determination 
of the tin. There can be no certainty about any method not in- 
volving complete solution of the ore. 

The originator of the Ferric Chloride titration for estimating tin 
is unknown to the writer, who became acquainted with the process 
many years ago ; at that time its application was rather limited, and 
the greater number of assay methods involving this titration, and 
described in Chapter V., as well as many important points about 
the titration itself, were elaborated in detail by the writer in con- 
junction with Mr. H. Hocking. 

Chapter I. 

Miscellaneous Facts bearing on the Assay of 
Tin and Antimony. 

Sn = 1 19. Sb = 120. (O = 1 6.) 

(i.) OXIDES. Oxides of tin formed in the wet way are soluble 
in HC1. Oxides of antimony are much less easily soluble. The 
oxides of both metals are soluble in alkalies (soda and potash, not 
ammonia), and in alkaline sulphides. 

SnO 2 obtained in the wet way by the action of HNO 3 on metals 
or sulphides is liable to hold Sb, As, P, as oxides, also PbO, Fe 3 O 3 , 
CuO, WO 3 , SiO 2 , and Bi 2 O 3 . Oxides of tin and antimony are almost 
insoluble in acids after ignition. Oxide of tin exhibits many modi- 
fications according to the method of preparation. Thus the rapid 
action of dilute HNO 3 on tin (with the aid of heat) yields a product 
different from that obtained by the slow action of dilute HNO 3 in 
the cold, which yields an unstable compound containing nitric acid. 
The action of HNO 3 on SnS 2 , the addition of KOH to SnCl^, the 
evaporation of SnCl 2 , with excess of HNO 3 , the cautious addition of 
HC1 to solutions of alkaline stannates, and the addition of NH 4 NO 3 
or Na 2 SO^ to SnCl 4 , all yield some form-of stannic oxide. Insoluble 
stannic oxide results from the ignition of any of these products or 
by burning stannic sulphide or tin, in air or oxygen. The hydroxide 
precipitates obtained by adding AmOH or NaOH to solutions of 
SnCl 2 or SnCl 4 are soluble in excess of NaOH, and partly soluble 
in acetic acid, but are insoluble in AmOH. Soluble in tartaric acid. 
Stannic acid is slightly soluble in HNO 3 , and therefore to ensure the 
complete separation of SnO 2 by HNO 3 it is sometimes necessary to 
evaporate to complete dryness, and is always safer. The dilute nitric 
acid extract from this will be quite free from tin. 

Two acids derived from stannic oxide are generally recognised ; 
ordinary stannic acid, H 2 SnO 3 , which can exchange all its hydrogen 
for metals, is formed by neutralising solutions of alkaline stannates 
with acids, and also by the addition of CaCO 3 to SnCl 4 ; metastannic 
acid, which results by oxidising tin with nitric acid, is H 10 Sn 5 O 15 , 
5 H 2 O at ordinary temperatures and H 10 Sn 5 O 15 when dried at iooC., 
and only one-hfth of its hydrogen is replaceable by metals. Both 
these substances are soluble in H 2 SO 4 or in HC1. The H 2 SO 4 
solution contains stannic sulphate, and on dilution a hydroxide pre- 
cipitate forms ; on boiling the diluted solution, all the tin is said 
to be thrown down as metastannic acid. The HC1 solution is not 
precipitated by dilution. Ignited and native stannic oxides are only 

partly acted on by strong H 2 SO 4 or boiling alkalies, and cassiterite 
is not completely opened up even by fusion with potassium bisulphate. 
When oxides of tin and antimony are dissolved in fixed alkaline solu- 
tions, alkaline salts of the particular oxides are formed. When the 
oxides are dissolved in alkaline sulphides, mixtures of sulpho and 
oxy- salts are formed. 

(2.) SULPHIDES. Sulphides of tin, arsenic, antimony, are all 
soluble in alkalies and alkaline sulphides, from which solutions they 
are all precipitated by cautious addition of HC1. 

Sulphide of arsenic is insoluble even in strong HC1 and is com- 
pletely precipitated by H 2 S on standing, from a strong HC1 solution 
of either As 2 O 3 or As 2 O 5 . Sulphides of antimony are precipitated 
from fairly concentrated HC1 solutions, but on boiling with strong 
HC1 both dissolve with formation of SbCl 3 . 

SnS is an inconvenient form in which to precipitate tin, and 
stannic sulphide SnS 2 is precipitated completely only from fairly 
dilute solutions. These sulphides readily assume a colloidal form 
and pass into solution when washed with pure water ; hence, when 
washing is necessary,_they should always be washed with some salt 
solution (NaCl or NaAc) see Ostwald, " Foundations of Analytical 
Chemistry." Sulphide of tin (SnS 2 ) is not precipitated from solutions 
containing excess of oxalic acid whilst sulphides of As and Sb are 
precipitated under these circumstances. Sulphides of tin are readily 
soluble in HC1 and are oxidised to SnO 2 by treatment with HNO 3 . 

(3.) OXYCHLORIDES. Solutions of BiCl 3 and SbCl 3 in HC1 
are precipitated by dilution with water, the oxychlorides being formed ; 
they redissolve on the addition of more HC1. A solution of SnCl 4 
in HC1 is not precipitated by dilution, but solutions of SnCl 2 readily 
absorb oxygen from the air, and in a neutral or only faintly acid 
solution a deposit of tin oxychloride forms ; if the solution of SnCl 2 
is freely acid with HC1 no precipitate is formed, but the solution 
becomes converted into SnCl 4 . A solution of SbCl 5 in HC1 is pre- 
cipitated by dilution with water, some form of antimonic acid (said 
to be probably orthoantimonic acid H 3 SbO 4 ) being thrown down ; 
the precipitate is only redissolved by HC1 with difficulty, especially 
after standing some time. Oxychloride of tin which has been long 
precipitated is also difficult to dissolve in HC1. The presence of 
tartaric acid or of alkaline tartrates prevents the precipitation of 
oxychlorides of tin and antimony. A solution of SbCl 3 in HC1 gives 
Sb 2 O 3 with KOH, soluble in large excess, easily soluble in presence 
of tartaric acid, forming potassium antimony 1 tartrate, which is tartar 
emetic (8HOH;c C o6(Sbo))- The use of organic acids in inorganic analyses 
is much too frequent, and is simply a fad with many chemists. Com- 
plications which are not always properly understood are very often 
introduced, and it is a safe rule to avoid the use of such compounds 
as much as possible at any rate in commercial work. 

alkaline solution of SnO (in potash) reduces a solution of cupro- 
potassic tartrate with precipitation of Cu 2 O. An alkaline solution 
of As 2 O 3 acts in the same way, but no other metallic oxide. A 
solution of SnO in KOH also reduces Bi(NO 3 ) 3 with deposition of 
suboxide, according to Tilden. Further, such a solution appears to 
reduce itself, to use a rather loose expression, depositing tin and 
giving a solution of stannate. 

SnO in either alkaline (bicarbonate) or acid (HC1) solution reduces 
a solution of iodine, forming a stannate or stannic chloride and HI. 

SnCl 2 in HCi solution reduces AsCl 3 with precipitation of a 
brown deposit of arsenic containing 4 to 5 per cent, of Sn ; reduces 
solutions of AuCl 3 and PtCl 4 to metals, solution of HgCl 2 to first 
Hg 2 Cl 2 and then Hg ; (in presence of KI, SnCl 2 does not reduce 
HgClJ ; reduces CrO 3 and Mn^O 7 to Cr 2 O 3 and MnO ; reduces SbCl 5 , 
CuClo, and FeCl 3 to SbCl 3 , Cu 2 Cl 2 , and FeCl 2 respectively. Also 
reduces BiCl 3 to metal. 

(5.) ACTION OF KMnO AND K Cr 9 7 .In acid solution 
oxidise Sb 2 O 3 , As 2 O 3 , SnO, FeO, Cu 2 O. 

'Liberates iodine from a solution of KI, converts Cu 2 Cl 2 into CuCl 2 , 
converts SnCl 2 into SnCl 4 and dissolves ppted Sb, As, and Cu, with 
formation of SbCl 3 , AsCl 3 , CuCl 2 . Does not oxidise HCI solutions of 
Sb 2 O 3 or As 2 O 3 , but on the other hand FeCl 2 reduces HCI solutions 
of Sb 2 O 5 and As 2 O 5 under some conditions. FeCl 2 does not reduce 
HgCl 2 . SbCl 5 FeCl 3 CuCl 2 are in the order of reducibility by 
SnCl 2 , which in a hot HCI solution containing all three of the above 
chlorides, reduces first the SbCl 5 , then the FeCl 3 , and lastly the CuCl 2 . 
FeCl 3 does not under similar conditions oxidise HCI solutions of 
SbCl 3 or As 2 O 3 , but oxidises solutions of Cu 2 Cl 2 or SnCl 2 . CuCl 2 
in such solutions does not oxidise SbCl 3 or FeCl 2 , but oxidises SnCl 2 . 
Cu 2 Cl 2 reduces SbCl- before FeCl 3 , and has very little, if any, action 
on HCI solutions of As 2 O 5 . In alkaline solution As 2 O 3 reduces CuO 
as does SnO also. In acid solution As 2 O 3 does not reduce CuO. A 
solution of SbCl 3 in HCI seems to be permanent, a solution of FeCl 2 
or As 2 O 3 gradually oxidises in the air, and solutions of Cu 2 Cl 2 or SnCl 2 
rapidly oxidise we should expect from the above that SnCl 2 would 
be less stable than Cu 2 Cl 2 , but it is not ; we should also expect that 
FeCl 2 would reduce SbCl 5 and As 2 O 5 , and to a certain extent and 
under some conditions this appears to be the case. KI reduces SbCl 5 , 
FeCl 3 , CuCl 2 , and As 2 O 5 in HCI solution, but not SnCl 4 , whilst iodine 
oxidises Sb 2 6 3 , As 2 O 3 , SnO, in alkaline solution. The behaviour of 
arsenic in HCI solutions is very peculiar, depending much on the 
amount and concentration of the HCI present. FeCl 3 does not oxidise 
As 2 O 3 in HCI solution, and excessof FeCl 2 reduces As 2 O 5 in a strong 
(^ saturated) HCI solution (Fischer-Hufschmidt distillation process), so 
that the place, of arsenic would appear to be before iron in order of 

reducibility. SbCl 5 is certainly more rapidly and completely reduced 
by KI in HC1 solution than is As 2 O 5 , the reaction in the case of 
As 2 O 5 only moving to completion as fast as the liberated-kxiine is 
removed, and being reversible unless the iodine is removed ; 
further, HC1 solutions of As 2 O 3 (in which, by the way, the arsenic 
is supposed to exist as As 2 O 3 unless the liquid is saturated with 
HC1, when it exists as AsCl 3 ) show a strong oxidation tendency, so 
that we should arrange the order of reducibility thus : SbCl 5 As 2 O 5 
(in HC1) FeCl 3 CuCl 2 SnCl 4 , and the order of permanency or 
stability of the lower chlorides would, of course, be SbCl 3 AsCl 3 
FeCl 2 Cu 2 Clo SnCl 2 . But, on the other hand, if a little FeCl 3 or 
CuCl 2 be added to a solution of arsenic acid in fairly strong HC1, and 
the solution titrated with SnCl 2 , the FeCl 3 or CuCl 2 are at once reduced, 
and would therefore appear to be under these conditions more readily 
reduced than As 2 O 5 ; it also follows that FeCl 2 does not reduce As 2 O 5 
under these conditions. Again, the reducing effect of SnCl 2 on HC1 
solutions of arsenic acid is very doubtful, and, further, As 2 O 3 reduces 
even CuO in alkaline solution. From this we should arrange the 
order of reducibility thus: SbCl 5 FeCl 3 CuCL As 2 O 5 SnCl 4 . 
SO 2 reduces Sb, As, Fe, from -ic to -ous in HC1 solutions, but neither 
Cu (unless in presence of KCNS) nor Sn. As 2 O 5 may be also reduced 
by PC1 3 . Also, it would appear from the foregoing that if we have 
SbCl 5 , As 2 O 5 , CuCl 2 in hot strong HC1 solution and titrate with SnCL, 
the SbCl 5 is first reduced, then any trace of FeCl 3 , then CuCl 2 , at 
which point the solution becomes colourless ; this suggests an obvious 
and very easy estimation of Sb in alloys containing Cu and As, when 
the Cu is known, the As making no difference. The strength of solu- 
tions of SnCl 2 may be checked with FeCl 3 , CuCl 2 , K 2 Cr 2 O 7 , KMnO 4 
or with iodine. Some of the foregoing reactions are applied in the 
"Weil" assays of Fahl Ores, described in " Sutton's Volumetric 
Analysis." These oxidising and reducing actions are extremely com- 
plicated, and for the elucidation of a satisfactory theory of them much 
investigation is needed, but the subject is well worthy of it. 

t (7.) METALLIC PRECIPITATION. From HC1 solutions of 
ic chlorides. 

Copper precipitates Hg, Ag, As, Sb, not Sn. 

Iron Hg, Bi, Sb, Cu, and (in presence of SnCl 4 ) As, 

and reduces FeCl, and SnCL to Fed., and 


Tin As, Sb, Cu, Hg. 

Lead ,, Cu, Hg, Sb, Bi. 

Aluminium ,, Cu, Sb, and most of the common metals. 
Zinc Sn, Sb, Cu, Pb, As, Cd, Hg. 

Magnesium ,, Fe, Zn, Co, and Ni, and most of the common 


Zinc evolves a considerable proportion of the As and Sb as 
hydrides, as does iron, though to a less extent, very little SbH 3 being 


formed with iron. Even magnesium does not appear to completely 
evolve As and Sb as hydrides, though Crookes states that it does. 

(8.) SOLUTION OF METALS. Arsenic, antimony, copper, 
bismuth, mercury, are insoluble in HC1 either strong or dilute, hot 
or cold, when in a coherent form ; from alloys a certain amount of 
As is evolved as AsH 3 , and certain tin copper alloys are completely 
soluble in HC1 giving a solution of SnCL, and Cu 2 Cl 2 ; in presence 
of oxidising agents such as free Cl the above metals are all soluble. 
Finely divided Sb and Cu readily oxidise in contact with air and 
moisture, and the oxides are, of course, then readily dissolved by any 
HC1 present, thus often giving rise to the appearance of solution of 
the metals themselves in HC1. The above metals are all soluble in 
aqua regia. 

Tin, zinc, cobalt, nickel, iron, cadmium, aluminium, are readily 
soluble in strong HC1, and lead is also completely soluble, though 
slowly. When antimony is dissolved in HC1 and KC1O 3 it gives 
SbCU ; when it is dissolved in HC1 and iodine, SbCl 3 is formed. 

Zinc, iron, cobalt, nickel, cadmium, readily dissolve in dilute 
H 2 SO 4 . Tin, copper, mercury, and finely divided arsenic and an- 
timony are soluble in hot strong H 2 SO 4 , forming in the case of arsenic 
and antimony solutions of the lower oxides. Sulphides of As, Sb, Sn, 
are also soluble, forming in the case of Sb and As solutions of Sb 2 O a 
and As 2 O 3 . In the case of Sn a solution of stannic sulphate is pro- 
duced unless the metal is in excess. 

Copper, lead, zinc, iron, cobalt, nickel, bismuth, mercury, cadmium, 
are dissolved by HNO 3 with formation of nitrates; tin and antimony 
(and in presence of tin) phosphorus and arsenic remain as insoluble 
oxides, though a little tin may go into solution if the liquid is not 
evaporated to dryness, and the residue is also liable to hold varying 
amounts of other metallic oxides, especially those of lead and iron. 
In absence of tin, arsenic and phosphorus go into solution as arsenic 
and phosphoric acids. 

Aluminium is not easily soluble in H 2 SO 4 or HNO 3 . 

TIN ASSAYS. When Sn Tv and As v or P v in solution in HC1 are 
treated with iron, the precipitate holds tin. In the case of As, the 
ppted metal holds 3 or 4 per cent. Sn. When Sn and As, or Sn and 
P, in any soluble form, or as metal or sulphide, are evaporated with 
HNO 3 insoluble 2 SnO 2 , As 2 O 5 , and 2 SnO 2 P 2 O 5 are formed, soluble 
in strong HC1. 

(10.) Arsenic acid is not reduced by HC1, as is sometimes asserted, 
at any rate not under ordinary circumstances, and HC1 solutions 
of arsenic acid may be boiled freely without any loss of arsenic by 

Ferric chloride does not convert precipitated Sb into SbCl.-, but 
into SbCl 3 . 


The low boiling point of SnCl 4 (i 14 C.) is no bar to the boiling or 
evaporation (unless carried very low) of HC1 solutions of SnO 2 as 
experiment has shown that any such loss is entirely negligible under 
all ordinary circumstances, and the possibility of such loss can always 
be checked. Crookes states that when a Sn Sb As alloy is boiled 
with HC1 the residue contains only Sb, that all the As is evolved as 
AsH 3 , but the statement requires considerable qualification. 

(n.) PURITY OF CHEMICALS used in the ensuing tin and 
antimony assays : Sheet zinc must be absolutely free from tin, and 
must not contain more than small traces of arsenic or antimony. 

Hydrochloric acid should be as concentrated as possible and 
free from As, HNO 3 , or Cl. 

Ferric chloride should be free from Cl, HNO 3 , As, or FeCl 3 . 

Soda ash and caustic soda used for fusions should be free from 

Water used for diluting should be boiled and free from oxygen. 
Iron used should be either best piano wire or soft iron rod. 

ARSENIC from one another. 

Separation of Tin. 

i. In a moderately strong warm HC1 solution, containing excess 
of oxalic acid (20 grms. oxalic acid for each grm. of tin) 
H 2 S pptes. As and Sb, Sn remains in solution. This 
method, which was devised by Mr. F. W. Clarke, is tedious 
and unsuitable for commercial work, though accurate. For 
Carnot's modification, using thio-sulphate as a precipitant, 
see " Crookes' Select Methods." 

2. Iron wire in an HC1 solution of As, Sb, Sn, precipitates As 
and Sb as metals, though a certain amount of SbH 3 and 
AsH 3 are formed. The arsenic precipitated contains 3 or 
4 per cent. Sn. Except where more than 10 per cent, of 
As is present, this is the best practical method of separating 
tin from arsenic and antimony. 

3. Strong boiling HC1 dissolves tin and lead, but not arsenic or 
antimony. This is a good rough separation, provided not 
more than 5 per cent. As and Sb are present. 

4. Electrolytic separation (see " Classen "). 

5. Fractional separation with H 2 S. As is ppted in a concen- 
trated HC1 solution and Sb on slight dilution. Sn not 
ppted in a strong solution. (Doviton's Method ?) 

6. Winkler's separation with CaCO 3 and KCN (see Menschutkn). 

Method unpractical and not suitable for commercial work. 
7- Fusion of oxides of As, Sb, Sn, with caustic soda. Stannate 


and arsenate soluble in water, antimonate insoluble. This 
is a thoroughly unpractical and unreliable method. 

In practice, methods 2 and 3 are the most useful, methods i and 
4 rarely. 

Separation of Antimony and Arsenic. 

N.B. Antimony can be readily estimated in presence of tin. 

i. When an alloy or an arsenious compound is distilled with 
a solution of FeCl 3 and CaCl 2 in HC1, the arsenic is all 
evolved as AsCl 3 at a temperature of i25C. By adding 
a solution of ZnCl 2 the antimony can then be all evolved 
as SbCl 3 at a temperature of about i9oC. (Gibbs' method). 
In "distilling off arsenic there is no loss of SnCl 4 . 

(a) When a solution of arsenic acid saturated with HC1 

is distilled with FeCl 2 the arsenic is quantitatively 
evolved as AsCl 3 (Fischer-Hufschmidt process). By 
distilling with a mixture of FeCl 3 and FeCl 2 in con- 
centrated HC1 and CaCl 2 solution the whole of the 
arsenic may be obtained as AsCl 3 , not only from alloys 
but from any arsenic compound. These distillation 
separations are chiefly used in the assay of arsenic. 

(b) Arsenic may also be separated from HC1 solutions with 

metallic copper, and the copper-arsenic then distilled 
with ferric-chloride mixture. 

2. HC1 dissolves sulphides of antimony and tin on boiling, whilst 
sulphide of arsenic is left undissolved. An imperfect method, 
though very convenient sometimes. 

3. As 2 S 3 is completely precipitable by H 2 S in a strong HC1 
solution, leaving Sb and Sn in solution. This is a good 
and useful method in special cases. (Rose's method.) 

4. By addition of magnesia mixture to an ammoniacal tartarate 
solution of Sn, Sb, As (ic), the As is precipitated as Mg, 
NH 4 , AsO 4 . It may also be precipitated from an alkaline 
sulphide solution, with MgO mixture. 

5. Sulphide of arsenic is soluble in Am 2 CO 3 solution. Sul- 
phides of tin and antimony are insoluble. A rough practical 
method, though imperfect. 

6. The gallic acid separation of antimony is unsuitable for com- 
mercial work except in special cases, but is a good method. 

7. Electrolytic separation (see "Classen "). 

8. Strong H 2 SO 4 dissolves As, Sb, Sn the Sb compound 
separates on cooling. 


9. KHSO 3 dissolves freshly precipitated As 2 S 3 or As 2 S 5 . Sul- 
phides of Sn and Sb are not dissolved. Method is un- 

I0 . AsH 3 and SbH 3 when passed into AgNO 3 act differently, 
AsH 3 forms H 3 AsO 3 , and Ag is precipitated. SbH 3 gives 
a deposit of SbAg 3 . (Houzeau's method. 1 ) This reaction 
is of doubtful value except as a qualitative test, as it is 
seldom possible to depend upon the quantitative evolution 
of arsenic and antimony as hydrides. 

In practice, the first four are the most useful methods. 

We might add here that most of the text-book methods of 
separating arsenic are quite useless in presence of tin, as they 
usually involve solution of the arsenic in HNO 3 , which is impossible 
from a tin arsenic mixture. 

Chapter II. 

The Assay of Tin. 


When necessary, tin is best separated from accompanying elements 
as follows : 

i. From Chlorine 

(a) Evaporation with excess of HNO 3 . 

(b) Boiling with Na 3 CO 8 . 

2. From Sulphur 

(a) Evaporation with HNO 3 (tin ore). 

(b) Solution in aqua regia. 

3. From Tungsten 

(a] Reduction of HC1 solution with iron. 

(b) Solution of unigmted WO 3 in Am^COg or AmOH 

4. From Antimony, Copper, Bismuth 

(a) Boiling the metals with HC1 (rough). 

(b) Reduction of HC1 solution with iron. 

5. From Cobalt, Nickel, Iron, Phosphorus 

By pption with H 2 S in dilute HC1 solution. 

6. From Arsenic 

(a) Iron wire in HC1 solution. 

(b) HoS in HC1 oxalic solution. 

(c) H 2 S in strong HC1 solution. 

(d) MgO in tartaric solution. 

(*) Distillation of metals with FeCL. 

7. From Lead, Zinc 

No need to separate for volumetric assay, but Sn and Pb 
are best separated by alkaline sulphide pption, and 
Sn and Zn by HNO 8 in cases where it is necessary to 
estimate Pb or Zn. 

8. From Silica - 

By evaporation with HF. 

9. From PbO, Fe 3 O 8 , CuO 

Stannic oxide may be roughly separated by evaporation to 
dryness with nitric acid and extraction with dilute nitric. 
All the SnO 2 is in the residue, but is very impure. 


i. Dry Assays 

(a) Ferrocyanide assay. 

(b) Cyanide assay. 

(c) Carbon reduction assay in carbon-lined crucibles. 
2. Gravimetric determination as SnO 2 . 

3. Gravimetric assay by electrolysis. 

4. Volumetric assay by titration of SnCl 2 with ferric chloride. 

5. Volumetric assay by titration of SnCL with iodine in acid 

6 and 7. Volumetric assay by titration of SnCl 2 with KMnO 4 
or K 2 Cr 2 O 7 , with or without addition of FeCl 3 . 

8. Titration with iodine in alkaline solution. 

9. Solution of metallic tin in FeCl 3 and titration with KMnO 4 
or K 2 Cr 2 O 7 . 

10. Titration of stannic chloride with ferrocyanide. 

The dry assay should, of course, in every case be combined 
with a wet assay of the button of metal obtained, otherwise it is not 
an assay at all, but a mere guess of a somewhat greater degree of 
approximation than a vanner's test for "black tin." la is a safe 
buyer's assay for tin ashes and tin slags, is quick, convenient, and 
fairly reliable within certain limits. In some cases Ib is more 
accurate for tin ashes e.g., in the case of irony tin dross. Ib and 
Ic are fairly good assays for tin ore if the button is assayed for tin 
by a volumetric method. The dry assay of tin ore is rapid, and the 
results are fairly consistent, but of course always rather lower than 
the actual percentages of tin present. If the buttons are not assayed 
for tin, the results are quite unreliable, and generally too high. 


Method No. 2, though occasionally useful, is a thoroughly un- 
reliable method for general use, for reasons given in the introduction. 
Method No. 3 is accurate, but tedious, and more suitable for a clean, 
quiet research laboratory than for ordinary use in commercial work. 
Method No. 4 is at once the most accurate and practical method of 
estimating tin, involving in general only one filtration, and that from 
a slight precipitate, or in some cases two filtrations, is rapid and for 
large numbers of assays is cheap and convenient. Its accuracy is 
inherent, and is probably almost absolute when the assay is worked 
under proper conditions, the titration figures being strictly propor- 
tional to the amounts of Sn present. 

Method No. 5 is a very good one, but it is unreliable to the 
extent that an assumption not justifiable a priori has to be made, 
and it is one that is not readily capable of direct verification viz., 
that all the tin tetra : ch]oride in every assay is reduced to stannous 
chloride by 20 to 30 minutes' reduction with iron in warm dilute 
HC1 solution. It is a most elegant method requiring but little 
apparatus and few chemicals and no filtrations, is rapid and in practice 
it is found to yield results which as a rule agree closely with those 
got by ferric chloride assay, though it does not possess the inherent 
accuracy of that method. 

Methods 6 and 7 are workably accurate but not as convenient as 
the two previous methods, though titration with KMnO 4 is useful 
for checking the working strength of standard solutions of stannous 

Methods 8, 9, and 10 are not to be recommended in practice. 


Tin occurs as (a) metal, (b) sulphide or arsenide, (c) soluble 
oxide or soluble salts, (d) silicate, (e) insoluble oxide. 

SOLUTION can always be effected as follows : 

(a) Removal of combined silica by evaporation with hydro- 

fluoric acid (seldom necessary except with slags). 

(b) Treatment of residue by boiling with HC1 and HNO 3 . 

All the tin not present as insoluble oxide is then ob- 
tained in solution as stannic chloride. 

(c) Reduction of the residue of stannic oxide to metal, in 

a stream of hydrogen or coal gas, at a low red heat. 

(d) Solution of reduced metal in HC1 and HNO 3 . 

(e) Fusion of any slight siliceous residue with KNaCO s 

or soda ash, and borax, in a platinum crucible, and 
solution of the melt in HC1. 

The method of effecting solution of insoluble tin oxide by fusion 
with caustic alkalies or caustics, alkalies and sulphur in porcelain 
vessels is a dirty, tedious, and unreliable method. 



1. As, Sb, Cu, VV are separated by heating the acid solution 

with iron, when As, Sb, Cu are precipitated in the metallic 
form and tungsten as blue oxide. 

2. When it is desired to separate tin from Fe, P, Co, and Ni, 

precipitate with H 2 S in dilute HC1 solution. The sulphides 
of Sn, As, Sb, Cu are redissolved in HC1 and KC1O 8 , and 
the As, Sb, Cu separated with iron. 


(i/) For titration with ferric chloride the filtrate from the 
iron reduction is precipitated with strip zinc, the pre- 
cipitated tin and lead and undissolved zinc are dis- 
solved in pure HC1, and the solution containing the 
tin as SnClo is' titrated at the boiling point with ferric 

(b) For titration with iodine, the solution is not filtered 
from the iron deposit, but is cooled, the iron rod with- 
drawn, and the solution containing the tin as SnCl 2 
titrated in the cold with iodine, using starch paste as 

Chapter III. 

The Dry Tin Assay. 


The fire assay gives low results with ashes which contain chlorine, 
grease, sulphur, or zinc, or much lime, slag, or fine carbonaceous 

The general method is to fuse. 

50 100 grms. ashes with 5 30 per cent, potassium ferrocyanide, 

10 20 per cent, soda ash, 
5 10 per cent, borax. 

in a G Cornish crucible, adding a little fluor and white arsenic if 
necessary. A high temperature is necessary, and the best fuel is 
gas carbon. When the fusion is complete the assay is allowed to 
cool in the crucible, which is broken up when cold. The button of 
metal usually consists of a soft or non-irony portion and of a hard or 
irony portion It is weighed, and the soft portion is melted away 
in a hand ladle and poured into a small hemispherical mould ; the 
iron button is weighed and the weight of the soft portion calculated 

by difference. Both portions are assayed for tin by the direct 
ferric chloride assay (see Wet Assay of Tin, Chapter V., A.), the iron 
portion being pounded in a mortar and the non-irony portion broken 
in two and filed across the centre. As a rule the soft portion holds 
about twice the percentage of tin that the iron holds, but if the iron 
button is very arsenical, it is always very poor in tin, and if only 
small in amount may be sometimes neglected altogether. 

Speaking generally, the iron portion is an iron tin antimony 
arsenic alloy, while the soft metal is a lead tin copper anti- 
mony alloy, which, however, may contain 10 to 20 per cent. Fe in 
the absence of lead. If lead is present in any quantity the soft portion 
is as a rule free from iron. If sulphur is present a matte-speise may 
be formed as well as a soft portion. In ashes containing chlorine, 
sulphur, zinc, there is usually considerable loss of tin by volatilisation. 
If thought desirable, chlorine may be removed by boiling the ashes 
with carbonate of soda, and the effect of sulphur removed by evapora- 
tion with dilute HNO 3 previous to the fusion in the crucible. The 
poorer the ashes and the greater the amount of lime and silica they 
contain, the greater the loss of tin in the slag. Very carbonaceous 
ashes also give low results, and if free from zinc and chlorine may 
with advantage be calcined previous to reduction. The best results 
are obtained from ashes which are mainly composed of metal and 
metallic oxides. Ashes which are practically pure oxide of tin, and 
irony tin dross containing S and As may be with advantage assayed 
by the cyanide method as in the case of tin ores. 

This combination of fusion with ferrocyanide and direct ferric 
chloride assay of the button is at best a rough method, though it 
is a safe buyer's assay ; the result will always average i or 2 per cent, 
lower than a complete wet assay both on account of loss by volatilisa- 
tion and in slag, and to a slight extent on account of the rough nature 
of the separation of the tin from Sb, As, Cu, adopted in the wet assay 
of such buttons. It is a purely commercial matter, whether in any 
particular instance it is worth while to adopt a complete wet assay 
of the ashes in place of the dry assay, depending mainly on the size 
of the lot. It is certainly not worth while from any buyer's point 
of view to adopt a complete wet assay for a lot of two or three hundred- 
weight of solder ashes. In ashes poor in lead the iron button, being 
the heavier, is the bottom portion ; but if the ashes are leady the 
iron button is lighter and is found above the soft. The irony portion 
does not, as a rule, contain copper unless much arsenic or sulphur, cr 
much copper, is present. Copper gives a white crystalline fracture 
to the buttons from tin ashes, easily recognisable after a little ex- 
perience. Antimony gives a white lustrous, crystalline fracture in 
rich tin buttons rather different in appearance from the copper fracture. 
Zinc gives a bluish crystalline fracture to the metal, and a bluish 
flame during the progress of the assay. Arsenic gives a graphitic- 
looking fracture to solder buttons. In the absence of Cu and Sb. 
the fracture of a rich tin button is dull grey and holocrystalline. 



50 grms. slag. 5 grms. white arsenic. 

30 grms. ferrocyanide. 5 grms. fluor. 

5 grms. soda ash. 

Mix, transfer to a G crucible, add a little borax, and fuse in the 
hottest fire possible. When in a state of tranquil fusion, remove 
from the fire and allow to cool in the crucible. When cold, break 
the crucible, knock off the slag, weigh the button of iron tin- 
arsenic alloy and assay for tin. This is an excellent comparative 
method for works purposes, but the results are of course low, as it 
is quite impossible to reduce all the tin to metal. 


The dry assay of tin ore is only to be recommended for works 
purposes, mines, or for prospectors. It is not accurate enough for 
buying and selling on equitable terms, having regard to the amount 
of money hanging on a single assay and the increasing keenness of 

THE CYANIDE ASSAY is of the most general applicability, but 

(a) where the ore is poor and contains much Fe 2 O 3 it should be 
digested with strong HC1 provided the tin is present as cassiterite. 

(b) If much pyrites is present it should be evaporated to dryness with 
HNO 3 before extraction with HC1. These operations may be con- 
veniently performed in a 6-in. evaporating basin with a clock glass 
cover. Oxide of lead, if occurring in tin ore, is very difficult to 
extract completely even with strong hot HC1, whilst HNO 3 often 
removes only half of it. Take 20 grms. dried ore, 20 c.c. of strong 
HNO 3 , and about the same amount of water, and evaporate cautiously 
to complete dryness. Add 100 c.c. strong HC1 and digest for half- 
an-hour just below the boiling point. Dilute with an equal bulk of 
water, filter and wash by decantation until the washings are free 
from HC1. Ignite the filter paper and add the ash to the cleaned 
residue of ore in the dish. The HC1 extract will usually contain 
most of the iron, arsenic, antimony, &c., and will in general be free 
from tin, but should always be tested or assayed for tin (see Wet 
Assay). It is never safe to assume that the cleaned residue is free 
from metallic oxides other than SnO 2 , though the assumption is 
frequently made. Dry the cleaned ore and mix it with an equal 
weight of cyanide (98 per cent. Au not commercial cyanide). Take 
a small dry crucible and charge in 10 grms. cyanide, then the above 
mixture, and finally 10 grms. cyanide as a cover. Place in a fire 
at a low red heat and fuse gradually, increasing the heat to bright 
redness at the finish of the fusion, which should not take more than 
ten minutes. Allow the assay to become quite cold in the crucible, 
and when cold break out the button and either remelt it by dropping 
it into a crucible containing melted borax, or under palm oil in a 
ladle, or cut it in two and boil out the adhering cyanide with 


water. The button should in every case be assayed by a wet method 
for tin, otherwise the result is a mere guess and may be very mis- 
leading, no matter how much the ore has been cleaned, or however 
pure the ore is supposed to be, or however clean the metal appears 
to the eye. The use of the term " fine tin " has been already re- 
ferred to in the introduction, and it is only necessary to add here 
that Sn is a definite entity, while " fine tin " is an expression which 
awaits definition. Further, it is quite as bad to assay such a button 
by oxidation with HNO 3 and weighing the oxide residue as SnO 2 
that would be merely making the same unjustifiable assumptions (in 
another form) as to absence of certain impurities, which are involved 
by weighing the prill as tin. The button must be assayed positively 
for tin by a method which shall ensure the elimination of the inter- 
ference of every possible impurity, and that, in practice, means assay- 
ing the button for tin by a volumetric method. Of course, if this 
procedure is systematically followed (and any other is illogical as an 
assay, and so uncertain as a valuation basis as to be inexcusable on 
account of the high price of tin), the results will always favour the 
buyer. The remedy is. not to attempt to counteract this " low " 
tendency, which is inherent in any dry assay, by balancing an un- 
known " high tendency " (impurities) against it (the net result of which 
is, in practice, to favour the seller), but to use a wet assay throughout. 

Of other dry methods of assaying tin ore, the Cornish tin assay 
is no doubt useful on mines as a comparative test, where the quality 
of the ore remains fairly uniform, whilst the vanning test is also 
most useful to prospectors and on mines. 

The German assay of tin ore by mixing with oxide of copper and 
fusion for white metal, does not appear to possess any advantages 
over the cyanide assay as regards accuracy, and is an exceedingly 
complicated method. 

The method of fusing tin ore witlj Na 2 CO 3 and borax in a luted 
carbon lined crucible, in the muffle, is said to give very perfect re- 
duction of the tin. 

Hallet's method fusion with KHF 2 , solution in H 2 SO 4 and pre- 
cipitation of the tin as metastannic acid on dilution and boiling 
seems to be a good assay, but is in reality a wet method. 

Chapter IV. 

The Wet Assay of Tin. 


Bi, Pb, As, Sb, Fe, W, Si, should be absent. 

The tin from i grm. of material, separated either as metal or 
sulphide, is treated with 20 c.c. of dilute HNO 3 (i : i) in an evapo- 
rating dish, and evaporated almost to dryness. It is diluted with 

B 2 


50 c.c. hot water and boiled, then filtered, the residue well washed 
with hot water, dried, and ignited in the muffle in a small porcelain 
dish. When cold, weigh the SnO 2 . It contains 78.7 per cent. Sn. 

The tin in bronze coins and tin copper alloys free from Sb, 
Pb, As, may be estimated this way, but its application to solder, 
metal from crucible assays of tin ore, and the sulphide precipitate 
from tin slags is inaccurate. 

If solder is treated in the above manner the residue consists of 
SnO 2 and SbO 2 and some PbO. Multiply the weight of residue 
from i grm. of solder by 78.7, and the result, less i per cent, de- 
duction for lead, may be taken as the sum of the percentages of 
tin and antimony in the solder. 


This assay is fully described in Classen's "Chemical Analysis 
by Electrolysis." 


When ferric chloride is added to a strong hot HC1 solution of 
stannous chloride it is immediately reduced to ferrous chloride, and 
stannic chloride is formed at the same time. One drop in excess of 
the ferric chloride gives a decided yellow colour to the previously 
colourless solution, provided the solution is hot and strongly acid. 
The nearer the boiling point and the greater the concentration of 
HC1 in the solution the more rapid is the completion of the reaction. 
A solution of FeCl 3 in dilute HC1, of which 100 c.c. = 2 grms. Sn, 
is employed. In the assay the addition of FeCl 3 from the burette 
cools the solution somewhat, so that the finish is rather slower than 
the commencement of the reaction, both owing to dilution and con- 
sequent lowering of the temperature, and to the presence, in increasing 
concentration, of ferrous chloride in the solution, but in any case the 
titration should never take more than a minute, if worked as directed. 
FeCl 3 gives a far stronger colouration in a hot, strongly acid solution 
than in a cold, faintly acid solution. The titrations cannot be done 
by gas light or electric light, and should always be effected in the 
daytime. In an emergency they may be done by magnesium light. 

The equation representing the chemical change is 2 FeCl 3 -f- 
SnCl 2 = 2 FeCl 2 + SnCl. t . The presence of chlorides of lead, zinc, 
aluminium, iron (ous), cobalt, nickel, antimony (ous), copper (ous), 
cadmium, does not affect the quantity of FeCl 3 required ; the presence 
of FeCl 2 in quantity somewhat retards the finish and lessens the 
delicacy of the colour indication ; Cu 2 Cl 2 reduces FeCl 3 with forma- 
tion of CuCl 2 , but SnCl 2 reduces CuCl 2 , and the net result of this is 
that not a trace of CuCl 2 is formed until all the SnCU is converted 
into SnCl 4 the next drop of FeCl 3 forms a trace of CuCl 2 , which 
gives a similar colour indication to that of FeCl 3 itself ; CoCl 2 and 
NiCL, give highly coloured solutions which render the recognition 
of the end point difficult one way of remedying this is to dilute 


the solution somewhat, with boiling water, which removes the blue 
colour, but of course renders the reaction slower and lessens the 
colour intensity of the drop or two excess of FeCl 3 ; SbCl 3 is not 
converted into SbCl 5 by FeCl 3 , and under the conditions of the 
a^say neither SbCl s nor SbCl 5 ever occur in the solution ; Cu Cl 2 , 
CoCl 2 , NiCl 2 are rarely present, also CdCl 2 and A1 2 C1 6 ; FeCl 2 is 
often present to begin with, and ZnCl 2 and PbCl 2 are generally present 
in greater or less amounts. BiCl 3 and HgCl 2 are reduced to metal 
by SnCl 2 , but Bi and Hg would be separated with iron. Precipitated 
Sb, Cu, As, are attacked by hot acid ferric chloride and blue oxide 
of tungsten is affected by it, but the assay method excludes the 
presence of these substances during titration. Acid solutions of 
SnCl 2 very readily oxidise by exposure to air ; the method of dis- 
solving the tin from the state of metal by boiling with HC1 in an 
atmosphere free from oxygen excludes the formation of SnCl 4 , if the 
operation is carried out as subsequently directed, and if the solutions 
are titrated as soon as ready and at the boiling point the oxidation 
tendencies are completely eliminated. Briefly, the best conditions 
are : (i) Solution from the state of metal as rapidly as possible in a 
non-oxidising atmosphere, the solution being brought to the boiling 
point before the solution is complete. (2) Use of strongest and 
purest HC1. (3) Bulk 150 250 c.c. (4) Titration rapid and at the 
B. Ft. (5) Strength of FeCl 3 100 c.c. = 2 grms. (6) Absence of 
precipitated Sb, As, Cu in the solution. 

From the dilute peroxidised HC1 solution of Sn, Sb, Hg, Bi, As, 
Cu, Pb, Cd, Zn, Co, Ni, Fe, P, the Sn, Sb, As, Cu, Hg, Bi, and 
some Pb and Cd are separated as sulphide by H 2 S if it is desired to 
separate from Co, Ni, Fe, P. The sulphide precipitate is redissolved 
in HC1 and KC1O 3 , and the solution reduced by heating with iron 
wire. The As, Sb, Cu, Hg, Bi are precipitated in the metallic form, 
and the solution (which must be strongly acid to avoid precipitation 
of SnOCl 2 ) is filtered and neutralised with thin strips of zinc. The 
action finished, the mother liquor, after testing for tin with H 3 S water, 
is poured off as completely as possible, and the residue of spongy 
metallic tin and lead and undissolved zinc, is dissolved in the same 
flask in about 200 c.c. of pure HC1, the flask being provided with 
a rubber cork and leading tube, and the liquid is brought to a boil 
as rapidly as possible ; a piece of pure zinc about the size of a 
pea is added to assist in preserving a non-oxidising atmosphere of 
hydrogen and hydrochloric acid in a flask until the liquid clears 
and boils. As soon as everything is in solution and the liquid is 
boiling, the flask is removed from sandbath or plate and titrated 
immediately with ferric chloride. The ferric chloride should be free 
from ferrous chloride, nitric acid, chlorine and arsenic, and the solution 
should contain 300 500 c.c. HC1 in two litres. It is best made up 
from a concentrated stock solution in HC1, made by dissolving piano 
wire as directed subsequently. If the assays turn dark greenish after 
titration the FeCl 3 solution is contaminated with HNO 3 . The FeCl 3 
may be made up also by dissolving 180 grms. of the yellow com- 


mercial lump salt, which is Fe.,Cl 6 12 H 2 O, in about 200 c.c. HC1 
and evaporating it to dryness. The residue is dissolved in 300 c.c. 
HC1 and diluted to two litres. The solution is standardised against 
i grm. of the purest tin obtainable, filed with a fine file. This is 
weighed into an 8 oz. flask, and the flask is about three parts filled 
with pure HC1, rubber cork, and leading tube inserted, and boiled (but 
not too rapidly) until solution is complete ; then titrated at once. 

Pure tin is more readily obtained from smelters of tin ore than 
from dealers in chemicals the writer o?ice ordered some " pure tin 
for standardising purposes " from a firm of wholesale chemists and 
received metal holding 3 per cent. Sb. 

It is rarely necessary to complicate the assay by separating the 
tin as sulphide having once got everything in solution, reduce with 
iron wire, filter, and precipitate on zinc. This method has been re- 
peatedly checked on made up metals of known composition containing 
varying amounts of Sn, Pb, Cu, Sb, and the results are in every case 
so close as to leave no doubt whatever that the method is extremely 
accurate ; indeed, it is much more accurate than the electrolytic assay 
on account of the complicated separations which the latter involves, 
and is incomparably quicker. 

The favourite objection to the ferric chloride assay is the oxida- 
tion tendency of solutions of SnCl 2 the method if properly worked 
overcomes this completely. It has also been objected that five or 
six drops of ferric chloride solution are necessary to give a perceptible 
colour indication. This is quite incorrect one drop in excess of 
FeCl 3 is ample if the operator possesses normal colour vision. It is 
also stated th,at Sb dissolves in HC1 this is not the case. It is true 
that finely divided Sb in contact with air and HC1 slowly dissolves, 
but even then, FeCl 3 does not oxidise SbCl 3 , and, further, under the 
conditions of the assay the absence of Sb is ensured by the iron wire 
separation. In direct ferric chloride assays on solutions from metal 
filings, the assays so far from being too high through Sb dissolving, 
are too low because of Sn retained with the black powder (Chapter V., 
A.), and it should also be remembered that metallic tin precipitates 
antimony from solution. 


The dilute acid solution of metallic chlorides, which should not 
be more than about 50 c.c. in bulk, and should be contained in a 
4-inch beaker, is reducted by heating with a clean piece of iron rod 
resting in the solution against the side of the beaker, which is 
covered by a watch glass. The whole is heated to 80 or 90 C. (not 
to boiling) over a Bunsen flame ; five or six assays may be conveniently 
heated in a small frying-pan sandbath. The assays are heated for 
20 30 minutes after they have lost their original red, yellow, or 
greenish colour. The Sb, As, Cu, are precipitated, and the SnCl 4 
is assumed to be reduced to SnCl 2 ; in practice this assumption is 
found to be justified, though it is really one of the weak points of 


the assay. The assays are cooled in a basin of cold water, and when 
cold the watch glass and rod are rapidly washed with a little cold 
boiled water, starch paste added, and the solution titrated rapidly 
with iodine. It is not necessary to remove the black precipitate of 
Sb, As, Cu, as the finishing point in the case of SnCl 2 and iodine is 
so sharp ; but the finely-divided metallic precipitate sometimes seems 
to slowly remove the blue colour. The iodine solution is made up by 
dissolving 21.32 grms. iodine and 45 grms. pure KI in about an inch 
of water in a small beaker and diluting to one litre. 100 c.c. = 
i grm. Sn. Not more than .5 grm. Sn should be present in the assay. 
It has been proposed to increase the accuracy of this assay by 
titrating in an atmosphere of CO 3 . 

Mr. A. H. Low states ("Technical Methods of Ore Analysis," 
2nd Edition, page 185) that copper and iron in contact precipitate 
tin. He does not, however, state the conditions which obtained when 
he made the observation, and the writer wishes to emphasise the fact 
that in a freely acid (HC1) solution (a sine qua non in tin assays), no 
such precipitation of tin ever takes place. 

Titanium and tungsten both interfere with the ferric chloride and 
tungsten with the iodine titrations for tin, as titanium sesquichloride, 
TioCl 6 , and the blue oxide of tungsten, WO 2 , are peroxidised in HC1 
solution by ferric chloride, and WO 2 by iodine. Titanium sesqui- 
chloride is not affected by iodine, and titanium does not interfere with 
the iodine titration. Hence Ti and W, if present, must be separated 
for the FeCl 3 assay, and W must be separated for the iodine assay. 
WO 2 is precipitated with antimony by iron, and Ti 2 Cl 6 remains in 
solution when tin is precipitated on zinc provided the liquid is dis- 
tinctly acid with HC1 when poured off from- the precipitated tin. 
Uranium does not interfere with either iodine or ferric chloride assays. 
Uranic salts in HC1 solution are reduced by iron, but uranous chloride 
is not oxidised by iodine. Uranous chloride which makes boiling 
HC1 solution greenish, is peroxidised slowly by FeCl 3 after the tin 
has been peroxidised ; as in the case of Cu 2 Cl 2 , the colour change 
to yellow takes place as soon as the tin is peroxidised, so that uranium, 
if present, does not affect the titration reading. In this case, how- 
ever, the colour due to FeCl 3 slowly goes as the uranium is per- 
oxidised, and is replaced by the much fainter yellow of uranic 
chloride ; so that the exercise of considerable care is required in noting 
the finish of the tin titration. Further, zinc precipitation leaves the 
uranium in the mother liquor if this is kept sufficiently acidulated. 
Molybdenum interferes with both iodine and ferric chloride assays, 
and tin requires to be separated from it by zinc precipitation, the 
mother liquor, if sufficiently acid after separation of the tin, containing 
the molybdenum. Molybdous compounds colour a boiling HC1 solu- 
tion reddish ; on titration with ferric chloride the colour changes to 
green, finishing a strong yellow. 

2 4 

Chapter V. 

Specific Tin Assays. 

A. DIRECT FERRIC CHLORIDE ASSAY. Applicable to alloys of 
lead, tin, zinc, aluminium, iron, arsenic, antimony, copper, cobalt, and 
nickel, with less than 5 per cent. Sb, As, Cu, and to certain tin copper 



The sampling of alloys of lead and tin and antimony is quite as 
important as the assay, on account of the pronounced liquation 
phenomena which these alloys exhibit. Wherever possible the sample 
should be taken from the molten kettle of metal. Where there is no 
option but to sample from the pigs, chips may be taken from the centre 
of opposite top and bottom longitudinal edges, and the chips melted in 
a ladle. The sample for assay should be cast in a. small hemispherical 
or rectangular mould (not in long thin strips) and cut transversely in 
two with a chisel. Filings are taken with a clean and fairly fine cut 
file across the surface of fracture, and the filing should be done gently. 
Very brittle alloys such as irony arsenides may be pounded in a steel 
mortar. The objection sometimes raised against filing, that the use 
of a file involves the contamination of the sample with particles from 
the file, is more imaginary than real, and in practice may be ignored 
quite safely. 

THE ASSAY. Weigh up i grm. of filings into a clean 8-oz. flask 
and add about 200 c.c. of pure HC1 (s.g. 1.16). Close the flask 
with a rubber cork with leading tube attached as shown, and boil 
the assay on a sandbath enclosed in a fume 
cupboard, over a moderate fire. As the object 
is to obtain the tin in solution as stannous 
chloride, precautions are necessary in order to 
prevent oxidation. The assay should not boil 
too rapidly, or the HC1 becomes weakened in 
strength before it has had time to thoroughly 
attack the metal, and any black residue is 
broken up so finely that it settles afterwards 
only with great difficulty. Neither should the 

assay come to a boil too slowly, as in that case the metal may be com- 
pletely attacked while there is still air in the flask, with attendant 
formation of SnCl 4 . The tin goes into solution as SnCl 2 tin, 
lead, zinc, aluminium, iron (forming FeCU), cobalt, nickel, all dis- 
solve, and in some cases copper (forming Cu 2 Cl 2 ) whilst arsenic, 
antimony, copper (also bismuth and mercury) are left undissolved as 
a black powder, also often considerable quantities of iron, and a 
certain amount of unattacked tin and lead occluded by the powder. 


Some As is evolved as AsH 3 , but in general the As remains as a 
brownish flocculent residue, while only faint traces of SbH 3 are formed. 
Certain tin copper alloys are completely soluble in HC1, giving a 
colourless solution of SnCL and Cu 2 Cl 2 . Cn 2 Cl 2 is oxidised to 
CuCL by FeCl 3 , but its presence does not affect the tin titration, as 
was explained in the previous chapter. The presence of SbCl 3 , PbCl 2 , 
FeCl 2 , CoCljj, NiClo, ZnCl 2 , has no effect on the titration figures. 
Alloys containing only lead, zinc, tin, and aluminium go completely 
into solution in fifteen to twenty minutes, and may then be immediately 
titrated with ferric chloride ; such alloys yield perfectly accurate results 
by this method. 

In the presence of a residual black powder the results will always 
be too low, though when the percentage of antimony and copper 
is low the results are near enough for many purposes, as, for example, 
in the assay of buttons got in the fire from tin ashes and solder 

Thus solders with 20 60 per cent. Sn and i| 4 per cent. Sb 
yield results for tin by this method which are ^ i per cent, too low. 
Alloys of iron and tin are also difficult to attack completely, and 
further the presence of much FeCl 2 in the solution retards and ob- 
scures the finish somewhat. CoCl 2 and NiCl 2 give coloured solutions 
in HC1 which also obscure the colour finish ; this may be oxercome 
by adding hot boiled water before titration. 

In all cases where solution in boiling HC1 is incomplete, the assay 
is allowed to boil until the metal is completely attacked. This 
is a matter for the judgment of the operator, and requires experience 
to determine, but in general the fine stream of gas bubbles is replaced 
by a slower stream of larger bubbles as solution proceeds. The assays 
must not be allowed to boil below 150 c.c. or the FeCl 3 used in 
titration may cool the assay so much as to impair the sharpness of 
the colour indication at the finish of the titration. It is an improve- 
ment to allow the leading tubes to dip under water this may be 
effected by attaching a straight vertical tube to the ordinary leading 
tube, with a piece of rubber tubing, or the leading tubes may be made 
longer and with a second right angle bend ; or a rubber tube valve 
may be fitted on the glass leading tube to prevent air finding its way 
into the flask. 

As soon as the metal is completely attacked, remove the assay 
to the titrating bench and allow it to stand for a minute or two until 
the black powder has settled. A slight rotatory jerk, given to the 
flask when it is removed from the sandbath, assists the powder to 
collect in the centre of the flask. Remove the cork and leading tube 
and carefully pour the liquid off into another clean flask, from any 
black powder pour oft as closely as possible, but so as not to carry 
over any powder into the other flask. (The black residue of Sb, Cu, 
As is soluble in FeCl 3 ). Titrate the hot acid solution of tin rapidly 
with ferric chloride solution from a fast running burette with glass 
stopcock. The assays must not be allowed to stand off the fire before 
titrating, for more than two or three minutes. After five minutes the 


effect of oxidation begins to be perceptible, and the oxidation increases 
very rapidly on further standing. 

The ferric chloride solution should be made up two litres at a 
.time from a concentrated stock solution made by dissolving piano 
wire in HC1, peroxidising with HNO 3 and evaporating twice with 
HC1, to dryness, and then dissolving in HC1, and is standardised 
.against i grm. of purest tin dissolved by boiling with HC1 as usual. 
The standard solution may also be prepared by dissolving 180190 
grams of the yellow lump salt (which is, roughly, Fe.,Cl 6 , 12 H 2 O) 
in 200 c.c. HC1 and evaporating the solution just to dryness. The 
residue is dissolved cautiously, and in a fume cupboard, in 300 c.c. 
HC1, and the solution diluted to two litres. Evaporation with HC1 
is necessary to free the solution from HNO 3 and arsenic. Only the 
purest and strongest HC1 should be used in these assays. 

As previously stated, in the case of alloys which dissolve com- 
pletely in HC1 the above method is perfectly accurate, but where 
.solution is incomplete (i) the black powder holds Sn, (2) oxidation 
possibilities are introduced in pouring off into the second flask, 
(3) a little powder may get over into the second flask, (4) it is im- 
possible to pour off quite clean though the error from this cause is 
much less than might be supposed, and may be practically eliminated 
by pouring a little hot boiled water on to the residue in the first flask 
immediately after decantation, and decanting off the washings into the 
second flask at the end of the titration. Admitting that in cases of 
incomplete solution the results are too low, they are in all cases 
sufficiently close to render the method very reliable, especially when 
'One considers the extent to which antimony and tin are allowed to act 
as understudies to one another in so many assays. Briefly, in cases of 
complete solution, this direct assay is quite accurate, while with in- 
complete solution the results are low, but provided not more than 
5 per cent. Sb and Cu are present they are reliable and within i per 
cent, of the Sn percentage. In the case of plumber's solder and 
tinman's solder it is a fairly safe rule to add ^ per cent, in the case 
of plumber's and f per cent, in the case of tinman's; by doing this 
it is possible to arrive more accurately at the percentage of Sn in solder 
in half-an-hour than is otherwise possible (except by the next assay 
to be described) in a week. Tinman's solder very commonly contains 
46 per cent. Sn and 3 per cent. Sb. This metal assays 50 per cent. 
Sn by oxidation with HNO 3 and weighing residue as SnO.,. Plumber's 
.solder often holds 30! per cent. Sn and 2 per cent. Sb. 

All tin assays for estimation with ferric chloride are finished in 
the manner just described. The precipitated tin and excess zinc are 
dissolved up together, over a good fire, in a flask with leading tube, 
and a granule of zinc is added to assist in keeping a non-oxidising 
atmosphere in the flask until the assay comes to a boil. In the 
general tin assay, As, Cu, Sb are always absent, being separated in 
the iron wire stage, so there is no error through decantation. 

Alloys containing zinc or aluminium effervesce violently in the 
cold, with strong HC1. Tungsten gives a bluish tint to the solution. 


Cobalt gives a blue colour much weakened by dilution, and nickel a 
weaker green colour which goes on dilution. 

B. ALLOYS OF LEAD TIN ANTIMONY holding muchSb and but 

little Cu and As. 


Weigh up i or 2 grms., according to the probable amount of 
tin present, into an 8-oz. flask, and boil gently with 50 75 c.c. HC1 
until action ceases, when most of the Pb and Sn are in solution. 
Complete solution is effected by the cautious addition of a saturated 
solution of KC1O 3 . Boil off the excess of chlorine, remove the 
assay from the bath, and add at once two bunches of fine piano 
wire. The action in the hot, strongly acid liquid is very rapid, and 
one minute after the solution turns colourless add a thin strip of 
copper foil. As soon as a small piece of copper remains bright 
(alter a minute or two) dilute with 30 or 40 c.c. of hot boiled water 
and boil for a minute. Filter very rapidly into another similar flask, 
keeping the precipitate as far as possible out of contact with the air, 
and wash the flask and paper once, with hot dilute HC1. Neutralise 
the solution (which should be freely acid) with thin strips of zinc 
use excess of zinc straight away, as if the neutralisation is effected 
too slowly tin is apt to remain unprecipitated. 

When the action ceases pour a little of the liquor off into a 
beaker containing a little H 3 S water; if the precipitate is white, all 
the tin has been precipitated. Pour away the mother liquor as closely 
as possible after allowing any floating particles of metal to settle. 
The best plan is to empty the liquid from the flask into a beaker, 
which is then rapidly filled with water from the tap. Larger pieces of 
spongy tin are pressed against the side of the beaker with a glass 
rod, after which they readily settle ; the addition of a drop or two of 
ammonia or pouring the liquid to and fro from one beaker to another 
will generally ensure the settling of the lighter particles. The metal 
in the beaker is washed back into the flask with 180 c.c. HC1 and the 
liquid brought to a boil as rapidly as possible with the addition of a 
granule of zinc and using a cork and leading tube. When solution 
is complete titrate at once with ferric chloride. 

Black powder should be absent, as all the As, Sb, Cu are pre- 
viously separated. This method has been repeatedly checked on 
made up alloys of known composition, and is perfectly accurate. 
From start to finish it need not take more than one hour, but the 
assays require unremitting attention. Copper is best precipitated 
on iron in a dilute HC1 solution, whilst antimony comes down im- 
mediately in a strong HC1 solution. The arsenic is not really pre- 
cipitated by iron but by the SnCL, formed by the action of Fe on 
SnCl 4 ; iron precipitates Cu and Sb. Copper precipitates As and 
Sb ; tin precipitates As, Sb, and Cu. Phosphorus is said to bring 
down some tin on iron wire, and in the case of arsenic the pre- 
cipitated metal holds 3 or 4 per cent. Sn. It may be asked, why 

reduce the tin to metal ; why not stop at the half-way stage when 
the SnCl 4 is reduced to SnCl 2 ? This is actually done in the iodine 
assay, but the procedure described affords a far surer way of obtaining 
the tin in solution as SnCl 2 , and further it is necessary to obtain 
the SnCl 2 in strong boiling HC1 solution free from precipitated As, 
Sb, and Cu, and this can only be ensured by the above method. 

c - In alloys which contain much Cu, and with non-ferruginous 

mattes, a little HNO 8 may be used instead of KC1O 3 to effect complete 
solution, and the reduction by iron wire prolonged in dilute solution. 

D - ALLOYS AND MATTES rich in Fe, Co, Ni, P. 

Weigh up i grm. and dissolve in 2030 c.c. HC1 and i c.c. 
HNO 3 . Heat until solution is complete, boil off red fumes, dilute to- 
200, and precipitate with H 2 'S. Allow to stand for an hour or two, 
filter, but do not wash, as SnS 2 is decomposed by water. (In cases 
where the filtrate is required wash with NaCl solution.) Test the 
filtrate with a further stream of H 2 S. Wash the sulphide precipitate 
back into the flask, add 30 40 c.c. HC1 and some KC1O 3 , boil, add 
iron wire, and heat to 70 or 80 for half-an-hour to two hours, or 
until reduction is complete. Filter, precipitate the tin on zinc, dis- 
solve and titrate with ferric chloride. 

E- ALLOYS, MATTES, AND SPEISES rich in arsenic and iron. 

i. In many cases methods C or D are quite satisfactory provided 
the assay is well boiled with plenty of iron wire, and is kept 
freely acid. The assay is too low for tin by 5 per cent, of 
the arsenic percentage. 

2. Method D is adopted, but instead of dissolving the mixed 
sulphides in HC1 and KC1O 3 they are boiled with 50 c.c. 
HC1 alone, down to about 30 c.c., diluted and filtered from 
the insoluble sulphide of arsenic. The filtrate is heated 
with iron wire to precipitate the Sb, filtered and precipitated 
with zinc as usual. This is a rough and ready separation, 
as the As 2 S 3 always holds some SnS 2 , and if much copper 
is present the loss is increased by the Cu 2 S which is un- 
dissolved in HC1. 

3. The arsenic may be distilled off with a solution of FeCl 3 and 
CaCl 2 , and the tin separated as sulphide from the diluted 
solution. This is sometimes convenient where the arsenic 
has to be estimated, as there is no loss of SnCl 4 . 

4. One grm. 25 c.c. HC1 and 2 c.c. HNO 3 . Heat till dissolved 
and boil off the fumes. Add sodium sulphide solution very 
cautiously until solution is alkaline and precipitate is black, 
and pass H 2 S. Warm, dilute, warm again, filter and wash 
well with hot dilute Na 2 S. (The precipitate if very bulky 
should be redissolved and reprecipitated.) The alkaline 
filtrates are cautiously acidified with HC1, the liquid allowed 

2 9 

to stand for half-an-hour in a warm place, and the sulphides 
of Sn, Sb, As are filtered, washed well with NaCl, and 
washed back into an 8-oz. flask, and boiled down with 
50 c.c. HC1 to 30 c.c., then diluted and filtered from the 
As 3 S 3 . Boil with iron wire, filter and precipitate on zinc. 

5. Or separate with H 2 S in acid solution first, and digest the 
precipitate with Na 2 S. Then proceed as before. 

^.Dissolve in 30 c.c. HC1 and a little KC1O 3 . Boil off the 
chlorine, and precipitate with H 2 S in strong HC1 solution. 
Allow the solution to stand for some time in a warm place 
and again pass H 2 S. As 2 S 3 alone is precipitated (and 
Cu 2 S). Filter through asbestos, dilute and precipitate the 
SnS 2 , &c., with H 2 S. 

7. Or having obtained the sulphides of As, Sb, Sn, separate the 
antimony by electrolysis, the arsenic with magnesia mixture, 
and the tin by electrolysis. (See electrolytic assay.) 

8. The oxalic acid separation may also be used with arsenical 
material. One grm. is dissolved in 20 30 c.c. HC1 and 
i c.c. HNO 3 . Add 20 grms. oxalic acid dissolved in 100 c.c. 
water and gas the solution with H 2 S, while at the boiling 
temperature. Two separate gassings are necessary. Filter 
the sulphides of As and Sb on the water bath and wash 
with a hot concentrated solution of oxalic acid. Neutralise 
the filtrate with ammonia, acidify with acetic acid, dilute 
to i litre, warm, and precipitate the tin completely with 
H 2 S. Filter, dissolve in HC1 and KC1O 3 , and precipitate 
with zinc as usual. 

F. GENERAL ASSAY applicable to everything not containing the 

tin as silicate ; suitable for calcined matte and calcined speise, bronze 
ashes, many tin copper alloys, chloriny and leady tin ashes, sulphide 
ores, metallic copper. 

i to 5 grms. is evaporated to complete dryness with 10 20 c.c. 
HNO 3 and some water, in a covered porcelain dish. It is then 
digested with dilute HNO 3 and filtered. The residue will contain 
all the tin, and is dried and ignited with the filter paper in the same 
dish, the ignition being best done in the muffle. The calcined residue 
is carefully transferred to a porcelain boat and reduced at a low red 
heat in a current of coal gas for an hour and a half or two hours (see 
Wet Assay of Tin Ore). The boat and its contents are allowed to 
cool in the tube in a current of coal gas, and when cold are trans- 
ferred to an 8-inch beaker, and the metal dissolved in HC1 and a little 
HNO 3 , the nitrous fumes boiled off, and the solution filtered. The 
residue of carbon and silica is dried, ignited, and examined for Sn 
as a matter of precaution by fusing with KNaCO 3 and borax in a 
platinum crucible, dissolving the melt in HC1, and precipitating with 
zinc, &c. 


The main solution if from 5 grms. is made up to 500 c.c., and 100 
c.c. are pipetted into an 8-oz. flask. 

(a) In presence of much arsenic, see Section E. Most of 

the arsenic is, however, volatilised in the reduction tube. 

(b) In presence of a little arsenic, precipitate the As and 

Sb on iron, and the tin on zinc as usual. 

(c) In presence of much iron or of phosphorus separate the 

SnS 3 with H 2 S in acid solution. (See Section D.) 

(d) In presence of tungsten, the uncalcined residue obtained 

by evaporation with HNO 3 is extracted with dilute 
AmOH or Am 2 CO 3 , when WO 3 dissolves. 

(e) Copper, cobalt, nickel, zinc, mostly go into the HNO 3 

solution, also much of the lead, whilst usually most of 
the iron and sometimes half the lead remain with the 
SnO 2 . Some Co and Ni may remain with the tin, in 
which case a separation with H 2 S is advisable as the 
coloured chlorides of Co and Ni interfere with the 
appearance of the colour finish in the ferric chloride 
In any tin compound or mixture of substances containing tin 

(except tin slag), the tin may be brought into solution by the above 



i. TIN PLATE SPELTER TEA LEAD. 5 grms. is boiled with 
300 400 c.c. HC1 in a i6-oz. flask provided with cork and 
leading tube. When the action is complete pour off from 
any residue and titrate with ferric chloride. In the case of 
spelter use less acid or more spelter, and bring rapidly to 
a boil. See article by R. Job, " The Mining Journal," 
December 12, 1908, p. 743. 

2. TIN FOIL CAPSULES BORINGS. 50 or 100 grms. is dropped 
into a crucible three parts full of melted borax with a little 
charcoal sprinkled on top. The button of metal is assayed 
for tin by method A or C, according to composition. The 
presence of zinc or aluminium in borings is easily recognised 
by the ease with which they burn in the crucible, and by 
the violent effervescence with cold HC1. 

or 40 grms. is dissolved in a large beaker in 100 500 c.c. 
HC1 and the minimum amount of HNO 3 . When solution 
is complete, dilute to 500, 1,000, or 2,000 c.c. and measure 
out the equivalent of i grm. Reduce with iron, filter, and 
precipitate on zinc. 

4. ALLOYS OF TIN AND ALUMINIUM. May be assayed by 
method A. 

5. METALLIC COPPER. Use method .4. separation with Na 2 S, 
or, better, method F. treatment with HNO 3 and reduction 
of residue. Take 5 to 10 grms. for assay. 

3 1 

6. TIN AMALGAM AND FUSIBLE ALLOYS. Weigh up to 5 grms. 
Treat with dilute HNO 3 and evaporate to dry ness- -extract 
with dilute HNO 3 filter calcine and reduce in coal gas. 
Dissolve as usual in HC1 and a little HNO 3 , separate Sb, 
&c., with iron, and precipitate tin on zinc. 


Samples of tin ashes should in general be tried for moisture and 
the tin assay done on the dried sample. In sampling material of 
this kind a sample weighing from 14 to 28 Ibs. is broken up and 
made to pass through a quarter inch sieve. Large pieces of iron are 
picked out and weighed and allowed for in making up final calcula- 
tions. 100 grms. of the dry ashes are pounded in an iron mortar 
and passed through a sieve with 16 meshes to the linear inch to 
separate the metallics. The metallics are weighed and the difference 
from 100 is the weight of the " fine " which is not separately weighed 
owing to slight loss of dust in pounding. 5 to 10 grms. of the ashes 
(with fine and metallic in proportion) is weighed out and treated by 
one of the following methods. Most tin ashes are best assayed by 
method i. Method 2 is suitable for chloriny or leady ashes (and for 
tin ore). Finely divided ashes, if free from chlorine, grease, sulphur, 
and metallic e.g. tin oxides, may be tried by method 3. Siliceous 
ashes in which silicate of tin is present are treated by method 4. 

i. GENERAL METHOD. 5 to 10 grms. ashes is weighed into a 
400 c.c. beaker, treated with 100 c.c. HC1 and 5 or 10 c.c. HNO 3 , 
and allowed to stand in a moderately warm place until the action 
abates. It is then boiled until the further action is completed. 
Dilute with an equal bulk of water and filter. If the ashes are 
leady and PbCl 2 clogs the filter, wash with hot sodium acetate, 
when the PbCl 2 dissolves (alternative to the use of method 2). Wash 
the residue well, firstly with dilute HC1, then with hot water, dry, 
and calcine with the paper in a porcelain dish in the muffle. Transfer 
the calcined oxides to a porcelain boat and reduce to metal in a 
current of coal gas at a low red heat. Time of reduction, i| 2 
hours. Transfer boat and contents to the same beaker when cold, 
and add 50100 c.c. HC1 and 2 or 3 c.c. HNO 3 ; boil off the nitrous 
fumes and dilute. Filter into the first HC1 solution and well wash 
the residue which is dried, calcined, and fused with four or five times 
its bulk of a mixture of KNaCO* and borax, the fusion being done 
in a platinum crucible in the muffle. The melt is dissolved in HC1 
and the residual tin precipitated on zinc as usual. The solution 
is made up to 500 or 1,000 c.c. and the equivalent of i gram is 
measured out with a pipette and reduced with iron wire in an 8-oz. 
flask, filtered and precipitated with zinc as usual, prior to solution 
in HC1 and titration with FeCl 3 . 

2. EVAPORATION with dilute HNO 3 and reduction of the residue 
in coal gas (see method F). Leady or chloriny ashes. 

3. FINE ASHES free from Cl, S, grease, moisture, and metallics. 
5 grms. is weighed into a porcelain boat and reduced for 2 hours in 


a current of coal gas. The reduced meta's are dissolved as in 
method i. 

4. SLAGGY ASHES. 5 grams is weighed into a platinum basin 
and evaporated just to dryness with HF. (If the ashes are also 
chloriny they must be first boiled in a porcelain dish with Na 2 Co 3 , 
and the liquid filtered off and, of course, examined for tin.) To 
the residue from HF add 40 50 c.c. HC1, heat to loosen from the 
basin and transfer to a 400 c.c. beaker. Add 50 c.c. HC1 and 5 c.c. 
HNO 3 and proceed as in i. 

which the tin all exists as soluble oxide, sulphide, oxychloride, or 
as metal. 

10 grams is dissolved in 100 c.c. HC1 and a little HNO 3 diluted 
and filtered. The residue which is small in amount is fused with 
KNaCO ;3 and borax. The solution is treated as in i unless Sb, Cu, 
As are known to be absent when the reduction with iron wire is 
omitted, the measured portion being at once treated with zinc. 

6. TIN GREASE. 5 grms. is washed with benzene in a dish, 
then filtered, dried, and treated like ordinary tin ashes. Slightly 
greasy ashes should be treated by method 2. 

7. IRONY SULPHARSENIDE TIN DROSS 20 grms of the dross is 

treated with 300 c.c. HC1 and 20 c.c. HNO 3 as in the case of ordinary 
tin ashes. The solution is diluted and filtered and the residue well 
washed. In this case the residue is washed into a dish without 
opening out the paper and evaporated to dryness with a little HNO 3 , 
extracted with dilute HNO 3 , and filtered through the same filter 
(not, of course, into the main solution). The residue is then dried 
and ignited as usual, and may be then reduced in coal gas, or if 
small in amount may be fused at once with KNaCO 3 and borax. The 
tin is determined in the usual manner reduction with iron precipi- 
tation on zinc solution in HC1, and titration with ferric chloride. 

8. VERY CHLORINY ASHES which often contain much ZnCL and 
large fragments of metal (" tin scruff") should not be assayed on the 
dry material, as this is extremely hygroscopic. Weigh out 100 or 
200 grams of the ashes, wet, on sampling, boil with NA.,CO 3 and filter. 
The washings are made acid with HC1 and a measured fraction pre- 
cipitated on zinc. The residue is dried and separated into fine and 
metallic. The fine is dried by method i or 2. The metallic is melted 
with reducing agents in a clay crucible and the button assayed 
for tin by A or C. If the ashes are chloriny but without large 
fragments of metal, weigh up 10 20 grms. wet and treat with 
200 c.c. HC1 and 100 c.c. HNO 3 in a large beaker. 


Tin ores are often exceedingly complex ; they may contain in 
addition to stannic oxide, ferric oxide, and silica, some or all of the 
following substances bismuth, copper, pyrites, iron pyrites, mis- 


pickel, wolfram, titanic acid, lead oxide, antimony oxide. Antimony 
and arsenic are common impurities, especially in South American ores. 
Contrary to a statement in " Crookes' Select Methods," the writer's 
experience is that antimony is almost invariably associated in small 
quantities with tin ore. It should be apparent that complete solution 
of the ore is absolutely essential in every case, in order to systematically 
ensure the complete extraction of the tin and its quantitative deter- 
mination. Any method which does not involve complete solution of 
the ore is quite unreliable as a method, although it may often yield 
correct results. Further, the final determination of the tin should 
always be effected volumetrically ; if a gravimetric estimation is adopted 
the assay develops into an academic research, owing to the number 
and complicated nature of the separations necessary to ensure that no 
possible impurity may score as tin ; while if such thorough separa- 
tion is neglected, the results obtained are quite unreliable. 

The following methods have been proposed and used for the wet 
assay of tin ore : 

i. Fusion with alkalies or alkalies and sulphur in nickel, silver, 
platinum, or porcelain crucible. 

2. Continued agitation with zinc and HC1. 

3. Reduction of the cleaned stannic oxide in hydrogen and cal- 
culation of the tin from the loss in weight, which is assumed 
to represent only the oxygen of the stannic oxide. 

4. Cleaning with HC1 and reduction with cyanide in a porcelain 

5. Fusion with potassium hydrogen fluoride, solution in sulphuric 
acid and precipitation as metastannic acid by dilution and 

6. Reduction in coal gas or hydrogen and extraction with HC1 and 
HNO 3 , combined with fusion of the siliceous residue with 
Na 2 CO 3 and borax in a platinum crucible and solution of 
the melt in HC1. 

7. Fusion with caustic soda in an iron crucible. 

The first method is now hardly ever used, being tedious and un- 
certain. The second method, agitation with zinc and HC1, is very 
slow ; it may occasionally give all the tin, but there can be no 
certainty whatever about such a method, and the same remark applies 
to the third method, calculation of the tin from the weight of oxygen 
lost by reducing the stannic oxide in hydrogen after presumably 
purifying it from other metallic oxides by cleaning the ore with acids ; 
rapidity is claimed for this method, but the assumptions as to com- 
plete reduction of the tin and absence of other metallic oxides reducible 
by hydrogen, are vital objections, and render the method unsound 
from either a scientific or commercial standpoint. Method four, 
reduction of cleaned ore with cyanide, is in reality a dry assay, not- 
withstanding the use of the porcelain crucible, and when the metal 


obtained is assayed for tin, must necessarily give results which are 
lower than the tin contents. It is not denied that in many cases 
the results will only be very slightly under the actual percentage, 
but one can never be certain of this ; often, indeed, results obtained 
in this way are 2 or 3 per cent, too low. If the button of metal 
is not assayed for tin, the results obtained may be either a little too 
low, correct, or up to as much as 4 or 5 per cent, too high. The 
method is not sufficiently certain for the commercial valuation of tin 
ores. Method 5 (Hallet's method) is logically admissible provided 
that the precipitated metastannic acid is not merely ignited and weighed 
as stannic oxide, but is redissolved and the tin carefully separated. 
This, however, complicates the method somewhat. Method 6 is the 
most practical method of assaying tin ore, which ensures at the same 
time accuracy and reliability as a method, and it alone will be con- 
sidered here in detail. Method 7 (the Pearce-Low method), which 
consists of fusion with caustic soda in an iron crucible, solution in HC1, 
reduction with iron, and titration with iodine, is a workable and an 
exceedingly rapid method for isolated assays; one assay can be done 
in an hour easily (see Low, " Technical Methods of Ore Analysis," 
2nd Edition). The above italics are mine, and I consider it necessary 
to emphasise this point, as Mr. Low claims that the method is at 
once the quickest and most accurate assay of tin in tin ores. It is 
undeniably quicker for isolated assays, though it is not as economical 
of time as the reduction method, since one can do seven or eight re- 
duction assays in conjunction with other work in the course of a 
working day, which is scarcely possible with the above fusion method. 
With regard to accuracy, the method compares favourably with most 
other assays, but is scarcely as accurate as the reduction assay. 

The foregoing remarks have reference to the assay of dressed 
tin ores in the main ; in previous editions it was omitted to clearly 
point this out. It is for dressed tin ores that fusion assays in general 
are so objectionable, owing to the possibilities of slight losses by 
spurting, creeping, volatilisation, &c., which on rich material intro- 
duce errors of commercial importance ; on poor material this objec- 
tion does not exist, and the writer must certainly admit that for poor 
mine stuff up to 5 or 6 per cent, of tin the Pearce-Low method is cer- 
tainly the method, and as slight losses in fusing do not here appre- 
ciably affect the result, a batch of such assays may be worked off 
very rapidly. On the other hand, the writer must still maintain his 
opinion that for dressed tin ores, the reduction assay, with fusion of 
the siliceous residue, as herewith described, is the only admissible 
method from a commercial standpoint, as .it favours neither buyer 
nor seller. 


Tin ores may be either pyritic or non-pyritic. Pyritic ores must 
be, and non-pyritic ores may be, treated with NHO 3 before reduc- 
tion, as sulphide of tin is volatile at a red heat. 5 grms. ore, ground 
as finely as possible in an agate mortar, is treated in an evaporating 


basin with clock glass cover, with 20 c.c. dilute HNO 3 and carefully 
evaporated to complete dryness. The residue is digested with dilute 
HNO 3 and filtered. The washed residue is dried, ignited in the 
dish, transferred to a porcelain boat and heated to a low red heat for 
2 hours in a current of hydrogen or coal gas. Coal gas is much 
more convenient to use and quite as effective as hydrogen. The boat 
is i\ inches by \ inch, and two at a time are placed in a porcelain or 
glass tube 12 inches long and f inch bore, which is then placed in 
the reduction furnace. A very convenient form of gas reduction 
furnace with clay body, brass gas jets, and asbestos rings to fit over 
the ends of the tube against the clay covers, is made in 6-inch lengths 
by Messrs. Fletcher, Russell, & Co., of Warrington. The ends of 
the tube should project about 3 inches from each end of the furnace 
and should be closed with rubber corks fitted with glass tubes as 
shown ; the escaping gas (about 2 bubbles per second) is passed 
through dilute HC1. 

The water 'through which the gas escapes should not be thrown 
away, but saved, and every now and then the tube should be washed 
out with HC1 and KC1O 3 , and the two solutions tested for tin as a 
check, against, for instance, loss by volatilisation through sulphur in 
the coal gas. A two-way gas branch is used; one jet supplies gas to the 
tube, the other supplies the gas for heating to the jets of the furnace. 
The boats are allowed to cool in the furnace, and when cold each boat 
and its contents is transferred to a 400 c.c. beaker and treated with 
100 c.c. HC1 and 5 c.c. HNO 3 , the assay being allowed to stand in a 
warm place until the action abates, when it is boiled for a few 
minutes, diluted with an equal bulk of water and filtered. The residue 
is well washed with hot acid water, then with hot water, is dried, 
ignited, and fused with four or five parts of a mixture of fusion mix- 
ture or sodium carbonate (free from chlorine) and borax, in a platinum 
crucible, and the melt dissolved in HC1 and precipitated with zinc 
as usual. The residue rarely holds more than \ per cent, of the 
total tin in the ore. 

Instead of fusing the residue with KNaCO 3 and borax in a 
platinum crucible, it may be fused with about 2 in. of stick 
caustic soda in an iron crucible over a bunsen burner. The melt is 
dissolved up in HC1 and precipitated on zinc as usual. (See K i, 
paragraph 2.) 

The main solution is made up to 500 c.c. and the equivalent 
of i grm. is pipetted into an 8-oz. flask, reduced with iron wire and 

c 2 


filtered ; the filtrate is precipitated with strips of sheet zinc as usual, 
and the metallic sponge dissolved in HC1 and titrated with ferric 

Except with impure ores the reduction with iron wire may be 
omitted, as it is always an advantage to save a filtration when 
possible. In this case the aliquot part of the solution is at once 
precipitated with zinc. The boiling HC1 solution for titration with 
FeCl 3 must, however, be free from black powder of Cu, Sb, &c., and 
from tungsten blue in suspension or solution. (N.B. Lower oxides 
of tungsten in solution give a brownish pink or light claret -coloured 
solution.) During precipitation with zinc the solution should be 
freely acid to start with, and in presence of Ti, or Mo, should be 
distinctly acid when poured off. (See Section B, p. 27.) 

Any copper in the tin ore is found in the HNO 3 solution, though 
traces may remain with the SnO 2 . In the iron wire stage, the 
arsenic which escaped extraction with HNO 3 and volatilisation in 
the reduction tube, is partly evolved as AsH 3 and partly precipitated 
with antimony in the metallic form. It comes down as a brown 
flocculent deposit which contains 3 or 4 per cent, of its weight of tin. 
As there is generally only a few per cent., at most, of As in a tin 
ore, the loss of tin from this cause is quite negligible, but as a check 
one should save the iron wire and precipitates and filter papers and 
examine them from time to time for tin ; it will be found, as in the 
case of the deposit in the tube and the dilute HC1 through which 
the escaping gas bubbles, that only the merest traces of tin are lost 
in these operations. Further, the HC1 solution of the reduced metal 
may be done in a conical flask with rubber cork and leading tube 
dipping under water, to assure oneself that there is no appreciable 
lose by volatilisation of SnCl 4 . 

If the ore contains wolfram, the tungsten is mostly found as WO 3 
in the residue from HC1 and HNO 3 extraction of the reduced metal, 
from which it may be removed before fusion, with AmOH. Any 
tungsten which gets into the main solution comes down as blue oxide 
with the iron wire precipitate, and any which is fused with KNaCO 3 
and borax should be removed by reducing the HC1 extract of the 
melt with iron wire, before precipitating with zinc. In general, all 
the antimony and some of the lead in the ore will be found in the 
main HC1 solution, whilst some of the lead will be obtained in the 
HNO 3 extract. 

Note. After the HNO 3 evaporation the residue may be boiled 
with HC1 (40 or 50 c.c.), diluted and filtered, though in this case the 
extract must be tried for tin as a matter of precaution. It will in 
general hold all the copper and most of the arsenic, antimony, 
lead, and iron, though one can never be sure that the residue is 
free from the oxides of these metals. Occasionally this HC1 
extract will hold a little tin. The residue is reduced in the usual 



Siliceous material very poor in tin is best assayed by a fusion 
method of which either of the following will serve : 

i. .5 i grm. of finely-powdered ore is fused in a platinum crucible 
with soda ash (free from chlorine) or NaHCO 3 , and borax. 
The melt is dissolved in dilute HCl, transferred to an 8-oz. 
flask, and heated with ten 2|-in. iron nails until the colour 
goes ; it is then allowed to stand in a moderately cool place 
for 20 minutes to half-an-hour, poured off into a clean flask 
containing CO 2 , and titrated with one-tenth strength iodine. 

la. Or, the HCl solution of the melt is precipitated with zinc, for 
the usual ferric chloride titration. 

2. Mr. A. H. Low's method is a very useful one. .5 grms. fine 
ore is fused over a bunsen burner in an iron crucible with 
3 or 4 inches of stick caustic soda. . The melt is extracted 
with water and solution effected with HCl. The solution is 
reduced with iron nails as above, and titrated with iodine 
as above. 

A blank assay with iron nails and dilute HCl must be 
done, as it has been found in practice that when using 
ten or a dozen 2|-in. iron nails and digesting these with 
HCl as above, the liquid requires .1 to .2 c.c. iodine, even 
in absence of tin. 

The solution of the melt may also be precipitated with 
zinc and the metallic residue dissolved in HCl and titrated 
with FeCl 3 . 


In order to avoid the preliminary evaporation of pyritic black 
tin with nitric acid, the following procedure may be employed : 
The ore is weighed direct into the porcelain boat and reduced as 
usual. After reduction the deposit in the tube is washed out with 
HCl and KClO 3 and mixed with the water (which should have been 
acidified with HCl before starting the assay), through which the gas 
bubbled, and the joint solution reduced with iron or zinc and assayed 
for tin by either the iodine or ferric chloride assay ; the amount 
so found being calculated on to the percentage determined from the 
solution of the reduced ore. 

This procedure applies to all dressed tin ores holding not more 
than 2 per cent, sulphur ; that is to say, it applies to ninety-nine out 
of every hundred buying samples of black tin or tin barilla. In such 
exceptional samples as may hold more sulphur than this, evaporation 
with nitric acid must precede reduction. 

As we have already mentioned in these notes, the reduction of the 
main assay solution with iron wire may in most cases be omitted, and 


it will be found that in this amended procedure we attain with the 
gas reduction and ferric chloride assay the following conditions : 

1. The minimum of manipulation of the main portion of the 

assay both before and after solution. 

2. Complete solution of the ore. 

3. The use of a method of determination by which only tin can 

score as tin. 

The first two of these conditions which ensure the elimination of all 
sources of loss, protect the seller's interests; while the third, which is 
at the same time absolutely fair to the seller, protects the buyer. 


i. HYDROFLUORIC ASSAY. The slag is pounded up finely in a 
steel mortar ; the more siliceous the slag the finer it should 
be powdered. 2 grms. slag is weighed into a platinum 
basin and about 20 c.c. HF (pure and strong) added in a 
fume cupboard and the assay evaporated to bare dryness. 
Most of the SiO 2 is removed and the tungsten remains as 
blue oxide, other metallic oxides being left in a form easily 
soluble in acids. The residue is treated in the basin with 
30 or 40 c.c. HC1, and after warming, the contents of the 
basin are transferred to a 400 c.c. beaker, the basin being 
cleaned out with a rubbered glass rod and as small a 
quantity as possible of hot water. Add / c.c. of HNO 3 and 
boil for 2 or 3 minutes, dilute to about 350 c.c. and pass a 
rapid stream of H 3 S for about 10 minutes. The SnS 2 should 
come down granular if these instructions are adhered to. 
Allow to stand for a few minutes and filter, but do not wash. 
(If the SnS 2 comes down in a very finely divided form, the 
assay must be allowed to stand for two hours and again 
gassed.) Tin should never be separated as SnS. Wash 
back the precipitate into the beaker and dissolve by boiling 
with 30 c.c. HC1 and a little KC1O 3 . Boil with iron wire, 
filter and precipitate on zinc as usual. Dissolve in HC1, 
adding a granule of pure zinc and bringing rapidly to a boil, 
and titrate with ferric chloride. 

2. HYDROFLUORIC ACID AND REDUCTION. In the case of certain 
very rich slags the tin may be present either wholly or in 
part in a form insoluble in HC1 after removal of SiO 3 
with HF. In such cases dilute the HC1 extract to 75 c.c. 
and filter; reduce the residue in coal gas, dissolve, the re- 
duced metal in HC1 and a little HNO 3 and filter the solution 
into the first HC1 extract. If there is still a residue it may 
be fused with KNaCO 3 and borax. The main solution 
(if from 5 grms. slag) is diluted to 250 or 500 and i grm. 
pipetted out and precipitated with H 2 S. Filter, dissolve in 
HC1 and KC1O 3 , reduce on iron and precipitate with zinc 
as usual. If the insoluble residue is small in amount it 


may be fused at once with KNaCO 3 and borax, after removal 
of WO 3 with dilute ammonia. In" the case of leady slags 
the HF residue may be treated with HNO 3 according to 
general method F. In the case of slags containing prills of 
metal the metallics must be sieved out, from a portion of 
loogrms. Weigh the fine and metallic in proportion. 

3. FUSION ASSAY. Melt 5 -10 grms. NaOH in a nickel (or, 
better, iron) basin and dust in 2 grms. of the finely powdered 
slag. Cover and heat to bright redness for 10 minutes with 
the aid of a foot blowpipe. Cool, loosen the melt, and 
transfer to a beaker; dissolve in HC1 and a little HNO 3 , 
precipitate with H 2 S and proceed as before. 


The general method consists in 

(a.) Obtaining the tin in dilute HC1 solution as SnCl 4 , bulk 

50 60 c.c. in a small beaker. 
(b.) Reduction of this solution to SnCL, by a piece of soft iron 

with precipitation of As, Sb, Cu, as metals. 
(c.) Titration with iodine and starch indicator in the cold. 

The method is reliable, quick, neat, and cheap, requires little 
apparatus or room, but has not the certainty of the ferric chloride 
assay. The preceding general methods and separations remain un- 
altered in most cases, as far as obtaining the tin in HC1 solution 
as SnCl 4 . 

i. In the case of ALLOYS and MATTES, &c., .5 i grm. filings or 
powder is weighed into a 4-in. beaker with watch glass 
cover and covered with about an inch of HC1 ; then boil 
gently until action ceases, on a frying pan sandbath over a 
bunsen burner. Add a crystal of two of KC1O 3 to complete 
solution, boil off the chlorine, add an equal bulk of water 
and a piece of soft iron (3 in. by 3-16 in.), cover with 
watch glass and allow to simmer gently for 20 30 minutes 
after the solution becomes colourless (which is an indication 
that all the iron in solution is present as FeCl 2 ). At this 
point we assume (a weak point of the assay) that all the tin 
is reduced to SnCl 3 . The Sb, Cu, and As are, of course, 
precipitated by the iron. Cool the assay quickly in a basin 
of cold water, wash the rod and cover with a rapid stream 
of cold boiled acid water saturated with CO 2 , add a little 
starch paste and titrate at once with iodine, made by dis- 
solving 21.32 grms. pure iodine, and 45 grms. KI free from 
iodate, in a little cold water, and diluting the solution to 
i litre .100 c.c. = i grm. Sn. In standardising take .5 
grms. tin, and dissolve in i in. of HC1 in a small beaker. 
Dilute, insert a piece of iron, reduce gently for five minutes, 
cool, wash cover and rod rapidly and titrate at once. The 


oxidation tendency is the important source of error in these 
assays. Precipitated Sb and Cu are not dissolved by 
iodine (except slowly) in a cold dilute solution, and therefore 
their presence as precipitated metal does not matter except 
that they darken the liquid, if in any quantity, and obscure 
the starch blue until the assay has stood for a moment. 
The iodine is more safely standardised by comparison with 
KMnO 4 of known strength, or against pure As 3 O 3 . 

2. ASHES AND ORES. The measured portion of the HC1 solution 
should not be too bulky ; it is reduced in a beaker by an 
iron rod as in the process just described. 

3. SLAGS, (a.) HF ASSAY. Tungsten blue should be filtered 
off. The HC1 solution of the HF residue may be reduced 
by iron, filtered, and again reduced. 

(b.) FUSION ASSAY. The HC1 solution is reduced and filtered 
from the tungsten blue, then reduced again and titrated. 
H 2 S separation seems to be unnecessary with iodine assays. 

Chapter VI. 

The Assay of Antimony. 


i. Gravimetric estimation as SbO 2 (unreliable). 

2. Gravimetric estimation by electrolysis. 

3. Titration of Sb 2 O 3 with iodine in alkaline solution. 

4. Titration of iodine liberated by action of SbCl 5 on KI in 
HC1 solution. 

5. Solution of precipitated Sb in H SO 4 and titration with 
KMn0 4 . 

6. Titration of precipitated Sb with FeCl 3 in a boiling HC1 

7. Titration of precipitated Sb with bichromate after solution 
in FeCl 3 . 

8. Dry assays (a) iron reduction (b) carbon reduction. 

The electrolytic assay is useful in special cases, as are 5, 6, 7. 
The dry assays are also useful in their proper place. The ordinary 
gravimetric estimation is a bad method. Methods 3 and 4 are of 
the most general applicability, and are both accurate and practical. 

SOLUTION from speises, mattes, metals, is easily effected by HC1 
and KC1O 3 or HNO 3 ; from oxides by reduction to metal with 
KCN or coal gas and solution of the reduced metal in HC1 and 
KC1O 3 , by fusion in a silver basin with caustic alkali and solution 

4 1 

in HC1, by extraction with alkaline sulphides (with or without a 
sulphurising fusion), and sometimes with the help of tartaric acid 
which dissolves the oxides and oxychloride; also by oxidising metal 
or sulphide with HNO 3 and solution of the oxide in alkalies or 
alkaline sulphide. 

SEPARATION. When necessary, HoS in a not too concentrated 
HC1 solution separates from Fe, Zn, Co, and Ni. Extraction with 
Na 2 S dissolves sulphides of As, Sb, Sn. Sulphides of As, Sb, Sn, 
are'dissolved in HC1 and KC1O 3 and HT, AmOH, and AmCl added ;, 
then magnesia mixture, when the arsenic is separated as Mg, NH 4r 
AsO 4 . Antimony may be readily estimated in presence of tin. 
Arsenic may also be distilled off from a metal, with a solution of 
FeCl 3 , and CaCL, in HC1. As 2 S 3 may be precipitated in a concen- 
trated HC1 solution, Sb remains in solution. Sb may be precipitated 
in the metallic form by iron. 

Chapter VII. 

Specific Antimony Assays. 

A. Alloys of Tin Lead Antimony Copper with but little 


SbCl 5 in HC1 solution oxidises KI. The liberated iodine may be 
titrated with stannous chloride not with Hypo. 

In this method the presence of tin (as SnCl 4 ) and lead has no- 
effect whatever on the titration ; indeed, the titration is done with 
SnCl 2 . Copper, arsenic, and iron, when present in solution in their 
highest state of oxidation, score as antimony, and should be separately 
estimated and allowed for, this being preferable to separating the 
Sb from them. Fe is rarely present, but in the case of alloys with 
more than i per cent. As, it is advisable to dissolve in aqua regia 
and separate the Sb and As as sulphides, as it appears that in some 
cases when an alloy rich in arsenic and tin is boiled with HC1 there 
is a considerable evolution of H 3 As. This matter is under investiga- 
tion. Arsenic, however, is rarely present in the above alloys in 
quantities greater than i per cent, i 3 grms. of fine filings is 
weighed into a i6-oz. flask and boiled gently with 150 c.c. HC1 until 
action ceases and most of the Pb and Sn are dissolved ; to complete 
solution add cautiously a cold saturated solution of KC1O 3 , which 
also peroxidises the metals. After solution is complete, add a little 
KC1O 3 and make sure that excess of chlorine is present. The solu- 
tion is diluted with rather less than an equal bulk of water and boiled 
until free from chlorine. In practice half-an-hour is a sufficient 
length of time ; the most practical and most delicate test of the 


absence of free chlorine in the cold solutions is the sense of smell. The 
assays should not be boiled below 150 c.c. as a matter of precaution, 
on account of the volatility of chloride of antimony. Allow the assay 
to cool; when quite cold, fill the flask with CO 2 from a Kipp charged 
with marble and HC1, add 20 c.c. of a fresh 20 per cent, solution 
of KI, and titrate as rapidly as possible with stannous chloride. This 
is made by dissolving 10 grms. of tin, or 20 grms. SnCl 2 , 2 aq., in 
300 c.c. HC1, and diluting to i litre. It should be kept under CO 2 , 
and must be standardised every time it is used, against a standard solu- 
tion of bichromate made by dissolving e xact ly 1 6 grms. of pure K.->Cr a O 7 
in i litre of distilled water. 100 c.c. Bic. = 2 grms. Sb, and looc.c. 
SnClo = about i grm. Sb. The assays, if overdone, are brought 
back with the standard bichromate (rapidly), but should only require 
a few drops at most, otherwise the accuracy of the assay is impaired. 
The assays have a great tendency to absorb oxygen, after titration, 
and reoxidise ; hence the necessity for a CO 2 atmosphere and rapid 
titration, especially in presence of As and Cu.~ Further, the reaction 
in the case of As is not instantaneous, and only proceeds to comple- 
tion as fast as the liberated iodine is reduced by SnCL, being to some 
extent reversible. Acid solutions of AsCl 3 , CugCl 2 , FeClg, and KI, 
readily absorb oxygen from the air ; SbCl 3 is scarcely affected. The 
bichromate remains quite constant, and it may be standardised against 
pure iron as a check on the weighing up. In standardising the 
stannous chloride, pour a little KI solution and starch paste into an 
8-oz. flask, fill the flask with CO 2 and run in rapidly, 20 c.c. SnCL 
from the burette. Then titrate rapidly with K 2 Cr 2 O 7 . 

As = Sb = Cu 2 = Fe 2 = O. 

i per cent. As= 1.6 per cent. Sb. i per cent. Cu = o_945 per cent. Sb. 
i per cent. Fe = 1.03 per cent. Sb. 

HC1 solutions of Sb (ic) As (ic) Cu (ic) and Fe (ic) oxides are 
all reduced by KI to the corresponding ous compounds, with libera- 
tion of an equivalent amount of iodine. These ( ous) solutions, as 
well as a solution of KI in HC1 and of SnCl 2 , absorb oxygen from 
the air, and therefore necessitate a CO 2 atmosphere. The solution 
requires to be more strongly acid in the case of arsenic. SnCl 4 is 
not reduced by KI, hence the liberated iodine may be titrated with 
SnCLj. In assaying type metal for Sb the assays generally have a 
slight yellow colour after boiling off all the chlorine. This may be 
due to either Cu or traces of Fe, but is not due to SbCl 5 . Rich type 
metals sometimes contain a little Fe, and a little iron may get in from 
the file, but it has been proved that any error arising from the use of 
a file is quite negligible. Alloys of the above class which are rich 
in As appear, in some cases at any rate, to lose much or most of their 
As or AsH 3 , during the action of HC1. The matter is being in- 
vestigated, but the possibility of such loss of As may be guarded 
against by adding solution of KC1O 3 before the HC1, and by keeping 
the solution saturated with chlorine until solution is complete. 


The arsenic in such alloys may be always estimated by distillation 
with a solution of FeCl 3 and CaCl 2 in HC1 (see Beringer, " Text- 
book of Assaying "). According to Mr. A. Gibb, after removing the 
As as AsCl 3 in this manner, the solution, if mixed with ZnCl 2 and 
redistilled yields all Sb (as SbCl 3 ) at 184 C. Since SnCl 4 boils at a 
much lower temperature than either AcCl 3 or SbCl 3 , it seems strange 
that SnCl 4 is not distilled off in this assay, but the probability is 
that it forms a thick double salt. 

The above method is both rapid and accurate, but it requires con- 
siderable judgment and experience in performing the titrations. The 
titration must be done as rapidly as possible, but not too rapidly ; it 
must be borne in mind that the finish is more or less gradual, and 
especially so in presence of arsenic. Sometimes in running in a rapid 
stream of SnCl 2 the blue colour vanishes, but reappears like a flash 
the moment after. In such cases, one should go on titrating, as the 
assay is still unfinished. If the flask is not properly filled with CO 2 , 
after-bluing will take place in any case, and it requires long experience 
to distinguish in every case between a finished and unfinished titration. 
Duplicate assays should always be done ; the bichromate solution 
itself should be standardised against pure iron wire, which contains 
99.6 per cent. Fe. Take the at. wt. of Fe as 56 and that of Sb 120 
in calculating the strength of the bichromate. 

B. Alloys of antimony and iron with arsenic and copper. 
(a.) The As is estimated by the distillation assay. 
(b.) The Cu by the usual means. 

i. i grm. metal. Dissolve in 20 30 c.c. HC1 and i or 2 c.c. 
HNO 3 in an 8-oz. flask. Add sodium sulphide till alkaline 
and pass H 2 S. Warm, dilute, and filter. Redissolve the 
Cu 2 S, PbS, FeS, &c., and reprecipitate. Filter, mix the 
filtrates, and precipitate the sulphides of As, Sb, Sn by 
cautious addition of HC1. Filter, dissolve the precipitate in 
100 c.c. HC1 and KC1O 3 . Boil until solution is complete, 
dilute, filter, add KC1O 3 , and boil off the excess of chlorine. 
Cool, add KI and titrate with SnCl 2 , making the equivalent 
deduction for As. 

2. i grm. metal, 40 50 c.c. HC1 and a little HNO 3 . Dilute to 
1 20 when solution is complete, and pass H 2 S, thus separating 
from Fe but not from Cu. Dissolve the precipitate in HC1 
and KC1O 3 , and proceed as in i. 

C. Mattes and sulphurous compounds generally, of Fe, Cu, Pb, 
As, Sb, Sn. Antimony ore, &c. (if impure). 

In these the As cannot be estimated by the direct distillation 
assay, and must be separated. Solution is effected as usual. The 
sulphides of As, Sb, Sn, obtained by precipitation of an alkaline 
polysulphide solution with HC1, are filtered and dissolved in 20 c.c. 
HC1 and a little KC1O 3 and the Cl boiled off. 5 grms. AmCl and 


5 grms. HT dissolved in 20 c.c. water are added, and then AmOH 
until the solution is alkaline. (If a precipitate forms on adding 
AmOH, more AmCl and HT are needed.) The solution, which 
should not be more than 60 or 70 c.c. in bulk, is mixed with 20 c.c. 
of magnesia mixture and allowed to stand for 24 hours. The Mg, 
NH 4 , AsO 4 , is filtered off and the filtrate acidified with HC1, diluted 
to i litre and precipitated with H 2 S. Allow to stand in a warm 
place for an hour or two. Again pass H 3 S and filter the sulphides 
of Sn and Sb. Dissolve the precipitate in HC1 and KClo 3 as usual. 
Add KI and titrate with SnCl 2 . 

D. ANTIMONY ORE (with "but little As). i grm. ore, 30 c.c. 
HC1, 2 c.c. HNO 3 . Add NaOH till alkaline, as soon as 
the ore is completely attacked, and pass H 2 S. Warm, filter, 
and wash with hot dilute Na 2 S. The precipitate if bulky 
should be redissolved and reprecipitated. Precipitate the 
filtrate with HC1, filter, wash with NaCl, boil the precipitate 
well with HC1 alone (As 2 S 3 left undissolved), dilute, filter, 
peroxidise with KC1O 3 , boil off the free chlorine, cool, add 
KI and titrate with SnCl 2 , or boil off the iodine, cool, add 
Rochelle salt, neutralise, make alkaline with bicarbonate 
and titrate with Iodine. 

E- OXIDES. Calcined antimonial material. Boil with HC1 and 
KC1O 3 , dilute and filter, (a] Dry, ignite, and fuse the residue 
with NaOH in a silver dish. Extract with HC1 and mix 
with the main solution ; (b) or reduce the insoluble oxide 
to metal in coal gas at a very low red heat, or in a small 
porcelain crucible with KCy, and dissolve the reduced 
metal in HC1 and KC1O 3 , and mix with the main solution. 
Proceed as in C if arsenic is present or as in B 2 if no arsenic 
is present. 

F. The electrolytic assay has been already referred to. (See 
Chapter IV.) 


This method is chiefly useful as a rapid approximate determina- 
tion of antimony in solder, and is used as an adjunct of the direct 
ferric chloride assay of tin in such alloys. It depends upon the 
fact that FeCl 3 dissolves finely divided Sb with the formation of 
FeCl 2 and SbCl 3 . After pouring off the hot acid solution of SnCl 2 
from "the black powder (see Chapter V., A.), pour on to the latter, 
immediately, a little hot boiled water to prevent aerial oxidisation of 
the Sb. When the tin titration is finished, pour back on to the black 
powder and retitrate. (Cu if present is also converted into Cu 2 Cl 2 
as soon as any CuCl 2 is formed it gives the colour indication.) The 


percentage of Sb is two-thirds of the apparent tin percentage, equiva- 
lent to the extra ferric chloride used in absence of copper. 



Useful for type metal and solder. Cu does not interfere, but As 
should be absent. The method is used as an adjunct of the iodine 
tin assay. The antimony adhering to the iron rod is washed off, 
allowed to settle, and the liquid decanted into another beaker ; the 
residue washed by decantation with hot dilute HC1 and the washings 
added to the rest of the tin solution, which is again reduced before 
titration. The precipitated Sb and Cu are heated with a few c.c. of 
strong H 2 SO 4 until solution is complete and white fumes evolved. 
Then cooled, diluted, and titrated with KMnO 4 . 
Fe 2 = Sb. 

K. Titration of Sb 2 O 3 in alkaline bicarbonate solution with iodine. 
(Mohr's method.) This is a useful assay in the absence of 
Cu, and is fully described in Sutton's " Volumetric Analysis." 

WITH K 2 CR 3 O 7 OR KMN<D 4 . 

This method may be useful as an adjunct of the iodine tin assay 
as in H, and is useful for type metal and solder in the absence of 
copper. The precipitated Sb is dissolved in a small basin in a few 
c.c. of FeCl 3 and HC1 (free from FeCl 2 ), and the solution titrated with 
bichromate until all the FeCl 2 is peroxidised, using spots of ferricyanide 
indicator on a plate. 


3 FeCl 3 + Sb = SbCl s + 3 FeCl 2 
3 FeClo + 3 HC1 + Oil = 3 FeCl 
SbClg + 2 HC1 + O = SbCl 5 + 

o 3 i = 3 e 3 i 

g + 2 HC1 + O = SbCl 5 + H 2 

l 3 + ii H 2 O. 

That is to say, when dissolving antimony in FeCl g and titrating with 
K 2 Cr 2 O 7 , Sb requires altogether 2% O, or is equivalent to 5 Fe in 
terms of the strength of the bichromate solution when standardised by 
titrating FeO to Fe 2 O 3 . 

Although FeCl 3 does not oxidise SbCl 3 , yet the SbCl 3 is all 
oxidised by the Bic. before the FeCl 2 ; before the disappearance of 
the FeCl 2 is shown by the spot indications which is taken as the finish 
of the titration. The development of the colour is, however, slow 
towards the finish. SbCl 3 will reduce the brown solution obtained by 
mixing FeCl 3 and K 3 FeCy 6 with formation of Prussian blue. Indeed, 
SbCl 3 may be thus titrated with Bic., using ferric ferricyanide solution 
as an outside indicator, or FeCl 2 as an inside indicator and K 3 FeCy 6 
as outside indicator. When KMnO 4 is used, the SbCl 3 is also 
oxidised in the titration as well as the FeCl 2 . The above method is not 
to be recommended except for rough purposes, as it is hard to wash 
pptd Sb (by iron) free from FeCl 2 without redissolving some Sb by 
aerial oxidation. 

4 6 


i. Antimonial litharge. 

(a) 50 litharge roughly pounded, 

10 15 soda ash, 10 15 charcoal, 

5 borax, and a little fluor. 
or (b) 50 litharge, 

50 100 cyanide, 
10 argol. 

In either case fuse in a clay crucible at a moderate heat, and 
assay the button for Sb. 

2. Sulphide Ore. This if mixed with much gangue should be 
concentrated by liquating 1 1,000 grams in a double luted 
Fuse 25 grms. ore, 5 soda, 

12 grms. iron filings, 5 borax, 

in an E crucible, and assay the button, which should con- 
tain Fe, for antimony. 

3. Type Ashes usually carbonaceous. Fuse with 15 to 20 per 
cent, soda ash and a little ferrocyanide and borax. Melt the 
button away from iron shots in a ladle, and pour into a hemi- 
spherical mould. Assay the metal for tin and antimony. 

out separation). 

We have seen that Sb may be estimated with SnCl 2 in presence 
of SnCl 4 and As 2 O 5 by titrating the iodine liberated from KI in 
HC1 solution and deducting the equivalent of the As 3 O 5 present. 
It is, however, possible to estimate Sb by direct titration with SnCl 3 
in a fairly strong and hot, but not too strong HC1 solution ; in such 
a solution arsenic acid is not affected at all until all the SbCl 5 , FeCl 3 , 
and CuCL, have been reduced, and therefore if we titrate until the 
colour goes we have the measure of the Sb + Fe + Cu. This method 
may be applied to type metals, which contain only traces of Fe, and 
usually under i per cent. Cu. The Cu must be known and deducted 
for, but it serves the purpose of an internal colour indicator. The 
method is beautifully simple and rapid, as the Cu may be estimated 
with sufficient accuracy in type metals and most anti-friction metals in 
a few minutes. In the case of solders, which are, as a rule, free from 
Cu or Fe, a drop of FeCl 3 is added to the assay as an indicator. With 
regard to the accuracy of the method, there can be no doubt that it 
is sufficiently accurate for most commercial purposes. 


in alloys, or when separated together as sulphides. 

i. The tin is determined in the usual way; solution in HC1 and 
KC1O 3 , separation of Sb and As on iron wire, filtration, 
precipitation with zinc, solution in HC1, and titration with 
ferric chloride. Add 5 per cent, of the arsenic percentage. 


2. The antimony is determined by solution in HC1 and KC1O 3 , 
boiling off the excess of chlorine, and titration of the hot 
solution with SnCl 2 . In absence of Cu (the amount of 
which must be known and allowed for), add a drop of 
FeCl 3 as an indicator. 

3. The arsenic is determined by (a) distillation with FeCl 3 and 
CaCl 2 in HC1, and titration of the evolved arsenic with 
iodine in alkaline bicarbonate solution ; (b) sulphides are 
dissolved in HC1 and KC1O 3 , the solution concentrated and 
distilled with a ferric chloride mixture containing also ferrous 
chloride, the evolved arsenic being estimated as before by 
titration with iodine in an alkaline bicarbonate solution ; 
(c) or the solution of sulphides in HC1 and KC1O 3 , from 
which the free chlorine has been driven off, is boiled with 
copper foil, which, with the precipitated Sb and As is after- 
wards distilled with ferric chloride mixture. The deter- 
mination of the three metals in an alloy should not take 
more than 2 hours by these methods. 

30. Or, better, the fine filings are dissolved by slow digestion with 
HC1 and KC1O 3 in a warm place, keeping always present a 
slight excess of free chlorine. When solution is complete, 
add a little more chlorate, dilute, boil off the chlorine, cool, 
add KI and titrate with SnCl = As + Sb. 

4 8 


AT the suggestion of the Cornish Correspondent of "The Mining 
Journal," and with the permission of the Editor of that paper, we 
subjoin a table showin'g a series of analyses of Cornish tin ores. 
Part of the article which accompanied the table in question, and 
which appeared in "The Mining Journal" for August 19, 1905, is 
also given herewith. The analyses were done by the writer of this 
book : 

These analyses were carried out by Mr. L. Parry, A.R.S.M., 
Assayer and Consulting Metallurgist, of Union Bank Yard, 

The wet assay adopted for tin is the one given in Mr. Parry's 
book, " The Assay of Tin and Antimony " viz., reduction in a 
current of coal gas combined with a volumetric estimate of the tin. 

In the cyanide assays the ores were extracted with aqua regia, 
and the solutions were tested for tin. In one or two cases a little 
tin (under .25 per cent.), was found in these solutions, and is 
included in the results given. The cleaned residues were reduced 
with cyanide (98 per cent. Au*), in clay crucibles. The assays were 
done on 20 gram charges, and the buttons of metal obtained were 
assayed for tin by the same volumetric method employed in the 
wet assays. The net cyanide percentages thus obtained average 
ij per cent, under the wet assays. 

It is possible, by adopting a different procedure, to arrive at 
results by the cyanide assay that will more closely approximate to 
the wet assay than those given above. The ores, either cleaned 
with aqua regia or not, are reduced with cyanide in porcelain crucibles 
in the muffle, 5-gram portions being taken for assay. When the 
fusion is complete, the assays are cooled, and the crucibles and con- 
tents are boiled in water until the cyanide and cyanate of potassium 
are dissolved out. The residue is then extracted with HC1, and the 
tin in the solution then determined volumetrically. Although rather 
better results are obtained, the assay is more tedious, and there is 
little saving of time over the wet assay, whilst there always remains 
the possibility of tin being lost as alkaline stannate. In cyanide 

* "98 per cent. Au " is the trade mark of the purest cyanide made, which is 
used for electro-plating, hence the "98 per cent, gold." This cyanide holds 
98 per cent, of pure KCN. 


assays the cyanide should be finely powdered and well mixed with 
the ore, otherwise some of the ore is liable to escape reduction. 

The wet assays were done throughout in duplicate. The arsenic 
and antimony were estimated in the aqua regia extract, and the 
results thus obtained are likely to be somewhat under the actual 
contents. The iron, silica, and tungstic acid were merely determined 
approximately. The crop ores having already been roasted, were 
not assayed for sulphur, but the unroasted slimes, Nos. 2, 5, 7, 8, 
and 1 1, were all rather strongly attacked by HNO 3 ,and were, therefore, 
assayed for sulphur. 

It has been necessary to detail the tests made for the simple reason 
that the majority of our mine managers, and others who control the 
industry apart from the smelters will now, for the first time, secure 
reliable data as to the actual constituents of the black tin they have 
so long handled, and sold on an assay (Cornish method), which, to 
put it mildly, affords only sufficient indication of values to keep the 
buyer from blundering. In this connexion, the necessity for assaying 
the buttons obtained, by the cyanide method, for tin that is, for 
purity of metal was amply demonstrated. Notwithstanding the clean- 
ing with aqua regia, only seven out of nineteen buttons viz., Nos. i, 
3, 4, 14, 15, 1 8, 19 were pure tin. The others varied from g6| 
to 99 per cent. Sn, with the exception of Nos. 9, 10, and n, which 
came out 90, g2j, and 93! per cent. Sn, respectively. The balance 
of the buttons was iron, showing that it is not possible to extract all 
the FE 2 O 3 from all ores direct by extraction with aqua regia. The 
percentage weight of the buttons was greater than the actual tin 
percentages shown by wet assay in the cases of Nos. 6, 8, 10, u, and 
12, which held g6J, 90, 92.5, 93.6, and 97.2 per cent. Sn respec- 
tively. In Cornwall the purity of the metal produced does not much 
trouble the seller of black tin. Later on the importance of paying some 
attention to this matter may be deemed worthy of the consideration 
of every mine manager in the county, and more especially of those 
whose products, as indicated by the above table, call for special 



Trace of Cobalt. 
Trace of Cobalt and Zinc. 
Unroasted Slimes. 

Trace of Cobalt. Unroasted 
A little MnO. 
Slimes Unroasted. 
Slight trace of Cobalt. Un- 

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Alloys, Assay of for Tin 24 31 

Assay of for Antimony ... ... ... ... 40 47 

Ashes, Wet Assay of 31 32 

Dry Assay of 16 17 and 24 

Actions, Oxidising and Reducing 89 

Anti-friction Metals 27 28 and 41 43 

Cornish Tin Ores, Analysis of 48 

Ferric Chloride Titration for Tin 2028 

Gravimetric Estimation of Tin ... ... ... 3 5 and 20 

Iodine Titration for Tin 2223 an d 39 

Metallic Precipitation 9 

Mattes and Speises, Assay of 28 29 and 43 

Purity of Chemicals ... ... ... ... ... ... u 

Rapid Determination of Arsenic, Antimony, and Tin in 

Alloys 46 

Solder, Assay of ...4,20, 2426, 41 44 

Separation of Tin, Antimony, Arsenic n 13 

Solution of Metals 10 

Sulphides, Oxides, Oxy chlorides 6 7 

Summary of Tin Assays ... ... ... ... ... 13 16 

Type Metal, Assay of 27 31 and 41 43 

Tin Ore, Assay of 45, 1820 and 32 37 

Tin Slags, Assay of 38 and 40 




Return to desk from which borrowed. 
This book is DUE on the last date stamped below. 

JAN 4 1949 

LD 21-100m-9,'47(A5702sl6)476 

YL JJ004