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Iodine (Fit. MOHR *).

The oxidation is effected in alkaline solution and proceeds as
follows : Sb,0, + 41 + 4NaOII = Sb,O 6 + 4NaI + 2H a O. The
method gives results which are serviceable only under very defi-
nite conditions, because the antimonous oxide has not always an
equal tendency to change to antimonic oxide in alkaline solution,
but this tendency is greater in the presence of much alkali car-
bonate than when little is present, and becomes constant only with
a certain excess of carbonate. According to my investigations it
is best to proceed thus :

Dissolve a quantity of the compound containing about 0*1
grm. of antimonous oxide in about 10 c. c. of an aqueous
solution of tartaric acid, then add sufficient sodium-carbonate
solution to about neutralize the liquid. Add now 20 c. c. of
a cold, saturated solution of sodium bicarbonate and (the liquid
remaining clear) some starch paste, then titrate with iodine
( 146) until the fluid just remains blue on being stirred. The
fact that the blue color soon disappears, however, must not induce
the operator to add more iodine. 4 eq. of iodine corresponds to
1 eq. of antimonous oxide.

The results so obtained are entirely satisfactory (Expt. No. 76).
I cannot recommend the use of sodium carbonate as employed
by FR. MOHR in his experiments, because it itself has the prop-
erty of fixing a considerable quantity of iodine, varying, moreover,
according to the quantity of water used (Expt. No. 77), whereas
this is not the case with sodium bicarbonate (Expt. No. 78).
Compare also 127, 5, a, 1, and Expt. No. 79.

a. Conversion of Antimonous Chloride to Antimonic Chlo-
ride by Hydrochloric Acid and Potassium Chromate or Perman-
ganate.

F. KKssLER'sf first description of this method was so wanting
in precision, that it could not be depended upon. However, he
has since:): determined most accurately the conditions under which
antimony in acid solution may be satisfactorily titrated either with
potassium eliminate (tlic excess of the standard solution being
determined with ferrous sulphate) or with potassium permanganate.

* Lehrbuch der Titrirnwthode, 3. Aufl., 276. f Pogg AnnaL, xcv, 204.
i lb., cxviu, 17, and Zeitschr.f. analyt. Chem., 11, 383.



I 124.] ANTIMOtfY. 401

I. Titration with Potassium Dichromate.

* 1. REQUISITES.

a. Standard Solution of Arsenous Acid. Dissolve exactly
5 grin, pure arsenous oxide by the aid of some soda solution, add
hydrochloric acid till slightly acid, then 100 c.c. more of hydro-
chloric acid of 1*12 sp. gr., and dilute to 1000 c.c. Each c.c. con-
tains 0-005 grm. arsenous oxide and corresponds to 0-007293
antimonous oxide.

ft. Solution of Potassium Dichromate. Dissolve about 2'5 grm.
to 1 litre.

y. Solution of Ferrous Sulphate. Dissolve about I'l grm. iron
wire in 20 c.c. dilute sulphuric acid (1 to 4), filter, and dilute to
1 litre.

S. Solution of Potassium Ferficyanide. Should be tolerably
dilute and freshly prepared.

2. DETERMINATION OF THE SOLUTIONS.

a. Relation between the Solution of Chromate and the Solution
of Ferrous Sulphate. Run into a beaker 10 c.c. of the chromate
solution from the burette, add 5 c.c. of hydrochloric acid and 50
c.c. water, and then add iron solution from a burette till the fluid
is green. Continue adding the iron solution, a c.c. at a time, test-
ting after each addition whether a drop of the fluid, when brought
in contact with a drop of the potassium ferricyanide, on a porcelain
plate, manifests a distinct reaction for ferrous iron. As soon as
this point is attained, add 0'5c.c. of chromate solution and then iron
solution two drops at a time, till the blue reaction just occurs.
Now read off both burettes, and calculate how much chromate
solution corresponds to 10 c.c. of iron solution. This experiment
is to be repeated before every fresh series of analyses, as the iron
solution gradually oxidizes.

ft. Relation between the Chromate Solution and the Solution of
Arsenous Acid. Transfer 10 c.c. of the arsenous solution to a
beaker, add 20 c.c. hydrochloric acid of 1'2 sp. gr., and 80 100
c.c.* water, run in chromate solution till the yellow color of the
fluid shows an excess, wait a few minutes, add excess of iron solu-
tion, then again '5 cliroinate solution, and finally again iron solu-
tion till the end-reaction appears (see above). Deduct from the

* The water must be measured, for the action of chromic acid on arsenous
acid (and also on antimonous chloride) is normal only if the fluid contains a*
least one sixth of its volume of hydrochloric acid of 1-12 sp. gr.



402 DETERMINATION. [ 125.

total quantity of chromate solution employed, the amount corre-
sponding to the iron used, and from the datum thus afforded calcu-
late how much antimony corresponds to 100 c.c. of chromate solu-
tion ; in other words, how much antimony is converted by the
quantity of chromate mentioned from SbCl 3 into SbCl 6 .

3. THE ACTUAL ANALYSIS.

In the absence of organic matter, heavy metallic oxides, and other
bodies which are detrimental to the reaction, dissolve the antimo-
nous compound at once in hydrochloric acid. The solution should
contain not less than ^ of its volume of hydrochloric acid of 1*12
sp. gr. It is not advisable, on the other hand, that it should con-
tain more than , otherwise the end-reaction with potassium fern-
cyanide is slower in making its appearance arid loses its nicety.
Tartaric acid cannot be employed as a solvent, since it interferes
with the action of chromic acid on ferrous salts. Now proceed as
directed in 2. If the direct determination of antimony in the
hydrochloric acid solution is not practicable, precipitate it with
hydrogen sulphide. Wash the precipitate, transfer it, together
with the filter, to a small flask ; treat it with a sufficiency of hydro-
chloric acid, dissolve by digestion on the water-bath, add a suffi-
cient quantity of a nearly saturated solution of mercuric chloride
in hydrochloric acid of 1*12 sp. gr. to remove the hydrogen sul-
phide, and then proceed as directed.

II. Titration with Potassium Pennanganate.

Here also the fluid must contain at least -J- of its volume of
hydrochloric acid of 1.12 sp. gr. The permanganate solution,
which may contain about 1/5 grm. of the crystallized salt in a litre,
is added to permanent reddening. The end-reaction is exact, and
the conversion of antimonous to antimonic chloride goes on uni-
formly, although the degree of dilution may vary, provided the
above relation between hydrochloric acid and water is kept up.
It is not well that the hydrochloric acid should exceed of the
volume of the fluid, as in that case the end-reaction would be too
transitory. Tartaric acid, at least in the proportion to antimony in
which it exists in tartar emetic, docs not interfere with the reac-
tion. Hence the permanganate may be standardized by the aid of
solution of tartar emetic of known strength.

If you have to analyze antimonous sulphide, proceed as directed
I. 3 ; make the fluid mixed with mercuric chloride up to a certain



126.] TIN IN STANNOUS AND STANNIC COMPOUNDS. 403

volume, allow to settle, and use a measured portion of the perfectly
clear solution for the experiment.

My own experiments* have shown that KESSLER'S methods are
also suitable for the estimation of very small quantities of anti-
mony.

1). Volumetric Estimation ly determining the Hydrogen Sul-
phide yielded ly the Sulphide (K. SCHNEIDER f).

Both antimonous and antimonic sulphides yield under the
action of boiling hydrochloric acid 3 mol. hydrogen sulphide for
uvery 2 atoms of antimony. Hence, if the amount of the gas
evolved under such circumstances is estimated, the amount of anti-
mony is known.

For decomposing the sulphide and absorbing the gas, the same
apparatus serves as BUNSEN employs for his iodimetric analyses
( 130). The size of the boiling-flask should depend on the quan-
tity of sulphide ; for quantities up to 0*4 grm. Sb a S 3 , a flask of 100
c.c. is large enough; for 0*4 1 grm,, use a 200 c.c. flask. The
body of the flask should be spherical, the neck rather narrow,
long, and cylindrical. If the sulphide of antimony is on a filter,
put both together into the flask. The hydrochloric acid should
not be too concentrated.

The determination of the hydrogen sulphide is best conducted
according to the method given in 148, b. The results obtained
by SCHNEIDER are satisfactory. If the precipitate contains anti-
monious chloride, the results are of course false, and this would
actually be the case if on precipitation with hydrogen sulphide the
addition of the tartaric acid were omitted.

126.
4. TIN IN STANNOUS COMPOUNDS, and 5. TIN IN STANNIC COMPOUNDS.

a. /Solution.

In dissolving compounds of tin soluble in water, a little hydro-
chloric acid is added to insure a clear solution. Nearly all the
compounds of tin insoluble in water dissolve in hydrochloric acid,
or in aqua regia. The hydrate of metastannic acid may be dissolved
by boiling with hydrochloric acid, decanting the fluid, and treating
the residue with a large proportion of water. Ignited stannic oxide,

* Zeitschr. /. analyt. Chem,, vin, 155. \Pogg. Annal., ex, 634.



404 DETERMINATION. [ 126

and stannic compounds insoluble in acids, are prepared for solution
in hydrochloric acid, by reducing them to the state of a fine pow-
der, and fusing in a silver crucible with potassium or sodium
hydroxide, in excess. Metallic tin is dissolved best in aqua regia;
the solution frequently contains metastannic chloride mixed with
the stannic chloride (Tn. SCHEERER*). It is generally determined,
however, by converting it into stannic oxide, without previous
solution. Acid solutions of stannic salts, which contain hydrochlo-
ric acid, or a chloride, cannot be concentrated by evaporation, not
even after addition of nitric acid or sulphuric acid, without volatili-
zation of stannic chloride taking place.

b. Determination.

Tin is weighed in the form of stannic oxide, into which it is
converted, either by the agency of nitric acid, or by precipitation
as stannic (or metastannic) acid, or by precipitation as sulphide. A
great many volumetric methods of estimating tin have been pro-
posed. They all depend on obtaining the tin in solution in the
condition of stannous chloride, and converting this into stannic
chloride either in alkaline or acid solution. A few only yield saris-
factory results.

We may convert into

STANNIC OXIDE:

a. By Treatment with Nitric Acid. Metallic tin, and those
compounds of tin which contain no fixed acid, provided no com-
pounds of chlorine be present.

b. By Precipitation as Stannic (or Metastannic) Acid. All tin
salts of volatile acids, provided no n on volatile organic substances
nor ferric salts be present.

c. By Precipitation as Sulphide. All compounds of tin with-
out exception.

In methods a and c, it is quite indifferent whether the tin is
present as a stannous or a stannic compound. The method b
requires the tin to be present as a stannic salt. The volumetric
methods may be employed in all cases ; but the estimation is simple
and direct only where the tin is in solution as stannous chloride
and free from other oxidizable bodies, or can readily be brought
into this state. For the methods of determining stannous and
stannic tin in presence of each other, I refer to Section Y.

*Journ.f. prakt. Chem. N. F., in, 472.



126.] TIN IN STANNOUS AND STANNIC COMPOUNDS. 405

1. Determination of Tin as Stannic Oxide.

a. By Treatment with Nitric Acid.

Tliis method is resorted to principally to convert the metallic
tin into stannic oxide. For this purpose the finely-divided metal
is put into a capacious flask, and moderately concentrated pure
nitric acid (about 1*3 sp. gr.) gradually poured over it ; the flask is
covered with a watch glass. When the first tumultuous action of
the acid has somewhat abated, a gentle heat is applied until the
metastannic acid formed appears of a pure white color, and further
action of the acid is* no longer perceptible. The contents of the
flask are then transferred to a porcelain dish and evaporated on a
water-bath nearly to dryness, water is then added, and the precipi-
tate is collected on a filter, washed, till the washings scarcely red-
den litmus paper, dried, ignited, and weighed. The ignition is
effected best in a small porcelain crucible, according to 53 ; still
a platinum crucible may also be used. A simple red heat is not
sufficient to drive off all the water ; the ignition must therefore be
finished over a gas blowpipe. Compounds of tin which contain no
fixed substances may be converted into stannic oxide by treating
them in a porcelain crucible with nitric acid, evaporating to dry-
ness, and igniting the residue. If sulphuric acid be present, the
expulsion of that acid may be promoted, in the last stages of the
process, by ammonium carbonate, as in the case of acid potassium
sulphate ( 97) ; here also the heat must be increased as much as
possible at the end. For the properties of the residue, see 91.
There are no inherent sources of error.

b. By Precipitation as Stannic (or Metastannic) Acid.

The application of this method presupposes the whole of the
tin to be present in the state of stannic salts. Therefore, if a solu-
tion contains stannous salts, either mix with chlorine water, or con-
duct chlorine gas into it, or heat gently with chlorate of potassa,
until the conversion of the stannous into stannic salts is effected.
When this has been done, add ammonia until a permanent precipitate
just begins to form, and then hydrochloric acid, drop by drop, until
this precipitate is completely redissolved ; by this means a large
excess of hydrochloric acid in the solution will be avoided. Add
to the fluid so prepared a concentrated solution of ammonium
nitrate (or sodium sulphate), and apply heat for some time, where-
upon the whole of the tin will precipitate as stannic acid. Decant
three times on to a filter, then collect the precipitate on the latter,



406 DETERMINATION. [ 126.

wash thoroughly, dry, and ignite. To make quite sure that the
whole of the tin has separated, you need simply, before proceeding
to filter, add a few drops of the clear supernatant fluid to a hot
solution of ammonium nitrate, or sodium sulphate, when the for-
mation or non-formation of a precipitate will at once decide the
question. The tin is also precipitated from metastannic chloride
by the above reagents.

This method, which we owe to J. LOWENTHAL, has been repeat-
edly tested by him in my own laboratory,* is easy and convenient,
and gives very accurate results. The decomposition is expressed
by the equation, SnCl 4 + 4^a 3 SO 4 + 3Ii a O = H 2 SnO 3 + 4NaCl
-|- 4NaHSO 4 , or in precipitating with ammonium nitrate : SnCl 4
+ 4NH 4 N0 3 + 3H 2 = 1-I.SnO. + 4NH 4 C1 + 4HNO 3 .

Tin may also, according to H. EosE,f be completely precipi-
tated from stannic solutions by sulphuric acid. If the solution
contains metastannic acid or metastannic chloride, the precipitation
is effected without extraordinary dilution ; the other stannic com-
pounds, however, require very considerable dilution. If free
hydrochloric acid is absent, the precipitation is rapid ; in other
cases 12 or 24 hours at least are required for perfect precipitation.
Allow to settle thoroughly, before filtering, wash well (if hydro-
chloric acid was present, till the washings give no turbidity with
silver nitrate), dry and ignite, at last intensely with addition of
some ammonium carbonate. The results obtained by OESTEN, and
communicated by II. ROSE, are exact.

c. By Precipitation as Stannous or Stannic Sulphide.

Precipitate the dilute moderately acid solution with hydrogen
sulphide water or gas. If the tin was present in the solution as a
stannous salt, and the precipitate consists accordingly of the brown
stannous sulphide, keep the solution, supersaturated with hydrogen
sulphide, standing for half an hour in a moderately warm place,
and then filter. If, on the other hand, the solution contain a stan
nic salt, or metastannic acid, and the precipitate is yellow and consists
of stannic sulphide mixed with stannic oxide, or yellowish brown
and consists of hydrated metastannic sulphide mixed with meta-
stannic acid (BARFOED, TH.. SCIIEERER ;[), put the fluid, loosely
covered, in a warm place, until the odor of hydrogen sulphide

*Journ.f. prakt. Chem.. LVI, 366. f Fogg. AnnaL, cxn, 164.

t Jvurn. f. prakt. Chem. N. F. t in, 472.



126.] TIN IN STANNOUS AND STANNIC COMPOUNDS. 407

lias nearly gone off, and then filter. The washing of the stan-
nic- sulpnide precipitate, which has a great inclination to pass
through the filter, is best effected with a concentrated solution of
sodium chloride, the remains of the latter being got rid of by a
solution of ammonium acetate containing a small excess of acetic
acid. If there is no objection to having the latter salt in the fil-
trate, the washing may be entirely effected by its means (BUNSEN*).
Transfer the dry precipitate as completely as possible to a watch
glass, burn the filter carefully in a weighed porcelain crucible,
moisten the ash with nitric acid, ignite, allow to cool, add the pre-
cipitate, cover the crucible, heat gently for some time (slight decrep-
itation often occurs), remove the lid and heat gently with access of
air, till sulphur dioxide has almost ceased to be formed. (If too
much heat is applied at first, stannic sulphide volatilizes, the fumes
of which give stannic oxide.) Now heat strongly, allow to cool,
and heat repeatedly with, pieces of ammonium carbonate to a high
degree, to drive out the last portions of sulphuric acid. When the
w r eight remains constant the experiment is ended (H. ROSE). For
the properties of the precipitates, see 91. The results are accu-
rate.

2. Volumetric Methods.

The determination of tin by the conversion of stannous into
stannic chloride with the aid of oxidizing agents (potassium dichro-
mate, iodine, potassium permanganate, etc. )offers peculiar difficulties,
inasmuch as on the one hand the stannous chloride takes up oxygen
from the air and from the water used for dilution, with more or
less rapidity, according to circumstances ; and on the other hand,
the energy of the oxidizing agent is not always the same, being
influenced by the state of dilution and the presence of a larger or
smaller excess of acid.

In the following methods, these sources of error are avoided or
limited in such a manner as to render the results satisfactory.

* Annal. d. Chem. u. Pharm., cvi, 13.



408 DETERMINATION. [ 126.

1. Determination of Stannous Chloride l)y Iodine in
Alkaline Solution (after LENSSEN *).

Dissolve the stannous salt or the metallic tin f in hydrochloric
acid (preferably in a stream of carbon dioxide), add sodium-potas-
sium tartrate, then sodium bicarbonate in excess. To the clear
slightly alkaline solution thus formed add some starch -solution,
and afterwards the iodine solution of 146, till a permanent blue
coloration appears. 2 at. free iodine used corresponds to 1 at. tin.
LENSSEN 's results are entirely satisfactory.

2. Determination of Stannous Chloride after addition
of Ferric Chloride.

The fact that stannous chloride in acid solution can be far more
accurately converted into stannic by oxidizing agents after being
mixed with ferric chloride (or even with cupric chloride) than
without this addition, was first settled by LOWENTHAL. J Sub-
sequently STROMEYER published some experiments leading to the
same results, together with practical remarks on the best way of
carrying out the method in different cases. The processes thus
originated, and which have been well tested, are as follows :

a. The given substance is a stannous salt. Dissolve in pure
ferric chloride (free from ferrous chloride) with addition of hydro-
chloric acid, dilute and add standard permanganate from the
burette. Now make another experiment with the same quantity
of water similarly colored with ferric chloride to ascertain how
much permanganate is required to tinge the liquid, and subtract
the quantity so used from the amount employed in the actual
analysis, and from the remainder calculate the tin.

The reaction between the tin salt and the iron solution isSnCl,
-f- Fe 2 Cl 6 =SnCl 4 +2FeCl 2 . The solution thus contains ferrous
chloride in the place of stannous salt, the former being, as is well
known, far less susceptible of alteration from the action of free
oxygen than the latter. 2 at. iron found correspond to 1 at. tin.

* Journ. f.prakt. Chem., LXXVIII, 200; Annul, d. Chem. u. Pharm., cxiv,
113.

f The solution of metallic tin is much assisted by the presence of platinum
foil, which is accordingly added. LENSSEN found this addition of platinum to
be objectionable ; but no other experimenter has observed that it interferes
with the accuracy of the results.

\ Journ. f. prakt. Chem., LXXVI, 484.

4-nnal d. Chem. u. Pharm., cxvir, 201.



8 127.] AfcSENOtTS AND A&SENIC ACIDS. 409

It must not be forgotten that the titration takes place in presence
of hydrochloric acid, and that hence the inconveniences mentioned
under 112, 2, y, may arise and impair the accuracy of the
method. The results cannot be considered accurate unless the
standardizing of the permanganate and the analysis take place
under similar conditions as regards dilution and amount of hydro-
chloric acid.

b. The given substance is metallic tin. Either dissolve in
hydrochloric acid preferably with addition of platinum and in an
atmosphere of carbon dioxide and treat the solution according to
, or place the substance at once in a concentrated solution of ferric
chloride mixed with a little hydrochloric acid; under these cir-
cumstances it will, if finely divided, dissolve quickly even in the
cold and without evolution of hydrogen. Gentle warming is
unobjectionable. Now add the permanganate. The reaction is
Sn + 2Fe 2 Cl 6 =SnCl 4 + 4FeCl 2 , therefore every 4 at. iron found
reduced correspond to 1 at. tin. The results are of course only
correct when iron is not present. Where this is the case, proceed
with the impure tin solution according to c.

c. The given substance is stannic chloride or stannic oxide, or a
compound of tin containing iron. Dissolve in water \vith addition
of hydrochloric acid, place a plate of zinc in the solution and allow
to stand twelve hours, then remove the precipitated tin with a
brush, wash it, dissolve in ferric chloride, and proceed as in b.

d. The given substance is pure stannic sulphide, precipitated
out of an acid stannic solution containing no stannous salt. Mix
with ferric chloride, heat gently, filter off the sulphur, and then
add the permanganate. 4 at. iron correspond to 1 at. tin, for
SnS, + 2Fe a Cl 6 = SnCl 4 + 4FeCl, + 2S. The results obtained by
STROMEYER are quite satisfactory. As regards the precipitated
stannic sulphide, see BARFOED, 91, c.

127.

6. ARSENOUS ACID, and 7. ARSENIC ACID.

a. Solution.

The compounds of arsenous and arsenic acids which are not
soluble in water are dissolved in hydrochloric acid or in nitrohydro-
chloric acid. Some native arsenates require fusing with sodium
carbonate. Metallic arsenic, arsenous sulphide, and metallic arsen-



410 DETERMINATION. [ 127.

ides are dissolved in fuming nitric acid or nitrohydrochloric acid,
or a solution of bromine in hydrochloric acid ; those metallic
arsenides which are insoluble in these menstrua are fused with
sodium carbonate and potassium nitrate, by which means they are
converted into soluble alkali arsenates and insoluble metallic oxides ;
or they may be suspended in potassa solution and treated with
chlorine ( 164, B, 7). In this last manner, too, arsenous sul-
phide dissolved in concentrated potassa may be very easily ren-
dered soluble. All solutions of compounds of arsenic which have
been effected by long heating with fuming nitric acid, or by warm-
ing with excess of nitrohydrochloric acid, or chlorine, contain
arsenic acid. A solution of arsenous acid in hydrochloric acid
cannot be concentrated by evaporation, since arsenous chloride
would escape with the hydrochloric-acid fumes. This, however,
less readily takes place if the solution contains arsenic acid ; in
fact, it only occurs in the presence of a large proportion of hydro-
chloric acid (for instance, half the volume of hydrochloric acid of
1*12 sp. gr.*). It is therefore advisable in most cases where a
hydrochloric-acid solution containing arsenic is to be concentrated
to previously render the solution alkaline.

b. Determination.

Arsenic is weighed as lead arsenate^ as ammonium magnesium



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