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acid, set aside in a warm place for the tin disulphide to settle,
and determine it according to 126, 1, c. Acids which are
stronger than acetic, and which liberate oxalic acid, must not
be employed. CLARKE recommends, in order to secure very
accurate results, to again dissolve the precipitate of antimony
and arsenic sulphides in an alkaline sulphide solution, add an
excess of oxalic acid, and boil with hydrogen -sulphide water,
thereby obtaining the last portions of tin in solution.
According to the investigations made by FR. PHILLIPS in my
laboratory, this last operation would appear to be absolutely
required in order to obtain any proper results whatever.



Chem. News^ XXT, 124 ; Zeitschr. f. analyt. Ghem., ix, 487.



724 SEPARATION. [ 165.

The very unfavorable results obtained by G. C. WITTSTEIN *
are perhaps referable to the fact that the solution used by
him contained too much free hydrochloric acid, whereby the
precipitation was rendered incomplete at the boiling heat,
and he was compelled to complete it in the cold. In the ex-
periments made by PHILLIPS the free hydrochloric acid was
neutralized as nearly as possible with potassa.

[CLARKE'S method, with some important modifications, has
been successfully applied to the separation of tin from anti-
mony in alloys by F. P. DEWEY,! wno proceeds as follows:

Dissolve in a mixture of 1 part strong nitric acid, 4 parts
strong hydrochloric acid, and 5 parts water. Since even small
quantities of free mineral acids prevent complete precipitation
of antimony, they are removed by evaporating to dryness on
a water-bath, with previous addition of enough potassium
chloride to form double salts with the tin and antimony chlo-
rides present. The presence of the potassium chloride entirely
prevents loss of tin and antimony by volatilization as chlorides
during the evaporation. Add to the salts thus obtained a
large quantity of pure oxalic acid (at least 20 parts crytallized
acid to 1 part tin) and dilute with water to about 125 c. c. per
0*1 grin, antimony present. The salts dissolve readily. Boil
and pass 11,8 through the boiling solution half an hour. Filter
immediately while hot, and wash the greater part of the soluble
matter out of the precipitate with hot water. The precipi-
tated antimonous sulphide will contain a little stannic sulphide.
Dissolve in ammonium sulphide, avoiding an unnecessary
quantity of the sol vent, and pour the solution into a strong hot
solution of oxalic acid. A liberal excess of oxalic acid should
be present after decomposition of the sulphur salts. Heat the
oxalic solution with the suspended precipitate of antimonous
sulphide to boiling and pass H a S gas ten minutes. Collect
the Sb,S, now free from tin on a weighed filter, wasli with
hot water, and proceed to determine the antimony as directed
in 125, 1, 5. To recover tin from the filtrate, evaporate
nearly to dryness, add strong sulphuric acid, and heat till all

* Vierteljahrcsschr. f. prakt. Pharm. xix, 551.
\Am. Chem. Journ., i, 244.



165.] * METALS OF GROUP VI. 725

the oxalic acid present is decomposed and removed. Dilute
largely and precipitate the tin with hydrogen sulphide
according to 126, 1, c.]

7. Methods based upon the Separation of the Metals
themselves, or, as the case may be, on their different
Deportment with Acids.

a. TIN FROM ANTIMONY (MODIFIED GAY-LUSSAC).
Heat a weighed portion of the finely divided alloy (or other 209
compound) with hydrochloric acid, add potassium chlorate in
small portions until solution is effected, and then divide the
liquid into two parts, a and 5. Precipitate both metals in a
by means of a zinc rod, wash them rapidly with hot water
containing some hydrochloric acid, wash with alcohol, then
with ether, dry at 100, and weigh. To I add a rather
large quantity of hydrochloric acid, immerse a strip of tin,
and heat for a long time. By this operation the antimony
is completely precipitated as a black powder, while the
gtannic chloride is reduced to stannous chloride. Wash oft'
the antimony from the tin with moderately dilute hydro-
chloric acid, collect it on a weighed filter, wash first with
diluted hydrochloric acid, then with alcohol, and finally with
ether, then dry at 100 and weigh. The quantity of tin. is
found by the difference. Since, according to investigations
made by A. W. CLASEN,* precipitated metallic antimony is
very perceptibly soluble in hydrochloric acid, either hot or
cold, of various strengths, a loss of antimony is scarcely
avoidable.

5. TIN FROM ANTIMONY (TooKEY,f improvements by
CLASEN (loo. cit.) and ATTFIELD :{.).

The hydrochloric solution should be oxidized if necessary 210
with a few drops of nitric acid or a little potassium chlorate.
Heat nearly to boiling and add iron so long as it dissolves.
Either hoop-iron or fine bright wire will answer the purpose;
it should dissolve in dilute hydrochloric acid, leaving little or
no residue. The antimony will be thrown down, the tin

* Journ, /. prakt. Chem., xcu, 477, Zeilschr.f. analyt. Chem., iv, 440.
f Journ. Chem, Soc., xv, 462. Journ. f prakt. Chem., LXXXVIII, 435.
%Zeitsc7ir.f. anaiyt. Chem., ix. 107.



726 SEPARATION. [ 165.

reduced to stannous chloride. As soon as all antimony ap-
pears to be precipitated and .the iron to be dissolved, add
more hydrochloric acid, allow to deposit, decant, and test
whether iron produces any further precipitate. In this way
you will ensure the absence of any metallic iron and the com-
plete precipitation of the antimony. Wash the antimony
with hot water, which should be at first acidified, then with
alcohol, finally with ether, drying at 100. Throw down the
tin with hydrogen sulphide ( 126, 1, <?). With care the re-
results are good. Compare CLASEN (loc. cit.}.

c. DETERMINATION OF ARSENIC IN METALLIC TIN (GAY-
LUSSAC *).

Dissolve the laminated or granulated metal in a mixture 211
of 1 eq. nitric acid and 9 eq. hydrochloric acid with the aid
of a gentle heat. Solution ensues without the disengagement
of any gas, stannous chloride and ammonium chloride being
formed. The arsenic remains behind as a powder.

2KNX), + 18IIC1 + 8 Sn = 8 SnCl a + 2NH 4 C1 + 6II.O.

The nitrohydrochloric acid must, hence, not be employed in
much larger proportion than will give 2 eq. of HNO 3 and 18
eq. of 1IC1 to 8 eq. of metal.

d. MUCH TIN FROM LITTLE ANTIMONY AND ARSENIC.

If an alloy of the three metals is treated in a very finely 212
divided condition in a stream of carbonic acid with strong
hydrochloric acid, the whole of the tin dissolves to stannous
chloride. A part of the arsenic and antimony escapes as
arsenetted and antimonetted hydrogen, while the rest remains
behind in the state of metal, or, as the case may be, of a solid
combination with hydrogen. Conduct the gas through several
U-tubes containing a little chlorine-free red fuming nitric
acid, whereby the arsenic and antimony will be oxidized.
When the solution is eifected, dilute the contents of the flask
with air-free water to a certain volume, mix, allow to settle,
and determine the tin in an aliquot part either gravimetrically
or volumetrically. Filter the rest of the fluid, wash the pre-
cipitate thoroughly, dry the filter with its contents in a porce-

* Annal. de Chim. et de P/iys., xxm, 228.



165.] METALS OF GROUP VI. 727

lain crucible, add the contents of the U-tubes, evaporate to
dryness, and in the residue separate the antimony and arsenic
as directed in 201. It is well to treat an aliquot part of the
hydrochloric solution with iron (210) to find and if necessary
estimate traces of antimony which may have passed into the
hydrochloric-acid solution.

e. TIN FROM GOLD.

Gold may be separated from excess of tin by boiling the 213
finely divided alloy with only slightly diluted sulphuric acid
to which hydrochloric acid has been cautiously added. The
tin dissolves as stannous chloride. Heat is applied till the
sulphuric acid begins to volatilize copiously. Stannic oxide
is formed which dissolves in the concentrated sulphuric acid,
while the gold remains behind. On addition of much water
the stannic oxide falls, mixed with finely divided gold, in the
form of a purple-red precipitate. On warming with concen-
trated sulphuric acid the stannic oxide finally redissolves,
while the gold is left pure (H. ROSE *).

/. PLATINUM FROM GOLD.

The aqua-regia solution is freed so far as possible from 214
nitric acid by evaporation with hydrochloric acid, and treated
with a solution of ferrous chloride, the gold being determined
as directed in 123, &. The platinum may be precipitated
from the filtrate by hydrogen sulphide according to 124, c.

8. Method ~based on the Extraction of Gold ~by
Mercury.

DETERMINATION OF GOLD IN PLATINUM ORE.

Treat the mineral for several hours with small quantities 215
of pure, boiling mercury, pour off and repeat the operation ;
then wash thoroughly with boiling mercury and distil off all
the mercury very cautiously. The gold remains behind
(DEVILLE and DEBEAY). Prudence requires that the residue
should be tested.

* Pogg. AnnaL, cxn, 173.



728 SEPARATION. [ 165.

9. Method l)ased on the Precipitation of the Indi-
vidual Metals as Sulphides ~by Sodium Thiosulphate.

ARSENIC AND ANTIMONY FROM TIN.

Add an excess of hydrochloric acid to the solution, heat 216
to* boiling, and add sodium thiosulphate until the precipitate
is no longer orange or yellow, but white, and the liquid is
opalescent from separated sulphur. Arsenic and antimony
are completely precipitated, while all the tin remains in solu-
tion (YoHL*). Estimate the former, if one alone of the
metals is present, according to 125, 1 and 127, 4. If
both together are present, separate according to 201 or 204.
The tin in the filtrate is best determined according to 1 26,
1, c. LENSSEN f employed this method with apparently good
results. My experience has not, however, been so favor-
able. As tin is also precipitated by sodium thiosulphate
unless free hydrochloric acid is present, the separation can be
successful only when hydrochloric acid present prevents pre-
cipitation of tin, while not hindering that of the antimony.

10. Method based upon the Precipitation of Tin
as Stannic Arsenate.

TIN FROM ARSENIC.

ED. HAFFELY J has proposed the following method of deter- 217
joining both the tin and the arsenic in commercial sodium
stannate, which often contains a large admixture of sodium
arsenate. Mix a weighed sample with a known quantity of
sodium arsenate in excess, add nitric acid also in excess,
boil, filter off the precipitate, which has the composition
2SnO a -As a O B -f 1011,0, and wash; expel the water by igni-
tion and weigh the residue, which consists of 2SnO,*As a O .
In the filtrate determine the excess of arsenic acid as directed in
127, 2. The amount of the stannic oxide is found from the
weight of the precipitate, that of the arsenic acid is obtained
by adding the quantity in the precipitate to the quantity in
the filtrate and deducting the quantity added.

* Annal. d. Chem. u. Pharm., XCVT, 240. \Ib. t cxiv, 118.

Mag., x, 220 ; Journ. /. prakt. Chem., LXVJI, 209.



165.] " METALS OF GROUP VI. 729

11. Method based on the Separation of Arsenic and
Antimony from their Hydrogen Compounds.

To determine both metals in a mixture of arsenic and 218
antimony hydrides, conduct the gas into a solution of neutral
silver nitrate. Antimony hydride yields silver antimonide,
whereas arsenic goes into solution as arsenous acid, with reduc-
tion of silver. This method was recommended by A. W.
HOFMAN * for the qualitative detection of arsenic and anti-
mony. Filter off the precipitate, consisting of silver and silver
antimonide, and wash it. To the solution add a slight excess
of hydrochloric acid, filter off the silver chloride, and pre-
cipitate with hydrogen sulphide. The precipitate is arsenous
sulphide containing a small quantity of antimonous sulphide,
which is to be separated according to 202 or 207. The pre-
cipitate of silver and silver antimonide heat with tartaric
acid and a very little nitric acid, and determine the antimony
according to 125, 1.

All methods of determining antimony and arsenic in solu-
tions, based on treating the solution with zinc and hydro-
chloric acid, passing the gas into silver-nitrate solution, etc.,
are unreliable, because only a certain part of the arsenic and
antimony are evolved as hydrides, while the balance remains
in the flask in the form of metals.

1 2 . Volumetric Methods.

a. ARSENOUS FROM AKSENIC ACID.

Convert the whole of the arsenic in a portion of the sub- 219
stance into arsenic acid and determine the total amount of this
as directed 127, 2 ; determine in another portion the arsen-
ous acid as directed in 127, 5, a, and calculate the arsenic
acid from the difference.

b. ANTIMONY OF ANTIMONOUS COMPOUNDS FROM ANTIMONIC
ACID.

Determine in a sample of the substance the total amount 220
of the antimony as directed 125, 1, in another portion esti-
mate the antimony present as an antimonous compound as

* Annal. d. Chem. u. Pharm., cxv, 287.



730 SEPARATION. [ 166.

directed 125, 3, and calculate the antimonic acid from the
difference.

c. TIN OF STANNOUS, FROM TIN OF STANNIC COMPOUNDS.

In one portion of the substance convert the whole of the 221
stannous into stannic salts by digestion with chlorine water or
some other means, and determine the total quantity of tin as
directed 126, 1, 5 ; in another portion, which, if necessary,
is to be dissolved in hydrochloric acid in a stream of carbonic
acid, determine the stannous tin according to 126, 2.

II. SEPARATION OF THE ACIDS FROM EACH OTHER.

It must not be forgotten that the following methods of
separation proceed generally upon the assumption that the
acids exist either in the free state, or as alkali salts ; compare the
introductory remarks, (p. 597. Where several acids are to be
determined in one and the same substance, we very often use
a separate portion for each. The methods here given do not
embrace every imaginable case, but only the most important
cases, and those of most frequent occurrence.



First Group.

ARSENOUS ACID ARSENIC ACID CHROMIC ACID SULPHURIC ACID

PHOSPHORIC ACID BORIC ACID OXALIC ACID HYDROFLUORIC

ACID SILICIC ACID CARBONIC ACID.

166.

1. ARSENOUS Aero AND ARSENIC ACID FROM ALL OTHER
Acros.

Precipitate the arsenic from the solution by hydrogen sul- 222
phide ( 127, 4, a or &), filter, and determine the other acids
in the nitrate. It must be remembered, that the arsenous
sulphide will be obtained mixed with sulphur if chromic acid,
ferric salts, or any other substances which decompose hydro-
gen sulphide are present. The estimation of sulphuric ucid
in the nitrate cannot be accurate unless air is excluded, and
oxidizers such as chromic acid are absent ; sulphuric acid is,
therefore, best estimated in a separate portion (223). From
those acids which form soluble magnesium salts, arsenic acid



166.] ACIDS OF GROUP I. 731

may be separated also by precipitation as ammonium magne-
sium arsenate ( 127, 2).

2. SULPHURIC ACID FKOM ALL THE OTHER ACIDS.*

a. From Arsenous, Arsenic, Phosphoric^ Boric, Oxalic,
and Carbonic Acids.

Acidify the dilute solution strongly with hydrochloric acid, 223
mix with barium chloride, and filter the barium sulphate from
the solution, which contains all the other acids. Determine
the barium sulphate as directed 132. If acids are present
with which barium forms salts insoluble in water but soluble
in acids, the barium sulphate is apt to carry down with it such
salts, and this is all the more liable to happen, the longer the
precipitate is allowed to settle. This remark applies especially
to barium oxalate, and tartrate, and the barium salts of
other organic acids (H. ROSE). In such cases I would recom-
mend, after washing, to stop up the neck of the funnel, and
digest the precipitate with a solution of hydrogen sodium car-
bonate, then to wash with water, with dilute hydrochloric
acid, and again with water. In every case, however, the
purity of the weighed barium sulphate must be tested as
directed 132, 1.

In the fluids filtered from the barium sulphate the other
acids are determined according to the directions of the Fourth
Section, after the removal of the excess of barium chloride.
Or the other acids may be estimated in separate portions of
the substan ce, which is indeed usually the best way, and for
carbonic acid is of course the only way.

l>. From Hydrofluoric Acid.

a. When sulphuric acid and hydrofluoric acid are present 224
in the free state in aqueous solution, it is best to estimate the
acidity in one portion by means of standard soda ( 215), and
the sulphuric acid in another ( 132, I., 1), finding the hydro-
fluoric acid by difference. The barium sulphate should be
purified by fusion with sodium carbonate ( 132, I., 1).



* With respect to the separation of sulphuric acid from selenic acid, corap.
WOHLWILL (Annal. d. Chem. u. Pharm., cxiv, 183).

f If metaphosphoric acid is present, it must first be converted into ortho
phosphoric by fusion with alkali carbonate.



732 SEPARATION. [ 166.

ft. To estimate both acids in minerals or other dry sub- 225
stances, it is safest, provided the fluoride can be decomposed
by sulphuric acid, to determine the fluorine in one portion
according to 138, 3, , and to fuse another portion for a
long time with four times its amount of sodium carbonate,
which will decompose the sulphate thoroughly, the fluoride
generally but partially. The fused mass is soaked in water,
the solution filtered, acidified with hydrochloric acid and pre-
cipitated with barium chloride. The barium sulphate thus
obtained generally contains barium fluoride, and must be
purified according to 132, I., 1, by fusion with sodium car-
bonate, &c.

y. An actual separation of both acids may be effected, 226
when both are in the form of alkali suits, by adding sodium
carbonate if necessary, and then precipitating the fluorine
according to 138, I., adding the calcium chloride cautiously
in very slight excess. The sulphuric acid is for the most part
found in the filtrate from the calcium carbonate and fluoride,
a very small part is generally also found in the calcium
acetate filtered from the calcium fluoride. Both filtrates arc
acidified and precipitated with barium chloride ( 132, I., 1.
H. EOSE).

ft. Insoluble compounds may also be decomposed by fusion 227
with six parts of sodium and potassium carbonates, and two
parts of silica. The fused mass, after cooling, is treated with
water, the solution is mixed with ammonium carbonate, and
heated, more ammonium carbonate is added to replace what
evaporates, the silicic acid thrown down is filtered off and
washed with water containing ammonium carbonate, a solu-
tion of zinc oxide in ammonia is added to precipitate the
remaining silica, the fluid is evaporated till all ammonia is
driven off, filtered and the process concluded as in y. The
precipitate produced by the zinc should be tested for sulphuric
acid.

c. From Chromic Acid.

Boil the dry compound with strong hydrochloric acid 228
(p. 357, ft) and estimate the chromic acid from the evolved
chlorine. Neutralize some of the acid with ammonia, dilute
and precipitate the sulphuric acid by long boiling with excess
of barium chloride. The barium sulphate thus obtained



166.] * ACIDS OF GROUP I. 733

retains chromic oxide (H. ROSE) and must always be fused
with sodium carbonate, &c. ( 132, I., 1).

d. From Hydrojluosilicic Acid.

First throw down the hydrofluosilicic acid according to 229
133, as potassium silicofluoride, then the sulphuric acid, in
the filtrate with barium chloride.

e. From Silicic Acid.
Compare 242.

3. PHOSPHORIC ACID FROM THE OTHER ACIDS.

a. From the acids of arsenic, see 222 ; from sulphuric 230
acid, see 223 ; from silicic acid, see 242.

b. From Chromic Acid.

Precipitate the phosphoric acid by adding ammonium
nitrate and ammonia, and then magnesium nitrate, and deter-
mine the chromic acid in the nitrate as directed 130, L, a,
ft or I., I.

c. Froin Boric Acid.

Precipitate the phosphoric acid with a solution of double 231
chloride of magnesium and ammonium ( 134, 6, a), wash the
precipitate partially, redissolve it in hydrochloric acid, repre-
cipitate with ammonia, adding a little magnesium and ammo-
nium chloride, and estimate the phosphoric acid as magnesium
pyrophosphate. In the nitrate estimate the boric acid as
magnesium borate ( 136, I., 1, tf).

d. From Oxalic Acid.

a. If the two acids are to be determined in one portion, 232
the aqueous or hydrochloric solution is mixed with sodium .
auric chloride in excess, heat applied, and the oxalic acid cal-
culated from the reduced gold ( 137, c). The gold added in
excess is separated from the nitrate by hydrogen sulphide, and
the phosphoric acid then precipitated by double chloride of
magnesium and ammonium.

ft. If there is enough of the substance, the oxalic acid is 233
determined in one portion according to 137, &, or d, and the
phosphoric acid in another portion. If the substance is solu-



734 SEPARATION. [ 166.

ble in water, and the quantity of oxalic acid inconsiderable,
the phosphoric acid may be precipitated at once with magne-
sium chloride, ammonium chloride, and ammonia: if not, the
substance is ignited with potassium carbonate and sodium car-
bonate, and the oxalic acid being thus destroyed, the phos-
phoric acid is determined in the nitric acid solution of the
residue according to 134, I., &, ft.

e. From Hydrofluoric Acid.

a. Phosphates and fluorides are frequently found together 234
in minerals. In the analysis of phosphorites, for instance, we
have to estimate small quantities of fluorine, often too in the
presence of aluminium and iron, which increase the difficulty.
According to my own experience,* it is always safest in such
cases to estimate in one portion the fluorine as silicon fluoride
( 138, II., 3, #), and in another portion the phosphoric acid.
Regarding the flrst estimation, it must be mentioned that car-
bonic acid if present must first be removed. To this end heat
the finely powdered weighed substance with water, add acetic
acid in slight excess, and also, if the fluoride present is soluble
in water, some calcium acetate ; evaporate to dryness on a water
bath, treat with water, filter, w r ash the insoluble matter, dry,
separate as far as possible from the filter, add the filter ash,
weigh, test a small portion for carbonic acid by heating with
hydrochloric acid, and weigh the rest for the fluorine estima-
tion. For the estimation of the phosphoric acid, dissolve the
finely powdered substance in hydrochloric acid, evaporate to
dryness on a water-bath, moisten with a little hydrochloric
acid, add nitric acid, warm, dilute, filter, evaporate filtrate and
washings to dryness, dissolve in nitric acid, and proceed
according to 134, I., 5, ft.

ft. Where you have an alkali phosphate and an alkali 235
fluoride together in aqueous solution the phosphoric acid may
be separated according to 135, II., d, ft, as silver phosphate,
or according to 135, II., &, as mercurous phosphate. The
fluoride will be all in the filtrate. If the former method is
adopted the silver is removed from the filtrate by sodium
chloride, and the fluorine estimated as calcium salt ( 138, 1.).

*Zeii*chr. f. analyt. Chem., v, 190, and vi, 403.



166.] - ACIDS OF GROUP I. 735

If the latter method is adopted, as the solution is always acid,
the use of glass and porcelain must be avoided. The mercury
is removed from the filtrate by neutralizing with sodium car-
bonate and without filtering passing hydrogen sulphide.
The fluorine is estimated in the filtrate as calcium salt, accord-
ing to 138, I. (II. KOSE).

y. Substances which are insoluble in water, and cannot 236
be decomposed by acids, are fused with sodium carbonate and
silica (227), the fused mass is treated with water, and Jie
solution with ammonium carbonate. In this way all the
fluorine and all, or nearly all, the phosphoric acid will be
brought into solution. The solution is treated as in 235, and
any remainder of phosphoric acid in the undissolved residue
is estimated according to 234.

tf. In compounds decomposable by water, fluorine may 237
be occasionally estimated indirectly also. Dissolve in hydro-
chloric acid, evaporate with a slight excess of sulphuric acid
until all the hydrofluoric acid has escaped (the amount must
not be increased to a point where the sulphuric acid will be
driven off, otherwise some phosphoric acid will also escape),



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