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tion that EBELMEN,! a long while ago, noticed the separation
of arsenic acid from sesquioxide of iron by ignition in a stream
of hydrogen sulphide.

10. Method based upon the Separation of Arsenic
as Mercurous Arsenate.

ARSENIC ACID FROM ALKALIES, ALKALI EARTHS, ZINC,
COBALT, NICKEL, LEAD, COPPER, AND CADMIUM.

Proceed exactly as in separating phosphoric acid by mer- 184
cury ( 134, 5, y). The arsenic acid can not be determined
in the insoluble residue as is done with phosphoric acid. If
it is to be estimated directly, it must be separated from the



* Zeitschr. /. analyt. Chem., i, 413.

f Annal. de Chim. et de Phys. (3), xxv, 98.



712 SEPARATION. [ 164

mercurous mercury by one of the methods given in this sec-
tion. Treat the filtrate as in 135, &, a (H. KOSE).

1 1 . Method based upon the Separation of Arsenic as
Ammonium Magnesium Arsenate.

ARSENIC ACID FROM COPPER, CADMIUM, FERRIC IRON, MAN-
GANESE, NICKEL, COBALT, ALUMINIUM.

Mix the hydrochloric acid solution, which must contain 185
the whole of the arsenic in the form of arsenic acid, with
enough tartaric acid to prevent precipitation by ammonia, pre-
cipitate the arsenic acid according to 127, 2, as ammonium
magnesium arsenate, allow to settle, filter, wash once with a
mixture of 3 parts water and 1 part ammonia, redissolve in
hydrochloric acid, add a very minute quantity of tartaric acid,
supersaturate again with ammonia, add some more magnesium
chloride and ammonium chloride, allow to deposit, and deter-
mine the now pure precipitate according to 127, 2. In the
filtrate the bases of Groups IY. and V. may be precipitated by
ammonium sulphide ; if aluminium is present, evaporate the
filtrate from the sulphides with addition of sodium carbonate
and a little nitre to dryness, fuse, and estimate the aluminium
in the residue. The method is more adapted to the separation
of rather large than of very small quantities of arsenic from the
above-named metals, since in the case of small quantities the
minute portions of ammonium magnesium arsenate that remain
in solution may exercise a considerable influence on the accu-
racy of the result.

12. Method based upon the Separation of Arsenic a
Ammonium Arsenio-molybdate.

ARSENIC ACID FROM ALL METALS OF GROUPS I. Y.

Separate the arsenic acid as directed in 127, 2, b ; long 186
continued heating at 100 is indispensable. The determination
of the basic metals is most conveniently effected in a special
portion.



164.] METALS OF GROUP VI. 713

13. Method based upon the Insolubility of Ferric
Ar senate.

ARSENIC ACID FROM THE METALS OF GROUPS I. AND II.,

AND FROM ZlNC, MANGANESE, NlCKEL, AND COBALT.

Mix the hydrochloric-acid solution with a sufficient quantity 187
of pure ferric chloride, neutralize the greater part of the free
acid with sodium carbonate, and precipitate the iron and arse-
nic acid together with barium carbonate in the cold or with
sodium acetate at a boiling heat. The precipitate should be so
basic as to have a brownish-red color. The method is espe-
cially suitable for the separation of arsenic acid when its esti-
mation is not required. However, the precipitate may be dis-
solved in hydrochloric acid and the arsenic determined by
precipitation with hydrogen sulphide.

14. Methods based upon the Insolubility of some
Chlorides.

a. SILVER FROM GOLD.

Treat the alloy with cold dilute nitrohydrochloric acid, 188
dilute, and filter the solution of auric chloride from the undis-
solved silver chloride. This method is applicable only if the
alloy contains less than 15 per cent, of silver ; for if it contains
a larger proportion, the silver chloride which forms protects
the undecomposed part from the action of the acid. In the
same way silver may be separated also from platinum.

b. . MERCURY FROM THE OXYGEN COMPOUNDS OF ARSENIC
AND ANTIMONY.

Precipitate the mercury from the hydrochloric solution by 189
means of phosphorous acid as mercurous chloride ( 118, 2).
The tartaric acid, which in the presence of antimony must 'be
added, does not interfere with the reaction (H. ROSE*).

15. Methods based upon the Insolubility of certain
Sulphates in Water or Alcohol.

a. ARSENIC ACID FROM BARIUM, STRONTIUM, CALCIUM, AND
LEAD.

Proceed as for the separation of phosphoric acid from the 190
same metals ( 135, b). The compounds of these basic radicals
with arsenous acid are first converted into arsenates, before

* Pogg. Annal., ex, 536,



714 SEPARATION. [ 164.

the sulphuric acid is added ; this conversion is effected by
heating the hydrochloric acid solution with potassium chlo-
rate or by means of bromine.

b. ANTIMONY FROM LEAD.

Treat the alloy with a mixture of nitric and tartaric acids. 191
The solution of both metals takes place rapidly and with case.
Precipitate the greater part of the lead as sulphate (116, 3),
filter, precipitate with hydrogen sulphide, and treat the sul-
phides according to 168, with ammonium sulphide, in order
to separate the antimony from the lead left unprecipitated by
the sulphuric acid (A. STRENG *).

16. Method based upon the Separation of Copper as
Cuprous Sulphocyanate.

COPPER FROM ARSENIC AND ANTIMONY.

From the properly prepared solution precipitate the cop- 192
per by 119, 3, #, as cuprous sulphocyanate, allow to settle,
filter, wash with water containing ammonium nitrate (to pre-
Tent the washings being muddy), and determine antimony
and arsenic in the filtrate, preciptating first with hydrogen
sulphide. Results good.

The following method, depending upon the precipitation
of the copper as an iodide, is not good. Dissolve in nitric or
sulphuric acid, taking care to have a slight excess of acid,
dilute with pure water (if antimony is present use water con-
taining some tartaric acid), add sulphurous acid, and precipi-
tate the copper with potassium iodide as cuprous iodide.
Arsenic and antimony remain in solution (FLAJOLOT).
Results are approximate, because the liquid retains some
cuprous iodide in solution in consequence of the presence of
the sulphurous acid. It is impracticable to use stannous
chloride for reducing the copper as recommended by
FLEISCHER,! because then the separation of the tin from the
arsenic and antimony would be too difficult.

17. Method based upon the Precipitation of Cop-
per as an Oxalate.
COPPER FROM ARSENIC.
Add ammonia to the nitric-acid solution until the blue 193
precipitate no longer redissolves, and then effect solution by

* Dingl. polyt. Journ., cu, 389. -\ Zeitschr. f. anatyt. Chem., ix, 256.



165.] METALS OF GROUP VI. 715

an excess of ammonium oxalate. Now add very cautiously
hydrochloric or nitric acid to- acid reaction, and allow to
stand. The copper separates almost completely as oxalate,
which is converted iuto oxide by ignition in the air. Make
the filtrate ammoniacal and precipitate the trace of copper
still in solution by adding a few drops of ammonium-sul-
phide solution (F. FIELD *).

18. Method based upon the different Deportment
with Potassium Cyanide.

GOLD FROM LEAD AND BISMUTH.

These metals may be separated in solution by potassium 194
cyanide in the same way in which the separation of mercury
from lead and bismuth is effected (see 147). The solution of
the double cyanide of gold and potassium is decomposed by
boiling with aqua regia, and, after expulsion of the hydro-
cyanic acid, the gold determined by one of the methods
given in 123.

II. SEPARATION OF THE METALS OF THE SIXTH GROUP

FROM EACH OTHER.

165.

INDEX. (The numbers refer to those in the margin.)
Platinum from gold, 195, 214, 215.

" tin, antimony, and arsenic, 196.

Gold from platinum, 195, 214, 215.

tin, 196, 213.

" antimony and arsenic, 196.
Tin from platinum, 196.

gold, 175, 196, 213.

arsenic, 199, 206, 207, 208, 211, 212, 216, 217.
" antimony, 197, 201, 208, 209, 210, 212, 216.

Tin in stannous from tin in stannic compounds, 221.
Antimony from platinum and gold, 196.

arsenic, 200, 201, 202, 203, 204, 206, 207.
tin, 197, 201, 208, 209, 210, 212, 216.
Antimony of antimonous compounds from autimonic

acid, 220.
Arsenic from platinum and gold, 196.

tin, 199, 206, 207, 208, 211, 212, 216, 217.
" antimony, 200, 201, 202, 203, 204, 206, 207, 218.
Arsenous acid from arsenic acid, 198, 205, 219.

* Chem. Gaz., 1857, 313; Journ.f. prakt. Chem., LXXII, 183.



716 SEPARATION. [ 165.

1. Method based upon the Precipitation of Plati-
num as Potassium Platinic Chloride.

PLATINUM FROM GOLD.

Precipitate from the solution of the chlorides the plati- 195
num as directed 124, 5, and determine the gold in the filtrate
as directed 123, I.

2. Methods based upon the Volatility of the Chlo-
rides of the inferior Metals.

a. PLATINUM AND GOLD FROM TIN, ANTIMONY, AND ARSENIC.

Heat the finely divided alloy or the sulphides in a stream 196
of chlorine gas. Gold and platinum are left, the chlorides of
the other metals volatilize (compare 160).

J. ANTIMONY FROM TIN.

The tin should be present wholly as a.stannous salt. 197
Precipitate with hydrogen sulphide, filter (preferably through
an asbestos filtering tube), dry the precipitate, and pass through
it a current of dry hydrochloric gas at the ordinary tempera-
ture. The sulphides are converted into the corresponding
chlorides ; the chloride of antimony alone escapes, and may
be received in water. Dissolve the residual stannous chloride
in water containing hydrochloric acid, and estimate the tin
according to 126 (C. TOOKEY*). The method can only be
used in rare cases, as it is difficult to obtain a precipitate quite
free from stannic sulphide.

c. ARSKNOUS ACID FROM ARSENIC ACID.

The amount of substance taken should not contain more 198
than 0*2 grin, arsenous acid. Heat with 45 grm. sodium
chloride, 135 grm. sulphuric acid (free from arsenic) of 1-61
sp. gr., and 30 grm. water in a tubulated retort containing a
spiral of platinum, and provided with a thermometer. The
temperature should rise to about 125. In order to condense
the arsenous chloride in the products of distillation, a LIEBIG'S
condenser is connected with the retort ; a tubulated receiver
is connected with the condenser ; a U-tube is connected with
the receiver, and finally a calcium chloride tube containing
fragments of glass moistened with weak soda solution is fixed

* Jour it. Chem. Soc., \\, 462.



165.] MfeTALS OF GROUP VI. 717

upright in the exit end of the U-tube. In the receiver and
U-tube water is placed. I can recommend the apparatus
shown in Fig. 78. At the end of the operation rinse
the calcium chloride tube, and mix with the contents of the
receiver. Determine the arsenic in the distillate according to
127, 4, #, in the residue according to 127, 4, b. The sul-
phide obtained from the former corresponds to the arsenous
acid, from the latter to the arsenic acid. Results satisfactory
(RiECKHEft*). If the substance given is a dilute fluid, render
slightly alkaline with sodium carbonate, and concentrate to
about 20 c.c., finally in a tubulated retort.

3. Methods based upon the Volatility of Arsenic
and Arsenous Sulphide.

a. AKSENIC FROM TIN (H. ROSE).

Convert into sulphides or oxides, dry at 100, and heat a 199
weighed portion with addition of a little sulphur in a bulb-
tube, gently at first, but gradually more strongly, conducting
a stream of dry hydrogen sulphide gas through the tube
during the operation. Sulphur and arsenous sulphide vola-
tilize; sulphide of tin is left. The arsenous sulphide is
received in U-tubes containing dilute ammonia, which are
connected with the bulb-tube in the manner described in 160.
When upon continued application of heat no sign of further
sublimation is observed in the colder part of the bulb-tube,
drive off the sublimate which has collected in the bulb, allow
the tube to cool, and then cut it off above the coating. Divide
the separated portion of the tube into pieces, and heat these
with a little solution of soda until the sublimate is dissolved;
unite the solution with the amrnoniacal fluid in the receivers,
add hydrochloric acid, then, without filtering, potassium
chlorate, and heat gently until the arsenious sulphide is com-
pletely dissolved. Filter from the sulphur, and determine the
arsenic acid as directed 127, 2. The quantity of tin cannot
be calculated at once from the blackish-brown sulphide of tin
in the bulb, since this contains more sulphur than SnS. It is
therefore weighed, and the tin determined in a weighed por-
tion of it, by converting it into stannic oxide, which is effected
by moistening with nitric acid, and roasting ( 12f>, 1, c).
* Pharm. t'entralhalle, xi, l>2.



718 SEPARATION. [ 165.

Tin and arsenic in alloys are more conveniently converted
into oxides by cautious treatment with nitric acid. If, how-
ever, it is wished to convert them into sulphides, this may
readily be effected by heating 1 part of the finely divided
alloy with 5 parts of sodium carbonate and 5 parts of sulphur,
in a covered porcelain crucible until the mass is in a state of
calm fusion. It is then dissolved in water, the solution filtered
from the ferrous sulphide, &c., which may possibly have
formed, and then precipitated with hydrochloric acid.

If the tin only in the alloy is to be estimated directly,
while the arsenic is to be found from the difference, convert
as above directed into sulphides or oxides, mix with sulphur
and ignite in a porcelain crucible with perforated cover in
a stream of hydrogen sulphide. The residual arsenic-free
stannous sulphide is to be converted into stannic oxide and
weighed as such.

b. ARSENIC FROM ANTIMONY IN ALLOYS.

Heat a weighed portion of the substance with two parts 200
of sodium carbonate and two parts of potassium cyanide in a
bulb-tube through which dry carbonic acid is being trans-
mitted. Heat at first gently, then more and more strongly,
and until no more arsenic volatilizes. (Take great care not
to inhale the escaping fumes. It is advisable to insert
the open end of the tube into a flask in which sublimed
arsenic will condense.) After cooling, treat the contents of
the bulb first with a mixture of equal volumes of alcohol and
water, then with water alone, and finally weigh the residual
antimony. The arsenic is found from the loss. The results
are only approximate. If it is desired to fuse the alloy,
itself not under a slag, in a current of carbonic-acid gas,
the heating must be very carefully done, otherwise much
antimony will volatilize. II. ROSE recommends the latter
process.

4. Met/tods based upon the Insolubility of Sodiin/i
Metantimonate.

a. ANTIMONY FROM TIN AND ARSENIC (H. ROSE).

If the substance is metallic, oxidize the finely divided 201
weighed sample in a porcelain crucible with nitric acid of



165.] METALS OF GROUP VI. 719

1'4 sp. gr., adding the acid gradually. Dry the mass on the
water-bath, transfer to a silver crucible, rinsing the last par-
ticles adhering to the porcelain into the silver crucible with
solution of soda, dry again, add eight times the bulk of the
mass of solid sodium hydroxide, and fuse for some time.
Allow the mass to cool, and then treat with hot water until
the undissolved residue presents the appearance of a fine
powder ; dilute with some water, and add one-third the volume
of alcohol of 0-83 sp. gr. Allow the mixture to stand for 24
hours, with frequent stirring; then filter, transfer the last
adhering particles from the crucible to the filter by rinsing
with dilute alcohol (1 vol. alcohol to 3 vol. water), and wash
the undissolved residue on the filter, first with alcohol diluted
with twice its volume of water, then with a mixture of equal
volumes of alcohol and water, and finally with a mixture of
3 vol. alcohol and 1 vol. water. Add to each of the alcoholic
fluids used for washing a few drops of solution of sodium
carbonate. Continue the washing until the color of a portion
of the fluid running off remains unaltered upon being acidi-
fied with hydrochloric acid and mixed with hydrogen- sul-
phide water.

Rinse the sodium metantimonate from the filter, wash the
latter with a mixture of hydrochloric and tartaric acids, dis-
solve the metantimonate in this mixture, precipitate with
hydrogen sulphide, and determine the antimony as directed
in 125, 1. In presence of much tin it is well to fuse the
metantimonate again with caustic soda, etc.

To the filtrate, which contains the tin and arsenic, add
hydrochloric acid, which produces a precipitate of stannic
arsenate; conduct now into the unfiltered fluid hydrogen
sulphide for some time, allow the mixture to stand at rest
until the odor of that gas has almost completely gone off, and
separate the weighed sulphides of the metals which contain
free sulphur, as in 199.

If the substance contains only antimony and arsenic, the
alcoholic filtrate is heated, with repeated addition of water,
until it scarcely retains the odor of alcohol ; hydrochloric acid
is then added, and the arsenic acid determined as magnesium
pyroarsenate ( 127, 2), or as arsenous sulphide ( 127, 4, J).



720 SEPARATION. [ 165.

I. Small quantities of the sulphides of arsenic arid anti- 202
niony mixed with sulphur are often obtained in mineral
analysis. The two metals may in this case be conveniently
separated as follows: Exhaust the precipitate with carbon
disulphide, oxidize with chlorine-free red fuming nitric acid,
evaporate the solution nearly to dryness ; mix the residue
with a copious excess of sodium carbonate, add some sodium
nitrate, and treat the fused mass as given in 201, a. If, on
the other hand, you have a mixture of sulphides of tin and
antimony to analyze, oxidize it with nitric acid of 1*5 sp. gr.,
and treat the residue obtained on evaporation as given in
201, a.

c. DETERMINATION OF ARSENIC SULPHIDE IN COMMERCIAL
ANTIMONY SULPHIDE (WACKENRODER).

Deflagrate 20 grm. of the finely triturated antimony sul- 203
phide with 40 grm. sodium nitrate and 20 grm. sodium car-
bonate by projecting the mixture in portions into a red-hot
Hessian crucible, then extract the strongly ignited mass by
repeated treatment with water, acidulate the filtrate with
hydrochloric acid, add some sulphurous acid, and precipitate
the arsenic together with a small part of the antimony by
means of hydrogen sulphide. Digest the still moist precipi-
tate with ammonium carbonate, filter, acidulate the filtrate,
conduct in hydrogen sulphide, and determine the arsenic as
arsenic sulphide according to 127, 4.

5. Methods based upon the Precipitation of Arsenic
as Ammonium Magnesium Arsenate.

a. ARSENIC FROM ANTIMONY.

Oxidize the metals or sulphides with nitrohydrochloric 204
acid, with hydrochloric acid and potassium chlorate, with
bromine dissolved in hydrochloric acid, or with chlorine in
alkaline solution, page 568, J; add tartaric acid, a large
quantity of ammonium chloride, and then ammonia in excess.
(Should the addition of the latter reagent produce a precipi-
tate, this is a proof that an insufficient quantity of ammo-
nium chloride or of tartaric acid has been used, which error
must be corrected before proceeding with the analysis.)



165.] * METALS OF GROUP VI. 721

Then precipitate the arsenic acid as directed in 127, 2, and
determine the antimony in the filtrate as directed in 125, 1.
As basic magnesium tartrate might precipitate with the
ammonium magnesium arsenate, the precipitate should
always, after slight washing, be redissolved in hydrochloric
acid, and reprecipitated with ammonia with addition of a
little magnesia mixture. An excellent method.

J. ARSEXOUS ACID FROM ARSENIC ACID.

Mix the sufficiently dilute solution with a large quantity 205
of ammonium chloride, precipitate the arsenic acid as directed
in 127, 2, and determine the arsenous acid in the filtrate by
precipitation with hydrogen sulphide (127, 4). LUDWIG* has
observed that if the solution is too concentrated, magne-
sium arsenite falls down with the ammonium .magnesium
arsenate, hence it is necessary to dissolve the weighed magne-
sium precipitate in hydrochloric acid and test the solution with
hydrogen sulphide. The presence of arsenous acid will be
betrayed by the immediate formation of a precipitate.

c. TIN AND ANTIMONY FROM ARSENIC ACID.



separated tin from arsenic acid with good 206
results by digesting the oxides obtained by oxidation with
nitric acid with ammonia and yellow ammonium sulphide, and
precipitating the arsenic acid from the clear solution accord-
ing to 127, 2, as ammonium magnesium arsenate. On acidify-
ing the filtrate the tin separates as stannic sulphide. The
method can only give good results when the whole of the
arsenic was present as arsenic acid before the addition of
ammonium sulphide, for the arsenic in a solution of arsenous
acid in yellow ammonium sulphide is not thrown down by
magnesia mixture. The method also answers for separating
antimony from arsenic.

* ArcMvfur P/tarm., xcvn, 24. f Annal. d. Chem. u. Pharm., cxiv, 116.



722 SEPARATION. [ 165.

6. Methods based on the different behavior of the
freshly Precipitated Sulphides towards Solution of
Potassium Hydrogen Sulphite or Oxalic Acid.

a. ARSENIC FROM ANTIMONY AND TIN (BuNSEN*).

If freshly precipitated arsenous sulphide is digested with 207
sulphurous acid and potassium sulphite, the precipitate is dis-
solved ; on boiling, the fluid becomes turbid from separated
sulphur, which turbidity for the most part disappears again on
long boiling. The fluid contains, after expulsion of the sul-
phurous acid, potassium arsenite and thiosulphate. The sul-
phides of antimony and tin do not exhibit this reaction. Both
therefore may be separated from arsenous sulphide by diluting
the solution of the three sulphides in potassium sulphide to about
500 c.c. and precipitating with a large excess (about a litre) of
saturated aqueous sulphurous acid, digesting the whole for
some time in a water-bath, and then boiling till one-third of
the water and the whole of the sulphurous acid are expelled
and the sulphur has disappeared ; this will take about an hour
and a half. The residuary sulphide of antimony or tin is arsenic-
free, the filtrate contains the whole of the arsenic and maybe
immediately precipitated with hydrogen sulphide. BUNSEN
determines the arsenic by oxidizing the dried sulphide together
with the filter with fuming nitric acid, diluting the solution
a little, warming gently with a little potassium chlorate (in
order to oxidize more fully the substances formed from the
paper), and finally precipitating as ammonium magnesium
arsenate.

With regard to the separation of stannic sulphide from
the solution of potassium arsenite, it is to be observed that the
stannic sulphide must be washed with concentrated solution
of sodium chloride, as, if water were used, the fluid would run
through turbid. As soon as the precipitate is thoroughly
washed with the sodium chloride, the latter is displaced by
solution of ammonium acetate, containing a slight excess of
acetic acid. These last washings must not be added to the
first, as the ammonium acetate hinders the complete precipita
of the arsenous acid by hydrogen sulphide.

*Annal. d. Chem. u. Pharm., cvi, 3.



165.] METALS OF GROUP VI. 723

The test-analyses adduced by BUNSEN show very satisfac-
tory results.

b. TIN FROM ARSENIC AND ANTIMONY (F. TV. CLARKE *).

Moist freshly precipitated tin disulphide completely dis- 208
solves on boiling for a moderate length of time with excess of
oxalic acid, and therefore tin in the form of dichloride is not
thrown down by hydrogen sulphide from a hot solution con-
taining excess of oxalic acid. The sulphides of arsenic are
barely affected by boiling with oxalic acid, and hydrogen sul-
phide immediately re precipitates the traces dissolved. Anti-
mony sulphide dissolves more copiously on boiling with oxalic
acid, but hydrogen sulphide reprecipitates the antimony
from the solution.

CLARKE hence recommends adding to the solution of the
three metals (and in which the tin is present in the form of
dioxide) oxalic acid in twenty times the weight of the tin.
The solution must be so concentrated that on cooling the oxalic
acid crystallizes out. Now conduct into the solution, main-
tained at a boiling heat, hydrogen-sulphide gas for 20 min-
utes, let stand for half an hour in a warm place and filter.
According to CLARKE all the arsenic and antimony, free from
tin sulphide, or at least nearly so, is thus precipitated, while
all the tin remains in solution. The tin is obtained by mak-
ing the solution weakly alkaline with ammonia and adding
ammonium sulphide until the precipitate first formed redis-
solves ; then decompose the sulphosalt with an excess of acetic



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