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oxide (chiefly copper oxide) the precipitate is fused with a mix-
ture of sodium carbonate and sulphur as described on p. 228,
The sulphides, remaining after the solution of the melt in hot
water, are filtered off, converted into oxides by ignition in the
air, and weighed. By subtracting this weight from that pre*
viously obtained, the weight of SnO 2 +P 2 5 is obtained. In order
to obtain the weight of the Sn0 2 a separate portion is analyzed
according to the method of Oettel as described below for phos-
phoric acid, and the amount of phosphoric anhydride subtracted
from the weight of Sn0 2 + P 2 O 5 .

The oxides obtained by the ignition of the insoluble sulphides
are dissolved in a little nitric acid (in case Fe 2 3 is present a
little hydrochloric acid is also necessary) and the solution of the
nitrates is added to the first filtrate from the impure metastannic
acid. To this solution an excess of dilute sulphuric acid is added,
and it is evaporated on the water-bath as far as possible and then
heated over a free flame until dense, white fumes of sulphuric
acid are evolved. After cooling, 50 c.c. of water and 20 c.c. of
alcohol are added, the precipitate of lead sulphate is filtered off
and its weight determined as described on p. 174. The filtrate
from the lead sulphate is heated to remove the alcohol and the
copper precipitated by means of hydrogen sulphide and weighed
as Cu 2 S according to p. 183. In the filtrate from the copper sul-
phide the iron, aluminium, and zinc (also manganese) will be
found. It is evaporated to a small volum.3 in order to expel the
hydrogen sulphide, oxidized by the addition of a few drops of
concentrated nitric acid, and the iron and aluminium separated
from the zinc by means of a double precipitation with ammonia,*
whereby the iron and aluminium are left behind as hydroxides

* If considerable zinc is present, the above separation is inexact. In this
case the filtrate from the copper sulphide is treated with sodium acetate,
heated to 60, saturated with hydrogen sulphide, and the iron and aluminium
determined in the filtrate, the zinc in the precipitate. If manganese is
present in the alloy, it should be separated from iron and aluminium as
described on pp. 149 to 155.



238 GRAVIMETRIC ANALYSIS.

and are separated and determined according to p. 107. The
zinc is precipitated from the filtrate after acidifying with acetic
acid, by passing hydrogen sulphide into the boiling solution. The
precipitated zinc sulphide is filtered off, dissolved in hydrochloric
acid, evaporated to dry ness in a weighed platinum dish, and trans-
formed to oxide by heating with mercuric oxide by Volhard's
method (cf. p. 142).

For the phosphorus determination Oettel * recommends the fol-
lowing procedure: From 25 gms. of the substance are dissolved,
as before, in nitric acid, and the impure metastannic acid with all
the phosphorus is filtered off, dried, and transferred as com-
pletely as possible to a porcelain crucible. The ash of the filter
is added, and the contents of the crucible ignited. After cooling,
the substance is mixed with three times as much solid potassium
cyanide, the crucible covered, and the contents fused; the stannic
oxide is reduced to metal,

SnO 2 + 2KCN - 2KCNO -f Sn,

while the phosphoric acid is converted into potassium phos-
phate.

By skilfully rotating the crucible during the fusion, it is possi-
ble to unite the small particles of molten tin into a larger button
whereby the subsequent filtration is greatly facilitated. After
cooling, the melt is treated with water and filtered. The filtrate
is cautiously treated with hydrochloric acid under a good hood
and boiled to remove the hydrocyanic acid. It is then saturated
with hydrogen sulphide in order to remove traces of copper and
tin which almost always remain in the solution. The filtrate is
freed from hydrogen sulphide by boiling, made ammoniac al, and
the phosphoric acid precipitated as magnesium ammonium phos-
phate oy the addition of magnesia mixture. After standing for
twelve hours, the latter is filtered off, washed with 2J per cent,
ammonia water, dried, and changed by ignition to magnesium
pyrophosphate, in which form it is weighed.

* Chemiker-Zeitung (1896), p. 19.



DETERMINATION OF PHOSPHORUS. 239

Ordinary bronzes may be analyzed very nicely in the fol-
lowing manner: The alloy is treated with nitric acid as described
above, the metastannic acid removed by filtration and the
filtrate electrolyzed ; using a dull platinum dish as cathode, and
a plate as anode, both of which are weighed. The electrolysis
is carried out with a current of 1 to 1.2 amperes at about 60 and
at the end of two and one-half to three hours the electrodes are
washed without breaking the circuit. On the anode will be found
all the lead as PbO 2 and on the cathode will be found the copper.
The siphoned solution contains the iron, aluminium and zinc,
which are determined as above. The phosphorus is determined
in a special sample.

Remark. The method just outlined will give exact results
only when the metastannic acid is purified and the recovered
solution of copper and lead nitrates added to the main solution.
In the electrolysis, the chief dangers to be feared are having
the solution so acid that the copper is not all precipitated, or
so dilute that a spongy deposit is obtained.

Method II.

An excellent method for the analysis of phosphor bronze con-
sists in dissolving the alloy as described under Tin, Method III., p.
231, and determining the tin as there described. The copp3r and
lead are determined in the nitric acid solution by electrolysis with a
current of 0.2 ampere, the copper being deposited on the cathode
and the lead as peroxide on the anode. The electrolysis is usually
finished in twelve hours, but it is well to clean the electrodes after
weighing the deposits and then to test the solution with the current
for an hour or so longer to see whether any lead or copper remains
in the solution. Often a little more copper will be found, especially
if the solution was a little too acid. During the electrolysis the
concentration of the acid gradually diminishes so that eventually
all the copper will be thrown down. The iron, aluminium, and
zinc remain in solution, and are determined as above outlined.

For the phosphorus determination,* one gm. of the borings
is weighed into a small beaker and dissolved in 20 c.c. of aqua

* Cf. Dudley and Pease, Eng. and R. R. Journ., March, 1894.



*40 GRAVIMETRIC ANALYSIS.

regia, made by mixing equal volumes of the concentrated acids
just previous to use. The beaker is covered with a watch-glass,
and, after solution is complete, the contents heated nearly to boil-
ing for fifteen minutes. After cooling, 25 c.c. of water are added,
and then just sufficient ammonia (sp. gr. 0.90) to redissolve
the copper hydroxide* and to produce a deep blue colored
solution; thereupon 50 c.c. of colorless ammonium sulphide are
introduced. This should be enough to precipitate the sulphides,
and the supernatant liquid should show no blue color. If it
does, more ammonium sulphide must be added. The solution
is digested at a temperature near the boiling-point for fifteen
minutes, the precipitated sulphides of copper and lead allowed to
settle, and then filtered into a 300 c.c. Erlenmeyer flask, decanting
the clear liquid carefully from the precipitate, and finally throwing
the precipitate upon the filter. When the filter has drained the
filter and precipitate is returned to the beaker, 50 c.c. of ammonium
sulphide wash water (one part colorless ammonium sulphide to
three parts of water) are added, and the mixture is heated, and
stirred occasionally, for ten minutes; it is then poured upon
another filter, washed with 50 c.c. of ammonium sulphide wash
water and allowed to drain completely. The total volume should
not be over 250 c.c., but it is not necessary to evaporate in case
this volume is slightly exceeded. To the filtrate 10 c.c. of mag-
nesia mixture are added and the solution shaken. The flask is
placed in ice water and allowed to stand with occasional shaking
for two hours. The precipitate of magnesium ammonium phos-
phate is filtered upon a small filter and washed with ammonia
water (one part 0.96 sp. gr. ammonia to three parts water) until
nearly free from sulphide. 10 c.c. of hydrochloric acid (one part
HC1, sp. gr. 1.20, to four parts water) are placed in the flask,
taking care that all of the precipitate adhering to the walls of the
flask is dissolved, and then poured through the filter, allowing the
solution to run into a No. 1 beaker. The flask and filter are
washed with 10 c.c. more of the same acid. 3 c.c. of magnesia
mixture are added to the filtrate, which is heated to boiling,
removed from the flame, and then treated with ammonia (sp. gr.
0.96 = 10 per cent. NH 3 ) until the latter is present in large excess.
* A precipitate of lead or tin hydroxide remains insoluble.



SEPARATION OF ARSENIC FROM ANTIMONY. 241

The solution is allowed to stand in ice water for two hours, and is
stirred occasionally. The precipitate is then filtered arid washed
with 2J per cent, ammonia water until free from chlorides, and
ignited with the usual precautions, weighing as



SEPARATION OF THE SULPHO-ACIDS FROM ONE ANOTHER.
i. Arsenic from Antimony.

(a) Method of Bunsen*

Principle. If a slightly acid solution of an alkali arsenate
and antimonate is treated with hydrogen sulphide in the cold
and the excess of the latter immediately removed by conduct-
ing air through the solution, the antimony is quantitatively
precipitated as pentasulphide, while the arsenic remains in solu-
tion.

Procedure. Assume the arsenic and antimony to be present
in the solution as arsenious and antimonous acids. Both ele-
ments are precipitated by hydrogen sulphide, filtered, and washed
with water. The greater part of the precipitate is transferred
by means of a spatula to a 200-c.c. porcelain casserole, and the
precipitate remaining on the filter is dissolved into the casserole
by dropping a solution of hot dilute pure caustic potash upon it.
From 3-5 gms. of pure solid caustic alkali are added, and the
precipitate dissolves to a clear solution. f

The casserole is now covered with a perforated watch-glass.
It is placed upon the water-bath, and chlorine is conducted into
the solution until all the alkali is decomposed; this takes from
one-half to three-quarters of an hour. By this operation the
arsenite and antimonite are oxidized to arsenate and antimonate
and a small amount of potassium chlorate is formed. Concen-
trated hydrochloric acid is now added to the warm solution drop by
drop from a pipette until all the chlorate is decomposed and no
more chlorine is evolved. The watch-glass is removed, the solu-
tion is evaporated to half its volume, and then an equal amount of
concentrated hydrochloric acid is added and the solution again

* Ann. d. Chem. imd Pharm., 102, 305.

f If alkaline earths were the only metals present besides the arsenic and
antimony, the first precipitation with hydrogen sulphide would be omitted.



242 GRAVIMETRIC ANALYSIS.

evaporated to half its volume. The contents of the casserole
are washed by means cf dilute hydrochloric acid into a large beaker,
diluted with water to a volume of 600 c.c. and for every decigram
or less of the antimony 100 c.c. of freshly prepared hydrogen
sulphide water are added. An orange precipitate of antimony
pentasulphide is formed at the end of a short time. A strong
current of air (filtered through a wad of cotton) is then passed
through the solution without delay until the excess of hydrogen
sulphide is completely removed; this usually requires about
twenty minutes. In order to avoid loss during this operation
a large beaker should be used to contain the solution and it should
be covered with a perforated watch-glass. The precipitate of
antimony pentasulphide is likely to contain traces of arsenic
pentasulphide so that it is dissolved once more in caustic
potash and the above operation repeated. The precipitate now
obtained will be pure antimony pentasulphide. It is filtered
through a Gooch crucible, dried at 280 C. in a stream of car-
bon dioxide as described under antimony, and weighed as
Sb 2 S 3 .*

For the arsenic determination, the combined filtrates are con-
centrated somewhat by evaporation, a few drops of chlorine;
water are added and hydrogen sulphide is passed into the warm
solution (being kept on the water-bath) for from six to eight
hours, after which it is allowed to cool in a rapid stream of
hydrogen sulphide. After allowing the precipitate to settle for
twenty-four hours, it is filtered through a Gooch crucible, washed
with water, then three- times with alcohol, four times with a mix-
ture of pure carbon bisulphide and alcohol (cf. p. 180), and
finally three times with pure alcohol. After drying at 110 C.,
the precipitate is weighed as As 2 S 5 .

Remark. If the solution contains no very large excess of
hydrogen sulphide, the precipitate will always contain trisulphide,
so that it is safer to dissolve it in ammoniacal hydrogen per-

* Bunsen weighed the antimony as pentasulphide after washing with car-
bon bisulphide. As, however, antimony pentasulphide is likely to be changed
to the trisulphide on treating with carbon bisulphide, the above procedure
is better. According to Braun, Sb 2 S 3 is reduced to Sb 2 S 2 on long-continued
treatment with CS 2 .



SEPARATION OF ARSENIC FROM ANTIMONY. ' 243

oxide * and then to precipitate the arsenic with magnesia mix-
ture as magnesium ammonium arsenate, as described on p. 206,
weighing it as Mg2As207.

Remark. The method gives very accurate results, but con-
sumes considerable time.



(6) Method of Fred. NeherJ

This, in the author's estimation, the best method for the separa-
tion of arsenic and antimony, depends upon the fact that arsenic
is precipitated from a solution strongly acid with hydrochloric
acid by a rapid stream of hydrogen sulphide, while antimony is
not.

Procedure. Starting with a precipitate consisting of the tri-
sulphides of arsenic and antimony, this is dissolved in caustic
potash solution and oxidized exactly as described under the
previous method. When free from chlorate, the acid solution
is washed into an Erlenmeyer flask and cooled by surround-
ing the flask with ice. In another flask some concentrated
hydrochloric acid (sp. gr. 1.2) is likewise cooled. When both
solutions are at C., the arsenic antimony solution is diluted
with twice its volume of the strong hydrochloric acid. Into
this cold solution a rapid stream of hydrogen sulphide is passed
for one and one-half hours. The flask is stoppered up and allowed
to stand one to two hours. The As 2 S 5 is filtered through a Gooch
crucible and washed with hydrochloric acid (1 vol. water, 2 vols. con-
centrated hydrochloric acid) until 1 c.c. of the filtrate after being
considerably diluted with water and tested with hydrogen sul-
phide shows no precipitation. It is then washed with water, and



* For this purpose as much of the precipitate as possible is placed in a
beaker, the portion adhering to the filter is dissolved by hot ammonia into
the same beaker, and this is warmed until the precipitate has entirely dis-
solved. After this, for every 0.1 gm. of As 2 S 5 , 30-50 c.c. of pure 3 per cent.
H 2 O 2 are added, the solution heated for some time on the water-bath and
then boiled ten minutes.

t Z. anal. Chem., 32, 45.



244 GRAVIMETRIC ANALYSIS.

finally with hot alcohol. After drying at 110 C., the precipitate
is weighed as A^Ss. *

The filtrate from the arsenic sulphide is diluted largely with
water and saturated with hydrogen sulphide. The Sb2S 5 is filtered
through a Gooch crucible, dried at 280 C. in a current of carbon
dioxide and weighed.

(c) The Tartaric Acid Method.

Principle. The separation is based upon the fact that if
magnesia mixture is added to a solution of an alkali arsenate
and antimonate containing tartaric acid, only arsenic will be pre-
cipitated.

Procedure. Tha sulphides are oxidized as described under
(a) by solution in aqueous caustic potash and introduction of
chlorine. The solution thus obtained is made acid, treated with
tartaric acid and an excess of ammonia added. This should
not cause any turbidity. If a precipitate is formed, it shows that
an insufficient amount of tartaric acid is present. In this case
the clear solution is decanted off, the precipitate is dissolved by
warming with tartaric acid, and the two solutions are mixed.
To the clear, ammoniacal solution, magnesia mixture is added
slowly with constant stirring (cf. p. 206. foot-note). After stand-
ing twelve hours, the precipitate of magnesium ammonium
arsenate is filtered off (it usually contains a little basic mag-
nesium tartrate), washed a few times with 2J per cent, ammonia,
dissolved in hydrochloric acid, and reprecipitated by the addi-
tion of an excess of ammonia. After standing for twelve hours
more, the precipitate is filtered, washed with 2 per cent,
ammonia, and weighed as magnesium pyroarsenate as described
on p. 206,

* If the solution was not cold, some arsenic trisulphide will be found
in the precipitate. The results are scarcely affected, however, when the
precipitate is merely washed with water and alcohol, because the free sulphur
is weighed with the sulphide of arsenic. If, however, the precipitate is
washed with CS 3 , it is evident that the results will be too low. For the
highest degree of accuracy, it is advisable to dissolve the precipitated sulphide
in ammoniacal hydrogen peroxide, or in fuming nitric acid, and to deter-
mine the arsenic as Mg 2 As 2 O 7 as described on page 206.



SEPARATION OF ARSENIC FROM ANTIMONY. 245

X

Remark. Arsenic can also be separated from tin according
to the above method, except that more tartaric acid is necessary
to prevent the precipitation of the tin than is the case when an-
timony alone is present (cf. p. 255).



(d) Method of E. Fischer.*



Principle. This separation depends upon the ready vola-
tility of arsenic trichloride in a current of hot hydrochloric acid
gas, under which conditions antimony chloride is not volatile. If
the arsenic is present as arsenic acid, which is usually the case,
the distillation must take place in the presence of some reducing
agent, f

Procedure. The apparatus shown in Fig. 47 is used for this
determination. In the course of analysis, the arsenic and anti-
mony, as a rule, are obtained first in the form of the sulphides, and
these are dissolved, as described under (a), in caustic potash
solution and oxidized by chlorine. Instead of using chlorine,
the alkaline solution may be boiled with hydrogen peroxide or
potassium percarbonate. If the latter method is used for the
oxidation, the boiling must be continued until there is no further
evolution of oxygen.

The oxidized solution is transferred, by means of a long-
stemmed funnel, to the 500-c.c. distilling flask, A, in which has
been placed 1.5 gms. of potassium bromide;}: the solution is
diluted in the flask with fuming hydrochloric acid to a volume of
about 200 c.c. The receiver, V, consists of a large flask of from

* Z. anal. Chem., 21, 266. The process as described is the modification
of M. Rohmer, Ber., 34, 33 and 1565 (1901).

t Fischer used a ferrous salt, O. Piloty and A. Stock used hydrogen
sulphide (Ber., 30, 1649), and Friedheim and Michaelis used methyl alcohol
(Ber., 28, 1414).

I Instead of the potassium bromide, hydrogen bromide may be used
which has previously been prepared by treating 1 gm. of bromine with
sulphurous acid. It is not permissible to introduce the bromine into the
flask, A, in order to convert it to hydrogen bromide by introducing sulphur
dioxide gas into the flask, because it is then possible for bromine vapors
to get into the receiver by means of the air which is first expelled from
the apparatus, and the bromine would oxidize the volatilized AsCl 3 , and
thus interfere with the subsequent determination of the arsenic by pre-
cipitation as the trisulphide, or by titration.



246



GRAVIMETRIC ANALYSIS.



1.5-2 liters capacity; it is kept surrounded by a current of cold
water coming from the condenser and contains at the start, 800 c.c.
of cold distilled water. Then, with the apparatus all connected
as shown in the drawing, the distilling flask is heated and its
contents partially distilled in a current of hydrogen chloride,*
meanwhile constantly passing a little sulphur dioxide into the
flask, until at the end of about forty-five minutes, the volume of




FIG. 47.

liquid in A is reduced to about 40 c.c. The flame is then removed
and the T-tube between the two evolution flasks removed in order
to prevent liquid from backing up into the wash bottles. The
adapter tube which connects the condenser with the receiver is
rinsed off and the receiver removed.

A new receiver is now placed at the end of the apparatus and
a seccond distillation is made in order to make sure that all of
the arsenic has been volatilized.! Then, for the determination of
the arsenic, the contents of the two receivers are each diluted to a
volume of about 1250 c.c., and the excess of sulphurous acid is
removed by heating to boiling and passing a stream of carbon
dioxide through the liquid as is. shown in Fig. 48. When the
sulphur dioxide has all been expeljed (as can be shown by insert-
ing a stopper with delivery tube into the flask so that the escaping
vapors can be led into a dilute sulphuric acid solution of deci-
ttormal permanganate which will be decolorized by sulphur
dioxide) , the solution is allowed to cool and the arsenic determined
as trisulphide according to the directions on p. 205 and weighed

* If there i.s any tendency to suck back, a little more sulphur dioxide
should be introduced.

f Rohmer found that as much as 0.15 gm. arsenic was volatilized com-
pletely by one distillation.



DETERMINATION OF ARSENIC BY TITRATION.



247



as As2Ss after treatment with 82 (pp. 170, 223), or it may be
titrated with iodine.

Determination of Arsenic by Titration.

The solution is treated with a few drops of phenolphthalem
and solid potassium hydroxide is introduced until a permanent
pink color is imparted to the liquid. Tho solution is then decolor-
ized by the addition of a few drops of hydrochloric acid; 5 gms.
of sodium bicarbonate are added, and the solution titrated with
decinormal iodine solution as described on p. 688.*

The antimony is determined by treating the contents of the
distilling flask with 2 or 3 gms. of tartaric acid, washing the




FIG. 48.

solution into an Erlenmeyer flask, expelling the sulphur dioxide
as above ,f and determining the antimony gravimetrically by
precipitating as the trisulphide according to the directions on
p. 218, or it is estimated volumetrically by titration with iodine
as described on p. 688.

* A blank determination should be made with all the reagents that are
to be used, and the iodine solution must be standardized in a solution as
dilute as that in which the analysis is made.

t The escaping gas will not decolorize a solution of 2-3 c.c. dilute sulphuric
acid and one drop of 0.01N. KMnO 4 , when all the SO 2 is expelled.



248 GRAVIMETRIC ANALYSIS.



Determination of Arsenic in Commercial Sulphuric Acid.

About 30 c.c. of concentrated hydrochloric acid and a little
potassium bromide, or hydrogen bromide, are placed in the dis-
tilling flask A (Fig. 47), whereupon 50 to 100 gms. of the acid to
be tested (the weight is determined by difference) is introduced
through a funnel that is fastened by means of rubber tubing to
the upper end of the delivery tube which enters the flask;* the
funnel is rinsed with concentrated hydrochloric acid, and the
distillation begun.

When the contents of the distilling flask have been concen-
trated so that concentrated sulphuric acid remains, the acid is
kept hot by means of a small flame until all of the arsenic has
been expelled. On account of the high temperature, 1 gm. of
arsenic will be driven over in about fifteen minutes. The analysis
is finished as described above.



Separation of Antimony from Tin.
(a) F. W. Clarke' s^ Method.

Of all the present known methods for the separation of anti-
mony from tin this is probably the most accurate. It depends
upon the fact that antimony is completely precipitated from a
solution containing oxalic acid, while stannic salts are not. Stan-
nous sulphide, however, is decomposed by oxalic acid, forming an
insoluble crystalline stannous oxalate, so that the tin must be in
the stannic form.



Online LibraryF. P. (Frederick Pearson) TreadwellAnalytical chemistry (Volume 2) → online text (page 20 of 74)