C. Remigius Fresenius.

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arsenate, as magnesium pyroarsenate, as uranylpy roar senate, or
as arsenous sulphide. The determination as ammonium mag-
nesium arsenate is sometimes preceded by precipitation as am-
monium arseno-molybdate. The method recommended by BER-
THIER and modified by v. KOBELL, of separating the arsenic as
basic-ferric arsenate, is only used in separations. Arsenic may be
estimated also in an indirect way, and by volumetric methods.

We may convert into

1. LEAD ARSENATE : Arsenous and arsenic acids in aqueous or
nitric-acid solution. (Acids or halogens forming fixed salts, with
lead, and also ammonium salts, must not be present.)



a. By direct Precipitation. Arsenic acid in all solutions
free from bases or acids precipitable by magnesia or ammonia.

* ZeiUchr.f. Chem., i, 448.


I. Preceded by Precipitation as Ammonium Arseno-molyb-
date. Arsenic acids in all cases where no phosphoric acid is
present, little or no hydrochloric acid, nor any substance which
decomposes molybdic acid.

3. UKANYL PYKO ARSENATE : Arsenic acid in all combinations
soluble in water and acetic acid.

4. AKSENOUS SULPHIDE : All compounds of arsenic without

Arsenic may be determined volumetrically in a simple and
exact manner, whether present in the form of arsenous acid or an
alkali arsenite, or as arsenic acid or an alkali arsenate. The volu-
metric methods have now almost entirely superseded the indirect
gravimetric methods formerly employed to effect the determina-
tion of arsenous acid.

1. Determination as Lead Arsenate.

a. Arsenic Acid in Aqueous Solution.

A weighed portion of the solution is put into a platinum or
porcelain dish, and a weighed amount of recently ignited pure lead
oxide added (about five or six times the supposed quantity of arse-
nic acid present) ; the mixture is cautiously evaporated to dryness,
and the residue heated to gentle redness, and maintained some
time at this temperature. The residue is lead arsenate + lead
oxide. The quantity of arsenic acid is now readily found by sub-
tracting from the weight of the residue that of the oxide of lead
added. For the properties of lead arsenate, see 92. The results
are accurate, provided the residue be not heated beyond gentle red-

b. Arsenous Acid in Solution.

Mix the solution with nitric acid, evaporate to a small bulk,
add a weighed quantity of lead oxide in excess, evaporate to dry-
ness, and ignite the residue most cautiously in a covered crucible,
until the whole of the lead nitrate is decomposed. The residue
consists here also of arsenic acid -|~ lead oxide. This method
requires considerable care to guard against loss by decrepitatior
upon ignition of the lead nitrate.


2. Estimation as Ammonium Magnesium Ar senate, or
Magnesium Pyroarsenate.

a. By direct Precipitation.

This method, which was first recommended by LEVOL, presup-
poses the whole of the arsenic in the form of arsenic acid. Where
this is not the case, the solution is gently heated, in a capacious
flask, with hydrochloric acid, and potassium chlorate added in
small portions, until the fluid emits a strong smell of chlorous acid;
it is then allowed to stand at a gentle heat until the odor of this
gas has nearly disappeared.

The arsenic-acid solution is now mixed with ammonia in ex-
cess, which must not produce turbidity, even after standing some
time ; magnesia mixture is then added ( 62, 6). The fluid, which
smells strongly of ammonia, is allowed to stand 24 or 48 hours in
the cold, well covered, and then filtered through a weighed filter.
The precipitate is then transferred to the filter, with the aid of
portions of the filtrate, so as to use no more washing water than
necessary, and washed with small quantities of a mixture of three
parts water and one part ammonia, till the washings, on being
mixed with nitric acid and silver nitrate, show no opalescence. The
precipitate is dried at 102 to 103, and weighed. It has the for-
mula (MgNH 4 AsO 4 ) 2 -|-H 2 O.* As the drying of ammonium mag-
nesium arsenate till its weight is constant, requires much time and
repeated weighings, it is a great advantage that we can now con-
vert it without loss of arsenic into magnesium pyroarsenate (Mg,
As a O 7 ), thanks to the researches of II. RosE,f WITTSTEIN^: and
PULLER. For this purpose first transfer the dried precipitate as
completely as possible to a watch-glass, saturate the filter with
a solution of ammonium nitrate, dry and burn it cautiously in a
porcelain crucible. After cooling, transfer the precipitate to the
crucible, heat in an air-bath to about 130, continue heating for 2
hours on a sand-bath, then heat for an hour or two on an iron plate
a little more strongly, and when the ammonia has been thus entirely

* If it is dried in a water-bath, the drying must be extremely prolonged, or
otherwise more than 1 eq. will be left. After brief drying in the water-bath
the compound contains between 1 and 3 eq. water. If it is dried between 103"
and 110 part of the 1 eq. water is lost.

f H(indl)uch der anal. Chem. 6. Aufl., n, 390.

\Zeitschr, f, anal. Chem., n, 19. %lb., x, 63-


expelled ignite strongly for some time over the lamp. The pro-
cess may be shortened by conducting the heating in a ROSE'S cruci-
ble in a slow current of oxygen. The ammonia may then be
driven off in 10 minutes, and after the precipitate has been at last
strongly heated it will be ready to weigh. For the properties of
the ammonium magnesium arsenate and magnesium pyroarsenate,
see 92. The method yields satisfactory results, since the small
loss of precipitate dissolved in the filtrate and washings is coun-
terbalanced by the presence of a trace of basic magnesium sulphate
(PULLER). PULLER with a quantity of 0'37 grm. ammonium mag
nesium arsenate lost only a fraction of a milligramme ; on the ad-
dition of a large proportion of ammonium chloride the loss rose to
about 0-002 grm. The correction for the solubility of the pre-
cipitate in the amrnoniacal nitrate containing excess of magnesia
mixture- is 0-001 grm. of (MgNH 4 AsO 4 ) a + H a O for 30 c.c.

b. Preceded by Precipitation as Ammonium Arseno-molyl-

Mix the acid solution, which must be free from phosphoric and
silicic acids, with an excess of solution of ammonium molybdate.
The ammonium molybdate solution should have been previously
mixed with nitric acid in excess, and the whole process is con-
ducted exactly as in the case of phosphoric acid see 134, 5, ft.
After dissolving the ammonium arseno-molybdate in ammonia,
neutralize the latter partially with hydrochloric acid. Treat th^
ammonium magnesium arsenate as in a. Results satisfactory.

3. Estimation as Uranyl Pyroarsenate.

This method was first proposed by WERTHER.* It has been
carefully studied by PULLER! in my laboratory, and gives thor<
oughly satisfactory results. Mix the arsenic acid solution with
potash or ammonia in excess, and then a good excess of acetic acid,
(If a precipitate of ferric or aluminium arsenate here remains
insoluble, the method would be inapplicable.) Add uranyl acetate
in excess, and boil. Wash the slimy precipitate of uranyl arsenatq
or of ammonium uranyl arsenate by decantation w r ith boiling water,
and then transfer to a filter. The addition of a few drops of chlo-
roform to the partly cool fluid will hasten the deposition of the pre-
cipitate. Dry, transfer the precipitate to a watch-glass, cleaning

* Journ. f. prakt. Chem., XLIII, 346. \Zeitschr.f. analyt. Chem., x, 72.


the filter as much as possible ; saturate the latter with ammonium
nitrate, dry it, incinerate in a porcelain crucible, and add the pre-
cipitate. If the precipitate contains ammonium, heat very cau-
tiously, finally adding nitric acid, or ignite in oxygen. (See 2, a.)
If the precipitate is free from ammonium, ignite in the ordinary
way. Ammonium salts do not interfere. Properties of the pre-
cipitate and residue, 92, e.

4. Estimation as Arsenous Sulphide.

a. In solutions of Arsenous Acid or Arsenites free from
Arsenic Acid.

The solution should be strongly acid with hydrochloric acid.
Precipitate with hydrogen sulphide and expel the excess with car-
bon dioxide. Pass the latter through the solution for an hour, a
longer time is useless. (See 125, 1.) "Wash the precipitate thor-
oughly and dry at 100 till the weight is constant. Particles of
the precipitate which adhere so firmly to the glass that they can-
not be removed mechanically are dissolved in ammonia and repre-
cipitated with hydrochloric acid. Properties of the precipitate,
92. Do not omit to test a weighed portion to see whether it
completely volatilizes on heating. If a residue remains it is to be
weighed and the proportional quantity deducted from the total
weight of the precipitate. Results accurate.

If the solution contains any substance which decomposes hydro-
gen sulphide, such as ferric chloride, chromic acid, etc., the precip-
itate produced in the cold contains an admixture of finely divided
sulphur. It should be collected in the same manner on a filter
dried at 100, and weighed, washed and dried. Extract the
admixed sulphur with purified carbon disulphide (which should
leave no residue on evaporation), continuing till the iluid which
runs through leaves no residue. Dry at 100 till the weight is
constant. From experiments made in my laboratory it appears
that the results thus obtained are quite accurate, even when the
amount of admixed sulphur is large ; but the precipitation must
have been effected in the cold. If, on the contrary, heat is used,
the sulphur is in the form of small agglutinated grains and cannot
be completely extracted by cold carbon disulphide on the filter.
However, it may be extracted by removing the precipitate from the
filter and repeatedly digesting it with the disulphide on a water-
bath (PULLER*).

*7,'it*<'7,r.f. tuuilyt. Cfiem., x, 46 et seq.


Instead of purifying the arsenous sulphide you may estimate
the arsenic in the mixture of the sulphide with sulphur as follows :
Dissolve the precipitate in strong potassa, and pass chlorine into the
solution ( 148, II. 2, J). The arsenic and the sulphur are con-
verted into arsenic and sulphuric acids respectively ; the former
may be estimated according to 2, a, or the latter according to 132.
In the latter case, deduct the sulphur found from the weight of the
arsenical precipitate. There is no loss of arsenic in this process
from volatilization of the chloride, as the solution remains alkaline.
The object may also be conveniently attained by the use of nitric
acid. A very strong fuming acid, of 86 boiling point, is
employed ; an acid of 1-42 sp. gr. which boils at a higher tempera-
ture does not answer the purpose, as the separated sulphur would
fuse, and its oxidation would be much retarded. The well dried
precipitate is shaken into a small porcelain dish, treated with a tol-
erably large excess of the fuming nitric acid, the dish immediately
covered with a watch-glass, and as soon as the turbulence of the
first action has somewhat abated, heated on a water-bath till all the
sulphur has disappeared, and the nitric acid has evaporated to a
small volume. The filter to which the unremovable traces of
arsenous sulphide adhere is treated separately in the same manner,
the complete destruction of the organic matter being finally effected
by gently warming the somewhat dilute solution with potassium
chlorate (BuNSEN*). Or the filter may instead be extracted with
ammonia, the solution evaporated in a separate dish, and the resid-
ual sulphide treated as above. In the mixed solution the arsenic
acid is finally precipitated as ammonium magnesium arsenate.
( 127, 2, a). Treatment of the impure precipitate with ammonia,
whereby the sulphide is dissolved, and the sulphur is supposed to
remain behind, only gives approximate results, as the ammoniacal
solution of arsenous sulphide takes up a little sulphur.

I. In solutions of Arsenic Acid, or of a mixture of the two
Oxides of Arsenic.

Heat the solution in a flask (preferably on an iron plate) to
about 70, and conduct hydrogen sulphide at the same time into
the fluid, so long as precipitation takes place. The precipitate
formed is always a mixture of sulphur and arsenous sulphide,
since the arsenic acid is first reduced to arseuous acid with separa-

* Anna I. d. Chem. u. Pharm ., cvj, 10.


tion of sulphur, and then the latter is decomposed (H. ROSE *).
Only in the case when a sulpho-salt containing pentasulphide of
arsenic is decomposed with an acid, is the precipitate actually
pentasulphide, and not merely a mixture of sulphur with arse-
nous sulphide (A. FUCHS f). To convert this mixture of arsenous
sulphide and granular sulphur into pure arsenous sulphide
suitable for weighing, treat it as follows : Extract the washed and
still moist precipitate on the filter with ammonia, wash the resid-
ual sulphur, precipitate the solution with hydrochloric acid with-
out heat, filter, dry, extract with carbon disulphide, dry at 100,
and weigh. Results accurate. The mixture of arsenous sulphide
and sulphur obtained by hot precipitation may, of course, also be
estimated directly or indirectly after one of the other methods
in 4, a.

5. Volumetric Methods,
a. Method which presupposes the presence of Arsenous Acid.

1. FR. MOHR'S method 4 This method depends upon the
principle already stated under Antimonous Oxide, 125, 3, #;
i.e., arsenous acid in alkaline solution is oxidized by iodine to
arsenic acid (As a O 8 + 4NaOH + 41 + = As 2 O 6 + 4NaI + 2H 2 O).

If, therefore, you have an aqueous solution of arsenous acid
or an alkali arsenite, weigh or measure off a quantity that will
contain about O'l grm. As 2 O 8 ; add to it 20 c. c. of a saturated
solution of sodium bicarbonate (previously purified by washing
with water), then add a little thin starch paste, and finally titrate
with standard iodine solution ( 146) until the starch-iodide reac-
tion appears ; 4 eq. of iodine correspond to 1 eq. of arsenous acid.
If the arsenous-acid solution is acid, neutralize it with pure sodium
carbonate; but, on the other hand, if alkaline, neutralize with
hydrochloric acid before adding the sodium bicarbonate. It is of
course understood that the solution contains no substances (sul*
phides or thiosulphates) that will act upon iodine. The results
are accurate. Compare Expt. No. 79; also WAITZ.

* Pogg. Annal., cvn, 186.
1[ Ziitschr.f. analyt. Chem., i, 189.
% Lehrbuch der Titrirmethode, 3. Aufl., 275.

%Zeitschr. f. analyt. Chem., x, 162. The attempts made by WAITZ to
convert the arsenic in arsenous sulphide into alkali arsenite were unsuccessful.


2. KESSLER'S method.* This method depends upon the
principle stated under 125, 3, 5; i.e., the oxidation of arsenous
to arsenic acid is effected in hydrochloric-acid solution by the use
of potassium dichromate, f and is carried out in exactly a similar
manner. The results are reliable only when care is taken that
the hydrochloric acid (sp. gr. 1-12) constitutes at least one sixth of
the volume of the liquid ; it should not, however, exceed one
half the volume, otherwise the end reaction, due to the formation
of iron ferricyanide, sets in more slowly and loses in sharpness.

If for any reason the direct titration of the hydrochloric-
acid solution is impracticable, precipitate with hydrogen sulphide
(if arsenic acid is present, at 70), wash the precipitate, transfer it
together with the filter to a stoppered flask, and treat it with an
almost saturated solution of mercuric chloride in hydrochloric acid
(sp. gr. 1*12), stopper tightly, digest at a gentle warmth until
the precipitate has. become white, dilute with a measured quantity
of w^ater (so that the proportion of hydrochloric acid of sp. gr.
1'12 does not fall below one sixth), add solution of potassium
dichromate, then iron solution, etc., as detailed under 125, 3, 5.
Results good. Compare also WAITZ.^:

3. BUNSEN'S method. This method is based upon the follow-
ing facts :

aa. If potassium dichromate is boiled with concentrated hydro-
chloric acid, 6 at. chlorine are disengaged for every 2 mol. chromic
acid ; 200 3 + 12HC1 = CrCl 3 + 6H 2 O + 6C1.

J5. But if arsenous acid is present (not in excess) there is not
the quantity of chlorine disengaged corresponding to the chromic
acid, but so much less of that element as is required to convert
the arsenous into arsenic acid (H 3 AsO 3 + 2C1 -f- H a O = H 3 AsO 4
-{- 2HC1). Consequently for every 2 at. chlorine wanting there,
is to be reckoned 1 mol. arsenous acid.

* Pogg. Amial., xcv, 204; cxm, 184; cxvin, 17; Zeitschr.f. analyt. Chem.,
II, 383.

f Oxidation may also be effected by potassium permanganate, an excess
being added, and this latter then determined with iron. The estimation is inac-
curate in hydrochloric-acid solution, hence the permanganate can only be used ia
sulphuric-acid solution. Compare WAITZ, Zeitschr.f. analyt. Chem., x, 174.
| Zeiischr.f. analyt. Chem., x, 169.

d. Chem. u. Pharm., LXXXVI, 2J)Q.


cc. The quantity of chlorine is estimated by determining the
quantity of iodine liberated by it from potassium iodide.

These are the principles of BUNSEN'S method. For the man-
ner of execution I refer to the Estimation of Chromic Acid.

I. Method which presupposes the presence of Arsenic Acid.

This method depends on the precipitation of the arsenic acid
by uranium solution and the recognition of the end of the reaction
by means of potassium ferrocyanide. It is therefore the same as
was suggested for phosphoric acid by LECOMTE, and brought into
use by NEUBAUEK,* and afterwards by PiNcus.f

BODEKER,;): who first employed the process for arsenic acid,
recommends the employment of a solution of uranyl nitrate, as
this is more permanent than the hitherto used acetate, which is
gradually decomposed by the action of light.

The uranium solution has the correct degree of dilution, if it
contains about 20 grm. of uranium in 1 litre. It should contain
as little free acid as possible. The determination of its value may
be effected with the aid of pure sodium arsenate or by means of
arsenous acid the latter is converted into arsenic acid by boiling
with fuming nitric acid. The solution is rendered strongly alka-
line with ammonia, and then distinctly acid with acetic acid. The
uranium solution is now run in from the burette slowly, the liquid
being well stirred all the while, till a drop of the mixture spread
out on a porcelain plate, gives with a drop of potassium ferrocya-
nide placed in its centre, a distinct reddish-brown line where the
two fluids meet. The height of the fluid in the burette is now
read off, the level of the mixture in the beaker is marked with a
strip of gummed paper, and the beaker is emptied and washed,
filled with water with addition of about as much ammonia and
acetic acid as was before employed, and the uranium solution is
cautiously dropped in from the burette, till a drop taken out of the
beaker and tested as above, gives an equally distinct reaction. The
quantity of uranium solution used in this last experiment is the
excess, which must be added to make the end-reaction plain for the
dilution adopted. This amount -is subtracted from that used in the
first experiment, and we thru know the exact value of the uranium
solution with reference to arsenic acid.

*Arc?iivfiir wissenchaftlichc ITcilkunde, iv, 228.

f Jour n. f. prakt. Chem., LXXVI, 104. \ Annal. de Ghem. u. Pharm., cxvn, 195.


In an actual analysis, the arsenic is first brought into the form
of arsenic acid, a clear solution is obtained containing ammonium
acetate and some free acetic acid,* and the process is conducted
exactly as in determining the value of the standard solution. The
experiment to ascertain the correction must not be omitted here,
otherwise errors are sure to arise from the different degrees of dilu-
tion of the arsenic acid solutions used in the determination of the
value of the standard solution and in the actual analyses. The results
of two determinations of arsenic given by BODEKER are satisfactory.
To execute the method well requires practice. The results are not
exact enough unless the conditions as regards amount and quality
of alkali salts are nearly similar in the standardizing of the uranium
solution and in its use. Compare "WArrz.f

6. Estimation of Arsenous Acid by Indirect Gravimet-
ric Analysis.

a. ROSE'S method. Add to the hydrochloric acid solution, in
the preparation of which care must be taken to exclude oxidizing
substances, a solution of sodium- or ammonium-auric chloride in
excess, and digest the mixture for several days, in the cold, or, in
the case of dilute solutions, at a gentle warmth ; then weigh the
separated gold as directed in 123. Keep* the filtrate to make
quite sure that no more gold will separate. 2 at. gold correspond
to 3 mol. arsenous acid.

J. VOHL'S^; method. Mix the solution with a weighed quan-
tity of potassium dichromate, and free sulphuric acid ; estimate the
chromic acid still present by the method given in 130, c, and
deduce from the quantity of that acid consumed in the pro ass, i.e.,
reduced by the arsenous acid, the quantity of the latter, after the
formula 3H 3 AsO 3 + 2CrO 3 = 3II 3 AsO 4 + CV 2 O 3 .

* Alkalies, alkali earths, and zinc oxide may be present, -but not such metals
as yield colored precipitates with potassium ferrocyanide, as, for instance,
\Zeitscfir.f. anal. Chem., x, 182. %Annal. de Chem. u. Pharm., xciv, 219.

420 DETERMINATION. [g 1 r,

Supplement to the Sixth Group.


Molybdic acid is converted, for the purpose of its determina-
tion, either into molybdenum dioxide, or into lead molybdate, 01
into molybdenum disulphide.

a. Molybdic anhydride (MoO 3 ), and also ammonium molybdate,
may be reduced to dioxide by heating in a current of hydrogen gas.
This may be done either in a porcelain boat, placed in a wide glass
tube, or in a platinum or porcelain crucible with perforated cover
(108, Fig 83). The operation is continued till the weight remains
constant. The temperature must not exceed a gentle redness,
otherwise the dioxide itself might lose oxygen and become partially
converted into metal. In the case of ammonium molybdate the
heat must be very low at first on account of the frothing. If you
have a platinum tube it is safer to ignite the molybdic acid in this
for 2 or 3 hours in a slow current of hydrogen, thus reducing it to
the metallic state. When reducing to dioxide the contents of the
crucible are frequently gray below, and brown above (RAMMELS-

J. The following is the best method of precipitating molybdic
acid from an alkaline solution : Dilute the solution, if necessary,
neutralize the free alkali with nitric acid, and allow the carbonic
acid, which may be liberated in the process, to escape, then add
neutral mercurous nitrate. The yellow precipitate formed appears
at first bulky, but after several hours' standing it shrinks ; it is
insoluble in the fluid, which contains an excess of mercurous
nitrate. Collect on a filter, and wash with a dilute solution of imr-
eurous nitrate, as it is slightly soluble in pure water. Dry, remove
the precipitate as completely as practicable from the filter, and deter-
mine the molybdenum in it as directed in a (II. ROSE) ; or mix the
precipitate, together with the filter-ash, with a weighed quantity
of ignited lead oxide, and ignite until all the mercury is expelled ;
then add some ammonium nitrate, ignite again and wrigh. The
excess obtained, over and above the weight of the lead oxide used,
is molybdenum trioxide

* Pogg. Annul, cxxvn, 281; Zeitschr.f. analyt. Chcm., v, 203.
\Journ.f.prakt. Chem., LXVII, 472.

138.] MOLYBDIC ACID. 421

o, CHATARU* recommends estimating molybdic acid in the solu-
tion of fts alkali salts by adding lead acetate in slight excess to the
boiling solution and boiling for a few minutes, The precipitate
which is at first milky becomes granular, deposits well, and may be
easily washed with hot water. It is dried, removed from the filter
as much as possible, ignited and weighed as PbMoO^ The method
is only applicable for solutions of pure alkali rnolybdates.

d. The precipitation of molybdenum as sulphide is always a

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