C. Remigius Fresenius.

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difficult operation. If the acid solution is supersaturated with
hydrogen sulphide, warmed, and filtered, the filtrate and washings
are generally still colored. They must, accordingly, be wanned,
and hydrogen sulphide again added, and the operation must after-
wards, if necessary, be repeated until the washings appear almost
colorless. The precipitation succeeds better when the molybdenum
sulphide is dissolved in a relatively large excess of ammonium sul-
phide, and, after the fiuid has acquired a reddish-yellow tint, precipi-
tated with hydrochloric acid. ZENKERf advises then to boil, until
the hydrogen sulphide is expelled, 'and to wash with hot water, at
first slightly acidified. To make quite sure that all the molyb-
denum is precipitated, treat the filtrate and washings again with
hydrogen sulphide and allow to stand for some time. The brown
molybdenum sulphide is collected on a weighed filter, and the
molybdenum determined in an aliquot part of it, by gentle ignition
in a current of hydrogen gas, as in a. The brown molybdenum
sulphide changes in this process to the gray disulphide (H. ROSE).

e. F. PISANI;); gives the following method for estimating moiyb-
dic acid volu metrically : Digest the molybdic acid with hydro-
chloric acid and zinc, dissolving any precipitate which may form
from want of acid and also the excess of zinc. The molybdic acid
is thus reduced to a molybdenum salt corresponding to molybdenum
sesquioxide. Convert the molybdenum in this solution again into
molybdic acid by standard potassium permanganate. The brown
color of the solution turns first green, and then disappears. EAM-
MELSBERG confirms the statements of PISA.NI.

* Sill. Amer. Journ. (3), i, 416. f Journ.f. prakt. C7iem., LVIII, 259.

t Compt. Rend., LIX, 301.

%Pogg. Annal., cxxvn, 281; Zeilschr. f. analyi. Chem., v, 20$.



422 DETERMINATION. [ 129, 130.

II. DETERMINATION OF ACIDS IN COMPOUNDS CONTAINING
ONLY ONE ACID, FREE OR COMBINED ; AND SEPARATION
OF ACID FROM BASIC RADICALS.

First Group.

FIRST DIVISION.

ARSENOUS ACID ARSENIC ACID CHROMIC ACID (Selenous
Acid, Sulphurous and Hyposulphurous Acids, lodic Acid).

129.
1. ARSENOUS AND ARSENIC ACIDS.

These have been already treated of among the bases ( 127) on
account of their behavior with hydrogen sulphide ; they are merely
mentioned here to indicate the place to which they properly be-
long. The methods of separating them from the bases will be
found in Section Y.

130.
2. CHROMIC ACID.

I. DETERMINATION.

Chromic acid is determined either as chromic oxide or lead
chr ornate. But it may be estimated also from the quantity of car-
bon dioxide disengaged by its action upon oxalic acid in excess,
and also by volumetric analysis. In employing the first method
it must be borne in mind that 1 mol. chromic oxide corresponds to
2 mol. chromic acid.

a. Determination as Chromic Oxide.

a. The chromic acid is reduced to the state of a chromic salt
and the amount of chromium in the latter determined ( 106). The
reduction is effected either by heating the solution with hydro-
chloric acid and alcohol ; or by mixing hydrochloric acid with the
solution, and conducting hydrogen sulphide into the mixture ; or
by adding a strong solution of sulphurous acid, and applying a gen-
tle heat. With concentrated solutions the first method is gener-
ally resorted to, with dilute solutions one of the two latter. With
respect to the first method, I have to remark that the alcohol must
be expelled before the chromium can be precipitated as hydroxide



130.] CHROMIC ACID. 423

by ammonia ; and with, respect to the second, that the solution
supersaturated with hydrogen sulphide must be allowed to stand in
a moderately warm place, until the separated sulphur has com-
pletely subsided. The results are accurate, unless the weighed pre-
cipitate contains silica and lime, which is always the case if the pre-
cipitation is effected in glass vessels.

ft. The neutral or slightly acid (nitric acid) solution is preeipb
tated with mercurous nitrate, after long standing the red precipitate
of mercurous chromate is filtered off, washed with a dilute solution
of mercurous nitrate, dried, ignited, and the residuary chromic
oxide weighed (II. ROSE). Results accurate.

b. Determination as Lead Chromate.

The solution is mixed with sodium acetate in excess, and acetic
acid added until the reaction is strongly acid ; the solution is then
precipitated with neutral lead acetate. The washed precipitate is
either collected on a weighed filter, dried in the water-bath, and
weighed; or it is gently ignited as directed 53, and then
weighed. For the properties of the precipitate, see 93, 2. Results
accurate.

c. Determination as Barium Chromate.

Moderately acidulate the alkali- chromate solution with acetic
acid, add a slight excess of barium chloride, allow the fine .pre-
cipitate to stand 12 hours, wash it with a solution of ammonium
acetate so far as possible by decantation, displacing the last por-
tion of the solution with ammonium nitrate (or the chromate may
be partially reduced on ignition), dry the precipitate, and ignite it
after removal, so far as is possible, from the filter.

Properties and composition of the barium chromate are given
under 93, 2, c (H. ROSE; PEARSON*). The test analyses given
by PEARSON are satisfactory.

d. Determination by means of Oxalic Acid (after YOHL).

When chromic acid and oxalic acid are brought together in
the presence of water and excess of sulphuric acid, chromic
sulphate and carbon dioxide are formed, 3H 2 C 2 O 4 -f- 2H a CrO 4 -f-
3H,SO 4 = 6CO, + Cr 2 (SO 4 ), + 8H.O. Accordingly the amount
of chromic acid can be calculated from the weight of carbon
dioxide evolved. The process is the same as in the analysis of
manganese ores (230). 1 part of chromic acid requires 2J

* Amer. Journ. of Science [2], XLV, 298; Zeitsclir.f. analyt, Chem,, ix, 108,



424 DETERMINATION. [ 130.

parts of sodium oxalate. If it is intended to determine potas-
sium or sodium in the residue, ammonium oxalate is used.
e. Determination ly Volumetric Analysis.
a. SCHWARZ'S method.

The principle of this very accurate method is identical with
that upon which PENNY'S method of determining iron is based
( 112, 2, 1). The execution is simple: Acidifj the not too
dilute solution of the chromate with sulphuric acid, add in excess
a measured quantity of solution of a ferrous salt, the strength of
which you have previously ascertained according to the direc-
tions of 112, 2, &, or ^, or the solution of a weighed quantity
of ammonium-ferrous sulphate, free from ferric salt, and then
determine in the manner directed in 112, 2 a, or J, the quan-
tity of ferrous iron remaining. The difference shows the amount
of iron that has been converted by the chromic acid from a
ferrous to a ferric salt. 1 grin, of iron corresponds to 0'5969 of
chromic anhydride (CrO 8 ). To determine the chromic acid in
lead chromate, the latter is, after addition of the ammonium
ferrous sulphate, most thoroughly triturated with hydrochloric
acid, water added, and the analysis then proceeded with.

j3. BUNSEN'S method.*

If a chromate is boiled with an excess of fuming hydrochloric
acid, there are disengaged for every atom of chromium 3 at.
chlorine; for instance, K 2 Cr 3 O 7 + 14HC1 = 2KC1 + 2CrCl, +
6C1 -f- 7H a O. If the escaping gas is conducted into a solution
of potassium iodide in excess, the 3 at. chlorine set free 3 at.
iodine. The liberated iodine may next be determined as de-
scribed in 146. 380'55 of iodine corresponds to lOOl of
chromic anhydride (CrO s ).

The analytical process is conducted as follows : Put the weighed
sample of the chromate (say 0*3 to 0'4 grm.)into the little flask e?,
Fig. 89 (blown before the lamp, and holding only from 36 to 40
c. c.), and fill the flask two- thirds with pure fuming hydrochloric
acid free from Cl and SO,), and add a compact lump of magnesite
to keep up a constant current of gas and prevent the fluid from
receding. Connect the bulbed evolution tube a with the neck

* Annal. d. Chem. u. Pharm., LXXXVI, 279.



130.] CHROMIC ACID. 425

of the ^ flask by means of a stout india-rubber tube c. As
shown in the engraving, a is a bent pipette, drawn out at the
lower end into an upturned point. A loss of chlorine need not be
apprehended on adding the hydrochloric acid, as the disengage-




. 89,



ment of that gas begins only upon the application of heat. Insert
the evolution tube into the neck of the retort, which is one-third
filled with solution of potassium iodide.* This retort holds about
160 c.c. The neck presents two small expansions, blown before
the lamp, and intended, the lower one, to receive the liquid which
is forced up during the operation, the upper one to serve as an
additional guard against spirting. Apply heat now, cautiously, to
the little flask. After two or three minutes ebullition the whole
of the chlorine has passed over, and liberated its equivalent quan-
tity of iodine in the potassium iodide solution. "When the ebulli-
tion is at an end, take hold of the caoutchouc tube c with the left
hand, and, whilst steadily holding the lamp under the flask with
the right, lift a so far out of the retort that the curved point is in
the bulb b. ~Now remove first the lamp, then the flask, dip the
retort in cold water to cool it, and shake the fluid in it about to effect
the complete solution of the separated iodine in the excess of potas-
sium iodide solution. When the fluid is quite cold, transfer it to a
beaker, rinsing the retort into the beaker, and proceed as directed
146. The method gives very satisfactory results. The apparatus
here recommended differs slightly from that used by BUNSEN, the
retort of the latter having only onef bulb in the neck, and the evo-
lution' tube no bulb, being closed instead, at the lower end, by a
glass or caoutchouc valve, which permits the exit of the gas from

* 1 part of pure potassium iodide, free from iodic acid, dissolved in 10 parts
of water. The fluid must show no brown tint immediately after addition of
dilute sulphuric acid.



426 DETERMINATION. [ 130.

the tube, but opposes the entrance of the fluid into it. I think the
modifications which I have made in BUNSEN'S apparatus are calcu-
lated to facilitate the success of the operation. Instead of this ap-
paratus, that described in 142 may also be very conveniently used.
y. There need be only mention made here regarding the
method by RUBE,* which is based on the equation 2CrO 8 4
6K 4 Fe(CN) fl +12HCl = 6KC1 + 2CrCl 3 +3K fl Fe a (CN) ] ,+ 6H 1 O >
and also regarding the method devised by ZULKOWSKY,! which ib
based on the direct (i.e., without distillation) estimation of the
iodine separated by chromic acid, and which is carried out ex-
actly as detailed under 113, /?, in estimating iron.

II. SEPARATION OF CHROMIC ACID FROM THE BASIC
RADICALS.
a. Of the First Group.

a. Reduce the chromic acid to a chromic salt, as directed in I.,
and separate the chromium from the alkalies as directed in g 155.

fi. Mix the potassium or sodium chromate with about 5 parts
of dry pulverized ammonium chloride, and heat the mixture cau-
tiously. The residue contains the chlorides of the alkali metals
and chromic oxide, which may be separated by means of water.

y. Precipitate the chromic acid according to I., #, /?, and sep-
arate the mercury and alkali metals in the filtrate by 162.

b. Of the Second Group.

a. Fuse the compound with 4 parts of sodium and potassium
carbonates, and treat the fused mass "with hot water, which dis-
solves the chromic acid in the form of an alkali chromate. The
residue contains the alkali earth metals in the form of carbonates ;
but as they contain alkali, they cannot be weighed directly. The
chromic acid in the solution is determined as in I. Strontium and
calcium chromates may be decomposed by boiling with potassium
or sodium carbonate. Barium chromate may also be decomposed
in the same way, but the boiling must be repeated a second time
with fresh solution of alkali carbonate (II. ROSE).

*Journ.f. prakt. C/iem., xcv, 53; Zeitechr.f. aualyt. Chem., iv, 444.
\Journ.f. prakt. Chem., cm, 351; Zeitschr.f. analyt. Chem., vin, 74.



130.] CHROMIC ACID. 427

j3. Dissolve in hydrochloric acid, reduce the chromic acid
according to I., a, and separate the chromium from the alkali
earth metals according to 156.

y. Magnesium chromate, as well as other chromates of the
alkali earth metals soluble in water, may be easily decomposed also,
by determining the chromic acid according to I., a, /?, or I., 5, and
separating the magnesium, etc., in the nitrate from the excess of
the salt of mercury or lead as directed 162.

d. Barium strontium and calcium chromates may also be
decomposed by the method described II., #, /?. Compare BAHR,
Analysis of barium and calcium dichromates, etc.*

H. HOSE recommends using 5 parts of ammonium chloride to
1 part of the very finely powdered substance. One single igni-
tion of the mixture usually suffices for complete decomposition,
but it is safer to repeat the ignition with ammonium chloride, to
make sure that the weight remains constant, before washing out
the barium chloride from the residue.

c. Of the Third Group.

a. From Aluminium.

If you have chromic acid to separate from aluminium in acid
solution, precipitate the aluminium with ammonia or ammonium
carbonate ( 105, a), and determine the chromic acid in the filtrate
according to I. If the washed aluminium hydroxide has a yellow
color, treat on the filter with ammonia, and wash with boiling
water ; this will remove the last traces of chromic acid. However,
a little aluminium hydroxide dissolves in the ammonia, therefore
heat the ammoniacal fluid in a platinum dish till it has almost lost
its alkaline reaction, and collect on a filter the flocks of aluminium
hydroxide which separate, and add them to the principal precip-
itate.

fi. From Chromium^

aa. Determine in one portion the quantity of the chromic acid
according to I., d, or I., 0, a, or /?, and in another portion the
total amount of the chromium, by converting it into sesquioxide
by cautious ignition with ammonium chloride, or by I., #, or by
converting it entirely into chromic acid by 106, 2.

55, In many cases the chromic acid may be precipitated accord -

* Journ.f. prakt. Chern., LX, 60.



428 DETERMINATION. [ 130.

Ing to I., , y#, or L, ~b. The cliromiuin and mercury, or lead, in
tlie filtrate, are separated as directed 162.

cc. The hydrated compounds of sesquioxide of chromium with
chromium trioxide, or chromic chromates, such as are obtained by
precipitating a solution of chromic salt with potassium chromate,
etc., may also be analyzed by ignition in a stream of dry air, in a
bulb tube, to which a calcium chloride tube is attached (Fig 44,
36). The loss of weight represents the joint amount of oxygen
and water that have escaped. If the increment of the CaCl a tube
is deducted, we shall have the oxygen. Now every 3 at. oxygen
correspond to 2 mol. CrO 3 . The amount of the latter being thus
calculated, we have only to subtract its equivalent quantity of ses-
quioxide from the weight of residue after the ignition, and the
remainder is the quantity of sesquioxide originally present. YOGEL*
and also STOKER and ELLIOT)- have employed this method.

d. Of the fourth Group.

a. Proceed as directed in J, a. Upon treating the fused mass
with hot water, oxides of the basic metals are left. In the case of
manganese the fusion must be effected in an atmosphere of carbon
dioxide. Apparatus, Fig. 83 in 108.

ft. Reduce the chromic acid as directed in L, #, and separate
the chromium from the metals in question, as directed in 160.

e. Of the fifth and Sixth Groups.

fx. Acidify the solution, and precipitate, either at once or after
reduction of the chromic acid by sulphurous acid, with hydrogen
sulphide. The metals of the fifth and sixth groups precipitate in
conjunction with free sulphur ( 115 to 127), the chromic acid is
reduced. Filter and determine the chromium in the filtrate, as
directed in L, a.

ft. Lead chromate may be conveniently decomposed by heating
with hydrochloric acid and some alcohol ; the lead chloride and
chromic chloride formed are subsequently separated by means of
alcohol (compare 162). The alcoholic solution ought always to be
tested with sulphuric acid ; should a precipitate of lead sulphate
form, this must be filtered oif, weighed, and taken into account.
Compare also 130, 1, d.

*Journ.f. prakt. Chem., LXXVII, 484.

f Proceedings of the American Academy, V, 198.



131.] SELENOUS ACID. 429

Supplement to the First Division.

131.
1. SELENOUS ACID.

From aqueous or hydrochloric- acid solutions of selenous acid,
the selenium is precipitated by sulphurous acid gas, or, in presence
of an excess of acid, by sodium sulphite, or ammonium sulphite.
The liquid containing the precipitate is heated to boiling for J hour,
which changes the precipitate from its original red color to black,
and makes it dense and heavy. The liquid is tested by a further
addition of the reagent to see whether any more selenium will sep-
arate ; the precipitate is finally collected on a weighed filter, dried
at 'a temperature somewhat below 100, and weighed. Since H.
ROSE* has shown that the presence of hydrochloric acid is an essen-
tial condition to the complete reduction of selenous acid, the for-
mer acid must be added, if not already present. To make quite
sure that all the selenium has been removed, the filtrate is evapo-
rated to a small volume, with addition of potassium or sodium chlo-
ride, boiled with strong hydrochloric acid, so as to reduce any sele-
nic acid to selenous acid, and tested once more with sulphurous
acid. If the solution contains nitric acid it must be evaporated
repeatedly with hydrochloric acid, with addition of sodium or
potassium chloride. If the latter were omitted there would be
considerable loss of selenous acid (RATHKE f ).

As regards the separation of selenous acid from basic radicals,
the following brief directions will suffice :

a. If the basic radicals are not liable to be altered by the action
of sulphurous acid and hydrochloric acid, the selenium may be at
once precipitated in the way just given ; the filtrate, when evap-
orated with sulphuric acid, yields the base as sulphate.

ft. From basic metals which are not thrown down from acid solu-
tion by hydrogen sulphide, the selenous acid may be separated by
precipitation with that reagent. The precipitate (according to
RATHKE, a mixture of SeS, , Se 2 S and S) contains 2 at. sulphur to
1 at. selenium. If it is dried at or a little below 100, the weight

* Zeitschr. f. analyt. Chern., i, 73.

\Journ. f. prakt. CJiem., cvm, 249; Zeitschr. f. analyt. Chem., ix, 484.

^Journ.f. prakt. Chem., cvin, 252.



430 DETERMINATION. [ 131.

of the selenium may be accurately ascertained. Should, however,
extra sulphur be mixed with the precipitate, the latter is oxidized
while still moist with hydrochloric acid and potassium chlorate, or
by treatment with potassa solution with simultaneous heating and
transmission of chlorine. It is necessary here to oxidize the sul-
phur completely, as it may enclose selenium, The solution now
containing selenic acid is heated till it smells no longer of chlorine,
hydrochloric acid is added, and the mixture is reheated. The sele-
nic acid is hereby reduced to selenous acid, and when the solution
has again ceased to smell of chlorine, the selenium is precip-
itated with sulphurous acid. Instead of this process you may digest
the precipitate of sulphur and selenium for some hours with con-
centrated potassium cyanide, which will completely dissolve it, and
then throw down the selenium from the dilute solution with hydro-
chloric acid as in c (EATHKE, loc. cit.).

c. In many selenites or selenates the selenium may also be
determined by converting first into potassium selenocyanate, and
precipitating the aqueous solution of the latter with hydrochloric
acid (OPPENHEIM*). To this end the substance is mixed with 7 or
8 times its quantity of ordinary potassium cyanide (containing
cyanic acid), the mixture is put into a long-necked flask, or a porce-
lain crucible, covered with a layer of potassium cyanide, and fused
in a stream of hydrogen. The temperature is kept so low that
the glass or porcelain is not attacked, and while cooling care must
be taken to exclude atmospheric air. When cold, the brown mass
is treated with water, and the colorless solution filtered, if neces-
sary. The liquid should be somewhat but not immoderately
diluted. Now boil some time (in order to convert the small quan-
tity of potassium selenide that may be present into potassium sele-
nocyanate, by the excess of potassium cyanide, allow to cool, super-
saturate with hydrochloric acid, and heat again for some time. At
the end of 12 or 24 hours all selenium will have separated, filter,
dry at 100, and weigh. The results obtained by this process are
accurate (II. RosEf). If the selenium agglomerates together on
heating, it may enclose salts. In such cases, by way of control, it
should be redissolved in nitric acid, and, after addition of hydro-
chloric acid, precipitated with sulphurous acid. The fluid filtered
from the selenium precipitate is, as a rule, free from selenium ; it

* Journ.f. prakt. Chem., LXXI, 280. ^ Ztitschr. f. analyt. Ctum., i, 73.



131.] SULPHUROUS ACID. 431

is, however, always well to satisfy one's self on this point by the
addition" of sulphurous acid.

d. From many basic radicals selenous acid (and also selenic
acid) may be separated by fusing the compound with 2 parts of
sodium carbonate and one part of potassium nitrate, extracting the
fused mass thoroughly by boiling with water, saturating the filtrate,
if necessary, with carbonic acid, to free it from lead which it might
contain, then boiling down with hydrochloric acid in excess (to
reduce the selenic acid and drive off the nitric acid), and precipi-
tating finally with sulphurous acid.

Selenium, if pure, must volatilize without residue when heated
in a tube.

2. SULPHUROUS ACID.

To estimate free sulphurous acid in a fluid which may contain
also other acids (sulphuric acid, hydrochloric acid, acetic acid), a
weighed quantity of the fluid is diluted with water, absolutely free
from air,* until the diluted liquid contains not more than 0-05 per
cent, by weight of sulphurous acid, the solution is poured witli
stirring into an excess of standard solution of iodine, the free
iodine remaining is titrated with sodium thiosulphate, and the
iodine used for the conversion of sulphurous into sulphuric acid is
thus found. The reaction is expressed by the equation, SO 3 + 21
+ 2H 2 O = H 2 SO 4 + 2HL According to FINKENEK, f if the iodine
is added to the sulphurous acid the reaction is not quite normal.
Anyhow this method of operating prevents any loss of sulphurous
acid. For the details, see 146. In case of sulphites soluble in
water or acids, water perfectly free from air is poured over the
substance, in sufficient quantity to attain the degree of dilution
stated above, sulphuric or hydrochloric acid is added in excess, and
then the titration is effected as above. The greatest care must be
taken in this method, to use, for the purpose of dilution, water
absolutely free from air.

Sulphurous acid may also be determined in the gravimetric way,
by conversion into sulphuric acid, and precipitation of the latter
with barium chloride, according to 132. This method is espe-
cially applicable in the case of sulphites quite free from sulphuric
acid. The conversion of the sulphurous into sulphuric acid is

* Prepared by long-continued boiling and subsequent cooling with exclusion
of air.
L ^Handb. der analyt. Chem. von H. ROSE, 6. Aufl. von FINKENER; n, 937.



432 DETERMINATION. [

effected in the wet way, best by pouring the dilute solution with
stirring into excess of chlorine or bromine water. Sulphites insolu-
ble in water are decomposed by boiling with sodium carbonate, and
the solution of sodium sulphite is treated as directed. After driv-
ing off the excess of chlorine or bromine by heating, the moderately
acid solution is precipitated with barium chloride. Sulphites may
be oxidized in the dry way by heating in a platinum crucible, with
4 parts of a mixture of equal parts sodium carbonate and potassium
nitrate.

3. THIOSULPHURIC ACID.

Thiosulphuric acid, in form of soluble thiosulphates, may be
determined by means of iodine, in a similar way to sulphurous
acid. The reaction is represented by the equation, 2Na a S a O 3 -f- 21



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