C. Remigius Fresenius.

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As soon as all the chromium is oxidized, precipitate with sodium
carbonate by ths aid of heat, and proceed for the rest according
to 55 (GIBBS *). Bromine instead of chlorine may be used;
however, the oxidation is but tardily effected by the mere addi-
tion of bromine water.

ft. Neutralize the solution with sodium carbonate, add
sodium hypochlorite, and heat, if necessary, with more hypo-
chlorite, until all the chromium is converted into chromate.
Then add again sodium carbonate, heat, decant the yellow
solution through a filter, boil the residue anew with a sodium-
carbonate solution and proceed as in 55.

2. CHROMIUM FROM BARIUM, STRONTIUM, AND CALCIUM. To 57
separate barium and strontium, precipitate the moderately acid,
hot, dilute solution with sulphuric acid in the presence of
strontium, allow to cool and add alcohol and when the pre-
cipitate has settled, filter. Chromium cannot be separated by
ammonia from the alkali-earth metals, since, even though car-
bonic acid be completely excluded, they are partially precipi-
tated along with the chromic hydroxide. From solutions
containing a salt of chromium, calcium cannot be precipitated
completely by ammonium oxalate ; but it may be by sulphuric
acid and alcohol ( 103, 1).

* Zeitschr. /. analyt. Chem., in, 328.



SEPARATION. [ 157.

3. CHROMIUM may also be separated from MAGNESIUM and 58
small quantities of CALCIUM by means of barium carbonate.
See 54.

III. SEPARATION OF CHROMIUM FROM ALUMINIUM.*
157.

a. Fuse the oxides with 2 parts of potassium nitrate and 4 59
parts of sodium carbonate in a platinum crucible, treat the fused
mass with boiling water, rinse the contents of the crucible into

a porcelain dish or beaker, add a somewhat large quantity of
potassium chlorate, supersaturate slightly with hydrochloric
acid, evaporate to the consistence of syrup, and add, during the
latter process, some more potassium chlorate in portions, to
remove the free hydrochloric acid. Dilute now with water,
and separate the aluminium and chromium as directed in 130,
II., <?, ot. If you omit the evaporation with hydrochloric acid
and potassium chlorate, part of the chromic acid will be reduced
by the nitrous acid in the fluid, and chromic hydroxide will
accordingly, upon addition of ammonia, be precipitated with
the aluminium hydroxide (DEXTER-)-).

b. Dissolve the oxides in hydrochloric acid, add soda or 60
potassa solution in sufficient excess and saturate the clear green
solution with chlorine gas. The chromium will be converted
into chromic acid, and the aluminium partially separated.
When the fluid has become of a pure yellow color, heat to
remove the excess of chlorine, add ammonium carbonate, and
digest to destroy the hypochlorous acid and precipitate the still
dissolved aluminium, and proceed according to 130, II., c, a



c. Nearly neutralize the acid solution with sodirxm carbonate, 61
add sodium acetate in excess, pass chlorine or add bromine and
warm. The chromium will readily be converted into chromic
acid, especially if sodium carbonate is added every uow and then
to keep the fluid nearly neutral. As soon as this is effected
proceed according to 130, II., c, a (GIBBS).

* The separation of aluminium from titanic acid will be gi /<m under the
Analysis of Silicates.

f Pogg. Annal., LXXXIX, 142.

\ Annul, d. Chtm. u. Pharm., cvi, 121. Zeitschr. f. analy^ Chem., HI, 327.



158.] BASES OF GROUP IV. 631

Fourth Group.

ZINC MANGANESE NICKEL COBALT FERROUS IRON FERRIC

IRON (URANIUM).

I. SEPARATION OF THE METALS OF THE FOURTH GROUP FROM
THE ALKALIES.

158.
A, General Methods.

1. ALL METALS OF THE FOURTH GROUP FROM AMMONIUM.

Proceed as for the separation of chromium and aluminium 62
from ammonium, 155 (41). It must be borne in mind that the
oxides of the fourth group comport themselves, upon ignition
with ammonium chloride, as follows : Ferric oxide is partly
converted into ferric chloride which volatilizes ; the oxides of
manganese are converted into manganous chloride and manga-
nous oxide with volatilization of some of the former ; * the
oxides of nickel and cobalt are reduced to the metallic state, no
chloride being lost by volatilization ; f oxide of zinc is converted
into chloride which volatilizes. It is, therefore, generally the
safest way to add sodium carbonate. The ammonium is deter-
mined in a separate portion.

2. ALL METALS OF THE FOURTH GROUP FROM POTASSIUM
AND SODIUM.

Mix the solution in a flask with ammonium chloride if 63
necessary, add ammonia till neutral or slightly alkaline, then
yellow ammonium sulphide saturated with hydrogen sulphide,
fill the flask nearly to the top with water, cork it, allow the
precipitated sulphides to subside, and then filter them off from
the fluid containing the alkalies. In performing this process
the precautionary rules given under the heads of the, several
metals in question ( 108 113) must be borne in mind.J; (If,
notwithstanding, the filtrate is brownish, acidify it with acetic

* Zeitschr. f. analyt. Chem., xi, 424. f Ib., xn, 73.

J Manganese may be separated from the alkalies according to 109, 2, b.
Nickel and cobalt may be separated from the alkalies according to 66, substi-
tuting ammonium acetate for sodium acetate.



632 SEPARATION. [ 158.

acid, pass hydrogen sulphide, boil, and filter off the small quan-
tity of the nickel sulphide which then separates.) Acidify the
filtrate with hydrochloric acid, evaporate, filter off the sulphur
if necessary, continue the evaporation to dryness, ignite the
residue to remove ammonium salts, and determine the alkalies
by the methods given in 152.

B. Special Methods.

1. ZINO FROM POTASSIUM AND SODIUM, by precipitating the 64
zinc from the solution of the acetates with hydrogen sulphide.
(See 87.)

2. NICKEL(OUS) AND COBALT(OUS) FROM THE ALKALIES, by
igniting the chlorides in a current of hydrogen and treating
the residue with water. Precipitation of the alkalies as silico-
fluorides ( 97, 5 STOLBA *) is less suitable for sodium than
for potassium.

3. FERRIC IRON FROM POTASSIUM AND SODIUM, by precip-
itating with ammonia, or by heating the nitrates. (See 45 and
46.)

4. MANGANESE FROM THE ALKALIES. Mix the neutral or 65
slightly acid solution with ammonium chloride and precipitate
the manganese with a slight excess of ammonium carbonate.
Allow the precipitate to settle in a warm place, filter through

a thick filter, wash with hot water, and weigh as protosesqui-
oxide (II. TAMM f). In the filtrate separate the alkalies from
ammonium salts by gentle ignition. The separation of man-
ganese as hydrated peroxide cannot be recommended, as the
precipitate retains alkali 4



* Zeitsc?ir. f. analyt. Chem. t ix, 100.
\Ib., xi. 425.
i 2b., xi, 298.



159.] BASKS OF GROUP IV. 633

<

II. SEPARATION OF THE METALS or THE FOURTH GEOTJP FEOM

THOSE OF THE SECOND.

159.

INDEX. (The numbers refer to those in the margin.)

Zinc from barium and strontium, 66, 67, 68, 73.
" calcium, 66, 68, 73.
" magnesium, 66, 68.
Manganese from barium and strontium, 66, 67, 70, 71, 72.

". calcium and magnesium, 66, 70, 71, 72.

Nickel and cobalt from barium and strontium, 66, 67, 73, 75.
'. calcium, 66, 73, 75.

41 magnesium, 66, 74.

Ferric iron from barium and strontium, 66, 67, 69.
" calcium and magnesium, 66, 69.

A. General Method.

ALL METALS OF THE FOURTH GROUP FROM THE ALKALI-
EARTH METALS.

Add ammonium chloride, and, if acid, also ammonia, and 66
precipitate with ammonium sulphide, as in 63. Take care to
use slightly yellow ammonium sulphide, perfectly saturated
with hydrogen sulphide, and free from ammonium carbonate
and sulphate, and to employ it in sufficient excess. Insert the
cork, and let the flask stand for some time to allow the precipi-
tate to subside, then wash quickly, and so far as practicable
out of the contact of air, with water to which some ammonium
sulphide has been added. Acidify the nitrate with hydro-
chloric acid, heat, filter from the sulphur, and separate the
alkali-earth metals, as directed in 1'54. If the filtrate is brown-
ish from a little dissolved nickel sulphide, make it slightly acid
with acetic acid instead of with hydrochloric acid, add some
alkali acetate, pass hydrogen sulphide, boil, and filter.

If the quantity of the alkali-earth metals is rather consider*
able, it is advisable to treat the slightly washed precipitate once
more with hydrochloric acid (in presence of nickel or cobalt it
is not necessary to effect complete solution), heat the solution
gently for some time, and then reprecipitate in the same way.

If we have merely to effect removal of nickel and cobalt,
we may also add ammonium sulphide, acidulate with acetic



634 SEPARATION. [159.

acid, add alkali acetate, heat, pass hydrogen- sulphide gas
through the solution while boiling, and filter. It is always
necessary to test the filtrate with ammonium sulphide, how-
ever, to make sure that all the nickel and cobalt are precipitated.
(Compare 90.)

In separating manganese from the alkaline earths the
method of precipitation described in 109, 2, I may be
employed, but even in this method a double precipitation is
advisable.

B. Special Methods.

1. BARIUM AND STRONTIUM FROM THE WHOLE OF THE METALS 67
OF THE FOURTH GROUP.

Precipitate the barium and strontium from the slightly acid
solution with sulphuric acid ( 101 , 102). The barium sulphate
should first be washed with water acidified with hydrochloric
acid, but even then you cannot be sure of getting it free from
iron. The sulphates, after weighing, must therefore always
be tested for iron, etc.

2. ZlNO FROM THE ALKALI-EARTH METALS. 68

a. Convert the basic metals into acetates, and precipitate
the zinc from the solution according to 108, 1, I.

b. Evaporate the solution of the chloride with an excess
of ammonium chloride, ignite the residue, and, if necessary,
repeat the operation. The zinc is completely volatilized as
zinc chloride, and the alkaline earths remain behind.

3. FERRIC IRON FROM THE ALKALI-EARTH METALS. 69
a. Mix the somewhat acid solution with enough ammonium

chloride, boil, add slight excess of ammonia, boil till the excess
of the latter is nearly expelled, and filter. The solution is free
from iron; the precipitate is free from calcium, barium, and
strontium, but contains a very slight trace of magnesium (H.
ROSE*). In delicate analyses, after moderately washing the
ferric hydroxide, redissolve it in hydrochloric acid, and repeat
the precipitation.

I. Precipitate the iron as basic ferric acetate or formate,

* Pogg. Annal., ex, 300.



159.] EASES OF GROUP IV. 635

t

compare 84 and 85. The method is good, and can frequently
be employed.

c. Precipitate the iron with ammonium succinate (86).

d. Decompose the nitrates by heat (46). A good method.*

e. Precipitate the diluted, weakly acid solution with ba-
rium carbonate, and filter after a short digestion in the cold
(54). Only applicable for the separation of iron from calcium
and magnesium.

4. MANGANESE FROM THE ALKALINE EARTHS.

a. Methods ~based upon the Precipitation of Man-
ganese as Sesqui oxide or Dioxide.

a. The methods of GIBBS, f SCHIEL,^: H. ROSE, and others 70
(described in the previous edition), in which the manganese is
precipitated as a hydrated manganic oxide by lead peroxide,
by adding sodium acetate and passing in a current of chlorine
gas, or by bromine, cannot be recommended, because notable
traces of the alkaline earths are precipitated with the hydrated
manganic oxide. Compare also R. FINKENER, || who has made
similar observations. According to GIBBS T the errors may
be decreased by double precipitation. As the precipitate,
however, as a rule contains alkali also,** and is therefore un-
fit for weighing after simple ignition, these methods can be
but rarely made use of.

ft. After DEviLLE.ff The bases must be present as 71
nitrates. Heat in a covered platinum dish to from 200 to
250 until all fumes cease to form, and the mass has become
black, then proceed as in 46. If a small quantity of organic
matter be present, or if the heat applied is too strong, some
manganese dioxide may be reduced and be dissolved in the
ammonium nitrate, hence the solution must always be tested
for manganese. According to my experience the precipitate
is not entirely free from alkaline earths.

* Compare LATSCHINOW, Zeitschr.f. analyt. Chem., vn, 213.

f Annac. de Chem. u. Pharm., LXXXVI, 54.

ISillim. Journ., xv, 275.

Pogg. Annal., ex, 305.

|| Handb. d. analyt. Chem., v. H. ROSE, 6. Aufl. v. R. FINKENER, II, 925.

^Zeitschr.f. analyt, Chem., ill, 331.

** 1W., xi, 298, tt Journ. f. prakt. Chem., LX, 11.



636 SEPARATION. [ 159.

I. Methods lased on the Volumetric Determination
of Manganese, according to BUNSEN and KRIEGER.*

a. MANGANESE FROM MAGNESIUM. Precipitate with soda 72
( 109, 1, b). Ignite and weigh the well- washed precipitate.
If the quantity of magnesium present is sufficient, the residue
has the composition Mii a O,*MgO + ccMgO. Treat a weighed
uample of it according to 142 ; this will give the quantity
of manganese (1 eq. of chlorine, corresponding to 1 eq. of
iodine liberated, is the equivalent of 1 eq. Mn 3 O 3 ) ; the dif-
ference will give the magnesia.

ft. From BARIUM and STRONTIUM. Precipitate with so-
dium carbonate ( 109, 1, a). The ignited precipitate has
the formula Mn a O, BaO + aBaCO 3 .

Treat a sample as in a, and so find the quantity of man-
ganese. The quantity of barium carbonate is found on de-
ducting the weight of the manganese sesquioxide from that of
the weighed precipitate, and adding to the difference as much
carbonic acid as has been expelled by the sesquioxide, i.e., for
each eq. of Mn,O, , 1 eq. of CO a .

y. FROM CALCIUM. Proceed as directed under barium
and strontium, but, after ignition, moisten repeatedly with
ammonium carbonate, dry, and ignite gently until the weight
remains constant. Here, however, it is advisable to ignite the
precipitate in a blast-lamp flame until calcium oxide has
formed.

N. B. This method of volumetric estimation of manga-
nese presupposes that more than 1 eq. of MgO'CaO, etc., is
present for every eq. of Mn a O s , otherwise the residue will
contain, besides Mn,O,, some Mn,O 4 also. In order to adapt
tin- method in such a case also, KRIEGER recommends to dis-
solve a sample of the weighed precipitate, add half its weight
of zinc oxide, precipitate with sodium carbonate, determine
the quantity of precipitate after long-continued ignition with
access of air, and then to use all or a part of the residue so
obtained for the volumetric determination. The precipitate
contains all the manganese as Mn,O 8 . As will be seen, how-
ever, this modification greatly complicates the process

* Annul, d. Chcm. u. Pharm., LXXXVII. 268.



159.] BASES OF GROUP IV. 637

4

"When employing the methods under 72 it must always be
remembered that the precipitation of manganese by caustic soda
or sodium carbonate can be complete only then when the pre-
cautions stated under 109, 1, a and J, are carefully ob-
served ; also that the precipitates are obtained free from alkali
only when again exhausted with boiling water after ignition.

. d. Solutions containing manganese, calcium, and magne-
sium must not contain ammonium salts. The manganese,
calcium, and magnesium may be present as chlorides, nitrates,
or acetates (or sulphates if but little calcium is in the solution
and care be taken to avoid deposition of calcium sulphate).
Neutralize any free acid which may be present by adding
sodium carbonate till a slight precipitate is formed. Redis-
solve this precipitate by the addition of just sufficient HC1.
Add next sodium acetate to the solution, then aqueous solution
of bromine. The solution should at this point be rather dilute.
Expose to a temperature of 50 to 70 a few hours, till free
bromine is all or nearly all expelled from the solution, and
filter. Test the filtrate by adding more sodium acetate and
more bromine water, and warming. The manganese is pre-
cipitated as hydrated dioxide, which is liable to contain soda.
If the quantity is very small, it may, unless great accuracy is
required, be converted by ignition, after careful washing with
hot water, directly into Mn 3 O 4 , and weighed. If, however,
the quantity is considerable, it should be dissolved in HC1 and
converted into some other suitable form for weighing.

According to FIKKENEK,* manganese dioxide precipitated
as above described (except using chlorine instead of bromine)
from a solution containing the alkali-earth metals, will not be
entirely free from the latter, especially from barium if that is
present. He recommends to dissolve the manganese precipi-
tate, and reprecipitate boiling hot with ammonium sulphide, by
which means pure manganese sulphide is obtained. GIBBS t
observes that when manganese is separated from zinc, calcium,
and magnesium by the above process (precipitation as dioxide),
a repetition of the process is necessary to secure good results ;

* Handbuch d. analyt. Chem. v. II. ROSE, 6. Aufl. v. FINKENER, n, 925.
f Zeitschr. f. analyt. Chem. , in, 321.



638 SEPARATION. [ 159.

but in case manganese is to be separated only from calcium and
magnesium, the second treatment may be omitted.*

5. COBALT, NICKEL, AND ZINC FROM BARIUM, STRONTIUM,
AND CALCIUM.

Add an excess of sodium carbonate, then add potassium 73
cyanide, warm very gently until all the precipitated cobalt,
nickel, and zinc carbonates are redissolved, and filter off the
carbonates of the alkaline earths from the solution of the metal-
lic cyanides in the potassium- cyanide solution. Dissolve the
former in diluted hydrochloric acid, and separate according to
154; separate the latter according to 160 (HAIDLEN and
FRESENIUS f).

6. COBALT AND NICKEL FROM MAGNESIUM.

Precipitate the solution with a mixture of potassium -hypo- 74
chlorite solution and potassa lye. Thoroughly wash the pre-
cipitate, consisting of the hydroxides of nickel, cobalt, and
magnesium, and while still moist digest it at a temperature of
from 30 to 40 with an excess of mercuric-chloride solution.
A double salt having the composition MgCl a + 3HgCl, is
formed, and the magnesium goes into .solution, while a corre-
sponding equivalent of basic mercury chloride is precipitated
(ULLOREN J). Evaporate the solution and the washings with
the addition of pure mercuric oxide, and determine the magne-
sium according to 104, 3, I. Eemove the nickel and cobalt
oxides by ignition, and separate the metals as detailed below.

7. COBALT AND NICKEL FROM BARIUM, STRONTIUM, AND
CALCIUM.

Ignite the chlorides of the metals in a current of hydrogen 75
and separate the reduced cobalt and nickel from the barium
chloride, etc., by treatment with water.



* E. A. COI.IIY (priv. contrib.) finds, by experiments made in the
Lal>oratory on the separation of Ca from MM, that by proceeding as above directed
only a slight unweighable trace of Ca goes down with the Mn ; while if the
amount of free acetic acid is moderately in.-icas, d, the manganese is precipitated
tntirely free from calcium. Too much acetic acid, however, prevents or delays
.tat ion of MM.

\Annal. d. Chen. u. Pha-rtn., xi.ui, 140.
j BERZELIUS' Jahretber., xxi, 146.



160.] BASES OF GROUP IV. 639

f

III. SEPARATION OF THE METALS OF THE FOURTH GROUP

FROM THOSE OF THE THIRD, AND FROM EACH OTHER.
160.

INDEX. (The numbers refer to those in the margin.)

Aluminium from zinc, 76, 77, 85, 86, 87, 97.

" manganese, 76, 77, 78, 85, 86, 108.

" nickel and cobalt, 76, 77, 80, 85, 86, 97.

" ferrous iron, 76, 77, 78, 85.

ferric iron, 77, 78. 84, 91, 92, 104.
CJiromium from zinc, manganese, nickel, cobalt, and

iron, 76, 77, 93, 94.
" ferric iron, 77, 91, 93, 94.

Zinc from aluminium, 76, 77, 85, 86, 87, 97.
" chromium, 76, 77, 93, 94.
manganese, 81, 87, 88, 109.
nickel, 88, 100, 101, 103.
" cobalt, 88, 96, 99, 101, 102.

ferric iron, 76, 82, 85, 86, 103, 106.
Manganese from aluminium, 76, 77, 78, 85, 86, 108.

chromium, 76, 77, 93, 94.
" zinc, 81, 87, 88, 109.

nickel, 81, 89, 90, 101.
cobalt, 89, 90, 96, 101.
ferric iron, 76. 82, 85, 86, 108.
Nickel from aluminium, 76, 77, 80, 85, 86, 97.
" chromium, 76, 77, 93, 94.

zinc, 88, 100, 101, 102.
manganese, 81, 89, 90, 101.
cobalt, 95, 96, 98, 110.
ferric iron, 76, 80, 82, 85, 86, 89, 106.
Cobalt from aluminium, 76, 77, 80, 85, 86, 87.
chromium, 76, 77, 93, 94.
zinc, 88, 96, 99, 101, 102.
manganese, 89, 90, 96, 101.
" nickel, 95, 96, 98, 110.

ferric iron, 76, 80, 82, 85, 86, 89, 106.
Ferrous iron from aluminium, 76, 77, 78, 85.
chromium, 76, 77, 93, 94.

ferric iron, 76, 83, 85, 105, 107, 111.
Ferric iron from aluminium, 77, 78, 84, 91, 92, 104.
" chromium, 77, 91, 93, 94.

zinc, 76, 82, 85, 86, 103, 106.
" manganese, 76, 82, 85, 86, 108.

nickel, 76, 80, 82, 85, 86, 89, 106.

cobalt, 76, 80, 82, 85, 86, 89, 106.

ferrous iron, 76, 83, 85, 105, 107, 111.



640 SEPARATION. [160.

A. General Methods.

1. Method lased upon the Precipitation of some
Basic Radicals ly Barium Carbonate.

FERRIC IRON, ALUMINIUM, AND CHROMIUM, FROM ALL OTHER
BASIC RADICALS OF THE FOURTH GROUP.

Mix the sufficiently dilute solution of the chlorides or 76
nitrates, but not sulphates, which must contain a little free
acid,* in a flask, with a moderate excess of barium carbonate
diffused in water ; cork, and allow to stand some time in the
cold, with occasional shaking. The ferric iron, aluminium, and
chromium are completely separated,! whilst the other basic
radicals remain in solution, with the exception perhaps of traces
of cobalt and nickel, which will generally fall down with the
precipitate. This may be prevented, at least as regards nickel,
by addition of ammonium chloride to the fluid to be precipi-
tated (SCHWARZENBERG:):). Decant, stir up with cold water,
allow to deposit, decant again, filter, and wash with cold water.
The precipitate contains, beside the precipitated metals,
barium carbonate; and the filtrate, besides the non-precipi-
tated metals, a barium salt.

If ferrous iron is present, and it is wished to separate it by
this method from ferric iron, etc., the air must be excluded
during the whole of the operation. In that case, the solution
of the substance, the precipitation, and the washing by decan-
tation are effected in a flask (A, Fig. 113) through which car-
bonic acid is transmitted (b). The washing water, boiled free
from air and cooled out of contact of air (preferably in a cur-
rent of carbonic acid), is run in through c and the fluid drawn
off by means of a movable tube, d\ all the tubes are fitted air*
tight into the cork and are smeared with tallow. The wash-
water, decanted off from the precipitate as much as possible,
is passed through the asbestos filter, e. It is evident that the
flow of the liquid through d is to be effected by the pressure



* If there is much free acid, the greater part of it must first be saturated
with sodium carbonate.

\ The separation of the chromium requires the most time.
\Annal. d. GKem. u. Pharm., xcvn, 210.



160.]



BASES OF GROUP IV.



641



of carbonic acid, hence this must be taken into considera-
tion when constructing the carbonic-acid evolution apparatus.




Fig. 113.

If e is placed sufficiently low, d y once filled, will act as a
siphon.



64S SEPARATION". [ 160,

2. Methodbased upon the Precipitation of the Metals
of the Fourth Group ~by Sodium Sulphide or Ammo-
nium Sulphide, from Alkaline Solution effected with
t/ie aid of Tartaric Acid.

ALUMINIUM AND CHROMIUM FROM THE METALS OF THE
FOURTH GROUP.

Mix the solution with pure normal potassium tartrate,* then 75
with pure solution of soda or potassa until the fluid has cleared
again ; f add sodium sulphide as long as a precipitate forms,
allow it to deposit until the supernatant fluid no longer exhibits
a greenish or brownish tint ; decant, stir the precipitate up
with water containing sodium sulphide, decant again, transfer
the precipitate, which contains all the metals of the fourth
group, to a filter, wash with water containing sodium sulphide,
and separate the metals as directed in B. Add to the filtrate
potassium nitrate, and evaporate to dryness ; fuse the residue
in a platinum dish, and separate the aluminium from the
chromic acid formed as directed 157. If you have merely to
separate aluminium from the metals of the fourth group, it is
better, after addition of potassium tartrate, to supersaturate
with ammonia, add ammonium chloride, and precipitate in a
flask with ammonium sulphide. When the precipitate has set-
tled it is filtered off and washed with water containing ammo-



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