C. Remigius Fresenius.

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simplicity and accuracy leave much to be desired.

1. Determination as Nickelous Oxide.

a. By Precipitation as Nickelous Hydroxide.

Mix the solution with pure solution of potassa or soda in excess,
heat for some time nearly to ebullition, decant 3 or 4 times, boiling
up each time, filter, wash the precipitate thoroughly with hot
water, dry and ignite strongly (avoiding contact with reducing
gases if the oxide is to be weighed) (BussELL*) ( 53). The pre-
cipitation is best effected in a platinum dish; in presence of
nitrohydrochloric acid, or, if the operator does not possess a suf-
ficiently capacious dish of the metal, in a porcelain dish ; glass
vessels, however, should be avoided. Presence of ammoniacal

* Journ. Chem. Soc., xvi, 58.

110.] KICKEL. 303

salts, or of free ammonia does not interfere with the precipita-
tion. For the properties of the precipitate and residue, see 79.
The method, if properly conducted, gives very accurate results.
Instead of weighing the oxide it may be reduced to metal accord-
ing to 110, 2. The thorough washing of the precipitate is a
most essential point. It is necessary also to ascertain whether the
weighed metal (or oxide) has not an alkaline reaction, and whether
it dissolves completely in nitric acid (or hydrochloric in case oxide
is weighed).

5. By Precipitation as Nickel Sulphide.

This process requirest the greatest care and attention. It
is best to proceed according 'to one of the three following
methods:* a. Neutralize with ammonia if necessary the moder-
ately dilute, cold solution (the reaction should be rather slightly
acid than alkaline) contained in not too large a flask, add
ammonium chloride if this or some similarly acting ammo-
nium salt, like the acetate, is not already present in suf-
ficient quantity, and then carefully add colorless or pale-yellow
ammonium sulphide thoroughly saturated with hydrogen sul-
phide, so long as a precipitate still forms. A large excess of
the reagent must be avoided. After mixing fill the flask with
water up to the neck, cork it, and allow it to stand for twenty-
four hours without warming, but in a moderately warm place.
The precipitate will then have settled, and the supernatant liquid
will be colorless or slightly yellow. Decant, filter, and wash as
directed in 109, 1, c. The filtrate and wash- water must be color-
less or only slightly yellow. Dry the precipitate in the funnel,
and transfer as completely as possible to a beaker. Incinerate the
filter in a coil of platinum wire or on a crucible-lid, and add the
ash to the dry precipitate. Now digest the mixture with concen-
trated nitrohydrochloric acid at a gentle heat until all the nickel sul-
phide is dissolved, and the residual sulphur appears of a pure yel-
low color. Add hydrochloric acid, evaporate to drive off the
nitric acid, dilute, filter, and precipitate as directed in a. The
properties of the precipitate are given in 79. If carefully exe-
cuted the method gives accurate results. If the solution, when
precipitating, contains free ammonia, or no ammonium salt, the

* Regarding another method of precipitation (with sodium thiosulphate), see
page 191.


liquid filtered off from the nickel sulphide is always colored more
or less brown, and contains nickel sulphide ( 79, e), which must
be regained by acidulating with acetic acid and boiling. If the
precipitate is not washed as directed, some nickel is very likely
to pass through with the wash-water. If the filter were not first
incinerated, but, together with the precipitate, treated at once
with nitrohydrochloric acid, the nickel could not be completely
precipitated by potassa or soda from the solution of nickel sul-
phide because of the organic substances which the solution would

/?. Add ammonium bicarbonate- to the slightly acidulated
solution of nickel so that the free acid may be neutralized, and
the solution contain a slight excess of ammonium bicarbonate to-
gether with free carbonic acid, and then pass in hydrogen sul-
phide. As soon as the nickel is precipitated, which is very soon
effected, filter and treat the precipitate as in a.

y. To the nickel solution first add ammonia to alkaline re-
action, then add a fairly large quantity of sodium (or ammonium)
acetate, ammonium sulphide in good excess, then acetic acid to
decidedly acid reaction, and finally heat to boiling. The precipi-
tate formed settles well and is treated as in a. Neutralize the
filtrate with ammonia, and test it by adding ammonium sulphide ;
if a black color develops acidulate with acetic acid and heat in
order to precipitate the last portion of nickel as sulphide.

It is not advisable to weigh the nickel sulphide obtained on
ignition with sulphur in hydrogen.

c. By direct Ignition.

The same method as described in 109, 1, e. (Manganese.)

2. Determination as metallic Nickel.

Ignite the oxide or chloride to be reduced in a porcelain cru-
cible in a slow stream of hydrogen (compare 108, 2) at first
gently, then more strongly till the weight is constant. For
properties of the residue, see 79, c. If on dissolving the metal
in nitric acid any silica remains, this must be weighed and de-

3. Determination as Nickel Sulphate.

The nickel solution should be free from other non-volatile
salts. Evaporate with a slight excess of pure sulphuric acid in a
platinum dish to dryness and heat for 15 or 20 minutes moder-

111.] COBALT. 305

ately, so as just to drive off the excess of sulphuric acid without
blackening the yellow sulphate at the edges. It is difficult to be
sure of hitting the exact point, hence we can place no dependence
on this method nor on that of GIBBS, which consists in dissolving
the sulphide in nitric acid and evaporating the solution with sul-
phuric acid. For the properties of the residue, see 79, d.

4. Determination of Nickel Volumetrically.
KUNZEL* precipitates with sodium sulphide, using sodium
nitroprussiate or an ammoniacal silver solution as an indicator of
excess of reagent. WICKE f and FLEISCHER J precipitate by boil-
ing with sodium hypochlorite and caustic soda, and determine the
quantity of oxide by its oxidizing action on arsenous acid or fer-
rous oxide. FR. MOHR determines the action on potassium
iodide. GIBBS || precipitates with oxalic acid and alcrhol, and
determines the oxalic acid in the precipitate with pota^e um per-



a. Solution.

Cobalt and its compounds behave with solvents like the corre-
sponding compounds of nickel. The protosesquioxide of cobalt
obtained by SCHWARZENBERG in microscopic octahedra does not
dissolve in boiling hydrochloric acid, or nitric acid, or iiitrohydro-
chloric acid; but it dissolves in concentrated sulphuric acid, and
in fusing potassium disulphate.

?>. Determination.

Cobalt is determined in the metallic state ( 80) or as sulphate,
being usually first precipitated as cobaltous hydroxide, sulphide, or
tripotassium-cobaltic nitrate. Cobalt may also be determined

"We may convert into

a. By direct reduction. All salts of cobalt, which can be
immediately reduced by hydrogen (chloride, nitrate, carbonate,

* Zeitschr. /. analyt. Chem. , n, 373. \ Ibid. , iv, 424. \ Ibid., x, 219.
%Lehrbuch der Titrirmethode, 3d edit., p. 303.
I Zeitschr. f. analyt. Chem., vn, 259.

306 DEf ERMINATION. [ 111 ,

J. By precipitation as cobaltous hydroxide. All salts of in-
organic acids soluble in water, and insoluble salts of such acids as
may be removed by solution. All salts of volatile organic acids.

c. By precipitation as sulphide. All compounds of cobalt
without exception.

d. By precipitation as tripotassium-cobaltic nitrite. All
compounds of cobalt soluble in water or dilute acetic acid.


a. By simple evaporation and ignition. The oxygen com-
pounds of cobalt and all cobaltous salts of acids which may be
completely expelled by evaporation and ignition with sulphuric

1. By precipitation as sulphide. All compounds of cobtilt
without exception.

The method 1, #, is preferable to all others when it can be
applied; it is quick and gives exact results. The method 1, Z>,
gives better results than it used to be credited with. The direct
conversion of suitable cobalt compounds into sulphate is also quite
satisfactory. The precipitations as sulphide and as tri potassium
cobaltic nitrate are rarely used except in separations. The volu-
metric methods are better adapted for technical than for scientific

1. Determination as metallic Cobalt.

a. By direct reduction.

Evaporate the solution of cobaltous chloride, or nitrate (which
must be free from sulphuric acid and alkali), in a weighed crucible,
to dryness, cover the crucible with a lid having a small aperture in
the middle, conduct through this a moderate current of pure dry
hydrogen, and then apply a gentle heat, which is to be increased
gradually to intense redness. When the reduction is considered
complete, allow to cool in the current of hydrogen, and weigh ;
ignite again in the same way and repeat the process until the
weight remains constant. The results are accurate. For the
properties of cobalt, see 80.

As regards the apparatus to be employed, see 108, 2.

1. By precipitation as cobaltous hydroxide.

The best material for the precipitating vessel is platinum,
porcelain may also be used, but not glass. First remove any large
excess of acid which may be piv-rnt by evaporation. Heat nearly

111.] COBALT. 307

to boiling, add pure potassa in slight excess, and continue heating
till the precipitate is brownish-black. Pour the supernatant fluid
through a filter, wash the precipitate by decantation with boiling
rater repeatedly, transfer it to the filter, and continue the washing
with boiling water till the washings are free from any trace of
dissolved substance. Dry, ignite in a porcelain crucible ( 52) till
the filter is thoroughly burnt, reduce in a current of hydrogen,
wash the metal several times with boiling water, dry, ignite again
in hydrogen and weigh. Test the weighed cobalt by dissolving in
nitric acid. If any silica remains, this must be weighed and de-
ducted. Mix the solution with ammonium chloride and ammo-
nium carbonate, if a small precipitate (alumina or perhaps a
trace of ferric hydroxide) forms, ignite and weigh this too and
deduct it. The results are excellent ; the amount of alkali which
remains with the metal when the work is done properly being ex-
ceedingly minute. Compare 80-, a.

c. By precipitation as sulphide.

Put the solution in a flask, add ammonium chloride, then
ammonia just in excess, then ammonium sulphide as long as a
precipitate is produced, fill up to the heck with water, cork and
allow to stand 12 or 2-i hours in a warm place. Decant, filter, and
wash as directed 109, 2. Finally, dry and proceed as directed
110, 5, <*, to redissolve the cobalt sulphide. Determine the
cobalt according to I. There are no sources of error in the pre-
cipitation with ammonium sulphide. For the properties of cobalt
sulphide, sec 80. It cannot be brought into a weighable form
by ignition in hydrogen, as the residue is a variable mixture of
different sulphides (II. HOSE). Cobalt may also be thrown down
as sulphide by the other methods given under Nickel. The
thorough precipitation of cobalt is much easier than that of nickel.

d. By precipitation as tripotassium cobaltic nitrate.

To the moderately concentrated solution of the cobalt salt add
potassa in excess, then acetic acid till the precipitate is just redis-
solved, then a concentrated solution of potassium nitrite previously
just acidified with acetic acid, and allow to stand 24 hours at a
gentle heat. Filter, wash with solution of potassium acetate (1 in
10) containing some potassium nitrite, till all foreign substances are
removed, dry, dissolve with the filter ash in hydrochloric acid,
filter and determine the cobalt according to 1, I. This method


was introduced by A. STKOHEYEK ; * the present modification, first
suggested by H. ROSE, and improved by FR. GAUHE, is the surest
to yield good results (GAUHE f). For the properties of the pre-
cipitates, see 80, e.

2. Determination as sulphate.

a. By direct conversion.

To the solution of cobaltous sulphate add a little more sul-
phuric acid than will suffice to form cobaltous sulphate with all
the cobalt present if a volatile acid is present. Evaporate, using
'a platinum dish or platinum crucible, at all events, to finish the
operation. Heat the residue cautiously over the lamp, gradually
increasing the temperature to dull redness, and maintain at this
point for 15 minutes. Should the edges blacken, moisten with
dilute sulphuric acid, dry, and ignite again with greater caution.
Properties of the precipitate, 80. Hesults quite satisfactory.^:

b. With previous precipitation as sulphide.

Precipitate the cobalt as sulphide according tol, 0, dissolve it
as directed, evaporate with excess of sulphuric acid in a porcelain
dish to dryness, take up the residue with water, transfer the solu-
tion to a weighed platinum dish and proceed according to 2, a.

3. Volumetric Methods of Estimating Cobalt.

1. According to CL. WINKLER.

Principle: On adding finely divided mercuric oxide, sus-
pended in water, to an aqueous solution of cobaltous chloride
( 60, 4), no decomposition takes place, and no cobaltous hydrox-
ide is precipitated. On adding a solution of potassium now, hy-
drated manganese dioxide and cobaltic hydroxide are precipitated.

(6CoCl 2 + 5HgO + 1 1H 2 O + 2KMnO 4 = 3Co 2 [OH] 6
+ 2MnO 2 [H 2 O] + 5IIgCl a + 2KC1).

This equation does not exactly express the change that occurs,
because with the cobaltic hydroxide there is always precipitated
a certain proportion of cobaltous hydroxide (or perhaps a com-
pound intermediate between Co 2 O 3 and CoO) ; hence the perman-

* Annal. d. Chem. u. Pharm., xcvi, 218. \Zcitschr.f. analyl. Chem., iv, 60.
J Compare GAT UK, Z< itschr. f. analyt. Chem., iv, 55.
\ZeitMhr.f. analyt. Chem., in, 265; in, 420; vn, 43.

111.] COBALT. 809

ganate sojution ( 112, 2) used must bo standardized, not against
iron or oxalic acid, but against eobaltous chloride, in order to
determine cobalt volumetrically.

Execution. Dissolve O'l or 0'2 grm. of pure metallic cobalt *
in warm hydrochloric acid, transfer to a stoppered 300-c. c. flask,
dilute to 200 c. c. , add an excess of mercuric oxide suspended in
water, and into the cold liquid run in from the burette the potas-
sium-permanganate solution (5 to 6 grm. of the pure crystallized
salt to the litre) in small portions and under constant agitation
until permanent redness of the liquid, in which the brown pre-
cipitate remains suspended. It is difficult at first to see the color of
the liquid, but it is readily observed towards the end, because the
precipitate settles the better the nearer the end of the reaction.
The addition of more mercuric oxide facilitates the deposition.
The operator must not be misled by the gradual disappearance of
the color which occurs on long standing. The c. c. of perman-
ganate solution used up correspond to the cobalt weighed off.
In applying the process to the estimation of unknown quantities
of cobalt, proceed similarly, taking pains to duplicate as nearly as
possible the conditions as regards the quantities of cobalt and the
mercuric oxide added, as well as the dilution.

If the cobalt solution contains sulphuric, phosphoric, or arsenic
acid, or oxygen acids of nitrogen or chlorine, or organic acids, the
method as above detailed is useless; ferric chloride, however, if
present, has no injurious action, because the mercuric oxide im-
mediately precipitates all the iron as ferric hydroxide.

The injurious influence of sulphuric acid may always be neu-
tralized by adding a slight excess of barium chloride ; that of
moderate quantities of phosphoric or arsenic acid may be destroyed
by first adding a sufficient quantity of ferric chloride, and only
then adding the mercuric oxide. If care be taken to add 1 part
of iron for every part of arsenic or phosphoric acid, the acid will

* According to WINKLER pure metallic cobalt may be obtained as follows:
Place a porcelain crucible, one-third filled with, purpureo-cobaltic chloride re-
peatedly recrystallized and free from nickel, within a large platinum crucible,
the lid of which is perforated and provided with a gas-conductmg tube, and ig-
nite in a current of hydrogen, gently at first; then, when most of the ammonium
chloride is expelled, increase the heat, and finally raise to the highest point, and
until no trace of hydrochloric acid is given off; then allow to cool in a current
of hydrogen.


be completely precipitated in the form of basic salts ; nor need
these salts or the barium sulphate formed be filtered off before
proceeding to titrate.

If the cobalt -chloride solution contains manganese, the method
is useless. Small quantities of nickel do no harm, but large are
injurious. Compare ICO (Separation of Cobalt and Nickel).
The results do not satisfy the highest requirements so far as
accuracy is concerned, but are perfectly satisfactory for technical

2. In regard to other methods of volumetrically estimating
cobalt, see Nickel. All the methods there given are applicable to
cobalt. FLEISCHER'S method will also be detailed under Separa-
tion of Cobalt and Nickel ( 160).



a. Solution.

Many ferrous compounds are soluble in water. Those which
are insoluble in water dissolve almost without exception in hydro-
chloric acid ; the solutions, if not prepared with perfect exclusion
of air, and with solvents absolutely free from air, contain invari-
ably more or less ferric chloride. In cases where it is wished to
avoid the chance of oxidation, the solution of the ferrous com-
pound is effected in a small flask, through which a slow current of
carbonic-acid gas is passed, the transmission of the gas being con-
tinued until the solution is cold. Many native ferrous compounds
cannot be thus dissolved. They are, indeed, rendered soluble by
fusing with sodium carbonate, but in this process ferric oxide is
formed. It is therefore advisable to heat such substances (in the
finest powder) with a mixture of 3 parts concentrated sulphuric
acid and 1 part water in a strong sealed tube of Bohemian glass
for 2 hours at about 210, or in the case of silicates to warm
them with a mixture of 2 parts hydrochloric acid and 1 part strong
hydrofluoric acid in a covered platinum dish (A. Mi ISCIIKKI.ICH *).
It is advisable to cover the water-bath on which the platinum dish is
heated with a ring of plaster of Paris about 0*1 metre high, and

* Journ. f. prakt. Chem.. \.\\\\, 116.

112.] FERROUS IRON. 311

to place on this a plaster plate having a section cut out at one
side through which carbon dioxide is conducted, so that solution
may take place in an atmosphere that will not oxidize the iron.*
Metallic iron dissolves in hydrochloric acid, and in dilute sul-
phuric acid, as ferrous chloride or sulphate respectively, with evo-
lution of hydrogen; in warm nitric acid it dissolves as ferric
nitrate, and in nitrohydrochloric acid as ferric chloride.

1). Determination.

Ferrous iron may be estimated 1, by dissolving, converting
into ferric iron, and determining the latter gravimetrically or
volumetrically ; 2, by precipitating as sulphide and weighing it as
such, or determining it after conversion into a ferric salt; 3, by a
direct volumetric method ; and 4, by treating with gold trichloride
and weighing the reduced gold.

The methods 1 and 2 are, of course, only applicable when no
ferric compound is present; the method 2 is scarcely ever used
except for separations. The methods included under 3 are adapted
to most cases, and, in absence of other reducing substances, are
especially worthy of recommendation. The method 4 will be
briefly treated of in the supplement to 112 and 113.

As the determination of iron as ferric oxide belongs to 113,
and as the process for precipitating ferrous iron as sulphide is the
same as that for precipitating ferric iron in this form, nothing
remains for us here but to describe the methods of converting
ferrous into ferric salts and the processes included under 3.

1. Methods of converting Ferrous into Ferric Iron.

a. Methods, applicable in all cases.

Heat the solution of the ferrous salt with hydrochloric acid and
add small portions of potassium chlorate, till the fluid, even after
warming for some time, still smells strongly of chlorine. Our
object may be also attained by passing chlorine gas or in the case
of small quantities by addition of chlorine water, or very con-
veniently by adding solution of bromine in hydrochloric acid. If
the solution is required to be free from excess of chlorine or
bromine, it is finally heated, till all odor of chlorine or bromine
has disappeared.

* Somewhat more complicated apparatus for attaining the object have been
described by COOKE (Zeitschr. /. analyt. Chem., vii, 98) and WILBTJR and
WHITTLESEY (ibid., x, 98),


b. Methods which are only suitable w.'ien the iron is to be subse
quently precipitated by ammonia, as ferric hydroxide.

Mix the solution of the ferrous salt in a flask with a little
hydrochloric acid, if it does not already contain any; add some
nitric acid, and heat the mixture for some time to incipient ebulli-
tion. The color of the fluid will show whether the nitric acid has
been added in sufficient quantity. Though an excess of nitric acid
does no harm, still it is better to avoid adding too much on account
of the subsequent precipitation. In concentrated solutions, the
addition of nitric acid produces a dark-brown color, which disap-
pears upon heating. This color is owing to the nitrogen dioxide
(N 2 O 2 ) formed dissolving in the portion of the solution which still
contains ferrous salt.

c. Methods which can be employed only when the ferric iron is
to be determined vohimetrically.

Add to the hydrochloric solution small quantities of artificially
prepared iron-free manganese dioxide, till the solution is of a dark
olive-green color from the formation of manganic chloride ; boil
till this coloration and the odor of chlorine have disappeared (Fit.
MOIIR) ; or you may add pure potassium permanganate (in crystals
or concentrated solution) till the fluid is just red and then boil, till
the red color and chlorine-odor have vanished. These methods
present the advantage of permitting complete conversion of ferrous
i:ito ferric salts without the use of any considerable excess of the
oxidizing agent.

2. Volumetric Determination.


If we add to a solution of ferrous salt, containing an excess of


sulphuric acid, potassium permanganate, the former is converted
into a ferric salt by the oxidizing action of the latter (10FeSO 4 +
8II a S0 4 + K a Mn a () 8 =: 5Fe a (SO 4 ) 3 + K a SO 4 -f 2MnSO 4 + 8II a O).
Now if we possess a solution of potassium permanganate, and know
how much iron 100 c.c. of it can convert from the ferrous to the ferric
condition, we can, with this, readily determine an unknown quan-
tity of iron; we have simply, for this purpose, to dissolve the iron
in acid, in the form of a ferrous salt, to oxidize the solution accu-
rately, and note how many c.c. of the solution of potassium per-
manganate have been used to accomplish that object.

11*2.] FERllOUS IRON. 313

It must be remarked here that tlie reaction takes place accord-

t - *

ing to the above equation only if the free acid present is sulphuric
acid, whereas in the presence of hydrochloric acid (see y), certain
changes occur (LOWENTHAL and LENSSEN*). The changes may,
however, be to some extent compensated by operating in a certain
manner, but the results cannot be considered as reliable (see y).

of. Titration of the Solution of Potassium Permanga-

Dissolve 5 grin, (roughly weighed) of pure crystallized potas-
sium permanganate in distilled water by the aid of heat, dilute to
1 litre, and preserve in glass- stoppered bottle. Action of direct
sunlight on the solution should be avoided. The solution if care-
fully kept does not alter, but still it is well to titrate it afresh
occasionally, while care must be taken to avoid contamination with
organic matter (as on opening the bottle).

aa. Titration l>y Metallic Iron.

Weigh off accurately about 1 grin, thin soft iron wire previ-
ously cleaned with emery paper, transfer to a J-litre measuring

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