cobalt, cobaltous oxide or carbonate are
dissolved in dilute sulphuric acid. It forms dark red crystals
isomorphous with ferrous sulphate, and readily soluble in water. By
dissolving it in concentrated sulphuric acid and warming the solution,
the anhydrous salt is obtained. Hydrated sulphates of composition
CoSO4.6H2O, CoSO4.4H2O and CoSO4.H2O are also known. The heptahydrated
salt combines with the alkaline sulphates to form double sulphates of
composition CoSO4.M2SO4.6H2O (M = K, NH4, &c.).

The cobaltic salts corresponding to the oxide Co2O3 are generally
unstable compounds which exist only in solution. H. Marshall (_Proc.
Roy. Soc. Edin._ 59, p. 760) has prepared cobaltic sulphate
Co2(SO4)3.18H2O, in the form of small needles, by the electrolysis of
cobalt sulphate. In a similar way potassium and ammonium cobalt alums
have been obtained. A cobaltisulphurous acid, probably H6[(SO3)6.Co2]
has been obtained by E. Berglund (_Berichte_, 1874, 7, p. 469), in
aqueous solution, by dissolving ammonium cobalto-cobaltisulphite
(NH4)2Co2[(SO3)6.Co2].14H2O in dilute hydrochloric or nitric acids, or
by decomposition of its silver salt with hydrochloric acid. The
ammonium cobalto-cobaltisulphite is prepared by saturating an
air-oxidized ammoniacal solution of cobaltous chloride with sulphur
dioxide. The double salts containing the metal in the cobaltic form
are more stable than the corresponding single salts, and of these
potassium cobaltinitrite, Co2(NO2)6.6KNO2.3H2O, is best known. It may
be prepared by the addition of potassium nitrite to an acetic acid
solution of cobalt chloride. The yellow precipitate obtained is washed
with a solution of potassium acetate and finally with dilute alcohol.
The reaction proceeds according to the following equation: 2CoCl2 +
10KNO2 + 4HNO2 = Co2(NO2)6.6KNO2 + 4KCl + 2NO + 2H2O (A. Stromeyer,
_Annalen_, 1855, 96, p. 220). This salt may be used for the separation
of cobalt and nickel, since the latter metal does not form a similar
double nitrite, but it is necessary that the alkaline earth metals
should be absent, for in their presence nickel forms complex nitrites
containing the alkaline earth metal and the alkali metal. A sodium
cobaltinitrite is also known.

Cobalt nitrate, Co(NO3)2.6H2O, is obtained in dark-red monoclinic
tables by the slow evaporation of a solution of the metal, its
hydroxide or carbonate, in nitric aci