CoCl2.2H2O and
CoCl2 . 4H2O have been described (P. Sabatier, _Bull. Soc. Chim._ 51,
p. 88; Bersch, _Jahresb. d. Chemie_, 1867, p. 291). Double chlorides
of composition CoCl2.NH4Cl.6H2O; CoCl2.SnCl4.6H2O and
CoCl2.2CdCl2.12H2O are also known. By the addition of excess of
ammonia to a cobalt chloride solution in absence of air, a
greenish-blue precipitate is obtained which, on heating, dissolves in
the solution, giving a rose-red liquid. This solution, on standing,
deposits octahedra of the composition CoCl2.6NH3. These crystals when
heated to 120 deg. C. lose ammonia and are converted into the compound
CoCl2.2NH3 (E. Fremy). The bromide, CoBr2, resembles the chloride, and
may be prepared by similar methods. The hydrated salt readily loses
water on heating, forming at 100 deg. C. the hydrate CoBr2.2H2O, and at
130 deg. C. passing into the anhydrous form. The iodide, CoI2, is produced
by heating cobalt and iodine together, and forms a greyish-green mass
which dissolves readily in water forming a red solution. On
evaporating this solution the hydrated salt CoI2.6H2O is obtained in
hexagonal prisms. It behaves in an analogous manner to CoBr2.6H2O on
heating.

Cobalt fluoride, CoF2.2H2O, is formed when cobalt carbonate is
evaporated with an excess of aqueous hydrofluoric acid, separating in
rose-red crystalline crusts. Electrolysis of a solution in
hydrofluoric acid gives cobaltic fluoride, CoF3.

Sulphides of cobalt of composition Co4S3, CoS, Co3S4, Co2S3 and CoS2
are known. The most common of these sulphides is cobaltous sulphide,
CoS, which occurs naturally as syepoorite, and can be artificially
prepared by heating cobaltous oxide with sulphur, or by fusing
anhydrous cobalt sulphate with barium sulphide and common salt. By
either of these methods, it is obtained in the form of bronze-coloured
crystals. It may be prepared in the amorphous form by heating cobalt
with sulphur dioxide, in a sealed tube, at 200 deg. C. In the hydrated
condition it is formed by the action of alkaline sulphides on
cobaltous salts, or by precipitating cobalt acetate with sulphuretted
hydrogen (in the absence of free acetic acid). It is a black amorphous
powder soluble in concentrated sulphuric and hydrochloric acids, and
when in the moist state readily oxidizes on exposure.

Cobaltous sulphate, CoSO4.7H2O, is found naturally as the mineral
bieberite, and is formed when