O, but the green solution on
evaporation leaves only an amorphous mass. Investigation has shown
that the change is due to the splitting off of sulphuric acid during
the process, and that green-coloured chrom-sulphuric acids are formed
thus--

2Cr2(SO4)3 + H2O = H2SO4 + [Cr4O.(SO4)4]SO4
(violet) (green)

since, on adding barium chloride to the green solution, only one-third
of the total sulphuric acid is precipitated as barium sulphate, whence
it follows that only one-third of the original SO4 ions are present in
the green solution. The green salt in aqueous solution, on standing,
gradually passes back to the violet form. Several other complex
chrom-sulphuric acids are known, e.g.

[Cr2(SO4)4]H2; [Cr2(SO4)5]H4; [Cr2(SO4)6]H6

(see A. Recoura, _Annales de Chimie et de Physique_, 1895 (7), 4, p.
505.)

Chromic sulphate combines with the sulphates of the alkali metals to
form double sulphates, which correspond to the alums. Chrome alum,
K2SO4.Cr2(SO4)3.24H2O, is best prepared by passing sulphur dioxide
through a solution of potassium bichromate containing the calculated
quantity of sulphuric acid,

K2Cr2O7 + 3SO2 + H2SO4 = H2O + K2SO4 + Cr2(SO4)3.

On evaporating the solution dark purple octahedra of the alum are
obtained. It is easily soluble in warm water, the solution being of a
dull blue tint, and is used in calico-printing, dyeing and tanning.
Chromium ammonium sulphate, (NH4)2SO4.Cr2(SO4)3.24H2O, results on
mixing equivalent quantities of chromic sulphate and ammonium sulphate
in aqueous solution and allowing the mixture to crystallize. It forms
red octahedra and is less soluble in water than the corresponding
potassium compound. The salt CrClSO4.8H2O has been described. By
passing ammonia over heated chromic chloride, the nitride, CrN, is
formed as a brownish powder. By the action of concentrated sulphuric
acid it is transformed into chromium ammonium sulphate.

The nitrate, Cr(NO3)3.9H2O, crystallizes in purple prisms and results
on dissolving the hydroxide in nitric acid, its solution turns green
on boiling. A phosphide, PCr, is known; it burns in oxygen forming the
phosphate. By adding sodium phosphate to an excess of chrome alum the
violet phosphate, CrPO4.6H2O, is precipitated; on heating to 100 deg. C.
it loses water and turns green. A green precipitate, perhaps
CrPO4.3H2O, is obtained on adding