Chromous oxide, CrO, is unknown in the free state, but in the hydrated condition as Cr04H 2 0 or Cr(OH) 2 it may be prepared by precipitating chromous chloride by a solution of potassium hydroxide in air-free water.
Chromous sulphate, CrS04 7H 2 0, isomorphous with ferrous sulphate, results on dissolving the metal in dilute sulphuric acid or, better, by dissolving chromous acetate in dilute sulphuric acid, when it separates in blue crystals on cooling the solution.
Chromic chloride, CrC1 31 is obtained in the anhydrous form by igniting a mixture of the sesquioxide and carbon in a current of dry chlorine; it forms violet laminae almost insoluble in water, but dissolves rapidly in presence of a trace of chromous chloride; this action has been regarded as a catalytic action, it being assumed that the insoluble chromic chloride is first reduced by the chromous chloride to the chromous condition and the original chromous chloride converted into soluble chromic chloride, the newly formed chromous chloride then reacting with the insoluble chromic chloride.
Cyanogen compounds of chromium, analogous to those of iron, have been prepared; thus potassium chromocyanide, K 4 Cr(CN) 6.2H 2 0, is formed from potassium cyanide and chromous acetate; on exposure to air it is converted into the chromicyanide, K 3 Cr(CN) 6, which can also be prepared by adding chromic acetate solution to boiling potassium cyanide solution.
Chromium forms three series of compounds, namely the chromous salts corresponding to CrO, chromous oxide, chromic salts, corresponding to Cr203, chromium sesquioxide, and the chromates corresponding to Cr0,, chromium trioxide or chromic anhydride.
On pouring a solution of chromous chloride into a saturated solution of sodium acetate, a red crystalline precipitate of chromous acetate is produced; this is much more permanent in air than the other chromous salts and consequently can be used for their preparation.