The fluoride, CrF3, results on passing hydrofluoric acid over the heated chloride, and sublimes in needles. The hydrated fluoride, CrF3·9H2O, obtained by adding ammonium fluoride to cold chromic sulphate solution, is sparingly soluble in water, and is decomposed by heat.
Oxyhalogen derivatives of chromium are known, the oxychloride, CrO2Cl2, resulting on heating potassium bichromate and common salt with concentrated sulphuric acid. It distils over as a dark red liquid of boiling point 117° C., and is to be regarded as the acid chloride corresponding to chromic acid, CrO2(OH)2. It dissolves iodine and absorbs chlorine, and is decomposed by water with formation of chromic and hydrochloric acids; it takes fire in contact with sulphur, ammonia, alcohol, &c., and explodes in contact with phosphorus; it also acts as a powerful oxidizing agent. Heated in a closed tube at 180° C. it loses chlorine and leaves a black residue of trichromyl chloride, Cr3O6Cl2, which deliquesces on exposure to air. Analogous bromine and iodine compounds are unknown, since bromides and iodides on heating with potassium bichromate and concentrated sulphuric acid give free bromine or free iodine.
The oxyfluoride, CrO2F2, is obtained in a similar manner to the oxychloride by using fluorspar in place of common salt. It may be condensed to a dark red liquid which is decomposed by moist air into chromic acid and chromic fluoride.
The semi-acid chloride, CrO2·Cl·OH, chlorochromic acid, is only known in the form of its salts, the chlorochromates.
Potassium chlorochromate, CrO2·Cl·OK, is produced when potassium bichromate is heated with concentrated hydrochloric acid and a little water, or from chromium oxychloride and saturated potassium chloride solution, when it separates as a red crystalline salt. By suspending it in ether and passing ammonia, potassium amidochromate, CrO2·NH2·OK, is obtained; on evaporating the ether solution, after it has stood for 24 hours, red prisms of the amidochromate separate; it is slowly decomposed by boiling water, and also by nitrous acid, with liberation of nitrogen.
Chromic sulphide, Cr2S3, results on heating chromium and sulphur or on strongly heating the trioxide in a current of sulphuretted hydrogen; it forms a dark green crystalline powder, and on ignition gives the sesquioxide.
Chromic sulphate, Cr2(SO4)3, is prepared by mixing the hydroxide with concentrated sulphuric acid and allowing the mixture to stand, a green solution is first formed which gradually changes to blue, and deposits violet-blue crystals, which are purified by dissolving in water and then precipitating with alcohol. It is soluble in cold water, giving a violet solution, which turns green on boiling. If the violet solution is allowed to evaporate slowly at ordinary temperatures the sulphate crystallizes out as Cr2(SO4)3·15H2O, 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