Chromic Fluoride, CrF₃

Chromic Fluoride, CrF₃, is obtained in the anhydrous condition either by passing hydrogen fluoride over heated chromic chloride, or by strongly heating a mixture of dried (but not ignited) chromic oxide and aqueous hydrofluoric acid. It is a green substance, of density 3.78, which melts at 1100° C. and sublimes at 1200° C. Several hydrates are known. The heptahydrate, CrF₃.7H₂O, which crystallises in green prisms, may be prepared by treating an aqueous solution of the anhydrous salt with alcohol. The tetrahydrate, CrF₃.4H₂O, which may be used as a mordant, can be obtained from a solution of chromic hydroxide in hydrofluoric acid. By addition of ammonium fluoride to a cold solution of chromic sulphate, the slightly soluble crystalline nonahydrate, CrF₃.9H₂O, results. In hydrochloric acid it yields a violet, and in potash a green, solution. On heating in the air it decomposes, leaving a residue of chromium sesquioxide, Cr₂O₃. The hydrates, CrF₃.6H₂O, 2CrF₃.7H₂O, and CrF₃.3H₂O, have been described, as also have complex fluorides of chromium and pyridine.

Double fluorides with the alkali metals, ammonium, copper, zinc, nickel, and cobalt, are known, namely:

CrF₃.2KF.H₂O; CrF₃.3KF;
CrF₃.2NaFH₂O;
CrF₃.2NH₄F.H₂O; CrF₃.3NH₄F;
CrF₃.CuF₂.5H₂O;
CrF₃.ZnF₂.7H₂O; CrF₃.NiF₂.7H₂O;
CrF₃.CoF₂.7H₂O.