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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.

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