Chromous Chloride, CrCl₂

Chromous Chloride, CrCl₂, may be prepared in the anhydrous condition by heating chromic chloride, CrCl₃, to incipient redness in a current of dry hydrogen quite free from oxygen, or at a high temperature in ammonium chloride vapour; by passing hydrogen chloride over chromium at a red heat; and, together with chromic chloride, by the action of chlorine on a mixture of chromium sesquioxide and carbon. Anhydrous chromous chloride can also be obtained by carefully heating the hydrated salt at 250° C. in a current of nitrogen. It forms white lustrous hygroscopic needles of density 2.751 at 14° C., stable in dry air. It volatilises with difficulty at a high temperature, the vapour density diminishing with rise in temperature, thus:

Temperature, ° C.	Vapour Density (Air = 1).
1300-1400	7.800
1400-1500	7.278
1500-1600	6.244

The theoretical vapour density corresponding to the formula CrCl₂ is 4.256. with production of an oxychloride, Cr₂Cl₄O. For this reason it has been proposed for use in gas analysis, but it is not a trustworthy reagent, the absorption of oxygen not being complete. The solution can readily be obtained by reduction, in absence of air, of a solution of chromic chloride, or of a mixture of potassium dichromate solution and fuming hydrochloric acid, by means of zinc; or, together with chromic chloride, by the electrolysis of a solution of potassium chloride, using a chromium anode.

With alkali hydroxide a solution of chromous chloride yields a yellowish-brown precipitate which absorbs oxygen, forming a hydrated chromo-chromic oxide, Cr₃O₄.H₂O.

The tetrahydrate, CrCl₂.4H₂O, is formed when chromous chloride solution is rapidly evaporated to dryness below 50° C. in a vacuum. The hydrate is capable of existing in two distinct modifications, namely, as dark blue and dark green needles respectively, the solution being green when hot, concentrated, and acidulated, or blue if cold and dilute. The tri- and di-hydrates, CrCl₂.3H₂O and CrCl₂.2H₂O, also exist, the former as pale blue crystals and the latter in a light green amorphous condition. A hexahydrate, CrCl₂.6H₂O, has also been described, but its existence has not been confirmed. In solution, chromous chloride is catalytically decomposed by platinum. A hydrochloride, 3CrCl₂.2HCl.13H₂O, is formed on passing hydrogen chloride free from oxygen into a concentrated solution of the salt. It is a fine powder, unstable at 20° C.