Chromic Bromide, CrBr₃

Chromic Bromide, CrBr₃, sublimes as dark green, lustrous hexagonal crystals, slightly dichroic, when pellets of an intimate mixture of carbon and chromium sesquioxide are heated in dry bromine vapour, or when strongly heated chromium is acted upon by bromine vapour in an atmosphere of nitrogen. When chromic bromide is heated in air, chromium sesquioxide is formed; it is easily reduced by hydrogen, and is attacked by fluorine at a red heat. The anhydrous salt is insoluble in water, but dissolves in the presence of a little chromous salt. A green solution is also obtained by dissolving chromic hydroxide in aqueous hydrobromic acid, or by digesting silver chromate in an alcoholic solution of hydrogen bromide. The hexahydrate, CrBr₃.6H₂O, exists in two isomeric modifications, as bluish-grey ("violet") and green crystals respectively, the former being stable in solution. The green variety, dibromtetraquochromic bromide [(H₂O)₄CrBr₂]Br.2H₂O, is obtained as green deliquescent needles by evaporating a mixture of a saturated solution of chromic acid with excess of 50 per cent, hydrobromic acid. The crystals are soluble in water and alcohol, but insoluble in ether; in the solid phase they are stable, but in solution, especially on warming, the violet modification is rapidly formed with evolution of heat. On boiling a concentrated solution of the green salt, and saturating with hydrogen bromide after cooling, the violet hydrate, hexaquochromium bromide [Cr(H₂O)₆]Br₃, is deposited in fine hygroscopic crystals, very soluble in water, but insoluble in alcohol. The heat of transformation of the violet to the green variety is 2.15 Calories per gram-molecule. An octahydrate, CrBr₃.8H₂O, is described in the form of violet cubic crystals, obtained by the interaction of potassium trichromate and hydrobromic acid. The double salt, [(H₂O)CrBr₅]Rb₂, is stated to exist.