Chromium hydroxide trihydrate

Cr2 03 - 112 0, of indefinite composition occurs. This compound is commonly misnamed as chromic or chromium (ITT) hydroxide [1308-14-1], Cr(OH)2. A tme hydroxide, chromium (ITT) hydroxide trihydrate [41646-40-6], Cr(OH)2 3H20, does exist and is prepared by the slow addition of alkaU hydroxide to a cold aqueous solution of hexaaquachromium(III) ion (40). The fresh precipitate is amphoteric and dissolves in acid or in excess of hydroxide to form the metastable Cr(OH). This ion decomposes upon heating to give the hydrous chromium (ITT) oxide. However, if the precipitate is allowed to age, it resists dissolution in excess hydroxide. 

The reaction of chromium hydroxide with hydrofluoric acid yields the trihydrate, not the anhydrous material. 

Chromium(lll) hydroxide trihydrate Cr(0H)3.3H20 1308-14-1 167.063 blue-grn powder i HjO s acid... 

Tris(ethylenediamine)chromium(III) chloride trihydrate (39.3 g., 0.10 mole) and 22.5 g. (0.15 mole) of (+)-tartaric acid, and a magnetic stirring bar, are placed in a 400-ml. beaker. A 100-ml. volume of water is added, and the mixture is dissolved with stirring. Immediately after the dissolution, 12.6 g. (0.30 mole) of IiOH-H20 is added with continued stirring. The hydroxide dissolves, and shortly after a yellow crystalline precipitate is formed. About 5 minutes after the addition of the hydroxide, 100 ml. of ethanol is added dropwise with continued stirring during a 15-minute period. The precipitate is filtered, and the filtrate is run directly into a mixture of 15 ml. (0.18 mole) of 12 M HC1 and 600 ml. of absolute ethanol. From this system a finely crystalline precipitate of impure (—)D-chloride separates out. The filter flask is replaced, and the crystals are washed with three 50-ml. portions of a 1 1 mixture of ethanol and water and dried in air. (Yield is 27-28 g.) The precipitate of (—)D-chloride from the filtrate is filtered, washed with three 50-ml. portions of ethanol, and dried in air. (Yield is 13-14 g.)... 

Potassium tris(malonato)chromate(IIl) trihydrate can be prepared by the method of Lapraik, reacting freshly precipitated chromium(III) hydroxide with malonic acid and potassium carbonate. Lapraik obtained the crystals by drying the aqueous solution over sulfuric acid, but the crystals are more easily obtained by precipitation in ethanol. 

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