Rubidium chromium alum

Crystalline rubidium chromium alum has been described. ... 

Biiltemann 2 observed that vanadium ammonium alum separates out in blue crystals from a solution containing sulphuric acid, but from solutions containing a weak acid, or from neutral solutions, red crystals are obtained. (The chromium alums can also be prepared in differently coloured modifications.) The analytical data, melting-point, electrical conductivity, rate of efflorescence, and general behaviour of both kinds of crystals are identical, so that it is difficult to ascribe different constitutions to them. Meyer and Markowitz3 have shown that both forms separate out when the molecular proportion of sulphuric acid in the solution is less than that theoretically required, and attribute the red colour to the presence of traces of vanadous oxide, V203, or its hydroxide, V(OH)3. Vanadium rubidium and vanadium ciesium alums behave in the same way. A vanadium guanidine alum has also been prepared.4... 

Alum, KAl(S0i)i-l2H20.—Ammonium, rubidium, cesium, univalent thallium, and in some cases sodium may replace potassium, while the aluminum may be replaced by trivalent iron, chromium, indium, gallium, titanium, vanadium but not by the rare-earth metals. 

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