SALTS OF OXIDE OF CHROMIUM. 611 



Sulphate of chromium, Cr 2 O 3 , 3SO 3 ; 2507.1 or 200.90. 

 Oxide of chromium is dissolved by sulphuric acid, but the salt 

 does not crystallize. When dried strongly, it loses its solubility. 

 It forms, however, a crystallizable double salt with sulphate of 

 potash, chrome alum, KO, SO 3 + Cr 2 O 3 , 3SO 3 + 24HO. This 

 salt appears when a mixture of its constituent salts, with a little 

 free sulphuric acid, is left to spontaneous evaporation. Its 

 octohedral crystals are of a dark purple colour, and of a beau- 

 tiful ruby red, when so small as to be transparent. The solution 

 of chrome alum is bluish purple, but when heated to 140 or 

 180 becomes green, a change of colour which indicates the 

 decomposition of the salt ; for when afterwards evaporated, it 

 no longer yields crystals of chrome alum, but of sulphate of 

 potash, and the sulphate of chromium dries up into a gummy 

 mass. Iron alum is often decomposed in the same manner, by 

 heating its solution, and is not reproduced on cooling. The 

 best mode of preparing chrome alum is to mix three parts of a 

 saturated solution of neutral chromate of potash, first with one 

 part of oil of vitriol, and then with two parts of alcohol, which 

 is added by small portions to the mixture of acid and chromate, 

 and not to apply artificial heat. The chromic acid is thus 

 deoxidised in a gradual manner, and large crystals of the double 

 sulphate are slowly deposited, (Fischer.) 



Oxalate of chromium and potash, 3 (KO, C 2 O 3 ) -f Cr 2 O 3 , 3C 2 O 3 + 

 6HO. This is another beautiful double salt of chromium. It 

 is easily prepared by the following process of Dr. Gregory. 

 One part of bichromate of potash, two parts of binoxalate of 

 potash, and two of crystallised oxalic acid are dissolved together 

 in hot water. A copious evolution of carbonic acid gas takes 

 place, arising from the deoxidation of the chromic acid, at the 

 expense of a portion of the oxalic acid, and nothing fixed re- 

 mains, except the salt in question, of which a pretty concen- 

 trated solution crystallizes upon cooling in prismatic crystals, 

 which are black by reflected light, but of a splendid blue by 

 transmitted light, when sufficiently thin to be translucent. The 

 oxide of chromium cannot be precipitated from this salt com- 

 pletely by an alkaline carbonate ; and it is remarkable that only 

 a small portion of the oxalic acid is thrown down from it by 

 chloride of calcium. When fully dried and then carefully 

 ignited, this salt is completely decomposed, and leaves a mix- 

 ture of chromate and carbonate of potash. The corresponding 



