When a slight excess of an alkali carbonate is added to a solution of a chromic salt, a light greenish-blue precipitate falls, soluble, when freshly formed, in excess of the alkali carbonate or of borax. The composition of the precipitate appears to vary with the conditions, the following compounds having been indicated: 4Cr2O3.CO2.H2O; 2Cr2O3.CO2.6H2O; 10Cr2O3.7CO2.8H2O; Cr2O3.CO2.4H2O; Cr2O3.2CO2. According to Lefort, the addition of an alkali carbonate to a solution of a green chromic salt precipitates chromic hydroxide, the carbonate only being deposited when violet salts are used; Parkmann maintains that both give the carbonate. The compound, Cr2O3.CO2.4H2O, when heated to about 75° C., loses 3 molecules of water, but the remaining molecule and carbon dioxide are only driven off at 300° C.
When pure chromic hydroxide is exposed to the air, it absorbs carbon dioxide until the saturation limit is reached. Analysis of the compound obtained points to the formula [Cr2(OH)5]2CO3.8H2O. It can be dried at 100° C. without loss of CO2, which, however, is liberated by acids. |