Chromic Carbonates

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.