The thermal decomposition of alkali metal chlorochromates, MCrO3Cl

Abstract

A systematic study of the thermal decomposition of potassium chlorochromate, KCrO3Cl, has been made, and less complete studies with the corresponding lithium, rubidium and caesium compounds. Three major processes occur, characterized by the formation of chromyl chloride, the mixed oxides, MCr308, and of chromium(III) oxide. In each case there is simultaneous production of dichromate and alkali chloride. ��� Chromyl chloride formation increases with rising temperature but decreases as the cationic radius increases. It is the major reaction when KCr3OCl is heated under vacuum at 400°C. Decomposition with the formation of M2Cr2O7 and MCr3O8 in equimolar amounts occurs below the melting point of KCrO3Cl, but, as the temperature of decomposition is raised, a reaction producing these products in the molar ratio 1 : 2 becomes more important. In the presence of air and alkali halide MCr3O8 can oxidize slowly to form dichromate, or at higher temperatures decompose to dichromate, chromium(III) oxide and oxygen. It can also react with alkali halide to form dichromate, chlorochromate and chromium(III) oxide. Decomposition with the formation of chromium(III) oxide is a secondary reaction involving the preliminary formation of MCr3O8 or chromyl chloride. ��� The temperatures and enthalpy increases for the solid state transitions and the fusion of KCrO3Cl, RbCrO3Cl and CsCrO3Cl are reported.