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.