Base decomposition of dichromate

Abstract

The rate of decomposition of dichromate, Cr₂O₇^2-, to form chromate, CrO₄^2-, is very dependent upon the choice of cation, and its concentration. At a total ionic strength of 1.0 mol l^-1 and at 25.0°C, the second-order rate constant decreases in the order Ca²⁺ (2600 mol^-1 l. s^-1) >> K⁺ + (960) > Na+ (920) >Li⁺ (700)>> bistet²⁺ (410) >> Me₄N⁺ (125) >> Et₄N⁺ (35) where bistet²⁺ is the N,N,N,N',N',N'-hexamethylethylene-1,2-diammonium ion. In the presence of sodium chloride the second-order rate constant increases as the ionic strength increases (200 and 1900 mol^-1 l. s^-1 at ionic strengths of 0 and 2.7 mol l^-1 respectively), whereas in tetramethylammonium chloride a maximum is reached at intermediate ionic strengths. The equilibrium between dichromate and hydrogen chromate is shifted towards the dichromate side in the presence of tetraalkylammonium ions, and towards the hydrogen chromate side in the presence of calcium ion.