Kinetics and mechanism of the electrode reactions of the Cr2+|Cr(Hg) system in lithium bromide + lithium perchlorate solutions of decreased water activity

The kineticsand mechanism of the electrode process Cr II ) + 2e − = Cr(Hg) was investigated in LiBr + LiClO 4 solutions having constant water activity equal to 0.764. The concentration of LiBr was changed from 0 to 4.0 M. On the basis of the results obtained, the equilibrium constant of the CrBr + complex ( K 1 = 0.3 ± 0.1 M) was calculated. This value is higher than that of the chloride and lower than that of the iodide complex, which proves that the Cr 2+ ion is a soft cation. During electroreduction of the Cr 2+ ion in solutions of different water activity in the presence and in the absence of bromide ions, the following cathodic reaction orders were observed: −7.2, −5.9 and −1 for water in the absence of bromide ions, water in the presence of bromide ions and bromide ions, respectively. Since the hydration number determined was equal to 9.2 in bromide-free solutions, therefore the complex which undergoes a direct reduction at the electrode is weakly hydrated Cr(H 2 O) 2 2+ . A comparison of the rate constants of the Cr(II)|Cr(Hg) electrode reaction in solutions of perchlorate, chlorides, bromides and iodides, with various water activity reveals that the water activity is the most important factor which determines the rate constant of this process.