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CrBr 2+ and CrI 2+ complexes formed during the electrooxidation of Cr 2+ on mercury electrodes in the presence of Br − and I − ions, respectively, aquate with rates ca. 10 3 higher than those measured by the spectrophotometric method in the bulk of the solution. The entropies of activation of these reactions are large and negative, which indicates that the transition states are highly organized. Changes in the H + concentration did not influence the rate of aquation. Since Cr 2+ ions are not oxidized on Pt, Au, glassy carbon or pyrolytic graphite electrodes, this high instability was explained in terms of the adsorption of the transition state on mercury during the inner-sphere electron transfer.

Supersaturated chromium amalgam as concentrated as 1.5 M, has been prepared electrochemically. The resulting amalgam has a mirror-like shine and behaves like plasticine. The stability of supersaturated (up to 10 mM) chromium amalgam was investigated in 1 and 5 M solutions of NaClO4, in the range of −20 to 60°C, using double potential step chronocoulometry (DPSC). The results enabled us to determine the conditions in which the oxidation of Cr(Hg) is diffusion controlled. In the millisecond time scale, this usually occurs at concentrations lower than 1 mM Cr3+ Cr(Hg) decomposes more rapidly at increased temperature and concentration of chromium in the amalgam, and, in the first stage, forms inactive clusters (probably Cr3). The energy of activation of this reaction was determined as Ea = 36.2 kJ mol .

The formal potentials and the kinetics parameters for the electrode process: Cr 2+ +2 e − = Cr o occurring at a mercury electrode in solutions of NaClO 4 , NaCI, NaBr, and NaSCN, were determined from the analysis of irreversible anodic and cathodic chronocoulometric waves. The interaction of Cr(II) with Cl − was found to be negligible (equilibrium constant K − and SCN − was weak ( K 1 (Br − )=1 M −1 and β 2 (SCN − ) = 25 M −2 ). The results of the analysis of the formal rate constant of this and other amalgam forming reactions suggested that the formation of amalgam was the most important step in the whole process.

Donnees sur le processus d'electrode Cr(II)+2e − =Cr(O) en presence d'une electrode de mercure, souvent observe comme 2eme etape de la reduction d'ions Cr(III), entre pH 2 et 4

Oxidation of Cr2+ at mercury electrodes in the presence of S2O2−3, thiosemicarbazide, 2,5-dimercapto-1, 3,4-thiadiazole, N-methylthiourea, thiourea and thioacetamide lead to Cr(III) complexes which feature a chromium(III)-sulphur bond. The equilibrium constants of these complexes are equal to 0.21, 0.14, 0.036, 0.0076, 0.0013 and 0.00015 M−1 for S2O2−3, TSC, DMTDA, NMTU, TU and TAA, respectively. The Cr(III)TU complex synthesized electrochemically aquates spontaneously, and the mechanism of this reaction is the same as that characteristic of other Cr(III)X(3-i)+ complexes. This complex is not adsorbed on the electrode surface.

Assuming a straight-line dependence of the chemical potential of anions (cations) on empirical Lewis acid (base) parameters of solvents, the values of the change of the chemical potential of ions and the change of the surface potential between water and non-aqueous solvents have been calculated and briefly discussed. On the basis of the results obtained and literature data the dependence of the standard emf of the cell Pt,H2 |HCl|AgCl|Ag|Pt′ on the function of empirical Lewis acid-base parameters has been analysed and discussed.