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Summary The electrode behaviour of nickel(II) in aqueous solutions of iodides at mercury electrodes has been investigated using polarographic and voltammetric methods. It was found that at 25°C in moderately concentrated iodides NiI + is formed which is reduced with the standard rate constant of the order 10 −5 cm s −1 , at 50°C. At higher temperatures, 50–85°C, and very high ionic strength the formation of NiI 2 was found which is reduced under such conditions with the standard rate constant of the order 10 −3 cm s −1 . The detailed mechanism of both processes is discussed.

Summary The electrode processes of the Ni 2+ |Ni(Hg) system in aqueous azide solution at mercury electrodes have been investigated. It was found that NiN 3 + reacts directly with the electrode. Consequently at low azide concentrations the electrode reaction is preceded by the process of formation of that complex from aquonickel (II) ion. At higher concentrations ([N 3 − ]>2 m ) the complex [Ni(N 3 ) 4 ] 2− is in equilibrium with NiN 3 + which is irreversibly reduced. The very high value of the apparent cathodic transfer coefficient suggests a two-step electrode reaction with a slow transfer of the second electron.

Summary The electroreduction of nickel(II) at the HMDE in thiocyanate solutions has been investigated. Chronovoltammetry, chronopotentiometry, a.c. voltammetry and large scale controlled-potential electrolysis with investigation of products were used in this study. It was found that if nickel exists as the nickel(II)-thiocyanate complex then the reduction proceeds in two parallel ways: the formation of nickel amalgam and the formation of NiS and Ni(CN) 4 2− as a result of reduction of thiocyanate ions. The participation of the second process in the overall electrode reaction is not large, however, due to adsorption of NiS on the electrode surface its influence on the total process is considerable. At full coverage of the electrode by NiS, a strong autoinhibition of the reaction occurs and at more negative potentials deposition of metallic nickel on the electrode surface was observed. It has been shown that the electroreduction of ammonia complexes of nickel(II) in the presence of adsorbed thiocyanates does not lead to their reduction. General suggestions concerning the mechanism of the process studied are presented.

Summary Dependences of the half-wave potential of the first reduction wave of nitrobenzene in 0.1 M Li + , Na + , K + and Et 4 N + perchlorates in DMF on varied amounts of water were studied polarographically. At fixed concentrations of water ( cf. Table 1) plots of E 1/2 against the logarithm of the Na + concentration have been made and respective composition of associate species were determined. Extension of the Peover-Davies method of evaluating the equilibrium constants of ion-pair formation from polarographic measurements has been given. From the analysis of polarographic data equilibrium constants have been determined for associates like (NB ⨪ , H 2 O), and partial equilibrium constants for the system (NB ⨪ , Na + )/(NB ⨪ , 2Na + ). The dependence of activity coefficients of H 2 O in DMF on the composition of the mixture is given.

Summary The electrode reaction Cr(III)+ e →Cr(II) was studied in concentratedsolutions of NaClO 4 , Ca(ClO 4 ) 2 , NaCl and CaCl 2 . In perchlorate solutions the formal potentials, standard rate constants and electrode reaction orders were determined. From these data the hydration numbers were estimated and the mechanism of the electrode system studied was discussed. The rate constant of the Cr(III)/Cr(II) system is practically independent of the perchlorate concentration. The electrode reaction proceeds much faster in concentrated calcium chloride. In addition the Cr(II)/Cr system at a mercury electrode in concentratedsolutions of sodium perchlorate and calcium chloride was studied.

Summary The overall mechanism of electroreduction of azoxybenzene in DMF is presented. The process is found to proceed in several steps. The absorption spectrum of the azoxybenzene radical anion obtained in the first, one electron, step of reduction is presented. The identity of the final step of reduction of azoxybenzene and azobenzene is also evident. Considerable dependence of the mechanism of reduction of azoxybenzene on the nature and concentration of cation of the supporting electrolyte is shown.

Summary The effect of specific solvation of electrode and depolarizer by different constituents of mixed water-alcohol solvents on electrode kinetics was investigated for the case of the Mn 2+ /Mn/Hg system. By adding the complexing agent NH 3 , the cathodic process was shifted to the potential region where alcohols are desorbed from the electrode surface. Then the ions of depolarizer and the electrode surface are specifically solvated by water. The opposite condition of preferential solvation of the electrode surface by alcohol, and manganese ions by water, was realized during anodic dissolution of Mn(Hg) x . Little change in the cathodic rate constants and about 1 to 1.5 order of magnitude decrease in the speed of the anodic reaction by transfer from water to 8.1% alcohol demonstrated the importance of respective specific solvation as the main factor influencing charge transfer rate constant in mixed solvents. From potentiometric experiments stability constants for Mn(NH 3 ) x 2+ complexes were determined and average coordination numbers and complexes distribution as a function of ligand concentration presented. The detailed mechanism of the Mn 2+ discharge in presence of NH 3 was discussed.

Summary The kinetics of the deactivation of manganese reduced on mercury electrodes has been investigated using cyclic chronovoltammetry and potentiometric measurements. On the basis of the theory of Nicholson and Shain, the parameter K /( k f + k b ) 1/2 was found to be 2.8 s 1/2 . To evaluate the rate constants k f and k b , the equilibrium constant of this reaction, K , was estimated.

Summary The electroreduction of nickel(II) at the HMDE in 6 M aqueous solutions of CaCl2 and 5 M LiCl in glycerine in the temperature range 20–230°C has been investigated by chronoamperometry, stationary electrode voltammetry and potentiometry. The formation of a relatively stable homogeneous supersaturated nickel amalgam for concentrations lower than 10−3 and 10−2 M, at temperatures 20° and 200°C respectively, was found. From the diffusion coefficients of nickel in mercury determined in the temperature range 20–230°C it was concluded that nickel atoms in the homogeneous amalgam are solvated by approximately three mercury atoms. It was found that in the crystallization of the homogeneous amalgam three compounds NiHg2, NiHg3 and NiHg4 are formed. Thermodynamic parameters for the formation of these compounds were determined and a mechanism for the crystallization process was suggested.

The mechanism and kinetics of nickel(II) reduction in thiocyanate solutions have been investigated using polarography and cyclic voltammetry. The process was found to be moderately rapid, and the rate constants determined by two methods for higher thiocyanate concentrations varied between 2 and 3.10−4 cm sec−1. Three waves observed at low thiocyanate concentrations are associated with the reduction of [Ni(H2O)3(SCN)3]−, [Ni(H2O)5(SCN)]+ and hydrated nickel(II) ions (at the first wave, middle wave, and the wave at the most negative potential, respectively). In addition, studies were carried out on the mechanism of nickel(II) reduction in (thiocyanate and pyridine)-containing solutions.

Summary From polarographic measurements pKass values of ion pairs formed by the nitrobenzene radical anion N− and cations M+ have been estimated. Application of the function (R+δ)−1 distinctly improved the empirical relation between E1/2ipf and ionic potentials verifying the assumption of the considerable influence of solvation effects on ion pair formation. This is also supported by a thermocycle treatment of solvation and association of ions and ion pairs.

Summary The reduction and oxidation processes of uranium(VI), (V), (IV) and (III), in aqueous perchlorate solutions have been studied using the dropping mercury electrode, the hanging mercury drop electrode and the thin-layer technique. The experiments were performed in acidic media (0 −1 . The data obtained show that under certain conditions, the reduction process of uranium(V) is influenced by the chemical reaction of uranium(V) with uranium(III), resulting in the formation of uranium(IV). In the presence of this reaction the character of the polarographic and chronovoltammetric curves is changed. The disappearance of the uranium(III) oxidation peak on the chronovoltammetric curves obtained with the hanging mercury drop electrode can also be explained by this reaction. The kinetic parameters of the reduction of uranium(V) to (IV) were determined as k f,h 0 =(4.0±0.4)×10 −8 cm s −1 , and α=0.46±0.01. A detailed mechanism of the reduction of uranium(V) is proposed.

The double-layer capacitance of a mercury electrode in the solutions containing pyridine at different pH was measured. From the dependence of capacitance on pH the p K of pyridine was determined. This p K obtained for different potentials of the electrode characterizes the acid-base properties of pyridine in the field of the electrode. The p K of pyridine was also calculated (a) using the Frumkin isotherm and calculated adsorption parameters and (b) from the dependence of the pzc of the mercury electrode in pyridine solutions on pH. The p K values obtained are in good agreement with data reported in the literature.

The behaviour of nickel(II) in pyridine or picoline solutions has been investigated using polarography, cyclic chronovoltammetry, spectrophotometry and the a.c. bridge method. It was found that the electrode process is irreversible and that nickel-pyridine or nickel-picoline complexes are adsorbed on the electrode. This adsorption can explain a slope of the polarographic curves similar to those of the reversible process as reported earlier.

Chronovoltammetrisch werden Kinetik und Mechanismus der Reaktion Ti(IV) e - Ti(III) an der Hg-Elektrode in NaClO4- oder Ca(ClO4)2-Losung untersucht.

MERCURY electrodes used in electrochemistry are often prepared by plating mercury on solid electrodes. The formation of amalgams in these conditions has been in general disregarded. We have already shown1 that when gold or gold-plated electrodes are covered with mercury, there are electrode reactions different from those on pure mercury, owing to formation of intermetallic compounds.

IN the application of the hanging mercury drop electrode to analytical determination of trace metals we have used very dilute solutions1 in which no mutual interaction of metals was observed, even in the presence of more than one metallic species. We wish to report an investigation of the conditions in which such interaction effect cannot be neglected.

IT is usual to study electrode processes using hanging-mercury microelectrodes. Some authors prepare these electrodes by hanging a small mercury drop on a gold wire or a gold-plated platinum wire1,2, deliberately neglecting the presence of the gold for its electropositive potential. Such an electrode is however an amalgam electrode, and, as we have shown3, gold can influence the electrode processes of those metals which combine with it to form intermediate compounds.