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The rate of the interfacial charge transfer in the V(III)/V(II) system at the mercury electrode in H2O+DMPU 1 +0.4 mol dm−3 HClO4 mixtures was found to increase when x DMPU was raised from 0.1 to 0.8. The solvatochromism of the electronic d-d transitions within the vanadium ions was used to leam about their resolvation with DMPU molecules. It was found that, under the experimental conditions used, the charge transfer occurs at the electrode surface which is already largely covered by DMPU (θDMPU = 0.9 to 1.0) and the interfacial kinetics are determined mainly by the solvation state of reactant in the bulk solution. Our observation of a close correspondence between log ks, and the reactant inner-shell composition can be considered as a result of the prevailing non-continuum contributions to the activation energy arising from the elimination of specific short-range, quo-ligand-solvent (hydrogen bond) interactions by coordinated DMPU molecules which are not hydrogen-bond donors. It seems that generally such effects could be kinetically significant also in other aqueous solutions of aprotic solvents (e.g. DMF, AN, HMPA).

The electroreduction of cadmium(II) at a mercury electrode and the oxidation of cadmium amalgam in 4.0 M NaCl at pressures ranging from 0.001 to 10 kbar, using cyclic voltammetry and chronoamperometry, revealed a moderate decrease in the limiting currents and diffusion coefficients of CdII in solution and Cd in mercury. The partial molar volume of the diffusion activation calculated from these dependences is equal to 1.1–1.2 and 0.93 cm3 mol−1 for diffusion in 4.0 M NaCl and in mercury, respectively. The formal potentials of the CdII/Cd(Hg) system expressed versus the ferrocene-ferricinium ion electrode as a function of pressure led to a value of 10.7 cm3 mol−1 for the volume of the electroreduction reaction of CdII.

The mechanism of Ni2(CN)64− electro-oxidation at mercury electrodes is investigated using cyclic voltanunetric and chronocoulometric methods. Ni2(CN)64− (existing presumably in the form of various ion-pairs {Ni2(CN)64−, x Na+}) is surface-active and manifests itself by the formation of an anodic adsorption wave. In the absence of an excess of free ligand, surface-active nickel cyanide was formed as one of the products of Ni2(CN)64− electro-oxidation on mercury. It was found that one cyanide ion was consumed in the rate-determining step of the electrooxidation of each Ni2(CN)64− particle. Within the potential range of mercury oxidation to Hg(CN)p(2−p)+ and in the presence of both CN− and Ni2(CN)64− ions, small latent limiting currents are caused by the reaction 2 Ni(I) + Hg(II) → 2 Ni(II) + Hg(0) The scheme proposed for the participation of CN− ions in Ni2(CN)64− electro-oxidation was confirmed by the results of digital simulation of an appropriate model system.

The discharge of the lead citrate complex (PbL2−) on a mercury electrode covered by n-hexanol or n-heptanol molecules has been investigated by means of polarographic and chronocoulometric methods. The relation between the bulk surfactant concentration and the rate of the electrode reaction was examined. An average value of the steric factor r≠/ri equal to 4.0 ±0.2 was found for the cathodic electrode reaction occurring in aqueous solutions of sodium citrate containing aliphatic alcohols. The kinetic and mechanistic implications of an increase in the reactant size under such conditions are discussed.

Electrode processes involving the formation of a heterogeneous amalgam of iron during the electroreduction of Fe(II) ions on mercury and the oxidation of the iron amalgam were investigated by cyclic voltammetry, chronoamperometry and cyclic chronopotentiometry. The hanging mercury drop electrode and an iron plate placed on a mercury pool electrode were used in the experiments as working electrodes. It was established that particles of the electrodeposited iron are wetted by mercury and penetrate into the bulk of mercury electrodes forming a suspension. However, during electroreduction of Fe(II) ions from concentrated CaCl2 and Ca(ClO4)2 solutions a small amount of metallic iron remains on the surface of mercury. Oxidation of the heterogeneous amalgam of iron occurs at potentials about 100–150 mV less positive than the onset potential of mercury oxidation in the solutions studied. At these potentials oxidation of mercury may become significant enough to uncover some parts of the iron microcrystals. These iron particles become partially passivated and their surface energy at the Fe/solution interface decreases; consequently, the spreading coefficient of mercury on iron decreases, microcrystals of iron are expelled from mercury and finally oxidized.

The coordination model of ion solvation in mixed solvents was applied to evaluate solvation numbers of cations and reaction orders with respect to the organic solvent for electrode reactions in non-aqueous media. These values were calculated from linear dependences of the Gibbs energy of transfer and logarithms of electrochemical rate constants plotted against the logarithm of organic solvent activity. This method gives credible results, but only for highly basic solvents.

The role of the steric factors in the electrode reactions occurring on the interface covered by aliphatic alcohols and acids has been investigated, taking as an example the Cd 2+ /Cd(Hg) system. It has been shown that this process takes place inside the inner layer and the activated complex is created by the replacement of r ≠ water molecules from the surface. From studies carried out at surfactant concentrations corresponding to the plateau of the adsorption isotherm the steric factors r ≠ / r i have been determined ( r i is the number of solvent molecules replaced from the surface by one molecule of adsorbed surfactant). By comparison with investigations performed at lower coverages the magnitude of r ≠ and r i has been estimated. The influence of different factors on the experimental value of these parameters has been discussed.

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.

Using pulse chronocoulometric polarography the kinetic parameters of the Ni(II)/Ni(Hg) system in thiocynate solutions have be determined. Under the conditions of the experiments the influence of electroreduction of SCN− ions resulting in formation of NiS adsorbed on the electrode was minimized. The standard rate constants obtained are of the order of 10−3 cm s−1, several times larger than earlier reported in literature. The Tafel slopes indicate a two-step process with slow transfer of the second electron and NiSCN+ directly participating in the electrode reaction.

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.

Chronoamperometric, chronopotentiometric and chronovoltammetric experiments were performed for the reduction of zinc ions on hanging gold amalgam drop electrodes when the zinc ion concentration in the solution was considerably higher than the concentration of gold in the amalgam. The formation of a pre-peak was observed which corresponds to electroreduction of zinc(II) to the intermetallic compound AuZn in mercury. Chronovoltammetric experiments may be utilized for analytical determinations of the gold concentrations in homogeneous amalgams. Analysis of the chronoamperometric experiments enabled the determination of the diffusion coefficient of gold in mercury which was found to be (8.5±0.4)×10−6cm2 s−1 at 298.2 K.

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.

The relation between the rate of electrochemical reduction of metal cations and the mixed solvent composition has been studied. A generalized model of such processes has been proposed in which fast reactions of reactant ions with solvent molecules in the surface layer are followed by electroreduction of the various formes of surface complexes formed. Some simplifications of the general equations leading to the dependences obtained experimentally were presented. It has been found that the existing models are special cases of the model described in this work. The major emphasis was on explaining the electrode processes in mixtures of water with less basic organic solvents.

Addition of dimethylformamide (DMF) to aqueous solutions containing the V(II)/V(II) couple and perchlorates as supporting electrolyte causes (I) an increase of the heterogeneous rate constant, (II) a shift of the formal potentials of the V(III)/V(II) system towards more negative values, and (III) a decrease of the diffusion coefficients. These changes are partly due to the resolvation of both V(III) and V(II) ions when the DMF concentration in the mixture increases, as is also evident from spectrophotometric measurements. Since variations in the electrode kinetics are interrelated on the one hand to the resolvation of a reactant ion in the bulk solution and to the electrode surface coverage by DMF molecules on the other, it is difficult to separate the influence of these factors on the electrode kinetics. The mechanism of the electrode reaction is discussed in terms of the model proposed earlier.

Relationships have been established between the cathodic and anodic rate constants of electrode processes and the Gibbs energy of transfer, Δ tr of the reactant for ion transfer and Δ Δ G tr = Δ G tr,Ox -Δ G tr,Red for electron-transfer reactions in aqueous + organic mixed solvents. These dependences are different from those obtained for electrochemical reactions in pure solvents, this being explained quantitatively by the various compositions of the surface and bulk phases. The correlations obtained offer diagnostic criteria which are useful in the understanding of the mixed-solvent influence on the mechanism of electrochemical reactions occurring under such conditions. Equations have been devised which describe three ways of electroreduction in two-solvent systems: (i) removal of surface molecules by an activated complex, a mechanism similar to that in the presence of neutral organic inhibitors; (ii) selective resolvation with adsorbed organic molecules; and (iii) electron transfer outside the surface phase. Satisfactory agreement was found between the experimental results and the calculated data for selected electrochemical processes in mixed solvents.

A new understanding of the nature of electrode reactions at graphite paste electrodes has been obtained by studying electron-transfer rates of redox systems under conditions of carefully controlled electrode composition and pretreatment. Dry graphite gives electron-transfer rates which give an almost Nernstian response and approach those obtained with platinum. The addition of any pasting liquid decreases these rates materially. Both electrochemical and chemical oxidative pretreatments of such pastes increase the electron transfer in the direction of the “dry” graphite limit. This effect appears to correlate with the increased hydrophilic nature of the electrode surface and concomitant removal of organic layers from the electrode surface.

The differences in electrochemical behaviour of nitrobenzene and 3-nitropyridine connected with a change of supporting electrolyte are discussed. It is shown that in the presence of alkaline earch metal cations a deposit of composition (NB−. Me2+) is formed at the electrode surface. The cations: Ba2+, Ca2+ and Mg2+ cause the instability of the nitrobenzene radical anion formed in the first electroreduction step. It was found that in the presence of Ba2+ and Ca2+ the NOB/NO−. couple is stabilized.

Adsorption of benzoic acid at the mercury electrode was studied in a wide pH range. The adsorption isotherms of benzoic acid from electrocapillary and capacity measurements were calculated. From the dependence of capacitance and potential of zero charge on pH the p K el of benzoic acid was determined.

There is a growing interest in transition metal hexacyanoferrates [1-12], polynuclear inorganic materials, for modification of electrode surfaces. The electrochemical behaviour of the respective films resembles both redox and ion-exchange organic polymers [13,14], as well as ionically conducting, zeolitic, or intercalation systems [15,16]. The cyanometallate materials can be also viewed as analogues of Prussian BIue, which is, from the electronic point of view, a semiconductor or, more precisely, a class II mixed-valence system [17] with electrons and unoccupied states for them to move to. For potential applications such as electrodes in batteries, the development of mixed conductors with comparable contributions of electrons (holes) and ions to the conductivity is of interest. To the best of our knowledge, developments in mixed conducting solid electrolytes are, at present, rather limited. Recent studies [1,5,8-12,18] indicate that sparingly soluble transition metal hexacyanoferrate deposits on electrodes exhibit a large degree of selectivity toward the flux of an electrolyte cation, providing charge balance during redox transitions. Little is known about ionic mobilities in the dry (solid) state of these materials. Their solution electrochemistry, employing them in the form of films on electrodes, allows us, however, to hypothesize that the .ability to store charge in the bulk of materials as well as cation transport should be well pronounced in the K+-containing cyanometallate microstructures. In the present study, we have attempted experiments in the solid state, using cyclic voltammetry with chemically inert glassy carbon slide electrodes that sandwich either a dry powder or a film of the material under discussion. A complete concept of the sandwich electrode system, in which a polymer film is placed (sandwiched) between two metallic conductors, has been proposed by Murray and co-workers [13,19]. The possibility of extending this approach to Prussian BIue has been mentioned by Grabner [20]. In our design, there is, however, no external

The kinetics of homogeneous and heterogeneous electron exchange V 3+ /V 2+ was investigated in 1 M HClO 4 , water + t -butanol, mixed solvent solutions ( x t -but . varied from 0 to 0.65). Non-monotonic heterogeneous and monotonic homogeneous variations of the rate constants were observed. The solvent effect on the work of bringing the reactants from infinite separation to the reaction site was estimated with the Gouy-Chapman and Debye-Huckel approximations respectively. The homogeneous kinetics data were next used to calculate the reorganization energy in the heterogeneous reaction, λ het . Finally the influence of the solvent composition on the rate of the electrode reaction was separated into surface and bulk effects.

Using single and double potential step chronocoulometry the Ni(II)/Ni(Hg) electrode reaction was investigated in the electrolyte MgCl2 + Mg(ClO4)2, with the total concentration varied within the range 4.5–9.3 mol kg−1 and the temperature from 25 to 145°C. Coordination numbers of nickel complexes estimated from the analysis of formal potentials as a function of Cl− ion concentration suggest that for the total salt concentration within the range 4.5–6.5 mol kg−1 (for an MgCl2 content higher than 10%) NiCl+ and NiCl2 are preferentially in equilibria the solutions. The equilibria are shifted to higher complexes with an increase in the concentration both of mixed electrolyte and MgCl2. For the total salt concentration exceeding 6.5 mol kg−1 and MgCl content higher than 30%, the coordination number is equal to four. This, being in accordance with spectrophotometric data, suggests the existence of the tetrahedral form in the solution. The standard rate constant increases from 10−1 cm s−1 to 4 × 10−3 cm s−1 with the change in electrolyte concentration from 4.5 to 6.5 mol kg−1 and in temperature from 25 to 115°C. The order of the electrode reaction determined with respect to chloride suggests that the forms NiCl or NiCl−2 participate in the rate determining step of the second electron transfer, according to the electrolyte concentration.

Following Anson's views on the analysis of reaction orders of processes with specifically adsorbed reactants our previous experimental data on the heterogeneous oxidation of the homogeneous nickel amalgam in aqueous solutions of halogen and pseudohalogen ions were reexamined. Reaction orders close to zero and one in respect to iodides and azides, respectively, were found.

The electrode reactions of the Eu(II)/Eu(Hg) system in aqueous and mixed H2O + DMF solutions of different supporting electrolytes (TEAP, LiClO4, Ca(ClO4)2) have been investigated by single potential step chronocoulometry and dc polarography. It was found that both reduction of Eu(II) and oxidation of Eu amalgam in the presence of acidified 0.05 mol dm−3 Ca(ClO4)2 are simple processes controlled both by diffusion and electrochemical kinetics in the whole range of DMF concentration (0–100%). The kinetic data obtained are discussed according to the models of electrochemical kinetics in mixed solvents proposed earlier. It was shown that the difference between the activation energies for pure aqueous and DMF solutions is concordant with that expected for the electron transfer mechanism with transfer of the second electron as the rate determining step.

The electrode reaction of the Mn(II)/Mn(Hg) system in acetonitrile and its mixtures with water has been studied. Charge-transfer rate constants, diffusion coefficients and formal potentials were determined. The transfer energies of Mn(II) from water to water+acetonitrile mixtures were calculated, based on potentials expressed in the ferrocene electrode scale. The charge transfer between Mn(I) and Mn(Hg) was found to control the rate of the electrode process in the whole composition range. The rate constants of this process go through a flat minimum as the concentration of acetonitrile increases. The change of the kinetics was explained using a slight modification of Behr's model.

The electrode kinetics of the Fe(CN)63−/Fe(CN)64− system on a platinum rotating electrode was studied in the presence of different anions. At constant sodium salt concentration the rate of the electrode reaction increased in the following order of anions: F−

The mechanism of the electroreduction of Ni(II)-NCS − complexes to NiS and CN − anions was studied using ac voltammetry. By analysing the dependence of the alternating current on time, at constant dc potential, recorded at different concentrations of Ni(II) and SCN − ions, it was concluded that the reaction proceeds through an intermediate [Ni I -SCN-Ni I ] + which further decomposes to NiS and cyanide complexes of Ni(II).

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.

The adsorption of α-, ⨿- and γ-cyclodextrins on mercury electrodes from l M NaClO 4 and 0.5 M Na 2 SO 4 was investigated, using surface tension and differential capacity measurements. All three cyclodextrins adsorb on mercury and, in addition, α- and γ-cyclodextrins exhibit a behaviour typical of the two-dimensional condensation of organic adsorbates in both electrolytes. They also show strong reorientation effects in the adsorbed state. A comparison of possible models of electrode occupation by surfactant molecules with the experimentally determined surface excesses shows that at less negative potentials the molecules of cyclodextrins are oriented with the cavity perpendicular to the electrode surface, while at more negative potentials an orientation intermediate between the “parallel” and “perpendicular” prevails.

Visible absorption spectroscopy coupled with controlled potential electrolysis at SnO2-covered glass transparant electrodes show that, in the course of oxidation of ferrocyanides to ferricyanides, the latter species undergo, under the influence of the electrode field, decomposition to inhibiting, Prussian blue-like, deposits. Addition of an effective Fe(III,II)-complexing agent, namely of CN− at the 0.01-10 mM level either to LiCl or KCl electrolytes, increases the measured standard rate constants (ks) significantly for electron transfer in the Fe(CN)3−/4−6 system. Protection of a Pt indicator electrode from passivation has also been accomplished by intentional modification of its surface via irreversible chemisorption of monoatomic iodine. Both approaches proposed here yield excellently reproducible rotating disk or cyclic voltammograms which, upon kinetic analysis, produce ks values consistently exceeding 10−1 cm s−1. Taken together, the results provide strong evidence that complications with the Fe(CN)3−/4−6 system could be solved essentially by preventing the electrode surface from being blocked by sparingly soluble iron cyanide species.

The kinetics of the Mn(II)/Mn(Hg) electrode process have been studied in DMF and DMF+water mixtures. Diffusion coefficients, formal potentials and charge-transfer rate constants were determined. The influence of NaClO 4 concentration on the kinetic parameters in selected DMF mixtures with water was studied. The kinetic desolvation step preceding the charge transfer was sought in DMF at −30°C. Two models of the electrode process in mixtures of water with solvents more basic than water are discussed.

Parallel optical and electrochemical studies on the V(III)/V(II) system in H2O + acetonitrile (AN) + CF3SO3H mixtures have been performed. It was found, on the basis of the spectra of vanadium ions in the visible range, that V(III) was totally hydrated in mixtures up to xAN ⋍ 0.6 while V(II) was specifically solvated by AN molecules, even at a molar fraction of acetonitrile in H2O + AN mixtures as low as 0.02. In agreement with this, the formal potentials of the V(III)/V(II) system expressed versus the ferrocene electrode move to less negative potentials with an increase in AN concentration. Straightforward correlations of the electrode kinetics of the V(III)/V(II) system at a mercury electrode in H2O + AN mixtures with both the electrode surface coverage by AN molecules and the resolvation of vanadium ions in the bulk solution were found.

Summary The discussion of the present state of understanding of the inhibition of electrode reactions by adsorbed organic compounds has been presented. It has been shown that the different models of the mechanism of inhibition may be reduced to the concept of competitive adsorption of activated complex, solvent and inhibitor molecules with different adsorption isotherms taken to describe adsorption of activated complex. On the basis of a statistical analysis of the literature data it has been shown that the inhibition may be well described by three different model equations originated from Flory-Huggins, Frumkin or Blomgren-Bockris adsorption isotherms for activated complex. Because of marked experimental errors both for the determination of the rate constant of the electrode process and the surface coverages of the electrode by the inhibitor, as well as a result of the impossibility or the proper correction of the rate constants for double layer effect it has been proved that the choice of the model of inhibition is not an ambiguous procedure if the measurements are carried out in the range of surface coverages 0

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.

Using the method of stationary electrode voltammetry with hanging mercury drop and glassy carbon electrodes, the electroreduction of Co(II)-thiocyanate complexes occurring at smaller cathodic potentials than the formal potential of the Co(II)/Co(Hg) couple was studied. This process results in the formation of CoS and CN− ions, and occurs only on electrode surfaces modified by the presence of CoS with an admixture of metallic cobalt. The maximum efficiency of this reaction, calculated with respect to the charge passed through the cell, was found to be 75%, on the basis of macroelectrolysis carried out at a mercury pool cathode. The influence of various factors on the electroreduction of SCN− was also studied.

By combining coulometric quantitation with cyclic voltammetry and stripping-like analysis, the compositions of mixed transition metal hexacyanoferrate films on electrodes can be determined. The approach was demonstrated by (1) integrating the charges under the voltammetric peaks related to the film ferri/ferrocyanide redox transitions (to yield Fe-loading), and (2) cathodic preconcentration of the other metal (causing film destruction), followed by reverse electrolysis in a manner analogous to stripping voltammetry. The feasibility of the experimental scheme was tested for hexacyanoferrate deposits formed with metals other than iron, namely for the following Prussian Blue analogues: Cu(II) hexacyano-ferrate(III,II), In(III)hexacyanoferrate(III,II), and Ag(I)- “crosslinked” Ni(II)hexacyanoferrate(III,II). The experimental requirements and results are discussed within a framework of the films' chemical nature and electrochemical properties. In general, to facilitate the analysis, electrode coatings as thin as possible should be used.

For compositions such that Ni2+ and Ni(CN)2−4 can co-exist, there is very strong adsorption of Ni(CN)2 on mercury electrodes. The electrochemical characteristics of this adsorption are quantitatively accounted for by a Frumkin isotherm, with an adsorption constant, β=(4.1±0.2)×106 mol−2 dm6, and a positive (attraction) constant, A=3.9±0.1. The surface layer appears to consist of strongly interacting NiNi(CN)4 units. The most probable model of surface equilibria involves direct co-adsorption of Ni2+ and Ni(CN)2−4 from the bulk on mercury. Electroreduction of an adsorbate proceeds with the uptake of one electron per NiNi(CN)4 unit.

Summary The electrode kinetics of electroreduction of vanadium(III), cadmium(II), plumbite ion, Mn(NH3)x2+ and nitrobenzene free radical as well as oxidation of the manganese amalgam has been studied at mercury electrodes in a mixture of water with propanol, isopropanol and tert-butanol. With the exception of manganese(II) electroreduction the standard rate constants for other systems at first largely decrease with the increase of the organic component concentration and later slowly increase when the content of alcohol in the mixture increases. These changes of the rate constants were qualitatively explained. The influence of the couble-layer structure on the electrode kinetics in such systems was also studied. The measured formal potentials were compared with those calculated on the basis of the modified Born equation and the differences were explained in terms of selective hydration of ions and liquid junction potential.

With the use of the thermodynamics of the surface phase the distinction between inhibitive properties of the equilibrium and non-equilibrium surfactant adsorption on energetically uniform and non-uniform surface is shown. It was pointed out that these properties additionally depend on the position of the reactant inside or outside the adsorbed layer. The respective equations are derived. On the basis of thermodynamic arguments the critical reassessment of the present understanding of the inhibition phenomena is performed. The possibility of the appearance of a limiting rate at higher coverages of the electrode by surfactant is discussed.

The electrode reaction of the Eu(III)−EDTA/Eu(II)−EDTA system at mercury electrodes in mixed solvents, H2O + DMSO, H2O + AN and H2O + HMPT, was studied by using mostly chronocoulometry and other supplementary techniques. It was found that, in each case, addition of small quantities of non-aqueous solvents to water produces a pure inhibitive effect on the electrode process. At higher concentrations of both AN and DMSO, the non-aqueous solvent generates an increase of the standard rate constant. The mechanism of the electrode reaction of the Eu(III)−EDTA/Eu(II)−EDTA system in the mixed solvents used was compared with that of non-complexed solvated europium ions in these media.

The electroreduction of Ni(II)—thiocyanate complexes has been studied using large-scale electrolysis, cyclic voltammetry, chronoamperometry and pulse polarography. Electrolyses carried out at 95°C, and at potentials not sufficiently cathodic for nickel amalgam formation demonstrate that there is quantitative electroreduction of these complexes to NiS and Ni(CN)42−. Under such conditions, the voltammetric i-E curves also indicate the electroreduction of the SCN− ligands. Adsorption and desorption of the NiS produced at less negative potentials, as well as its electroreduction to nickel amalgam were studied by pulse polarography. The mechanism of NiS formation is discussed.

The literature data on the kinetics of cation electrodeposition on mercury in different solvents were analysed. For all cations considered in different solvents there was a linear decrease of the logarithm of the standard charge-transfer rate constant with increasing basicity of solvent and with more negative formal potential of the electrode reaction expressed in the scale of a solvent-independent electrode, as well as a linear dependence of the activation energy on the Gibbs energy of cation transfer. No dependence of the logarithm of the heterogeneous rate constant on the rate of exchange of solvent molecules from the first solvation sphere was observed. For the different electrode systems studied in one solvent, the dependences of the activation energy on (i) the cation solvation energy, (ii) the Gibbs energy of metal amalgamation, and (iii) metal solubility in mercury were analysed.

In cyclic voltammetric measurements of DMF solutions of nitroso-, azoxy- and azobenzene using HMDE it was found that besides the usual peaks of red-ox processes an additional, new system of peaks was observed. Its properties were nearly identical for all three depolarizers. Taking into account various possible interpretations, based on experimental facts observed, the most probable is to attribute this additional system of peaks to electrode reaction of the material of electrode (i.e. Hg) in the presence of adsorbed azobenzene dianion.

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.

The electrode reactions of the Ba 2+ /Ba(Hg) system in aqueous Ca(ClO 4 ) 2 and LiClO 4 solutions were investigated using various chronocoulometric methods, dc polarography and linear sweep cyclic voltammetry (LSCV). The reduction of Ba 2+ and the oxidation of Ba(Hg) under chronocoulometric conditions were found to be simple, controlled both by diffusion and electrochemical kinetic processes, while serious deviations from diffusional behaviour were observed during the oxidation of Ba(Hg) using LSCV. Possible origins of the above deviations are discussed. The formal potentials, kinetic parameters and diffusion coefficients of Ba 2+ were evaluated on the basis of chronocoulometric measurements and critically compared with data reported in the literature.

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.