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Voltocoulometric, chronocoulometric and cyclic voltammetric studies of the electrode reactions of the Pb(2,2,2) 2+ , Pb(2,2,1) 2+ and Pb(2,2) 2+ complexes were described ((2,2)-1,7,10,16-tetraoxa-4,13-diazacyclooctadecane, (2,2,1)-4,7,13,16,21-pentaoxa-1,10-12 diazabicyclo[5.8.8]tricosane, (2,2,2)-4,7,13,16,21,24-hexaoxa-1,10diazabicyclo[8.8.8]hexacosane). It was found that such Pb(L) 2+ complexes are adsorbed on the Hg electrode. The analysis of adsorption data for Pb(2,2,2) 2+ in the presence of excess of ligand (2,2,2) shows that the complex is adsorbed according to the competitive adsorption mechanism. Electroreduction processes of the studied complexes proceed via two parallel pathways: from the solution and from the adsorbed state. The electrode kinetic parameters of lead(II) complexes free of adsorption influence were found by analysis of the normal pulse voltocoulometric curves recorded with a 1 ms delay after application of the short potential pulse. From the analysis of kinetic parameters it follows that the electroreduction of Pb(L) 2+ is a two-step CE process with the rate determining step preceded by complex dissociation. The rate constants of the studied Pb(L) 2+ /Pb(Hg) systems are considerably lower than the rate constant of noncomplexed Pb 2+ ion in aqueous solution. We have found that the reaction rate constants of the systems Pb(L) 2+ /Pb(Hg) decrease linearly with increasing stability of the complexes. This dependence is also obeyed by cadmium and thallium complexes with the same ligands.

The electroreduction of nitromethane and 1-nitrohexane in both the absence and in the presence of n-octanol and n-decanol was studied. It was found that, under comparable conditions at smaller concentrations of inhibitors, nitromethane is inhibited more strongly than 1-nitrohexane by the n-alcohol adsorbed monolayer on the mercury electrodes. This was explained by the larger affinity of nitrohexane to the alcohol-rich surface phase and of nitromethane to the aqueous solution phase. Through the increased affinity of both reactants to the electrode surface phase at the inhibitor concentration, c/cs, exceeding 0.6, the independence of the electroreduction rate constant of both reactants was explained. When the bulk concentration of inhibitors approached saturation even the increase of the rate constant was observed. The change of the solvent composition, from pure water to 10 vol. % ethanol, increases the inhibition by increasing the affinity of the reactant to the solution phase.

The electrode reaction of the Cd2+ complex with acetylacetone (Acac) was studied on mercury electrodes in the absence and in the presence of n-pentanol and n-octanol. In the inhibitor free solution and under conditions where Cd(Acac)2 complex predominated, the standard rate constant of the cadmium reaction was found to be 6.2×10−2 cm s−1. In the presence of inhibitors, it was determined from cathodic and anodic reaction orders with respect to Acac, that the Cd(I)(Acac) species reacted directly with the electrode. In the presence of n-pentanol and n-octanol adsorbed on electrodes, the electrode reaction of the Cdaq2+ | Cd(Hg) system with hydrated Cd2+ ions proceeds more than one order of magnitude slower than the reaction of Cd-acetylacetone complexes. The ionophoretic effect of Acac is postulated to be due to the interaction of the reactant with the adsorbed surface layer.

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.

The electroreduction of 3-nitrophthalic acid anion on the mercury electrode in water + HMPA mixtures was studied with 0.5 M NaClO 4 and 0.05 M NaOH as electrolytes. A detailed kinetic study concerned one-electron reduction of 3-nitrophthalic acid anion to free radical. The change of the determined rate constants of this process on water + HMPA mixture composition was interpreted in terms of the solvent adsorption on the electrode and reagent solvation. An equation is proposed which describes the observed change of the kinetics.

The formal potentials and kinetic parameters for the electrode process Cr2+ + 2e−⇄Cr0 occurring at mercury electrode in solutions of LiClO4 + LiCl(aH2O = 0.741) and LiClO4 + LiI(aH2O = 0.853) were determined from analysis of irreversible anodic and cathodic chronocoulometric charge-potential curves. The interaction of Cr(II) with I− ions was found to be stronger (K1 = 1.6 M−1) than the very weak interaction with Cl− ions (K1 < 0.1 M−1). Double-layer properties were established for the solutions studied. On the basis of the rate constants, transfer coefficients, electrode reaction orders, double-layer properties and charges of the activated complex determined, a mechanism for the electrode reaction of the Cr2+/Cr(Hg) redox couple was proposed.

The orientation of adsorbed aliphatic alcohols on mercury electrode surface is discussed on the basis of literature data concerning steric factors of electrochemical reactions inhibited by these alcohols and on quantum chemical calculations using the mopac program. Steric factors were evaluated from dependences of rate constants of electrochemical reactions of several model systems on alcohol concentration at total surface coverage. Theoretical calculations were made for a single alcohol molecule interacting with a 28-atomic mercury cluster. These two sets of data point to a vertical orientation of adsorbed alcohol molecules, even at incomplete coverage of the electrode surface.

The kinetics of the [Mn(CN) 6 ] 4− + e − ⇌ [Mn(CN) 6 ] 5− system at mercury electrodes has been investigated using cyclic voltammetry, chronocoulometric and faradaic impedance methods. The standard heterogeneous rate constant k s of this process in 1 mol dm −3 NaCN is 0.26 ± 0.01 5 cm s −1 and the cathodic transfer coefficient α is 0.59 ± 0.01, similar to the kinetic characteristics of the [Cr(CN) 6 ] 3− /[Cr(CN) 6 ] 4− system. The diffusion coefficient of [Mn(CN) 6 ] 4− in 1 mol dm −3 NaCN is (5.6 ± 0.8) × 10 −6 cm 2 s −1 . The role of the composition of the inner coordination sphere of manganese complexes in their electroreduction mechanism and the possibility of the further electroreduction process [Mn(CN) 6 ] 5− + e − → [Mn(CN) 6 ] 6− at mercury electrodes are discussed using literature data.

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.

Voltocoulometric, chronocoulometric and cyclic voltammetric studies of the electrode reaction of Tl(2,2,2)+ cryptate at mercury electrodes are described. This process occurs via two parallel pathways – from the solution and from the adsorbed states which are energetically close. The kinetic and adsorption parameters of thallium(I) cryptate electroreduction were found by analysis of the normal pulse voltocoulometric curves recorded with a 1 ms delay after the application of the potential pulse. Such Qt−1-E dependences are characterized by a maximum that originates from the electroreduction of the adsorbed reactant. The model for numerical simulation of such curves was developed. Comparison of experimental and simulated curves allowed to determine apparent standard rate constant of Tl(2,2,2)+/Tl(Hg) couple as equal to 8×10−3 cm s−1, thus significantly lower than that for noncomplexed Tl+ /Tl(Hg) system. The stability constant of the Tl(2,2,2)+ complex (log K=6.70) and the surface coverage of the electrode by Tl+ were determined.

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.

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.

The electrode reaction of the Mn(II)/Mn(Hg) system in DMSO 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 + DMSO mixtures were calculated, based on potentials expressed in the ferrocene electrode scale. No minimum on the dependence of the standard rate constant on DMSO concentration was observed. To describe this dependence the proposed earlier model of the electrode reaction in the mixtures of water with more basic solvents was used. A decrease of cathodic Tafel slope with the increase of DMSO concentration and practically no change of the anodic one indicates that the rate of both steps of Mn(II) reduction in the mixtures rich in DMSO is rather comparable.

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.

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 Pb(II)/Pb(Hg) system has been studied in DMSO and DMSO-water mixtures using the faradaic impedance measurements. With the increase of the mole fraction of the organic component in the mixture rate constant decreases steadily from 2 cm s −1 in pure water to 5 × 10 −2 cm s −1 in DMSO in rough proportion to the coverage of the electrode by DMSO. The change of the kinetics with the solvent composition was discussed in the frame of existing models. Basing on the determined potentials of the Pb(II)/Pb(Hg) electrode expressed vs the ferrocinium ion/ferrocene electrode potential, the free energy of transfer of Pb(II) from water to DMSO and its mixtures was calculated and discussed.

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.

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.

The kinetics of the electrode reaction of the Ni(II)/Ni(Hg) system in DMF and its mixtures with water at different concentrations of several background electrolytes has been studied. Similarly to aqueous solutions, at higher background electrolyte concentrations the charge transfer step is preceded by a chemical reaction. It was deduced that this proceeding reaction is due to slow dissociation of the solvent molecule in the S 1 N process. The change of a solvent from water to 70 vol.% of DMF decreases the standard rate constant ( k s ) for about one order of magnitude. At larger DMF content k s is virtually independent of the solvent composition. The change of k s with the DMF content was explained by assuming the resolvation of Ni(II) ions at the electrode surface in proportion to the surface coverage of the electrode molecules of both solvents.

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.

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 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.

The electrode reaction of the Mn(II)/Mn(Hg) system was studied in Ca(ClO4)2 solutions in concentrations ranging from 1 to 13.5 mole kg−1 and temperature limits from −5 to 160°C using chronocoulometry and cyclic voltammetry. From the analyses of formal potentials is function of water activity the hydration numbers of Mn(II) ions were estimated. The changes of rate constants and transfer coefficients with the increase of electrolyte concentration and temperature above 5 mole kg−1 and 85°C, respectively, suggest a change in the rate-determining step from the second to the first electron. The calculated individual rate constants for both steps are presented and their changes with the increase in electrolyte concentration are discussed.

The electrode reaction of the Co 3 3+/2+ couple on mercury electrodes in water-acetonitrile (AN), water-DMSO and water-hexamethylphosphortriamide (HMPT) mixed solvents has been investigated. The differences between Gibbs energies of trasfer of the Co(en) 3 3+/2+ and Co(en) 3 2+ ions from water to mixtures studied were calculated from the formal potentials expressed in the ferrocinium ion/ferrocene (Fic + /Foc) scale. The data obtained point to selective solvation by more basic organic solvent in the second sphere. The determined dependence of electrochemical rate constant on mixed solvent composition is different from that observed in the case of simple solvated ions involving electron transfer, mainly due to be reactant adsorption from aqueous solutions. The results obtained were useful in extension of the model of electrochemical reactions in mixed solvents, proposed earlier for simple inorganic ions.

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.

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.

The kinetics of the Eu(III)/Eu(II) system at mercury electrodes has been investigated in water, acetonitrile (AN), N,N-dimethylformamide (DMF) dimethylsulphoxide (DMSO) and hexamethylphosphoramide (HMPA). Formal potentials Efo move towards more negative values in the order AN