Jadwiga Stroka

Kinetics of Zn2+ reduction at a Hg electrode from water-acetone and water-methanol mixtures

Summary The kinetics of the reduction of Zn 2+ ions at a mercury electrode from water-acetone and water-methanol mixtures has been studied. The dependence of the kinetic parameters on the solvent composition and on the kind and concentration of the supporting electrolyte has been discussed. It was pointed out that in the case of mixed solvents, if the electrolyte is strongly preferentially solvated by water, e.g. NaF in H 2 O + acetone, it can produce pronounced effects due to changes in the activities of the solvent components. The effect of solvent composition on the kinetics of electrode reactions has been discussed in terms of the equilibrium concentration of depolarizer in the surface phase σ adjoining the electrode, where the solvent composition is different from that in the bulk α. The equilibrium is determined by the free enthalpy of transfer α Δ σ G t of Zn 2+ ions from phase α to σ. A correction factor for the concentrations of Zn 2+ at the surface expressed as exp (− α Δ σ G t / RT ) included into the kinetic equation can account for the minimum at the log κ F vs. χ CH 3 COCH 3 curve. Such a correction term is similar in its nature to the generally used Frumkin correction for the potential drop in the diffuse double layer.

Electrochemical identity of copper hexacyanoferrate in the solid-state: evidence for the presence and redox activity of both iron and copper ionic sites

Abstract We describe the electrochemical behavior of solid copper hexacyanoferrate (powder) investigated in the absence of contact with a liquid external supporting electrolyte. In addition to a typical hexacyanoferrate(III,II) redox transition, the system is characterized by a second electrode process which appears at more negative potentials. Diagnostic experiments, which include spectrochemical and solid-state voltammetric characterization, support our view that the latter redox reaction most likely originates from reduction of the systems lattice copper(II) ions. The overall dynamics of charge transport in the material, when discussed in terms of effective diffusion, is moderately high (on the level of 10 −8 cm 2 s −1 ). Comparison has been also made to the electrochemical behavior of solid Prussian blue and to the voltammetric characteristics of conventional electrodes modified with thin films of copper hexacyanoferrate.

The free energy of transfer of Pb(II) ion and electrode kinetics of the Pb(II)/Pb(Hg) system in dimethyl sulfoxide (DMSO) and DMSO-water mixtures

Abstract 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.

High Affinity of Thallium Ions to Copper Hexacyanoferrate Films

High affinity of thallium(I) ions with respect to a copper hexacyanoferrate (CuHCF) film was found. Interaction between the CuHCF film and thallium(I) ions was investigated using the electrochemical quartz crystal microbalance and cyclic voltammetric techniques. In 0.5 M KNO 3 solution, at submillimolar concentrations of Tl(I) ions, the CuHCF film reversibly transforms, during electrochemical experiments, from the potassium into the thallium form. For slightly higher (several millimoles) Tl(I) concentrations, the electrochemical and gravimetric responses prove that the CuHCF film behaves as the thallium form only. Its formal potential (E f 0 ), calculated from the dependence of the E f 0 on [Tl(I)], is for [Tl(I)] = 1 M, 0.28 V more positive than that found for the potassium form. Experimental results obtained suggest that in both cases, potassium(I) and thallium(I) ions, exchange in interstitial positions takes place. Since the ionic radii and the hydration parameters of both ions are similar, we concluded that this high affinity of thallium(I) ions with respect to the CuHCF film, 250-100 times higher than that of potassium(I) ion, results from chemical interactions. In consequence, the solubility product of a thallium analogue of copper hexacyanoferrate is much smaller than that of the potassium form. For different forms they are equal to (pK values): 37.8 (K 2 Cu 3 [Fe(CN) 6 ] 2 ), 46.3 (Tl 2 Cu 3 [Fe(CN) 6 ] 2 ), and 17.2 (Cu 3 [Fe(CN) 6 ] 2 ).

A generalized model of electrode processes in mixed solvents

Abstract 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.

Electrode processes of the Pb(II)/Pb(Hg) and Zn(II)/Zn(Hg) systems in water + hexamethylphosphortriamide mixtures

Abstract The kinetic parameters of the Pb(II)/Pb(Hg) and Zn(II)/Zn(Hg) electrode processes have been determined in water + hexamethylphosphortriamide (HMPT) mixtures. Charge-transfer rate constants of both reactants drastically decrease for about three orders of magnitude in a narrow HMPT concentration range from 0 to 1 vol. %. At higher concentrations of the organic solvent the changes in the rate constant are small. Gibbs energies of transfer of Pb(II) and Zn(II) ions from water to water+ HMPT solutions have been calculated on the basis of potentiometric measurements using the ferrocene electrode. The Zn(II)/Zn(Hg) process has been analysed in detail and the results obtained suggest that the existing models of electrode reactions in mixed solvents do not apply to this system. An experimental equation describing the rate constant in terms of the dependence on solvent composition has been proposed.

Electrode processes of the Eu(III)/Eu(II) system in water + hexamethylphosphortriamide mixtures

Abstract The electrode reaction of the Eu(III)/Eu(II) system on a mercury electrode in water + hexamethylphosphortriamide (HMPT) mixtures was studied. Charge-transfer rate constants, diffusion coefficients and formal potentials were determined. The difference between the Gibbs energies of transfer of the Eu(II) and Eu(II) ions from water to water + HMPT mixtures was calculated from the potentials expressed in the ferrocene scale. The rate constants of the process investigated passed through a minimum as the concentration of HMPT was increased. In water-rich mixtures, the reaction rate depended only on the composition of the surface phase, and in mixtures of higher HMPT concentration, it depended on the state of reactant solvation.

Films of mixed nickel(II) and thallium(I) hexacyanoferrates(III, II): voltammetric preparation and characterization

Abstract Composite nickel/thallium hexacyanoferrates were prepared as thin films on electrode surfaces. Electrodeposition of composite films was achieved by potential cycling in the solution for modification containing, in addition to potassium salt electrolyte, nickel(II), thallium(I), and hexacyanoferrate(III). Depending on the relative amounts of thallium to nickel, either a mixed phase with respect to interstitial ions was formed in which Tl(I) cations substitute potassium countercations at interstitial positions of nickel hexacyanoferrate or mixed phases of hexacyanoferrates of nickel and thallium were produced. Due to large differences in ionic radii of Ni(II) and Tl(I), formation of heteronuclear nickel/thallium hexacyanoferrate, in which both nickel(II) and thallium(I) are nitrogen-coordinated within the cyanometallate lattice, was rather unlikely. On basis of the stripping-like determinations, we propose a stoichiometric formula, K 2− z Tl I z Ni II [Fe II (CN)] 6 ( z ≤1), for the system in which interstitial K + was substituted with Tl(I); and a more general K x Tl I z Ni II y [Fe II (CN)] 6 formula that also reflects the possibility of codeposition of separate nickel hexacyanoferrate and thallium hexacyanoferrate microstructures during the film growth. Mixed nickel/thallium hexacyanoferrate films showed good stability during voltammetric potential cycling. In addition to potassium salt electrolytes, well-defined and reversible cyclic voltammetric responses were also obtained in sodium and cesium electrolytes.

Electroreduction of nitrobenzene on the mercury electrode in water + HMPA solutions

Abstract The electroreduction of nitrobenzene on the mercury electrode has been studied in water+ hexamethylphosphotriamide (HMPA) mixtures. During this study the influence of HMPA on the electrode reactions occurring in the systems PhNO 2 /PhNO 2 .− , PhNO 2 .− /PhNHOH and PhNHOH/PhNO was investigated. The corresponding kinetic parameters have been calculated. The rates of these processes change in the series PhNO 2 /PhNO 2 .− > PhNHOH/PhNO PhNO 2 .− / PhNHOH. The standard rate constants of the PhNO 2 .− /PhNHOH and PhNHOH/PhNO processes decrease with increasing HMPA concentration. This influence is stronger for the PHNO 2 .− /PhNHOH process. The rate of the PhNO 2 /PhNO 2 .− process is practically independent of the solvent composition.

Electrode kinetics of the Eu(III)—Eu(II) system at mercury electrodes in wateracetonitrile (AN) and waterDMSO mixtures

Abstract The electrode reaction of the Eu(III)—Eu(II) system on the mercury electrode in waterAN and waterDMSO mixtures has been studied. Charge transfer rate constants, and formal potentials at different mixed solvent compositions are determined. The standard rate constants of the process investigated passed through a minimum, as the concentration of AN in waterAN mixtures increased, while in waterDMSO mixtures it increased with increasing DMSO content. Formal potentials expressed in the ferrocene scale were used to calculate the difference between Gibbs free energies of transfer of the Eu(III) and Eu(II) ions from water to waterAN and waterDMSO mixtures.