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Electrocatalytic activity of common noble metal (platinum, bimetallic platinum-ruthenium and palladium) nanoparticles (both unsupported and supported on Vulcan carriers) toward electrooxidation of formic acid in acid medium (0.5 mol dm–3 H2SO4) can be significantly enhanced by dispersing them over thin film of zirconia (ZrO2). The enhancement effects concern increases of the electrocatalytic current densities (normalized against the mass of noble metal catalysts) recorded under both cyclic voltammetric and chronoamperometric conditions. The observations can be rationalized in terms of the ability of zirconia nanostructures to provide active hydroxyl groups capable of inducing the removal of the inhibiting COOH or CO type adsorbates from their surfaces. The role of Vulcan carriers is in improving electronic conductivity, in addition to facilitating distribution of catalytic metal sites at the electrocatalytic interface. Regarding dissimilar mechanisms for HCOOH oxidations at Pt, PtRu and Pd nanoparticles, the activating capabilities of zirconia differ depending on the potential applied thus reflecting distinct nature of electronic or chemical interactions. Specific interactions between the zirconia support and noble metal nanoparticles are also possible, particularly in the case of Pd-based catalysts where the enhancement effects are the most pronounced. Finally, it should be emphasized that, historically, representatives of Russian school of electrochemistry, including V.S. Bagotsky, were one of the first world-wide who investigated the formic acid oxidation and emphasized the importance of metal—support interactions during electrooxidation of small organic molecules at noble metal nanoparticles.

The preparation of copper(II) hexacyanoferrate (CuHCF) films on the surface of gold electrodes as well as their characterization in solutions of various alkali metal and NH 4 + cations and in the presence of thallium(I) are described. The electrochemical quartz crystal microbalance and cyclic voltammetric techniques were used. In 0.50 M lithium nitrate, even at submillimolar concentration of Tl(I), the formal potential of CuHCF was shifted to more positive values. At higher Tl(I) concentrations, the formal potential of the CuHCF redox reaction changed linearly with the logarithm of Tl(I) concentration (in the 0.50 M solution of lithium or another alkali metal nitrate). From such dependencies, selectivity coefficients K Tl/M were calculated, and they show that the CuHCF film on the gold electrode interacts preferentially with Tl(I). High affinity of Tl(I) to copper hexacyanoferrate, that was observed in the presence of alkali metal cations, was explained by relatively strong donor–acceptor interactions of Tl(I) ions with nitrogen in CN groups of the CuHCF film. It was also shown for simple M 4 [Fe(CN) 6 ] metal ferrocyanate salts (where M = Li + , Na + , K + , Rb + , Cs + and Tl + ) that there is a preferential interaction of Tl + with CN group consistent with formation of a Tl–NC–Fe bridge.

Electrodeposition of well-adhering polypyrrole-based hybrid films containing hexacyanoferrate(II,III) anions from neutral solutions of pyrrole and potassium hexacyanoferrate(II) on medium carbon (0.48% C) steel has been described. The resulting polypyrrole coatings that are doped with hexacyanoferrate(II,III) anions show protective properties against pitting corrosion of carbon steel substrates in strongly acidic media containing chlorides (0.1 mol dm−3 HCl + 0.4 mol dm−3 NaCl). Polypyrrole acts as a stable host matrix for inorganic anions. The presence of negatively charged species (hexacyanoferrates) in the polymer backbone tends to block the access of pitting-causing anions (chlorides) to the surface of steel. The Fe(CN)63-/4− anions existing in the vicinity of steel substrate stabilizes its surface by forming an overcoating in the form of sparingly soluble metal hexacyanoferrate, mostly Prussian blue (PB), microstructures. It has been demonstrated that by applying cyclic voltammetry and X-ray photoelectron spectroscopy, the presence of traces of free cyanide anions promotes the formation of PB on carbon steel surface which results in increasing the adherence of polypyrrole-based films to the metallic substrate. Morphology of the protective composite films is also addressed.

Electrochemical and electrochemical quartz crystal microbalance (EQCM) methods were used for the analysis of the growth and dissolution processes of sparingly soluble deposits. As a model of a sparingly soluble film, uranyl-hexacyanoferrate was selected. Additionally the rate of Prussian blue deposition was studied. We have shown that coupling of the EQCM technique with standard electrochemical methods may be useful in the study of mechanisms of electrodeposition phenomena when the solubility of the product of the electrode reaction is limited. The transformation of mass–time data into current–time dependences allows an analysis of the reaction in terms of electrocrystallization steps while other masking processes are superimposed during the polarization of the electrode. This approach may have general value and may be applied not only to hexacyanoferrate deposition but also to deposition of other products of the electrode reaction, which exhibit limited solubility. Analysis of growth of uranyl-hexacyanoferrate on the Au/quartz electrode under potentiostatic and cyclic voltammetry conditions, taken as an instance, reveals that the first step, limited by the growth of the crystallization nuclei, is clearly separated from the uniform growth, limited by diffusion. For thicker films, the slope of the frequency–charge dependence is dominated by the insertion/removal process of potassium ions, to and from the film. During the dissolution of the uranyl-hexacyanoferrate deposits, two clearly separated steps were observed. During the first, relatively fast oxidation of the deposit is observed which is accompanied by removal of potassium ions from the film. The second step corresponds to a chemical dissolution of the oxidized film. The rate coefficients corresponding to both steps were determined. Based on the gravimetric and electrochemical data, the molar mass of the deposited uranyl-hexacyanoferrate was estimated as equal to 690 ± 90 g/mol. From these and the EDX data, a chemical formula of the deposited uranyl-hexacyanoferrate was proposed as: K2(UO2)[Fe(CN)6].

The electrochemical behaviour of mixtures of phenazine (P) with iodine (I2) in the solid state was investigated. Three types of electrochemical experiments were conducted: (i) EMF measurements of the Ag/AgI/I2:P system for various I2 : P ratios, (ii) cyclic voltammetric (CV) experiments and (iii) potentiometric measurements of the open cell potentials after different cathodic charges were injected into the I2 : P pellets. In the first case, a detailed analysis of the observed electromotive force was presented. The Gibbs free energy of the I2 : P complex determined was in good agreement with earlier results. In the second case, the CV curves corresponding to the electrode processes of phenazine mixed with graphite in the solid state indicate that the mass and charge transfer, which occur in the pellets, are faster than expected. The potentiometric experiments confirm this conclusion. Additional experiments, conducted with pellets of different thickness, indicate that although, in comparison with an equilibrated pellet, the concentration profiles are modified in small layers (diffusion layers) neighbouring the electrodes, the electrode reaction probably occurs in the whole pellet. A tentative mechanism of these processes is discussed. Copyright © 2003 John Wiley & Sons, Ltd.

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 system's 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.

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.

The temperature dependences of the four transfer coefficients for cathodic and anodic electrode reactions of two different types of systems, namely Cr(III)/Cr(II) and Cr(II)/Cr(Hg), were determined in 5.0 M NaClO 4 solution. The results were obtained for a wide range of temperatures from −20 to 90°C. The α parameter determined from the log k fh on E dependence corresponding to the Cr 3+ +e − →Cr 2+ electrode process shows the highest dependence on temperature (∂ α /∂ T =1.1±0.3×10 −3 K −1 ), whereas the β parameter corresponding to the Cr 2+ →Cr 3+ +e − electrode process is virtually independent of temperature (∂ β /∂ T =0). The same results were obtained when the transfer coefficients were determined from dependence of the enthalpy and entropy of activation on overpotential. The observed temperature dependence of α is attributed to the changes of the ϕ 2 potential. The transfer coefficients for the Cr 2+/0 electrode reaction also show some dependence on temperature (∂ α /∂ T =8.7±2.0×10 −4 K −1 and ∂ β /∂ T =3.2±1.4×10 −4 K −1 ), which probably results from the influence of the ϕ 2 potential.

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.

Two different methods for solid-state electrochemical measurements in the absence of liquid electrolyte are discussed. The simplest approach is based on a two-electrode sandwich cell (without reference electrode), and it is suitable for ionically-conducting materials containing stable and chemically identifiable, mixed-valence redox sites (eg Fe(III,II) in Prussian Blue or Berlin Green). The second two-electrode design utilizes an ultramicrodisk working electrode which faces a large surface area silver-disk semireference/counter electrode. In order to discuss usefulness and limitations of these methods, we consider the following inorganic materials: Prussian Blue, Berlin Green, and silicotungstic acid single crystals. These structures contain mobile countercations (hydrated K + or H + ) in concentrations extensive enough to support diffusive transport of electrons hopping between Fe(III,II) or W(VI,V) centers (redox conduction). In the latter case, combination of potential step measurements in short-time and long-time experimental regimes provides an absolute electroanalytical method for determinations of redox centers and an effective diffusion coefficient for charge propagation.

The standard hydrogen electrode (SHE) acts as a primary reference in electrochemistry. The standard potentials of all other reference electrodes are linked to that of the SHE at the same temperature. The SHE contribution to the cell potential is by convention zero at all temperatures (see Chap. 1).

The electrooxidation of in situ produced electrolytically radical anion of 3-nitrophthalic acid in various water—ethanol solutions containing 0.5 M NaCIO4, 0.05 M NaOH and inhibitor has been studied. The inhibition of the electrode kinetics in the presence of n-octanol and n-decanol, used as inhibitors, at relative concentrations not exceeding c/cs ≤ 0.5 could be interpreted in terms of the competitive monolayer adsorption of reactant, inhibitor and solvent on the electrode surface. At c/cs > 0.8, on the capacitance-potential curves, a subtle minima caused by formation of two layers adsorbed on the mercury electrode were observed. The kinetics of the electrooxidation of the 3-nitrophthalic acid anion radical was additionaly influenced by this multilayer. An equation which describes the inhibition of the electrode process by two adsorbed layers of inhibitor is presented and used in the estimation of the surface coverage in the second layer.

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 electrochemistry of wet solid Berlin green (BG) containing Na+ and K+ has been studied by voltammetry and chronoamperometry. It has been found that the reactions of both electroactive systems in BG, FeII/III cyanide complex and Fe2+/3+, are diffusion controlled. The charge diffusion coefficients in BG containing K+ are high and have been estimated as 2.2 × 10−8 cm2s−1 and 1.1 × 10−7 cm2s−1 under transient and steady-state conditions respectively. Evidence is provided that almost all redox sites in BG are electroactive. BG has been found to be nearly 1:1 mixed hexacyanoferrate(II)/(III).

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 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 sections in this article are Double-layer Properties of Cadmium Electrodes Electrochemical Properties and Kinetics of the Cd(II)/Cd(Hg) Systems Electrochemistry in Aqueous Solutions Kinetics of Simple Ions Influence of a Catalyst Influence of Inhibitors Electrochemistry in Mixed and Nonaqueous Solvents Electrochemical Properties of the Cadmium Complexes Determination of Stability Constants Electrochemical Preparation of Cadmium Complexes Preparation and Electrochemical Properties of Cadmium Chalcogenides CdS CdSe CdTe Properties of Cadmium Intermetallic Compounds Cadmium Underpotential Deposition and Electrodeposition on Solid Electrodes Deposition of Cd on Cadmium Electrode Electrodeposition of Cd on Other Solid Substrates Gold Platinum Silver Copper Silicon Other Substrates Passivation and Corrosion of Cadmium Passivation of Cadmium Electrode Corrosion of Cadmium Applied Electrochemistry Batteries Cadmium–Mercuric Oxide Primary Cells Cadmium–Nickel Oxide (Ni–Cd) Secondary Cells Cadmium–Silver Oxide Secondary Cells Application in Electrochemical Sensors Potentiometric Sensors Amperometric-voltammetric Sensors

The sections in this article are Double-layer Properties of Pb Electrodes Properties of Solid Electrodes Properties of Liquid Electrodes Adsorption on Solid Electrodes Electrochemical Properties and Kinetics of Lead and Lead Compounds Pb(II)/Pb(Hg) System Studies in the Presence of Inhibitors and Organic Solvents Other Phenomena Complexes of Pb(II) and Related Phenomena Electrochemical Processes of Liquid Lead in Molten Salts Voltammetric Behavior of Lead Sulfuric Acid Solutions Nitric Acid Solutions Phosphoric Acid Solutions Sodium Hydroxide Solutions Different Salt Solutions Voltammetric Behavior of Several Lead Compounds PbO PbO2 Pb chalcogenides Electrodeposition and Underpotential Deposition of Lead on Solid Substrates Platinum Gold Silver Copper Carbon Silicon Germanium Other Materials Corrosion and Passivation Applied Electrochemistry Lead-acid Batteries Electrocatalytic Properties Pb PbO2 Electrochemical Sensors Potentiometric Sensors Amperometric/Voltammetric Sensors

The sections in this article are Preparation of Au Surfaces and Double-layer Properties Polycrystalline (pc-Au) and Single-crystal Au/H2O Interface pc-Au and Single-crystal Au/Nonaqueous Solutions Interface Adsorption of Inorganic Compounds on Gold Electrodes Sulfate Hydroxide Nitrate Chloride Bromide Iodide Cyanide Thiocyanate Other Inorganic Species Adsorption of Organic Compounds on Gold Electrodes Adsorption of Sulfur-containing Compounds Adsorption of Thiols Adsorption of Modified Thiols Adsorption of Thiourea Adsorption of Disulfides and Related Compounds Other Sulfur Compounds and Complex Adsorption Layers Adsorption of Selenium-containing Compounds Adsorption of Nitrogen-containing Compounds Pyridine and its Derivatives Pyrazine Amines and Other Nitrogen-containing Compounds Adsorption of Other Organic Compounds Alcohols Aminoacids Other Organic Acids Nucleic Bases and Other Biochemical Compounds Other Compounds Reconstruction of the Surface of Gold Electrodes Formation of Oxides on Gold Electrodes Underpotential Deposition of Various Metals on Au Cu Cd Pd S, Se, and Te Sulfur Selenium Tellurium Bi Pt Ni Co Sb Sn Al Tl Pb Zn Ge Rh Li Fe Ag Hg Electrochemistry of Coordination Compounds of Gold. Electrodissolution and Electrodeposition of Au Au Electrodeposition Au Electrodissolution Gold Clusters

The sections in this article are Properties of Ag Electrodes Polycrystalline Ag Electrodes Preparation Surface and Double-layer Properties Single-crystal Ag Electrodes Preparation Surface and Double-layer Properties Surface Dynamics Affinity of Water and Nonaqueous Solvent Molecules to Ag Surfaces Adsorption of Inorganic Species on Ag Electrodes Halide Ions pc-Ag Electrodes Single-crystal Ag Electrodes Pseudohalide Oxygen and its Compounds Sulfur Compounds Inorganic Carbon Compounds Fullerenes Other Inorganic Compounds Metalocenes Adsorption of Organic Species on Ag Electrodes Hydrocarbons Pyridine and Other Nitrogen Compounds Carboxylic Acids Alcohols Sulfur Compounds Thiourea (TU) Alkanethiols and Self-assembled Monolayers Biochemically Important Compounds Adsorption on Modified Ag Electrodes Other Organic Compounds Electrode Processes with Participation of Silver Electrodes Reactions Involving Oxidation of Ag Surface Oxygen and its Compounds Sulfur Compounds Carbon Compounds Electrode Reactions of Selected Organic Compounds, Adsorbed on Ag Electrodes Biochemically Important Species Underpotential Deposition Processes Ag UPD on Ag Ag UPD on Au and Pt Pb Tl Ni Chalcogenides Other Elements and Compounds Electrodeposition and Electrodissolution Processes of Ag Electrodeposition Processes Electrodissolution Processes Silver Compounds at the Oxidation States Higher Than I

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 ).

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.

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.

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 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.

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.

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.

Spectrophotomeric and electrochemical behaviour of V(III) ions was investigated in H 2 O + DMPU and H 2 O + HMPA mixed solvents. While optical studies have revealed the resolvation of V(III) already in H 2 O rich solutions, it has been found that the reduction of V(III) to V(II) at the mercury electrode yields the double polarographic wave in these mixtures. The double wave behaviour has been ascribed to the reduction of V(III) aquo-ions and mixed solvates of V(III) proceeding at different rates.

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 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).

The V(III)/V(II) system has been investigated polarographically in HClO 4 + NaClO 4 aqueous solutions of pH ranging from 0 to 4.6 at constant ionic strength equal to 1 mol dm −3 . Low concentration of vanadium ions, 0.47 mM, was used in order to avoid their polynuclear hydrolysis. While an oxidation wave of V(II) was practically-pH independent, large pH effects on a reduction wave of V(III) were found: (i) a shift toward more negative potentials; (ii) a split into several compositional waves; (iii) gradual decrease of the total wave with a rise in pH.These pH effects have been ascribed to the formation of reducible, mononuclear V(III) hydroxycomplexes such as VOH 2+ , V(OH) 2 + and V(OH) 3 as well as to a precipitation of V(III) hydroxide. Very slow equilibration of V(III) solutions hydrolysed above a pH ⋍ 3.8 has been found.

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 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.

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.

At high ionic strength the ion pair (NiPy2+4, nX−) or complex (NiPy4X2), n = 0, 1, 2; X− = Cl−, Br−, SCN−, N−3, F−, NO−3, ClO−4; is adsorbed at the surface of mercury electrode. Under specified conditions in chloride, bromide, and thiocyanates solutions the electroreduction is preceded by a crystallization of a complex on the electrode surface. The inductive role of specifically coadsorbed Cl− ions is discussed.