S. N. Ovchinnikova

Silver dissolution in acid thiocarbamide solutions containing sulfide ions

Regularities of silver dissolution in acid thiocarbamide electrolytes are studied. The kinetics of the process is shown to be severely affected by the admixture of hydrogen sulfide molecules that form upon inserting sodium sulfide or accumulate in electrolyte with the passage of time elapsed since its preparation. Catalytic effect increases with increasing length of time of the electrode’s contact with solution prior to the beginning of experiment or following an increase in the concentration of sulfide ions. Experiments with the surface renewed in the course of potential scans show that the catalytic effect is connected with the adsorption of sulfide ions on an interface. At large values of the surface coverage with sulfide ions, the dissolution rate increases so much that the dissolution process starts to be limited largely by the process of supply of thiocarbamide molecules toward the electrode surface.

Desorption of octanethiol from gold electrode surface during its electrochemical cleaning

The potentialities of electrochemical method of removal of adsorbed octanethiol layer from gold electrode surface are considered. The dependence of a degree of gold macro- and microelectrode cleaning on the electrolyte composition, potential cycling range, and the number of cycles is studied. It is shown that the commonly used method of cleaning by multiple potential cycling in the perchloric acid solution in the potential range from 0 to 1400 mV (SCE) is not sufficiently efficient. Cycling in the water-ethanol solution of perchloric acid promotes the octanethiol desorption and leads to the complete regeneration of surface even after 6–8 cycles. This is evidenced by comparing with the voltammograms, which were obtained on pure Au surface in 4 mM potassium ferri-ferrocyanide and 0.1 M HClO4 solutions. A range of ethanol concentrations, which provides the highest degree of gold surface cleaning, is determined. The electrochemical method of cleaning by using water-ethanol solution of perchloric acid enables one to perform multiple electrode regeneration without destruction of thin gold layer (for example, in the quartz microbalance), which takes place in the case of chemical regeneration in the “piranha” solution based on sulfuric acid and hydrogen peroxide.

Composition of magnetic layer and magnetotransport properties of electrodeposited layered nanostructures (Co-Cu)/Cu and (Ni-Co-Cu)/Cu

The magnetotransport characteristics of Co-Ni-Cu layered coatings containings 100 nanobilayers were measured at room temperature in a magnetic field of ±5 kOe. The coatings were deposited onto nickel sputtered on glass. It is shown that a decrease in the content of copper in the magnetic layer due to the conversion of copper ions in the solution into the multi-charge anionic complexes (by the example of sulfosalicylate electrolyte) leads to an increase in the giant magnetoresistance (GMR) effect. The stabilization of composition of Co-based magnetic layer by shifting the copper deposition potential to the equilibrium potential of Co or by introducing Ni, which assists the deposit passivation, leads to a decrease of the GMR effect. A benefit effect of partial dissolution of Co at the deposition potentials of nonmagnetic layer on the magnetotransport characteristics of multilayered coatings is first revealed. It is supposed that the effect is caused by the morphological factor of surface smoothing due to preferential dissolution of projecting areas of magnetic layer.

Kinetics of silver dissolution in sulfide containing thiocarbamide electrolytes

Effective values of reaction order with respect to ligand P, transfer coefficient α, and exchange current i0 at constant silver surface coverages θ by sulfide ions are measured. The employed solutions contained from 0.4 to 0.05 M thiocarbamide, 0.5 M HClO4, 10−4 M AgNO3, and from 10−5 to 10−4 M Na2S. It is shown that the exchange current grows approximately linearly from 10−5 to 1.5 × 10−4 A/cm2 at θ increase in the range from zero to 0.8, while α and P values grow negligibly in the ranges of 0.4–0.45 and 0.9–1.1, accordingly. The obtained results are compared with the data of similar studies of the gold behavior in acidic thiocarbamide solutions. The possible reasons for the different effects of sulfide ion chemisorption on the anodic dissolution of gold and silver in the studied solutions are discussed.

Mutual Influence of Electrode Processes during Electrodeposition of Layered Structures by the Single-Bath Method: The Effect of Nickel Deposition and Hydrogen Evolution on the Transport of Copper Ions in Acetate and Sulfamate Electrolytes

The effect of nickel deposition and hydrogen evolution on the transport of copper-containing ions is studied by numerically solving an electrodiffusion problem with use made of variational values of the formation constants of nickel and copper complexes in acetate and sulfamate electrolytes and thicknesses of the diffusion layer. It is concluded that the major contribution to the mass transfer is made by the effects of exaltation of the migration current and by the agitation of the near-electrode layer of electrolyte by evolving hydrogen. The possibility of employing migration effects in order to reduce the limiting current of copper in the region of nickel deposition and hydrogen evolution is substantiated. This will decrease the copper content in a layer of alloy during electrodeposition of layered structures.

Anodic Dissolution of Gold in Alkali–Cyanide Solutions: Effect of Lead Ions

Effect of the presence and the concentration of lead ions on the anodic dissolution of gold in cyanide electrolytes is studied. The ions dramatically accelerate the process in the potential region extending from –0.61 V vs. NHE (equilibrium potential of gold) to –0.2 V. The effect is attributed to a catalytic action of adsorbed lead ions on the process.

Comparative electrochemical study of self-assembly of octanethiol from aqueous and aqueous ethanol solutions on a gold electrode

The self-assembly of octanethiol (OT) on the surface of a polycrystalline gold electrode in aqueous and aqueous ethanol thiol-containing (1 × 10–4 М) 0.1 М NaClO4 solutions was studied. The blocking properties and electrochemical stability of monolayer OT films were studied by chronopotentiometry during OT adsorption under the open circuit conditions (chronoamperometry at a fixed potential) combined with cyclic voltammetry for modified Au/OT electrodes. It was found from the change in the rate of electrochemical reactions in the range of monolayer stability potentials that in aqueous media, compact insulating OT monolayer films formed at a open circuit potential within ~100 s, and the shift of the adsorption potential toward negative values (to–0.6 V) allowed a considerable decrease in the monolayer self-assembly time. The potential shift toward higher negative values (–0.9 V) leads to a removal of OT from the electrode surface during the reductive desorption, with a multipeak current signal recorded on the voltammograms. A transition from aqueous to aqueous ethanol solutions accelerated the formation of an insulating OT monolayer (≈6 s) and led to a change in the shape of the desorption current peak, whose value was almost independent of the ОТ accumulation time and potential.

Mutual Influence of Electrode Processes during Electrodeposition of Layered Structures by a Single-Bath Method: Effect of Nickel Deposition and Hydrogen Evolution on the Mass Transfer of Copper in a Sulfosalicylate Electrolyte

The work justifies the selection of a sulfosalicylate copper–nickel electrolyte and its composition for obtaining nanostructures comprising alternate magnetic and nonmagnetic layers by a single-bath method. According to calculations of an equilibrium composition of the electrolyte and electrodiffusion fluxes of discharging species, in a neutral region of pH copper exists in the form of multicharged anions CuSSA3–, and the migration effects must reduce the rate of the mass transfer of copper-containing species with increasing current of the parallel reaction of discharge of nickel ions. Dependences of the partial current density of copper deposition on the electrode potential are studied at various values of pH of the copper and copper–nickel electrolytes. Experimental results are analyzed on the basis of modeling concepts. It is shown that the drop of the partial current density of copper deposition below its limiting value at potentials of deposition of a magnetic layer is connected with the emergence of migration effects.

The Effect of Adsorption of Ions of the Hexacyanoferrate(II)/(III) Redox Pair on Self-Assembly of Octanethiol at Its Adsorption from Aqueous Solutions on Gold Electrode

The effect of components of the redox pair K3[Fe(CN)6]/K4[Fe(CN)6] on the dynamics of formation of octanethiol (OT) monolayers from aqueous thiol-containing solutions of 0.1 М NaClO4 is studied by cyclic voltammetry (CVA). The formation of OT monolayers is shown to depend on the presence of ions of hexacyanoferrate(II)/(III) in solution. Being added to solution, the components of the [Fe(CN)6]3–/4– redox pair sharply increase the time of formation of the insulating monolayer OT films and make them less stable. The destabilizing and inhibiting action of [Fe(CN)6]3–/4– ions becomes stronger as their concentration in solution increases. The adsorption activity of individual components of the redox pair is assessed. The strong and approximately equal adsorption activity of ions [Fe(CN)6]3– and [Fe(CN)6]4– on gold in the presence of octanethiol is observed. At the same time, OT and the hexacyanoferrate(II)/(III) ions can be placed in the following row: OT > [Fe(CN)6]3– ≈ [Fe(CN)6]4–. Recommendations are given on how to eliminate the interfering action of the K3[Fe(CN)6]/K4[Fe(CN)6] redox-pair ions when studying the insulating properties of thiol monolayers on gold.

Peculiarities of electrodialysis processing of mineralized human exometabolites

The possibilities of the electrodialysis method in processing solutions of mineralized human exometabolites, where the dialysate is used for watering plants and the concentrate is subjected to further processing to afford common salt, are considered. The effect of the degree of solution purification from organic compounds on the electrodialysis rate is studied. Solutions are purified on carbon sorbent Sibunit which can be repeatedly recovered. The optimal parameters of the sorption stage of purification, namely, solution pH 2–4; contact time 1.5–2 h, specific consumption of sorbent 300 g/L, are determined. It is shown that the preliminary enzymatic degradation (enzymolysis) of urea by urease and, especially, the sorption cleaning of solution considerably accelerate the transmembrane mass transfer, remove complications of this process (precipitation), and favor both the dialysis with the preset degree of desalination with respect to sodium ions (which inhibit the plant growth) and the formation of the organic-free concentrate that can be used in the further extraction of dietary salt.