Regularities of gold anodic dissolution in thiocarbamide solutions

It is shown that the gold dissolution in thiocarbamide electrolytes containing sulfide ions in the pH range from 1 to 4 proceeds with a current efficiently of almost 100%, and a change in solution acidity has a weak effect on the process. The oxidation of thiocarbamide to formamidine disulfide proceeds at the potentials around 1.1 V and depends on the pH of solution. When the pH value is raised, the thiocarbamide oxidation potential shifts in the negative direction and approaches the potential of gold dissolution. It is found that, in the absence of catalytically active species, the gold dissolution rate in thiocarbamide solutions in the pH range from 6 to 10 is vary low. At these pH values, as well as in the acidic solutions, an addition of less than 10⁻⁴ M sulfide ions to thiocarbamide electrolyte accelerates the gold dissolution at the potentials about 0.6 V. It is shown that sulfide ions in the concentration above 10⁻⁴ M inhibit the process. At pH 11, the gold dissolution proceeds also with no special addition of sulfide ions. This is associated with the formation of sulfide ions as a result of accelerated decomposition of thiocarbamide and formamidine disulfide with increasing pH value. It is found that in the pH range from 12 to 13, the processes of gold dissolution and thiocarbamide oxidation are inhibited as a result of the formation of passive film on the electrode surface. Probably, the film consists of elemental sulfur.     

Quartz crystal microbalance measurements of kinetic parameters of anodic dissolution of gold in thiocarbamide electrolytes in the presence of sulfide ions

As shown by quartz-crystal microbalance measurements, in the potential range from 0.0 to 0.55 V (NHE), sulfide ions adsorbed on the gold electrode surface accelerate the electrode reaction of anodic dissolution of gold in acidic thiocarbamide solutions. The microbalance determination of kinetic parameters at a constant electrode surface coverage with sulfide ions includes a special procedure developed for the determination of the gold dissolution rate. The conditions (the potential range and the potential scan rate) of independence of the dissolution rate from the diffusion limitations associated with the ligand delivery is determined. Under these conditions, the polarization curve is shown to be linear on semilogarithmic coordinates and correspond to the Tafel equation. In this potential range, the transfer coefficient α and the reaction order with respect to the ligand p are determined at a constant electrode surface coverage θ with adsorbed sulfide ions. It is shown that with the transition from the surface coverage with sulfide ions θ = 0.1 to θ = 0.8, the transfer coefficient α changes from 0.25 to 0.55, the exchange current (i ₀) changes from 10^?5 to 5 × 10^?5 A/cm², and the effective reaction order p with respect to the ligand changes from 0.2 to 1.3. The mentioned changes are associated not only with the acceleration of gold dissolution in the presence of chemisorbed sulfide ions but also with the changeover in the mechanism of this process. Quartz-crystal microbalance data on the gold dissolution rate qualitatively agree with the results of voltammetric measurements of a renewable gold electrode. A possible version of explanation of the catalytic effect of sulfide ion adsorption on the gold dissolution is put forward.     

The role of sulfide ions in the electrode processes involving gold in sulfite-thiocarbamide electrolytes

The regularities of electrochemical deposition and dissolution of gold in the mixed sulfite-thiocarbamide electrolytes in the absence and in the presence of sodium sulfide additive are studied by using the voltammetric measurements on a renewable electrode and quarts microgravimetry. It is shown that, in the cathodic metal deposition, an addition of sodium sulfide promotes the depolarization effect, which is caused by the presence of thiocarbamide in the solution. Under the anodic polarization of gold in the mixed sulfite-thiocarbamide solution with pH < 10, the gold dissolution rate is insignificant. An addition of 10^?5 M Na₂S to this solution dramatically accelerates the process. At pH > 10, the gold dissolution in the sulfite-thiocarbamide electrolyte is observed even in the solution free of Na₂S additive. It is evidenced that this is associated with spontaneous accumulation of sulfide-containing species in the solution, probably, as a result of thiocarbamide hydrolysis; the rate of hydrolysis steeply increases with increasing pH value.     

Kinetics of silver electrodeposition from thiocarbamide solutions at a regular surface coverage with sulfide ions

Kinetics of silver electrodeposition in the presence of sulfide ions is studied on electrodes renewed by cutting off a thin surface layer, at a controlled time of contact of the “fresh” surface with the electrolyte. Solutions containing 10^?2 M AgNO₃, 0.1 M thiocarbamide, 0.5 M HClO₄, and from 2 × 10^?6 to 1.5 × 10^?5 M Na₂S are studied. It is shown that under the studied conditions, the effect of silver electrodeposition on the surface concentration of sulfide ions is insignificant. As the concentration of sulfide ions in solution and their coverage on the electrode surface θ increase, the cathodic polarization decreases. Tafel curves plotted for θ = const are used in estimating the exchange current i ₀ and the transfer coefficient α. It is shown that α ≈ 0.5 and weakly depends on θ, whereas the exchange current increases with the increase in θ by an approximately linear law from 10^?5 A/cm² at θ ? 0 to 10^?4 A/cm² at θ = 0.43. The obtained data are compared with the results of kinetic studies of silver anodic dissolution in similar solutions.     

The effect of chemisorbed sulfide ions on the gold electrodeposition from acidic thiocarbamide electrolytes

Introducing sodium sulfide (about 10^?5 M) into acidic thiocarbamide solutions reduces the gold reduction overpotential. The reaction rate passes through a maximum at a potential of 0.1 V. The overpotential depends on the sulfide ion concentration and the time of electrode exposure to solution prior to the beginning of scanning. Transients of potential measured on a renewable gold electrode in thiocarbamide electrolytes containing catalytically active species served as the basis for calculations of the coefficient of trapping of sulfide ions by the growing gold deposit. The kinetics of gold electrodeposition at fixed surface coverages with adsorbed sulfide ions θ is studied. It is shown that at θ = const, the dependence of the reaction rate on the overpotential is described by the Tafel equation. It is shown that with an increase in θ, the effective values of exchange current and transfer coefficient increase from i ₀ ≌ 10^?5 A/cm² and α ≌ 0.25 in pure solutions to α ≌ 0.5 and i ₀ ≌ 10^?4 A/cm² at θ ≥ 0.3 and then remains virtually unchanged. The reaction order decreases in the absolute magnitude, remaining negative. Thus for θ ≌ 0, p _k = ?logi/?logc = ?1, whereas for θ ≥ 0.3, p _k = ?0.3. A possible explanation is proposed for the catalytic effect of the sulfide ion adsorption on the mechanism of the gold reduction from acidic thiocarbamide electrolytes.     

Gold Dissolution Electrocatalysis in Thiourea Electrolytes in the Presence of Chemisorbed Sulfide Ions

Adding a microscopic quantity of sodium sulfide (～10^?5 M) into acid solutions of thiourea leads to a dramatic acceleration of anodic dissolution of gold. The acceleration effect is greater at larger thiourea concentrations (c) and longer times of the electrode contact with solution (Δt) before the beginning of measurements. The effect diminishes after a polarization curve passes through a maximum at E ? 0.5 V. Regularities of the gold dissolution in a solution containing 0.1 M thiourea and 0.5 M H₂SO₄ at given values of c and Δt are studied with use made of the technique of renewing the electrode surface by cutting off a thin surface layer of metal. The discovered regularities are given an explanation which is based on the assumption that the dissolution process is catalyzed by sulfide ions adsorbed on the electrode surface.     

Estimation of degree of lability of gold(I) sulfite and thiocarbamide complexes

The supposition that gold thiocarbamide complexes can be involved in the reduction of electrochemically inactive sulfite complex gold ions as intermediate species is supported by investigating the kinetics of cathodic process in the solution of the following composition: 10^?3 M Na₃Au(SO₃)₂, 0.1 M Na₂SO₃, 0.1M thiocarbamide (TC), and 0.2 M Na₂SO₄. The study was performed by the potentiodynamic method in the potential scan rate range from 0.05 to 1 V/s using an electrode with mechanically renewed surface. It is found that both Au(SO₃) ₂ ^3? , and Au(TC) ₂ ⁺ are labile complexes. This confirms the hypothesis for the mechanism of the catalytic effect of thiocarbamide on the electrode processes involving gold ions in the sulfate solutions, which was proposed earlier.     

The effect of potential on the gold dissolution in cyanide solutions in the presence of sulfide ions

The effect of potential on the rate of gold dissolution in the cyanide solutions in the presence of sulfide ions is studied. The dependences of current on the time after the electrode surface renewal were measured under the potentiostatic conditions. The majority of experiments were performed in the solution of the following composition, M: 0.1 KCN, 0.1 KOH, 0.01 KAu(CN)₂, (1.5–2) × 10^?5 Na₂S at 23°C. It is shown that, at the potentials more positive than ?0.1 V (NHE), the rate of gold dissolution starts to increase as soon as the surface is renewed, which is associated with high-rate chemisorption of catalytically active sulfide ions. At E < ?0.1 V, the chemisorption proceeds slowly, and a considerable increase in the current takes much time. Therefore, in the potentiodynamic measurements, at E < ?0.1 V, no catalytic effect of sulfide ions is observed. When the ratio between the concentrations of sulfide and cyanide ions is decreased, the potential, which, by convention, bounds the aforementioned ranges, shifts in the positive direction. Plausible explanations for these regularities are proposed.     

Anodic oxidation of sulfide ions from chloride brines

The anodic oxidation of sulfide ions from chloride brines at graphite electrodes is shown to produce elemental sulfur, that was identified using XPS and EDS measurements. The process is controlled by charge transfer at the interface and diffusion of the sulfide ions in the electrolyte. Potentiostatic current transients reveal exponential decay of the current with time accompanied by gradual increase of the polarization resistance of the interface, at a rate which increases with increase of the potential in the noble direction. This behavior is attributed to passivation of the electrode by the elemental sulfur which deposits on its surface.     

Effect of solution pH on the electrochemical behavior of thiocarbamide at gold and platinum electrodes

The effect of solution pH on the electrochemical behavior of thiocarbamide at gold and platinum electrodes is studied by using cyclic voltammetry and quartz microgravimetry. It is shown that with the increasing of pH the half-wave potential of thiocarbamide oxidation shifts toward negative values in both cases. In the cathodic branches of the cyclic voltammograms, up to pH ≃ 4 the potential region of reduction of the thiocarbamide oxidation product remains practically unchanged; this allows concluding that the oxidation product is formamidine disulfide (at platinum) or its mixture with gold complex ions (at gold electrode). Further increasing of pH up to 9.5–10 results in the increase of the contribution of the reduction current of these ions to the cathodic signal. An explanation for this phenomenon is given. At the same time, new signals appear at E < −0.4 V; they may be interpreted as the current of elemental sulfur reduction. Increasing of pH over 10 leads to gold passivation by the elemental sulfur; this manifests itself as the electrode mass increase during the microgravimetry measurements. Introducing of sodium silicate (recommended in the literature as a solution stabilizer) to the solution does not eliminate the passivation of gold.     

Effect of time of electrode exposure to the solution on gold dissolution rate in thallium-containing thiosulfate electrolytes

The effect of TlNO₃ additive on the gold dissolution in the thiosulfate electrolyte is studied by using chronoammetry on the electrode, which was renewed by cutting-off a thin surface metal layer immediately in the solution. It is shown that the ascending branches of anodic polarization curve shift in the negative direction with increasing time of electrode exposure prior to the onset of potential scan Δt (from 0 to 120 s) to the solution containing thallium(I) ions. It is shown that solution stirring and a twentyfold increase in the concentration of thallium ions have a weak effect on the current transients. It is found that the duration of gold dissolution current transients decreases significantly with a potential shift in the positive direction. Based on this data, a mechanism of the effect of thallium(I) ions on the gold dissolution rate is proposed.     

Effect of hydrogen sulfide on the iron dissolution rate in a sodium sulfate solution under natural aeration conditions

The influence of hydrogen sulfide (10–100 mg/1) on the Armco iron anodic dissolution in an aerated 0.17 M Na₂SO₄ solution is investigated. During a potentiostatic anodic polarization, the hydrogen sulfide introduction makes the current increase stepwise. The magnitude of the increase depends on the duration of preliminary anodic polarization, electrode potential, and hydrogen sulfide concentration. The anodic metal dissolution activation by hydrogen sulfide is explained by chemical conversion of the oxide-hydroxide passivating film into iron sulfide that is generated at the metal surface in the form of a porous film and does not hinder the electrode dissolution. Dedicated to the ninetieth anniversary of Ya.M. Kolotyrkin’s birth.     

A comparison of the effects of chemisorbed sulfide ions on the electrochemical behavior of gold and silver in thiourea electrolytes

It is shown that, in the case of gold, the catalytic activity of sulfur adatoms is significantly higher in the anodic dissolution of metal than in its deposition. In both processes, the catalytic activity increases with increasing overvoltage. The catalytic activity of sulfur adatoms is considerably lower for silver than for gold both in the metal deposition and dissolution, and it is virtually independent of overvoltage. In the case of silver, the catalytic activity of sulfur adatoms in the electrodeposition is slightly higher than in the metal dissolution. There results are compared with the action of heavy metal adatoms on the dissolution and deposition of gold and silver in the cyanide solutions. It is shown that the above regularities are qualitatively true for both systems. The main distinction is that the catalytic activity of sulfide ions in the thiourea solutions reaches a plateau with increasing surface coverage with sulfur adatoms, whereas the activity of heavy metals passes through a maximum with increasing surface coverage. The results are explained in view of earlier determined regularities of kinetics of electrode processes in these systems and the effect of electrocatalysis on these regularities.     

硫化银/凹凸棒的Ag+选择电极

以硝酸银、凹凸棒石和硫代乙酰胺为原料制得硫化银/凹凸棒-Ag₂S/ATT电极,并探讨了硫代乙酰胺配比、增塑剂用量、膜厚度以及溶液pH值等因素对电极性能的影响. 结果表明,新型银离子选择电极有较好的能斯特响应,其响应斜率48.0 mV·decade^-1,Ag⁺浓度线性响应范围1.0×10^-1 ~ 1.0×10^-6 mol·L^-1. 在pH = 2.0 ~ 8.0溶液中该电极电势可稳定72 h,对常见阳离子如Na⁺、Ca²⁺、Fe²⁺、Cu²⁺等呈现较强的抗干扰能力.     

Microbalance study of gold dissolution in alkaline sulfite-thiocarbamide electrolytes

It is shown that gold does not virtually dissolve in alkaline (pH 12.5) solutions containing either thiocarbamide or sodium sulfite. Gold dissolves in alkaline solutions simultaneously containing thiocarbamide (0.1 M) and sodium sulfite (0.5 M). The gold dissolution rate increases with the increase in the contents of thiocarbamide and sodium sulfite. The methods of microbalance and voltammetry are used in studying the mechanism of gold dissolution in a solution containing 0.5 M sodium sulfite, 0.1 M thiocarbamide, and 0.03 M KOH. The found relationships are explained based on the assumption that the gold dissolution in alkaline sulfite-thiocarbamide electrolytes affords gold sulfite complexes.     

METHYLMERCURY: PHOTOCHEMICAL TRANSFORMATION OF MERCURIC SULFIDE INTO METHYLMERCURY IN AQUEOUS SOLUTIONS1

Abstract. The photochemical reaction of mercuric sulfide was studied in aqueous solutions. The photo-solubilization of mercuric sulfide occurred under irradiation with light of ≤650nm and was enhanced in the presence of oxygen and electrolytes. The resultant mercuric ions were easily methylated in the presence of acetate ions via the photosensitized reaction by the simultaneously produced "photo-sulfur" under irradiation with light of ≤420nm. The photoelectrochemical studies of mercuric sulfide electrode revealed that black mercuric sulfide was an n -type semiconductor and the photo-current due to the dissolution as mercuric ions was observed on anodic polarization. The photo-solubilization of mercuric sulfide may be interpreted in terms of the local-cell mechanism.     

Surface chemistry and potential response of copper sulfide based ion-selective membrane in ligand solutions

Dissolution of membraneous composite ions of copper sulfide precipitate-based ion-selective electrodes (ISEs) has been measured in various ligand solutions by ICP atomic emission spectrometry and atomic absorption spectrometry from the solution side and by X-ray photoelectron spectrometry on the electrode surface. Asymmetric dissolution, amount of copper dissolved is higher than that of sulfur, was generally observed. The copper dissolution is correlated with the potential response as well as conditional stability constants. The results are discussed in terms of the charge separation caused by the asymmetric dissolution and electrostatic interaction of charged ligands with the space charge on the electrode surface in view of microscopic characterization of ISEs in metal buffer solutions in general. The results were also used to clarify the reason for some discrepancies of complex stability constants obtained by precipitate-based ISEs and other methods.Dedicated to Professor W. Simon on the occasion of his 60th birthday     

铂电极在氯化钾溶液中电化学行为的STM和EMUV/VRS方法研究

用电化学现场扫描遂道显微镜(in situ STM)和电化学调制紫外可见反射光谱(EMUV/VRS)技术研究了铂电极在KCl溶液中的电化学行为, 结果表明,在KCl溶液中,Pt电极阳极氧化/阴极还原过程为阳极溶解/阴极沉积过程,阳极溶解可能经历吸附的Cl~-离子与Pt表面原子生成表面铂氯化物步骤,实验表明STM比EMUV/VRS具有更高的空间分辨率。     

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 i ₀ 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.     

Peculiarities of cathodic reduction of gold from sulfite-thiocarbamide electrolytes

The cathodic reduction of gold from mixed sulfite-thiocarbamide electrolytes is studied as a function of the electrolyte composition. In the absence of thiocarbamide in the gold-plating sulfite solutions, gold is deposited at the cathode at high overpotentials. The equilibrium composition of the electrolyte is calculated at various ratios between the amounts of sulfite and thiocarbamide; it is shown that an addition of 10⁻⁵ to 0.5 M thiocarbamide does not change considerably the solution composition, and gold ions are present, predominantly, in the form of the complex with sulfite ions. However, an addition of thiocarbamide to the solution leads to a decrease in the overpotential of metal deposition by approximately 0.5 V. A possible mechanism of the catalytic effect of thiocarbamide on the cathodic reduction of gold from mixed sulfite-thiocarbamide electrolytes is proposed. It is shown that sulfite ions have a stabilizing effect on the decomposition of thiocarbamide in the alkaline solutions.