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.     

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.     

Kinetics and mechanism of gold dissolution in thiourea solutions: Effect of sulfide ions

Chronoamperograms for gold in solutions containing 0.1 M thiourea, 0.5 M H₂SO₄, and catalytically active sulfide ions at the concentration c ₁ from 1 × 10^?5 to 4 × 10^?5 M are obtained at different potentials with the aid of an automated setup intended for renewing the electrode surface directly in the solution by cutting off a thin surface layer of the metal. It is shown that the results of measurements of the current practically coincide at a constant value of the product c ₁ t, where t is the time period elapsed after the renewal of the electrode surface. Such a coincidence testifies to a diffusion nature of processes that hamper accumulation of sulfide ions at the gold surface. This fact permitted the use of a procedure developed previously for the calculation of polarization curves at constant values of surface coverage θ by catalytically active ions. At θ = const, the voltammetric curves for gold in sulfide-containing thiourea solutions are shown to correspond to the Tafel equation. With the surface coverage increasing, the effective values of the exchange current i ₀, transfer coefficient α, and anodic reaction order with respect to thiourea P _a increase from the values i }~ 10^?5 A cm^?2, α }~ 0.12, and P _a = 0.2, which are characteristic of pure solutions, to 2 × 10^?4 A cm^?2, α }~ 0.5, and P _a = 1.1 (at θ }~ 0.5). An interpretation to the established regularities is given.     

The peculiarities of the effect of mercury ions on the kinetics of gold electrodeposition from cyanide solutions

The peculiarity of the catalytic effect of complex mercury ions (in the concentration of 10^?5 to 10^?4 M) on the gold electrodeposition from cyanide solutions are studied. It is shown that, all other conditions being the same, the catalytic activity of mercury ions is much lower than that of earlier studied thallium and lead ions. Another peculiarity of catalytic effect of mercury ions is the necessity of prolonged (for tens minutes) contact of gold with the mercury-containing solution. Possible explanation for the obtained regularities is proposed.     

Electrocatalysis by adatoms at the gold and silver dissolution in cyanide solutions

The peculiarities of the effects of upd thallium, lead, bismuth, and mercury on the dissolution rates of gold and silver in cyanide electrolytes are compared. In general, they feature the abrupt acceleration of the dissolution of gold and, to a lesser extend, silver in the chemisorption range of mentioned ions. As the potential increases, the gold dissolution rates passes through a maximum the height of which is comparable with the limiting current of this process associated with limitations in the delivery of cyanide ions to the electrode surface. The current decay after the maximum is due to desorption of catalytically active adatoms. The chemisorption rate of thallium, lead, and bismuth ions at potentials more negative than the current peak is controlled by their diffusion to the gold surface, whereas the chemisorption rate of mercury is controlled by the adsorption kinetics. With the increase in the surface coverage with adatoms θ, the catalytic activity of all considered adatoms passes through a maximum. The sharp increase in the effective transfer coefficient in the presence of these adatoms makes the main contribution into the acceleration of the gold dissolution, while the increase in the exchange current has a smaller effect. The chemisorption of mentioned atoms on gold not only accelerates the dissolution but also changes its mechanism. For gold dissolution, the catalytic activity of upd thallium, lead, and bismuth increases in the following sequence: Tl ? Pb < Bi and the effect is additive in their simultaneous presence. For silver, the increase in the exchange current makes the main contribution into the acceleration of dissolution, whereas the transfer coefficient and the reaction order with respect to the ligand change insignificantly. Explanation of the observed peculiarities is given.     

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.     

Comparison of catalytic activity of thallium and lead adatoms at the gold electrodeposition and dissolution in cyanide solutions

Catalytic activity of thallium and lead adatoms is compared in electrode processes involving gold in cyanide solutions. It is shown that in the presence of considered atoms on the interface, the gold dissolution is accelerated to the greater extent than its electrodeposition. Lead adatoms are shown to exert a substantially stronger effect on the anodic dissolution rate as compared with thallium adatoms, whereas the opposite trend is observed in the cathodic deposition. An explanation for the fact is proposed.     

Phenomenological kinetics of irreversible electrochemical dissolution of metal-oxide microparticles

The electrochemical dissolution of metal oxides and other stable solid phases composed of nano- to micro-crystalline particles is generally a complex process. It can be simplified by distinguishing two main contributions to the reactivity of the solid: the potential-dependent rate coefficient k(E), and the conversion-dependent function f(y). These contributions can be evaluated by a combination of potentiostatic and potentiodynamic experiments. Both k(E) and f(y) were obtained experimentally for the dissolution of iron and chromium oxides, and theoretical consequences of their particular forms are discussed. A peak-shaped function k(E) was observed in the case of γ-Fe₂O₃, whereas, for α-Cr₂O₃ and CrO₂, a different model based on intermediate surface complexes is proposed. This model also explains the complete electrochemical dissolution of metal oxides regardless of their low intrinsic electric conductivity. Received: 2 July 1997 / Accepted: 3 November 1997     

On the reason for gold passivation in thiourea electrolytes during its dissolution in the presence of sulfide ions

The effect of the gold surface renewal on the polarization curve is studied at various gold dissolution stages in electrolytes containing 0.1 M thiourea, (1.4–4.0) 10^?5 M sodium sulfide, and 0.5 M sulfuric acid, at 20°C. The behavior of the curves after the cutting-off of a surface layer of gold in the potential region where a current decay is observed (i.e. passivation of the process) and the dependence of the current in the maximum of a polarization curve on the thiourea concentration are explained by “deactivation” of catalytically active adsorbed sulfide ions.     

The kinetics of zinc dissolution in nitric acid

The dissolution of zinc in 0.48–1.49M HNO₃ was studied at 15–25°C, by following simultaneously the concentration changes of the reactants (Zn and HNO₃), intermediate (HNO₂) and product (Zn²⁺) with time. Explicit mechanisms were suggested for the dissolution of zinc in nitric acid. The kinetics of the dissolution process show that it is of the first-order with respect to [Zn] and [HNO₂]. The data obtained show that the dissolution process is diffusion-controlled. The mechanism of zinc dissolution is compared with the mechanism of copper dissolution.

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Die Kinetik der Auflösung von Zink in Salpetersäure

Zusammenfassung Die Auflösung von Zink in 0.48–1.49M HNO₃ wurde bei 15–25°C mittels gleichzeitiger Verfolgung der Konzentrationsänderungen der Reaktanden (Zn und HNO₃), des intermediären HNO₂ und des Produkts Zn²⁺ untersucht. Es wird ein Mechanismus vorgeschlagen. Die Kinetik der Auflösung ist erster Ordnung bezüglich [Zn] und [HNO₂]. Die Daten zeigen, daß der Auflösungsvorgang diffusionskontrolliert ist. Der Mechanismus der Auflösung von Zink wird mit dem der Kupferauflösung verglichen.

     

Anodic dissolution of gold in cyanide solutions containing ions of thallium(I)

The potential dependence of the gold dissolution rate in alkali-cyanide solutions with and without 1.5 × 10^-5 M T1NO₃ is studied using the technique of the electrode surface renewal prior to taking measurements. The thallium ions accelerate the gold dissolution at potentials below 0.2 V and inhibit it at more positive potentials. The effect depends on the time the electrode is in contact with solution before the measurements, the potential scan rate, and concentrations of KCN and KOH. Possible approaches to interpreting the discovered phenomena are suggested.     

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.     

The kinetics and mechanism of dissolution of gold in solutions of thiocarbamide with an oxidizer

The kinetics and mechanism of dissolution of gold in solutions of thiocarbamide (T) in the presence of Fe₂(SO₄)₃ as an oxidizer were studied. The dependences of the rate of dissolution of gold on the concentration ratio between iron(III) and T and pH were determined, and optimum solution compositions for the dissolution of gold were found. The compositions of gold(I) complexes formed in the boundary double layer ([Au{(NH₂)₂C=S}₂]⁺) and in the bulk ([Au{(NH₂)₂C=S}₃]⁺) were determined. The diffusion and kinetic components of the overall reaction of gold dissolution in solutions of T in the presence of the oxidizer were obtained by the rotating disc method. The first-order rate constants at 278–333 K, k _Au = 3.5 × 10^?5?2.73 × 10^?4 s^?1, and the activation energies at 278–295 K (E _a = 13.4 kJ/mol, which is evidence that dissolution value characteristic of kinetically controlled reactions) were determined for the dissolution of gold in solutions of T. The composition of the adsorption sulfide-containing film on the surface of gold was studied by Auger electron spectroscopy. The film, which inhibited gold dissolution, consisted of gold(I) hydrosulfide (AuHS) and sulfide (Au₂S). The solubility products of these compounds and their solubilities in aqueous solutions were calculated.     

Effect of dioctyl sulfide on the kinetics of oxidative dissolution of gold nanoparticles in triton N-42 reverse micelles

The effect of additions of hydrophobic dioctyl sulfide (L) on the kinetics of dissolution of gold nanoparticles in the interaction with a dispersed aqueous hydrochloric solution of H₂O₂ in Triton N-42 reverse micelles (decane was the dispersion medium) was studied spectrophotometrically. The process consists of a two-stage oxidation Au⁰ → AuCl₂⁻ → AuCl₄⁻ at the surface of gold particles; the first stage occurs in two ways: a spontaneous reaction and an autocatalytic reaction involving AuCl₄⁻ ions. With small additions of L (c _L < c _Au), only spontaneous oxidation of Au(0) to Au(I) takes place because Au(I) is completely bound in an inert complex AuLCl. When unbound L is exhausted, the newly formed AuLCl is accumulated in micellar shells, changes the properties of the medium inside the micelles, and affects the rate constant of the autocatalytic reaction, which increases with increasing c _L. At high concentrations of L, the coagulation of particles occurs instead of their dissolution, because of the deterioration of the protective properties of micellar shells as a result of the ingression and accumulation of dioctyl sulfide molecules on account of selective adsorption on gold particles. The rate constants of all stages of dissolution and coagulation are determined.     

Relation of lattice ion solution composition to octacalcium phosphate dissolution kinetics

The kinetics of dissolution of octacalcium phosphate (OCP) has been investigated under conditions of constant relative undersaturation with respect to OCP, σ_OCP=−0.57, at pH 4.500, and ionic strength (I), 0.15 mol l⁻¹. The molar calcium/phosphate ratio (R) in the solutions was varied from 0.1 to 10. The dissolution rate decreased by 160% as R increased from 0.1 to 10. The ζ-potential of the OCP surfaces was measured in solutions equilibrated with respect to OCP at pH values ranging from 5.0 to 11. Under stoichiometric conditions (R=1.33), OCP was positively charged at pH values from 5.0 to 10. As the solution calcium concentration was increased, ζ became more positive over the entire pH range studied. At R=0.1, two isoelectric points were apparent at pH values of about 6.3 and pH 9.5. This behavior may be related to the solubility product (K_sp) of OCP. The relationship between surface characteristics and dissolution rate are discussed in terms of kink density and the kinetic ionic ratio model developed previously (J. Zhang, G.H. Nancollas, J. Colloid Interf. Sci., 200 (1998) 131.     

The mechanism of pulse anodic activation of iron dissolution in acid solutions

Basing on the model of spatial separation of the dissolution and passivation of hydrophilic metals, formation of stationary surface morphology during pulse anodic activation of iron dissolution in sulfuric acid solutions is analyzed.     

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.     

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 kinetics of transformation of isopentenols in the presence of the Nafion-H catalyst

The kinetics of transformation of 3-methyl-1-buten-3-ol in the presence of the Nafion-H catalyst was studied. A comparison between the data obtained and those for the reaction catalyzed by sulfuric acid showed that under the same conditions, the macroscopic acidic properties of Nafion-H correspond to the catalytic activity of 8–15% sulfuric acid. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2090–2093, October, 1998.     

Study on the chemisorption kinetics of Methylene Blue using SERS technique

SERS technique was used to study the chemisorption kinetics of Methylene Blue (MB) on the HNO3-etched silver surface. The adsorption kinetic parameters were deduced from different vibrational modes at a low concentration of 3.5×10-6 mol/L, and it showed that MB adsorbed uniformly (monolayerly) on silver surface. However, the adsorptive behavior turned anomalous at relatively higher concentrations and a possible explanation was suggested. In addition, the influence of Cl- ions on the adsorption states of MB was investigated, and it was shown that MB molecules, adsorbed on the silver surface, tended to transform from the "lying-down" state to the "end- on"4 state after Cl- ions were added.