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.     

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.     

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 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.     

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.     

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 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.     

Extraction of Gold from ores and concentrates by leaching with the use of cyanides and alternative reagents

The problems associated with gold mining are considered from the standpoint of application of heap leaching, a method for gold recovery. Data on the kinetics and mechanism of gold dissolution in cyanide and alternative solvents [thiocarbamide solutions containing Fe(III) ions and hypochlorite-chloride bromine-bromide, iodine-iodide, and copper-thiosulfate solutions], which make it possible not only to improve the working efficiency in gold mining, but also to lessen the contamination of the environment with highly toxic compounds.Translated from Zhurnal Prikladnoi Khimii, Vol. 77, No. 10, 2004, pp. 1585–1604.Original Russian Text Copyright © 2004 by Kozin, Melekhin.     

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 the Dissolution of Gold in Solutions of Thiocarbamide

The kinetics and mechanism of the dissolution of metallic gold in solutions of thiocarbamide in the presence of iron(III) were investigated. The dependence of the dissolution rate of gold on the pH of the solution and the ratio of the concentrations of iron(III) and thiocarbamide was determined, and the optimum conditions for the dissolution of gold were determined. The first-order rate constant for the dissolution of gold and the equilibrium constants for the formation of the complex cations Au[SC(NH₂)₂]⁺ ₂ and the formation of formamide disulfide S₂C₂(NH)₂(NH₂)₂ were calculated.     

Specific features of iodometric methods for determining the content of sodium sulfite in fixing-bleaching photographic solutions

The conditions of oxidation of thiocyanate ions and thiocarbamide with iodine in aqueous solutions are discussed. The range of the pH values of the solutions, in which thiocyanate ions do not interfere with the iodometric determination of sodium sulfite in fixing-bleaching photographic solutions and the expenditure of iodine for oxidation of thiocarbamide can be taken into account correctly, is revealed.Translated from Zhurnal Prikladnoi Khimii, Vol. 77, No. 12, 2004, pp. 2056–2058.Original Russian Text Copyright © 2004 by Kiryushov, Skvortsova.     

Peculiarities of anodic behavior of gold electrode in thiosulfate electrolytes

Using the methods of quartz microgravimetry and voltammetry, the anodic behavior of gold electrode in thiosulfate electrolytes is studied in the pH range of 7 to 11. It is found that, in the potential range from 0.15 to 1.0 V (NHE), the anodic current is associated predominantly with the oxidation of thiosulfate ions, and the gold dissolution rate in this electrolyte is negligibly low (< 0.02 mA/cm²). It is shown that the study of anodic processes in the neutral thiosulfate electrolytes requires stabilization of solution acidity, because the near-anode layer can be acidified to the pH values, which are sufficient for the formation of elemental sulfur. It is found that the use of Britten-Robinson buffer solution with pH 7 as the supporting electrolyte changes significantly the polarization curve of thiosulfate ion oxidation, but does not raise the gold dissolution rate. An increase in the solution pH to 11 and an exposure of electrode at various potentials (−0.5 and 0.15 V) prior to the onset of potential scanning also do not accelerate considerably the gold dissolution in the thiosulfate electrolyte. A comparison between the regularities of gold anodic behavior in the thiosulfate solutions and earlier studied gold dissolution in the cyanide and thiocarbamide electrolytes showed that they are similar. It is supposed that the specific features of anodic processes in these cases are of a similar nature: the metal dissolution proceeds with the formation of two-ligand complexes with linear structure, which is typical for all aforementioned ligands.     

The effect of sulfide ion chemisorption on the kinetics of gold dissolution in cyanide solutions

The effect of sodium sulfide additions (from 5 × 10^?6 to 2 × 10^?5 M) on the kinetics of gold dissolution in cyanide solutions of the following composition, M: 0.1 KCN, 0.02 KAu(CN)₂, 0.5 K₂SO₄, pH 10–13 is studied. Hydrosulfide ions are shown to exert a strong catalytic effect on the dissolution kinetics of this metal in a potential range where their adsorption is accompanied by the formation of polysulfides (?0.2 < E < 0.4 V). The reaction acceleration depend on the potential and is 100-fold for E ? 0.1 V. The effect becomes more pronounced as the concentration of hydrosulfide ions increases to 10^?4 M and is almost pH-independent in the pH range from 10 to 13. An attempt to explain the found relationships is undertaken.     

Palladium(II) Extraction by Pyridinecarboxylic Acid Esters

The commercial extractant Acorga CLX-50 and model individual di-2-ethylhexyl pyridine-3,5-dicarboxylate and 2-ethylhexyl pyridine-3-carboxylate in toluene were used for palladium(II) extraction from aqueous HCl solutions. The studies of extraction rate and equilibrium were carried out in systems containing palladium(II) ions in 3.0, 0.1, and 0.1M HCl in the presence of 0.5M sodium chloride and in 0.1M HCl in the presence of 0.1–6.0M lithium chloride and in 0.1M HCl in the presence 0.1–3.5M sodium nitrate. The examined extractants can efficiently extract palladium(II) from aqueous hydrochloric acid and nitrate solutions. The extraction is slow and equilibrium is obtained after 2 hours. The best extraction of palladium(II) is observed from 0.1M HCl solution in the presence of 3.5M sodium nitrate. A spontaneous transfer of palladium(II) to the toluene phase without any phase mixing is also observed.     

Corrosion resistance of molybdenum silicides in aqueous solutions

The corrosion performance of Mo-22Si and Mo-25Si alloys in 0.5 M sodium chloride (NaCl) and 0.5 M sodium hydroxide (NaOH) solutions, at room temperature, was evaluated using electrochemical techniques. In 0.5 M NaCl, additionally, the effect of solution pH (3, 7 and 10) and concentration (0.1, 0.5 and 1.0 M) was studied using techniques such as potentiodynamic polarization curves, linear polarization resistance and electrochemical noise in current. The alloy contained either -Mo or Mo₅Si₃ phases in a Mo₃Si matrix. Polarization results showed that only the alloys containing 22Si developed a passive film in 0.5 M NaOH solution, whereas the alloy containing 25Si was passivated only in 0.5 M NaCl, pH 10 solution. In 0.5 M NaCl, pH 7 and 0.5 M NaOH solutions, the alloy with 25Si was the one with the highest corrosion rate, whereas the one containing 22Si was the most corrosion resistant. In NaCl solutions, the alloys exhibited a localized type of corrosion, but not in NaOH solutions. Alkaline NaCl solutions increased the corrosion rate of the 75Mo-25Si alloy with respect to acidic or neutral solutions, whereas diluted (0.1 M) or concentrated (1.0 M) NaCl solutions produced lower corrosion rates than the 0.5 M NaCl solution. Some localized type of corrosion occurred in the NaCl solutions, due to a selective corrosion of the -Mo and Mo₅Si₃ phases with respect to the Mo₃Si matrix.     

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.     

Electrooxidation of Silver in the Presence of Some Amino Acids

Data on the influence the solution's ionic composition and amino acids (histidine, methionine) have on the anodic dissolution of silver are presented. The dissolution is most effective in intensively agitated nitrate solutions containing methionine in excess of 0.05 M (current efficiency reaches 75%). By analogy with the gold dissolution, acceleration of the silver dissolution is attributed to oxidized methionine complexing with metals.     

Kinetics and Mechanism of the Electro-oxidation of Gold in Thiocarbamide Solutions

The kinetics and mechanism of the electrooxidation of gold and thiocarbamide in sulfuric acid solutions of thiocarbamide (TC) have been investigated. The potentials for the oxidation of gold in TC solutions to [Au(TC)₂]⁺ _ads and [Au(TC)₃]³⁺ _ads are 0.132 and 0.561 V (relative to the standard silver chloride electrode). The electrooxidation of thiocarbamide in sulfuric acid solution is characterized by two maximums on voltammograms at 0.983 V (formation of formamidine disulfide, FAD) and 1.437 V (oxidation of FAD, sulfides and hydrosulfides of gold(I)). The calculated rate constants for the electrosolution of gold at the maximum current of the voltammogram is k ₁ = 1.15·10^–5 cm/s and at the minimum current is k ₂ = 3.13·10^–6 cm/s in sulfuric solutions of TC. A mechanism is proposed for the electrosolution of gold and TC in sulfuric acid solutions of thiocarbamide.     

Effect of thiocyanate counterion condensation on poly(allylamine hydrochloride) chains on the buildup and permeability of polystyrenesulfonate/polyallylamine polyelectrolyte multilayers

In this study, we investigate the buildup of PEI-(PSS-PAH)(n) polyelectrolyte multilayers at pH 7.4 in the presence of either NaCl or NaSCN as a supporting electrolyte. It appears that in the presence of increasing thiocyanate concentrations (from 0.1 to 0.5 M), the thickness increment, obtained from optical waveguide lightmode spectroscopy experiments, increases whereas it stays practically constant for increasing sodium chloride concentrations (between 0.1 and 0.5 M). The hydration of the films differs also markedly between both electrolyte solutions. The differences in the construction of the polyelectrolyte multilayers in the presence of both supporting electrolytes are rationalized in terms of strong SCN(-) condensation on the PAH chains. The occurrence of this ion condensation is indirectly demonstrated by means of zeta potential measurements and directly demonstrated by means of attenuated total internal reflection infrared spectroscopy on the multilayer films. Moreover when the films are built up in the presence of SCN(-), these ions are only slowly exchanged by the Cl(-) ions introduced in the bulk. Conversely the thick films obtained from 0.5 M NaSCN solutions do not deswell when the buffer solution is replaced by a 0.5 M NaCl containing buffer. The permeability of the films constructed in the presence of both sodium salts is also studied by means of cyclic voltametry and is found to be markedly different in the case of films made from five bilayers at 0.5 M salt concentration. This difference is due to the different morphology and porosity of the films constructed in the presence of 0.5 M NaCl and 0.5 M NaSCN.     

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.