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.     

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.     

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.     

Anodic behavior of gold in acid thiourea solutions: A cyclic voltammetry and quartz microgravimetry study

It is shown that at potentials E < 0.5 V (NHE) gold undergoes practically no dissolution in thiourea solutions containing no catalytically active species. The dissolution at a perceptible rate (> 100 μA cm^?2) starts at E ≥ 0.65 V, with the primary process being the oxidation of thiourea, which gives rise a current peak at E ? 1.0 V. The thiourea oxidation at E ≥ 1.1 produces the appearance of catalytically active species, which drastically accelerate the gold dissolution process in the potential region extending from a steady-state value to 0.6 V, where the current efficiency for gold approaches 100% and a peak emerges at E ? 0.55 V. The peak’s height is commensurate with the value of the limiting diffusion current associated with the ligand supply. The species in question make no discernible impact on the thiourea oxidation process. Formamidine disulfide, which forms during the anodic oxidation of thiourea or which is added in solution on purpose, exerts no noticeable catalytic influence on the anodic gold dissolution. The catalytically active species is presumably the S^2? ion, product of decomposition and deep oxidation of thiourea and formamidine disulfide. Indeed, adding sulfide ions in solution has a strong catalytic effect on the gold dissolution, whose character is identical to that of the effect exerted by products of anodic oxidation of thiourea at E ≥ 1.1 V μA cm^?2.     

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.     

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.     

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.     

Multilayer film of thiourea and gold nanoparticles as an effective platform for immobilization of activated non-labeled-DNA and construction of an ultrasensitive electrochemical DNA biosensor

In this work, self assembly of thiourea and gold nano-particle multilayer built up on a thiourea modified gold nanoparticles Au electrode, has been used as a platform for immobilization of activated ss-DNA. Two NH₂ group of thiourea on a multilayer surface can interact with an activated phosphate group of non-labeled ss-DNA. Activated non-labeled ss-DNA was prepared using N-(3 dimethylaminopropyl)-N-ethyl-carbodiimide hydrochloride (EDC) and N-hydroxy-succinimide (NHS). The whole DNA biosensor fabrication process was characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) methods with the use of ferricyanide as an electrochemical redox indicator. Methylene Blue (MB) was used as the electrochemical indicator for monitoring the hybridization reaction after hybridized with the target ssDNA and the reduction current of MB intercalation decreased with increasing the concentration of target DNA, ranging from 7.9 × 10^–13 to 1.2 × 10^–8 M with a very low detection limit of 3.8 × 10^–13 M (S/N = 3).     

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.     

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.     

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.     

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.     

XPS studies on the gold oxide surface layer formation

The initial stage of gold oxide layer formation on the gold electrode surface was investigated in 0.5 M H₂SO₄. X-ray photoelectron spectroscopy (XPS) spectra of pure gold and the anodically polarized gold electrode surface were compared quantitatively. It was found that gold anodic polarization in the E range from ∼1.3 to 2.1 V causes increase in intensity of the XPS spectra at an electron binding energy ε_b=85.9 eV for gold and at ε_b=530 eV for oxygen. These ε_b values correspond to Au³⁺ and O²⁻ oxidation states in hydrous or anhydrous gold oxide. The larger the amount of the anodically formed surface substance the higher is the intensity of the spectrum at the ε_b values mentioned above. It was concluded that gold anodic oxidation, yielding most likely an Au(III) hydroxide surface layer, takes place in the E range of the anodic current wave beginning at E≈1.3 V. At E_B=1.7 V (the potential of the Burshtein minimum) the stationary surface layer consists of 2.5 to 3 molecular layers of Au(OH)₃. The theoretical amount of charge required for the reduction of one molecular layer of Au(OH)₃ is ∼0.15 mC cm⁻², since the Au(OH)₃ molecule is planar and occupies about four atomic sites on the electrode surface.     

Kinetics of electrode processes on a bismuth electrode in acid solutions containing thiourea and formamidine disulphide

A study of the mechanism by which bismuth is electrodissolved in an aqueous solution of thiourea on the background of H₂SO₄ demonstrated that, in the thiourea concentration range 0.001 M < c < 0.5 M, a current oscillation is observed in cyclic voltammograms at E ≈ 0.4–0.3 V when the potential is swept from the anodic to the cathodic region. This oscillation is due to the loosening of the passivating film formed in the anodic process. It is shown that thiourea is not oxidized to formamidine disulphide at the bismuth electrode. thiourea and formamidine disulphide have mutually opposite effects on the height of the cathodic peak: the peak current falls with increasing thiourea concentration and grows with increasing formamidine disulphide concentration. According to the results of an X-ray fl uorescence analysis, sulfur is formed on the bismuth electrode upon its prolonged polarization of in a 0.5 M solution of thiourea. An explanation is provided for the experimental facts observed in the study.     

Zur kinetik der metallionenadsorption: II. Galvanostatische einschaltmessungen an silber-, bleiund thalliumadsorbaten auf goldelektroden

The kinetics of the metal ion adsorption of Ag⁺, Tl⁺ and Pb²⁺ on polycrystalline and (111) orientated monocycrystalline gold electrodes were investigated using a low-level short-time polarization technique. In the present case the systems were potentiostatically prepolarized under steady state equilibrium conditions. The perturbation was realized by galvanostatic square pulses. The observed overpotential—time transients were analyzed by polynomial curve fitting. The results can be interpreted in terms of the developed polarization model [1]: (1) the process of metal ion adsorption is not reversible; (2) the hindrance is caused by charge transfer and can also be influenced by surface diffusion of the adsorbed metal ion species. In the case of Ag⁺ and Pb²⁺ the calculated mean values of the exchange current densities of the sorption processes are lower than the exchange current densities of the corresponding metal ion electrodes and they do not depend markedly on the potential. The data obtained are discussed.     

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.     

Reversed-phase foam chromatography : Chemical enrichment and separation of gold in the tributylphosphate-thiourea-perchloric acid system

Gold(III) is quantitatively extracted from acidic solution of thiourea into tri-n-butyl phosphate (TBP). The extracted species contains one molecule of thiourea and four molecules of TBP for each atom of gold. On short columns of TBP-loaded polyurethane foam, gold(III) can be separated from many other elements by retention from 0.1 M perchloric acid solution containing 3% thiourea and 1% sodium perchlorate. Flow-rates of 50–60 ml min^?1 are possible. Trace amounts of gold can be separated quantitatively from high concentrations of Zn²⁺, Co²⁺, Ni²⁺, Fe³⁺, Sb³⁺, Cu²⁺, Bi³⁺ and Pd²⁺, which have a negligible effect on the rate of adsorption of the gold-thiourea complex. The chemical enrichment of gold from dilute aqueous solutions is also possible.     

Radiochemical neutron activation analysis for determination of Au,Ag, Pt and Pd in geological samples

A procedure for separation of Au, Ag, Pt, and Pd in geological samples has been developed. After irradiation, samples were fused with Na₂O₂ and silver was separated by filtering through a PbCl₂ filter in 4M nitric acid solution. Au, Pt and Pd were concentrated with rhodium and thiourea as rhodium sulfide and the separation process of these elements was carried out by a chromatographic method. Au, Pt and Pd were retained on a Dowex-1×8 anion column in 1M HCl. Pd was eluted from the column by using a mixture of 75% HCl acid-25% acetone. Au was eluted by using a mixture of 10% HCl-90% acetone. In the gold fraction, Pt was also determined through the photopeak of¹⁹⁹Au radionuclide (158 keV). The method was simple and rapid.     

Surface-Confined Metallodendrimers: Isolated Nanosize Molecules

The incorporation of dialkyl sulfide side chains in metallodendrimers is a simple method for their insertion into a monolayer of decanethiol formed by self-assembly on a gold surface. The dendrimer binds through the sulfide group to a defect in the monolayer on the gold surface (see picture). The surface concentration of the isolated dendrimer adsorbate can be regulated by the adsorption time (for example, 55 adsorbates on a surface of 200×200 nm² after 20 h).     

Gold recovery onto poly(acrylamide-allylthiourea) hydrogels synthesized by treating with gamma radiation

Poly(acrylamide-1-allyl-2-thiourea) hydrogels, Poly(AAm-ATU), were synthesized by gamma irradiation using a ⁶⁰Co γ source at different irradiation dose rates and in a monomer mixture with different 1-allyl-2-thiourea contents. These hydrogels were used for the specific gold recovery from single and competitive media. It was observed that the gold adsorption capacity onto the hydrogels was high at low pHs and reached a maximum value at pH 0.5. It was found that the adsorption capacity of the hydrogels for gold ions in acidic media around pH 0.5 was high and about 940 mg g⁻¹ dry hydrogel. Adsorption of these hydrogels for gold ions was found to be very fast and also these hydrogels were showed extremely high selectivity to the gold ions in acidic media even when the concentrations of the other metal ions were extremely higher than that of the gold. Because of the high specificity of these hydrogels to gold ions compared with the other metal ions at low pHs, all matrix effects could be easily eliminated by adsorbing gold ions onto the hydrogels at around pH 0.5 and desorbing into 0.8 M thiourea in 3.0 M HCl. The swellability of the synthesized hydrogels varied with irradiation dose rates and increased at high irradiation dose rates. The minimum swellability of the hydrogels was found to be at least 1000% which made it attractive for gold to penetrate into the hydrogels and react with all the functional groups in the interior surface of the hydrogels.