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 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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

A highly sensitive and selective fluorescent probe for cyanide based on the dissolution of gold nanoparticles and its application in real samples

We report a simple fluorescence method for detection of cyanide sensitively and selectively based on the dissolution of polymer-coated gold nanoparticles by cyanide. The lowest concentration for quantification of cyanide ions was 3.0×10(-7) M, and other common anions nearly have no influence. Furthermore, several real water samples spiked with cyanide, including local groundwater, tap water, boiled water, and lake water, were analyzed, and the experimental results demonstrated that our sensing system worked well in the above water samples, with a good linear correlation.     

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.     

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.     

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.     

A new example of the three-component reaction: nucleophilic ring opening in cyclopropenones by cyanide ions in the presence of water or alcohols

3-Methyl-2,3-diphenyl-and 3-methyl-2-phenylcyclopropenones react with NaCN and water or alcohols as nucleophiles in the presence of NaHSO₄ to give novel compounds, namely, 3-cyano-2,3-diphenylpropionic acid or alkyl 3-cyano-2,3-diphenyl-and alkyl 3-cyano-2-methyl-3-phenylpropionates. A reaction mechanism was proposed. The anti-(R*R*)-diastereomer was found to be the major reaction product. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 2066–2069, September, 2005.     

The effect of thallium ions on the gold dissolution rate in thiosulfate electrolytes

The effect of TlNO₃ additions in the concentration (c ₁) range from 5 × 10^?6 to 1 × 10^?4 M on the anodic dissolution of gold in sodium thiosulfate solutions with the concentration (c ₂) from 0.005 to 0.2 M is studied by voltammetry on the electrode surface renewed by cutting off a thin metal layer immediately in solution and also by the quartz-crystal microbalance method. For c ₂ = 0.2 M, as c ₁ increases from 5 × 10^?6 to 1 × 10^?4 M, the gold anodic dissolution rate is observed to increase from 0.02 (in the absence of TlNO₃) to 0.75 mA/cm² for c ₁ = 7.5 × 10^?5 M according to a nearly linear law. The dissolution accelerates because the effective values of the transfer coefficient and the exchange current density increase from 0.2 and 4 ??A/cm² (in the absence of TlNO₃ admixtures) to 0.47 and 35 ??A/cm² (for c ₁ = 1 × 10^?4), respectively. Experiments with the renewal of the electrode surface in the course of electrolysis suggest that the gold dissolution is catalyzed in the presence of thallium ions by the adsorption mechanism and also as the result of the mixed kinetics of their adsorption on the electrode surface.     

基于金纳米棒刻蚀比色法检测铁、铜离子

采用种子生长法制备金纳米棒(AuNRs)以构建光学传感器,用于Fe³⁺和Cu²⁺的高选择性快速可视化检测。在酸性环境中,Fe³⁺和Cu²⁺通过与KI溶液反应,将I-氧化成I2。I2刻蚀AuNRs,导致其纵向表面等离子体共振(LSPR)吸收峰蓝移,从而实现对Fe³⁺和Cu²⁺的检测。结果表明,反应温度为50℃时,添加0.8 mL 0.1 mol·L^-1 HCl、2 mL AuNRs生长液和20 mmol·L^-1 KI溶液,与2 mL 500 μmol·L^-1 Fe³⁺或30 μmol·L^-1 Cu²⁺反应25或90 min,可将AuNRs刻蚀至LSPR吸收峰消失。该方法对Fe³⁺和Cu²⁺检测具有高选择性和准确性,对于Fe³⁺、Cu²⁺共存体系的检测,可通过加入适量F-与Fe³⁺生成配合物[FeF₆]^3-完成对Fe³⁺的化学掩蔽,消除Fe³⁺的干扰,实现共存体系中Cu²⁺的准确检测。     

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.     

基于金纳米棒刻蚀比色法检测铁、铜离子

采用种子生长法制备金纳米棒(AuNRs)以构建光学传感器,用于 Fe³⁺和 Cu²⁺的高选择性快速可视化检测。在酸性环境中,Fe³⁺和 Cu²⁺通过与 KI溶液反应,将 I^-氧化成 I₂。I₂刻蚀 AuNRs,导致其纵向表面等离子体共振(LSPR)吸收峰蓝移,从而实现对Fe³⁺和Cu²⁺的检测。结果表明,反应温度为50℃时,添加0.8 mL 0.1 mol·L^-1 HCl、2 mL AuNRs生长液和20 mmol·L^-1 KI溶液,与 2 mL 500 μmol·L^-1 Fe³⁺或 30 μmol·L^-1 Cu²⁺反应 25或 90 min,可将 AuNRs刻蚀至 LSPR 吸收峰消失。该方法对 Fe³⁺和 Cu²⁺检测具有高选择性和准确性,对于 Fe³⁺、Cu²⁺共存体系的检测,可通过加入适量 F-与 Fe³⁺生成配合物[FeF₆]3^-完成对 Fe³⁺的化学掩蔽,消除Fe³⁺的干扰,实现共存体系中Cu²⁺的准确检测。     

Catalytic effect of CO2 on peroxynitrite decomposition in the presence of cyanide complexes

The catalytic effect of CO2 on the kinetics and yields of peroxynitrite decomposition in the presence of cyanide complexes was investigated as a function of pH, substrate, bicarbonate and nitrite concentration. This revised version was published online in August 2006 with corrections to the Cover Date.     

铜离子在合金硫化镍矿浸出过程的化学动力学

建立了在常压氧化硫酸浸出合金硫化镍矿过程中通氧反应结束后铜离子浸出镍的动力学方程,反应的表观活化能为129.88 kJ.K-1.mol-1.根据该动力学方程,可以进一步对硫酸氧化浸出合金硫化镍矿过程中镍的浸出速率进行推导.