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.     

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 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.     

Kinetics of silver anodic dissolution in thiosulfate electrolytes

The regularities of silver anodic dissolution are studied by using the voltammetry (at the potential scan rates from 5 to 1000 mV/s) on the electrode, which was renewed immediately in the solution by cutting-off a thin surface metal layer, and quartz microgravimetry, for various concentrations of sodium thiosulfate (0.05–0.2 M). It is shown that, in the potential range from 0 to 0.4 V (normal hydrogen electrode), the polarization curves reflect the silver dissolution, whereas the contribution of oxidation of S₂O₃²⁻ ions is insignificant. At low potential scan rates, the process kinetics is of mixed nature. The kinetics and mechanism of anodic process are studied by using the measurements at high potential scan rates (100–200 mV/s) and the calculations of equilibrium composition of near-electrode layer. It is found that the exchange current in the electrolytes studied is 5 × 10⁻⁵ A/cm², the transfer coefficient α is approximately 0.5, and both parameters are virtually independent of the concentration of S₂O₃²⁻ ions. The reaction order of silver dissolution with respect to the ligand $ \left. {\frac{{\partial logi}} {{\partial logc}}} \right|_E $ \left. {\frac{{\partial logi}} {{\partial logc}}} \right|_E is close to unity and is independent of potential. With regard for the literature data on the adsorption of thiosulfate ions on silver, this result is interpreted as the evidence for the involvement of one S₂O₃²⁻ ion from bulk solution, along with adsorbed ligands, in the elementary act of metal dissolution.     

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 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.     

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.     

Relaxation kinetics of the current of hydrophilic metal dissolution activated by anodic pulses

Relaxation kinetics of the metal dissolution current after pulsed anodic activation are analyzed using a model of spatial separation of the dissolution and passivation of hydrophilic metals. The current rise is contributed to mainly by the formation of new step sources resulting from the desorption of blocking oxygen atoms from vertexes of trihedral pyramids that are formed in excess quantities during the activation pulse.Translated from Elektrokhimiya, Vol. 40, No. 12, 2004, pp. 1487–1493.Original Russian Text Copyright © 2004 by Lazorenko-Manevich, Sokolova.     

Effect of specific adsorption of anions on the kinetics of the current relaxation after pulsed anodic activation of dissolution of hydrophilic metals

Based on the model of spatial separation of the dissolution and passivation of hydrophilic metals, the effect of strong specific adsorption of anions on the shape of curves for the relaxation of anodic current of metals in acid solutions after switching an anodic activation pulse off is considered by using iron as an example.     

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.     

Kinetics of gold dissolution in thiosulfate electrolytes

The kinetics of gold dissolution in solutions containing Na₂S₂O₃ with the concentration c from 0.025 to 0.2 M and different supporting electrolytes is studied by the voltammetric method on renewable electrodes and the quartz crystal microbalance. It is shown that in the range from the steady-state potential to E = 0.3 V (from hereon, the potentials are related to the normal hydrogen electrode), the polarization curves are well approximated by straight lines in semilogarithmic coordinates. The exchange currents i ₀ and the transfer coefficients α are calculated. It is shown that for c = 0.025 M, the values of i ₀ and α are about 4 × 10⁻⁶ A/cm² and 0.2. With the increase in the Na₂S₂O₃ concentration, the exchange current increases weakly and the transfer coefficient remains virtually unchanged. The reaction order of gold dissolution with respect to ligand is calculated to have the value p = $ \left( {\frac{{\partial logi_a }} {{\partial logc}}} \right)_E $ \left( {\frac{{\partial logi_a }} {{\partial logc}}} \right)_E = 0.25 which is independent of E. With the changeover of supporting electrolyte, the exchange current increases in the following sequence: Li⁺ < Na⁺ < K⁺, but α and p remains unchanged. Data in thiosulfate solutions is compared with analogous data obtained earlier for the gold dissolution processes in cyanide and thiocarbamide electrolytes in which complexes of the similar structure were also formed. In electrolytes under comparison, the kinetics of gold dissolution is shown to exhibit common features.     

Multicycle chronoamperometry at rotating ring-disc electrode as a method of current separation into partial currents of silver ionization,Ag<Subscript>2</Subscript>O formation and its chemical dissolution in alkaline medium

A method of multicycle chronoamperometry at rotating ring-disc electrode is suggested for experimental separation of the disc polarization current into its components that correspond to the substrate metal ionization, an oxide formation, and the oxide chemical dissolution. The method was validated by the example of the Ag|Ag₂O|OH^?(H₂O) system. At moderate anodic potentials of Ag-disc (0.48–0.51 V), silver active dissolution from open areas of its surface and through film’s pores dominates; the phase-forming current, hence, the current efficiency of this process drops down rapidly. At the potentials of the maximum at voltammograms (0.52–0.53 V), when the silver active dissolution current is suppressed, the phase-forming currents dominate; they exceed the oxide chemical dissolution rate significantly. The Ag₂O film thickness increases rapidly, the current efficiency of the oxide formation process approaches 100% during the entire disc polarization period. The Ag(I)-oxide chemical dissolution rate constant practically does not depend on the anodic phase-formation potential; however, it somewhat varies depending on the oxide film thickness, thus reflecting changes in the film structure and, possibly, chemical composition (from AgOH to Ag₂O).     

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.     

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 Anodic Dissolution of a Hydrophilic Metal: Effect of the Surface Morphology

Current-voltage curves for the anodic dissolution of iron in acid chloride and sulfate solutions containing Cl^–, Br^–, and I^– ions are calculated. Calculations, made on the basis of a model of spatial separation of dissolution and passivation processes, account for the formation of pairs of kinks on stationary steps. The calculated curves quantitatively coincide with those found in literature. It is shown that portions of passivity in the curves are caused by specifically adsorbed anions, which hinder motion of kinks, while regions of nonpolarizability are connected not with the desorption of anions but with accelerated formation of vacancies on the steps, generating fresh active dissolution centers.     

Role of Hydrodynamic Conditions in the Distribution of Anodic Dissolution Rates in Cavity Etching Regions during Electrochemical Micromachining of Partially Insulated Surfaces

Experimental study of the distribution of local rates of electrochemical micromachining in the presence of photoresist masks in various hydrodynamic conditions (macroscopically nonuniform rotating disk electrode, sprayer flow, an electrode placed into a cell with chaotic bulk electrolyte mixing) shows that the maximum etching localization is achieved at the control of the dissolution rate by the mass transport rate (at achieving the anodic limiting current). The localization enhancement as compared to the primary current distribution takes place in the case of a turbulent flow at hydrodynamic conditions where the removal of dissolution products from the undercutting region is hindered. These conditions (electrochemical reaction limited by the ion mass transport rate, high resistance to the mass transport in the undercutting region) are necessary for the localization enhancement using a pulsed anodic–cathodic treatment.     

Biogenic fabrication of silver nanoparticles,oxidative dissolution and antimicrobial activities

Metallic silver nanoparticles (AgNPs) were prepared by using Foeniculum vulgare Mill seeds extract. The silver nitrate was used as silver precursor in an aqueous solution. The photooxidative dissolution of AgNPs with persulfate (K₂S₂O₈) under UV light was investigated. Effects of initial concentration of K₂S₂O₈, AgNPs, initial solution pH, and temperature were studied on dissolution of AgNPs. The 100% AgNPs dissolution was achieved in 60 min under typical conditions (pH = 4.0, 1.2 mM K₂S₂O₈, and 30 ⁰C). The experimental results showed higher temperature brought faster dissolution rate, and the activation energy was 65.2 kJ/mol. The effects of ethanol, tertiary butanol, and nitrobenzene were studied to establish the role of SO₄^? and HO radical species. AgNPs dissolution was inhibited by Cl^?, Br^?, I^?, and NO₃^? ions. Staphylococcus auerus (s. aureus), Escherichia coli (E. coli) and Candida albicans (C. albicans) were the effective human pathogens against the AgNPs. The lag phase, growth kinetics, minimum bactericidal concentration, death rate, and antimicrobial efficacy depend on the concentration of AgNPs.     

Macrodistribution of Anodic Dissolution Rate at a Rotating Disk Electrode with Partially Insulated Surface

Macrodistribution of etching rates of a macroscopically nonuniform, partially dipped, rotating disk electrode of low-carbon steel in 1 M NaCl at pH 1 with photoresist insulation and the density of active rectangular holes being 1.6 holes/mm² is studied at dimensionless etching rates i ⁰ _avg /i _l = 0.2–1.1, where i ⁰ _avg is the average current density per active surface and i _l the limiting anodic current density determined by the ionic transport rate. The distribution of the average etching rates in the normal direction is determined by the primary current distribution and is insensitive towards the type of the dissolution rate distribution in a cavity.     

Pt(110)/Sb电极上甲酸的电催化氧化特征和动力学

研究了Sb在Pt(110)晶面上不可逆吸附电化学特性及甲酸在Sb_ad修饰Pt(110)电极[Pt(110)/Sb]上的电催化氧化特征及其反应动力学.发现当扫描电位的上限E_u≤0.45V时,Sb_ad可稳定地吸附在Pt(110)表面上,从而有效地抑制了甲酸的解离吸附.与未修饰的Pt(110)上的结果相比,在Pt(110)/Sb上甲酸氧化的峰电位负移了0.35V.当θ_Sb=0.126时,Pt(110)/Sb电极对甲酸的电催化活性最高.还研究了Pt(110)/Sb上甲酸氧化反应的动力学,定量解析了不同θ_Sb下甲酸氧化的速度常数kf和传递系数β.