Kinetics of Anodic Dissolution of Gold in Cyanide Solutions Containing Thallium Ions: Potentiostatic Measurements

Effective reaction order by cyanide ions, p, effective transfer coefficient _ef, and exchange current i ₀of the gold dissolution process are measured at a constant surface coverage by thallium adatoms _Tl. The constancy of _Tlis ensured by constant time tof the electrode's contact with solution after renewal of its surface and before starting the measurements. Solutions containing 2.5 × 10^–6to 10^–5M TlNO₃, 0.1 M KCN, and 0.1 M KOH are studied. With an increase in t, quantities p, _ef, and i ₀increase from values typical for pure KCN solutions (0.17, 0.1, and 10^–5A/dm², respectively) to p 1.1, _ef 0.4, and i ₀ 3 × 10^–4A/dm². The increase in pby nearly unity is interpreted as indicating a change in the limiting stage in the presence of thallium ions: in the latter case, it involves not only adsorbed cyanide ions (as in pure cyanide solutions), but also those located beyond the adsorption layer. A possible mechanism explaining the acceleration of gold dissolution in the presence of thallium adatoms is put forward.     

Anodic Dissolution of Gold in Cyanide Solutions Containing Bismuth Hydroxy Compounds

The gold dissolution rate in alkali–cyanide solutions with and without bismuth hydroxy compounds is studied as a function of potential E, with the electrode surface renewed before taking measurements. At E < 0 (NHE), these ions accelerate the process. At more positive E, their influence is ambiguous. At negative potentials, the effect depends on the time of electrode contact with solution prior to beginning the measurements, the scan rate of E, and the KCN and KOH concentrations. Possible approaches are suggested to explaining the established regularities.     

Anodic Dissolution of Gold in Cyanide Solutions: Effect of Temperature

It is shown that a temperature increase has a complex effect on the anodic dissolution of gold. At negative potentials (in the standard hydrogen scale), in the absence of heavy metals, the effective activation energy for the dissolution is close to 48 kJ/mol and weakly depends on the polarization. At more positive potentials, the temperature dependence of the gold dissolution rate reflects the temperature effect on competitive adsorption of cyanide and hydroxide ions, the surface development during electrolysis, and the accumulation of the cyanide ion decomposition products and their passivating action. Near 0.6 V, the temperature affects transport processes.     

Kinetics of Anodic Dissolution of Gold in Cyanide Solutions Containing Bismuth Hydroxy Compounds

Effective values of order of the reaction of the gold dissolution process by cyanide ions p, transfer coefficient , and exchange current i ₀ are measured at constant values of the coverage of the gold surface by bismuth adatoms . The constancy of is ensured by maintaining a constant value of the time t during which the electrode is in contact with solution after the renewal of its surface until the instant of taking measurements. The solutions under study contain 2.5 × 10^–6 to 7 × 10^–6 M bismuth hydroxy compounds, 0.1 M KCN, 0.01 M KAu(CN)₂, and 0.1 M KOH. With increasing t, quantities p, , and i ₀ increase from 0.17, 0.1, and 10^–5 A cm^–2, respectively, which are the values typical for the gold dissolution process in plain potassium cyanide solutions, to p 1.1, 0.45, and i ₀ 2 × 10^–4 A cm^–2. The increase in the quantity p by approximately a unity is interpreted as an evidence in favor of the assumption that the presence of bismuth adatoms alters the nature of the limiting stage: the latter starts involving cyanide ions localized outside an adsorption layer, rather than the adsorbed cyanide ions only, as is the case in plain solutions. A comparison of obtained results with the data of similar investigations performed earlier in thallium- and lead-containing solutions shows that possible mechanism of acceleration of anodic process suggested in the previous works may be used for explaining regularities of the gold dissolution process observed in the presence of bismuth adatoms as well.     

Kinetic Parameters of Gold Dissolution in Cyanide Solutions: The Effect of Complex Cyanide Ions of Mercury

The kinetics of the gold dissolution in cyanide solutions is studied at constant values of the coverage () of the gold surface by mercury atoms. The constancy of is ensured by maintaining an identical value of the duration (t) of contact of electrode with solution (after renewing its surface by cutting off a thin surface layer of metal) at a potential of –1.3 V, at which the discharge of mercury ions is limited by their diffusion to the electrode. At t = const kinetic dependences of the gold dissolution process correspond to the Tafel equation. Effective values of exchange current i ₀, transfer coefficient , and reaction order by cyanide ions P are determined. With increasing value of their magnitude increases from values 10^–5 A cm^–2, 0.1, and 0.17 that are characteristic of purely cyanide solutions (composition 0.1 M KCN, 0.1 M KOH, and 0.01 M KAu(CN)₂) to i ₀ 2 × 10^–4 A cm^–2, = 0.46, and P 1 at t = 270 s. These results are compared with the data obtained earlier during similar investigations in solutions containing thallium, lead, and bismuth. Common and individual features in the behavior of mercury-containing electrolytes are revealed. It is shown that the possible mechanism of the acceleration of the gold dissolution process in the presence of catalytically active atoms, which had been proposed in these works, may be used also for explaining the action of mercury atoms on this process.     

Silver Electrodeposition from Cyanide Solutions: Effect of Lead Ions

Effect of lead hydroxy compounds on the process of electrodeposition of silver from cyanide electrolytes is studied on an electrode whose surface is renewed in solution by cutting off a thin layer of metal. This permitted to perform the study on both the freshly renewed electrode and at controlled values of the time of the electrode contact with solution t. Shown is that on the freshly renewed electrode (t<1 s) the presence in the solution of lead ions in concentrations c ₁ on the order of 10^–5 M leads to the process depolarization only in the initial portion of a polarization curve. With c ₁ increased to 10^–4 M the effect of depolarization extends on the entire polarization curve. Keeping the electrode in solution after the renewal of the metal surface magnifies depolarization, and the greater the concentration c ₁, the shorter the time period t required to achieve the same effect. These regularities are attributed to catalytic influence of lead adatoms, whose surface concentration depends on c ₁ and t, as well as on the intensity of their incorporation in the silver deposit.     

Effect of Lead Ions on the Kinetics of Gold Deposition from Cyanide Electrolytes

The depolarization of the gold electrodeposition in the presence of lead ions depends on their concentration and the duration of electrode contact with solution preceding a potential scan in an extremum fashion. At constant coverages of the gold surface by lead adatoms , the process rate depends on the overvoltage in accord with the Tafel equation. Effective values of the exchange current i ₀ and transfer coefficient increase with from i ₀ 3 × 10^–5 A cm^–2 and = 0.23 in pure solutions to 3 × 10^–4 A cm^–2 and 0.53 at 0.4. The reaction order by cyanide ions is independent of and equals nearly –0.9. Effects of lead adatoms on the kinetics of cathodic and anodic processes are compared and the obtained data may be brought to conformance given that their mechanisms in pure solutions differ and converge in the presence of lead adatoms.     

Catalytic Activity of Thallium, Lead, and Bismuth Adatoms in the Gold Dissolution Reaction in Cyanide Solutions: A Comparative Characterization

Quartz microgravimetry is used to determine the ratio between coefficients of mass transfer (1 : 0.54 : 0.48), which characterizes relative values of rates of diffusion of hydroxy complexes of thallium, lead, and bismuth in alkaline solutions. The ratio is used when refining the condition under which on a renewable electrode in solutions containing these ions at the concentration c _i the electrode coverage by relevant adatoms _i with increasing duration of contact of the electrode with solution reaches constant values: (c _Tl t) = (0.54c _Pb t) = (0.48c _Bi t). Measured are i,t curves on a renewable gold electrode at E = const in solutions containing 0.1 M KCN, 0.1 M KOH, 0.01 M KAu(CN)₂, and 8 × 10^–6 M compounds of thallium or 1.5 × 10^–5 M, lead, or 1.6 × 10^–5 M, bismuth. Shown is that at (, E) = const the currents of dissolution of gold in these solutions increase in the series Tl < Pb < Bi, which evidences an increase in this series of the catalytic activity of adatoms of these metals. Shown is that at = const the catalytic action of adatoms of thallium and bismuth has an approximately additive character. The obtained data are analyzed with allowance made for the explanation offered earlier for the catalytic effect of adatoms on the anodic dissolution of gold based on the hypothesis about the shift of the potential of the free zero charge in the negative direction after substituting a metal atom for chemisorbed cyanide ions.     

纯铝在KOH-乙醇溶液中的电化学行为

研究纯铝在KOH 乙醇溶液中的电化学行为,腐蚀失重、极化曲线、EIS和恒电流放电实验表明,该体系纯铝的腐蚀速率极小,因而有可能应用于密闭电池,并可在相当宽的电位范围内保持一定的活性.放电的终止可能是由于放电产物在电极表面堆积引起的.     

硫酸根离子对铁在弱碱性溶液中阳极溶解和钝化行为的影响

采用浸泡实验, 电化学测试和表面分析技术研究了硫酸根离子浓度对铁在稀碳酸氢钠溶液中开路状态和阳极极化行为的影响. 在无硫酸根离子及含有少量硫酸根离子的碳酸氢钠溶液中, 铁的开路电位约为(-0.225±0.005) V, 并呈现钝化状态, 其电化学阻抗很大, 腐蚀速率较低. 在含有较高浓度硫酸根离子的碳酸氢钠溶液中, 铁的开路电位为(-0.790±0.010) V并呈现活性溶解状态, 其电化学阻抗较小, 腐蚀速率较高, 同时阳极极化曲线上能观察到活化-钝化转变现象. 由于铁在含有较高浓度硫酸根离子的碳酸氢钠溶液中处于活化状态, 阳极极化曲线上存在数个电流峰. 足够高的硫酸根离子浓度会导致铁表面预先形成或转变而成的氧化膜失效. 相比于自然曝氧状态, 在除氧条件下较低的硫酸根离子浓度即可引起铁在碳酸氢钠溶液中由钝态向活性溶解态的转变.     

Electroreduction of Molybdate Ions in Solutions Containing Ammonium Ions

An incomplete reduction of molybdate ions in solutions of pH 3.0–9.0 is shown to be accelerated by ammonium ions: a film of hydrated oxides of molybdenum in lower oxidation states forms on the cathode in their presence. Products of the incomplete reduction adsorbed at the cathode block its working surface.     

Kinetics of Gold Dissolution in Thiosemicarbazide Solutions

The kinetics of gold dissolution in thiosemicarbazide solutions was studied relative to the ratio of ferric ion and thiosemicarbazide concentrations as well as pH. The composition of the complex formed was found to be [Au(NH₂)₂NHC=S₃]⁺. The rate constants for gold dissolution were determined at 278-298 K along with the stability, dissociation, and equilibrium constants at 288 K. The activation energy at 288 K was also found.     

Anodic Dissolution of Samarium in an Acetonitrile Solution in Acetylacetone

Electrochemical dissolution of metallic samarium is studied in an acetonitrile solution containing 0.1 M tetraethylammonium bromide and 0.9 M acetylacetone. The study is performed in an argon atmosphere, at a constant voltage of 3 V and various ratios between the cathode and anode areas. If the cathode area considerably exceeds the anode area, as a result of anodic oxidation of samarium, ions Sm³⁺ form in solution, undergo reduction to Sm²⁺, and 20–25 min later generate chelate complex Sm₄(AA)₈ · 3HAA, which is insoluble in acetonitrile. Simultaneously, Sm³⁺ interacts with deprotonated acetylacetone. Chelate complex Sm(AA)₃ · HAA is extracted from solution. If the anode area exceeds the cathode area, no reduction of Sm³⁺ to Sm²⁺ occurs on the cathode, and eventually adducted samariumtris–acetylacetonate Sm(AA)₃ · 4HAA is extracted from solution. The composition of the obtained compounds is confirmed by IR and mass spectrometry, thermogravimetry, isothermic heating, and elemental analysis for metal.     

Kinetic Parameters of the Dissolution of Gold in Thiourea Solutions

The temperature dependence of the rate constants for the dissolution of gold in thiourea solutions was determined. The activation energies calculated suggest that the rate of gold dissolution is diffusion-controlled at 5-22 °C but is kinetically-controlled at 25-60 °C (E _a = 42.83 kJ/mol). Auger spectroscopy was used to study the composition of the surface adsorption layers and a mechanism for the passivation of the reactive surface during the gold dissolution process was proposed.     

Effect of Cl~-ion on Anodic Iron Dissolution in H_2S-Containing Sulfuric Acid Solutions

ResearchershavestudiedtheeffectofCI-onthestructureandpropertiesofthecorrosionproductsinchloridesolutionscontainingH,S"'.IthasbeenfoundthattherearenocorrosioneffectsonironsurfaceswhicharesubmergedinconcentratedacidicchloridesolutionswithlowHZSlevel.Inthepast,theinteractionbetweentheCI-ionandFehasnotbeenstudiedcompletelyandintensively.TheanodicreactionofironinacidicsolutionscontainingHZSwastheironoxidationcatalyzedbyHS-3.WhenCI-ispresentinHZS-containingacidicsolutions,theeffectofCI-onir…     

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.     

Anodic Dissolution of Molybdenum and Tungsten in a Solution of Sodium Sulfate

The electrochemical dissolution of molybdenum and tungsten in a 0.5 M solution of sodium sulfate at various temperatures was studied. The conditions of the efficient recovery of molybdenum from the binary metal system Mo-W were determined.     

Complexation of Lead(II) Ions with Hydroxide Ions in Nitrate-Chloride Solutions

Complexation of lead(II) ions with hydroxide ions in nitrate-chloride solutions at 298 K was studied using potentiometric titration.     

十四烷基二甲基苄基氯化铵萃取Au(CN)2-的微观机理

通过萃取平衡、傅里叶变换红外光谱及分峰技术研究了季铵盐十四烷基二甲基苄基氯化铵(TDM-BAC)-磷酸三丁酯(TBP)-正庚烷体系萃取Au(CN)₂^-的机理及过程.提出了萃合物的结构模型是基于氢键的超分子体系,组成为[R₄N⁺]·[Au(CN)₂^-]·4H₂O·4TBP.当有机相金浓度大于3g·L^-1时,有机相中存在聚集现象.通过激光光散射技术测定了有机相中反向胶团的大小,表明萃取过程是分散在水相中的胶团在协萃剂(或助表面活性剂)作用下溶入有机相,转型为反向胶团或微乳液(W/O型)聚集状态.     

An Electrochemical Puzzle: The Catalyzed Anodic Dissolution of I-coated Palladium in Halide-Free Acid Solutions

Iodine adsorbed on a Pd electrode has been shown to catalyze the anodic dissolution of Pd in halide-free acidic solutions. An undergraduate instrumental analysis laboratory experiment has been developed that enables students to use basic electrochemical techniques (cyclic voltammetry and chronocoulometry) to detect and confirm this phenomenon. Cyclic voltammograms of clean Pd and I-coated Pd were done in 0.5 M sulfuric acid solutions containing iodide and iodine as well as halide free 0.5 M sulfuric acid. The large currents noted in the halide-free solution indicate an anodic process that is not related to the oxidation process in solutions containing iodine species. Chronocoulometry was used to determine the total charge that was passed in the 0.5 M sulfuric acid solutions for clean and I-coated Pd electrodes. The results of these experiments point to a unique catalyzed corrosion process for this otherwise inert metal.