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 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.     

Electrochemical nanogravimetric studies of sulfur/sulfide redox processes on gold surface

Electrochemical quartz crystal microbalance, combined with cyclic voltammetric, chronoamperometric, and potentiostatic measurements, was used to study electrodeposition/dissolution phenomena at a gold electrode in solutions containing Na₂S. Spontaneous, open-circuit deposition processes as well as dissolution of the deposits in sulfide-free solutions have also been investigated. The potential range, scan rate, sulfide concentration, and pH have been varied. The results of the piezoelectric nanogravimetric studies are elucidated by a rather complex scheme involving underpotential deposition of sulfur at approximately −0.85 V vs. sodium calomel electrode, reductive dissolution of the deposited sulfur-containing layer at potentials more negative than approximately −0.9 V, and formation of a sulfur-containing multilayer at potentials more positive than −0.2 V. During the reduction of sulfur deposited on Au, a mass increase due to the formation of polysulfide species in the surface layer, accompanied by incorporation of Na⁺ counterions, can be observed that starts at approximately −0.4 V. This is a reversible process, i.e., during the reoxidation, counterions leave the surface layers. Frequency excursions during the electroreduction and reoxidation processes reveal existence of several competitive dissolution–deposition steps. Spontaneous interaction between Au and HS⁻ species results in a surface mass increase at the open-circuit potential, and it also manifests itself in the substantial decrease of the open-circuit potential after addition of Na₂S to the supporting electrolyte.     

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.     

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.     

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.     

Anodic Dissolution of Gold in Cyanide Solutions Containing Lead Ions: Effect of the Solution pH

The effect of the electrode potential on the gold dissolution rate in alkali–cyanide solutions with and without 10^–5 M of hydroxy compounds of lead is studied. With the compounds, the process rate passes through a maximum, whose potential E _m shifts in the negative direction and whose height drops with increasing pH. The pH dependence of E _m is linear, with the slope dE _m/dpH = –71 ± 5 mV, and correlates with that of the potential at which lead adatoms start to undergo desorption from the gold surface in alkali solutions. Without the compounds, the gold dissolution rate in alkali–cyanide solutions is independent of the solution pH at E < 0. Thus, the effect of the solution pH in this potential range is connected not with a direct participation of hydroxide ions in the anodic process but is of a secondary nature caused by the dependence of the region of adsorption of catalytically active lead adatoms on the hydroxide ion content in solution.     

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.     

Electrochemical dissolution of gold in acidic medium

Polycrystalline gold dissolves at high potentials in acidic medium even without the presence of complexing agents. In the present work the dissolution of gold is quantitatively studied by an inductively coupled plasma mass spectrometer (ICP-MS) directly connected to an electrochemical scanning flow cell (SFC). It is shown that the onset of gold dissolution coincides with the onset of the formation of 2D surface oxide (ca. + 1.3 V_RHE). An increase in the upper potential limit (UPL) during potential cycling or an increase in the holding potential during potential steps leads to an enhancement in the amount of dissolved gold. Moreover, a change in the dominating mechanism for gold dissolution with increasing potential is found in transient experiments. At lower potentials, gold is dissolved mostly during the gold oxide reduction, while at higher potentials anodic dissolution is the dominating process. Finally, the possible application of gold for stabilization of PtAu-based fuel cell catalysts is discussed.     

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.     

A study of the differential capacitance of zinc in aqueous solution with potassium silicate additions

The differential capacitance of the polycrystalline zinc electrode has been studied in aqueous solutions of KCl, KNO₃ and KOH both with and without the addition of potassium silicate. Double layer capacitance measurements can be made in KCl and KNO₃ without the interaction of OH^? at low pH values < 3.0. The reduction of the nitrate ion takes place at the zinc electrode in aqueous potassium nitrate.The silicate ion is adsorbed on the zinc electrode in aqueous KOH solutions at a potential close to the dissolution potential. This results in inhibition of metal dissolution, due to limited interaction of OH^? with the metal surface. The electrode resistance is increased by this adsorbed layer of silicate ion. In alkaline solution the h.e.r. is stimulated by the addition of potassium silicate.     

Effect of Thallium Ions on Anodic Dissolution of Gold in Alkali–Cyanide Electrolytes at Potentials More Positive Than the NHE Potential

The gold dissolution rate iin solutions containing 0.1 M KOH, 0.1 M KCN, and 2.5 × 10^–7to 1.5 × 10^–5M TlNO₃is studied as a function of potential Eof the electrode whose surface is renewed prior to each experiment, the TlNO₃concentration c, and the time tof the electrode contact with solution. At cexceeding 0.5 × 10^–5M and t 0, the rate is 1.5–2 times that at c= 0. Initial portions of ivs. tcurves in the absence and presence of TlNO₃coincide only at cbelow 10^–6M. Potentiostatic and potentiodynamic measurements show that, at positive E, only small coverages of the electrode surface with thallium are obtained, which make no impact on iat E< 0 and heavily increase it at 0 < E< 0.3 V. The discovered effects are attributed to the formation, during the adsorption of oxidized thallium forms, of dipoles comprising thallium adions and gold atoms. Presumably, the dipoles face the gold with their negative ends and make the potential of zero free charge more negative.     

Catalytic Effect on Silver Electrodeposition of Gold Deposited on Carbon Electrodes

A new methodology, based on silver electrocatalytic deposition and designed to quantify gold deposited onto carbon paste electrode (CPE) and glassy carbon electrode (GCE), has been developed in this work. Silver (prepared in 1.0 M NH₃) electrodeposition at ?0.13 V occurs only when gold is previously deposited at an adequate potential on the electrode surface for a fixed period of time. When a CPE is used as working electrode, an adequate oxidation of gold is necessary. This oxidation is carried out in both 0.1 M NaOH and 0.1 M H₂SO₄ at oxidation potentials. When a GCE is used as working electrode, the oxidation steps are not necessary. Moreover, a cleaning step in KCN, which removes gold from electrode surface, is included. To obtain reproducibility in the analytical signal, the surface of the electrodes must be suitably pretreated; this electrodic pretreatment depends on the kind of electrode used as working electrode. Low detection limits (5.0×10^?10 M) for short gold deposition times (10 min for CPE and 5 min for GCE) were achieved with this novel methodology. Finally, sodium aurothiomalate can be quantified using silver electrocatalytic deposition and GCE as working electrode. Good linear relationship between silver anodic stripping peak and aurothiomalate concentration was found from 5.0×10^?10 M to 1.0×10^?8 M.     

Anodic Dissolution of Gold in Alkali–Cyanide Solutions Containing Microquantities of Mercury Ions

An admixture of mercury ions accelerates dissolution of gold at negative (in the hydrogen scale) potentials and hinders it at positive potentials. In contradistinction to a similar effect exerted by admixtures of thallium, bismuth, and lead ions, the influence of mercury ions, all other conditions being equal, manifests itself at much longer times of contact between gold and solution. This difference is due to a low rate of the act of adsorption (discharge) of mercury ions. The rate increases at more negative potentials, and at E –1.2 V (NHE) the act accelerates to such an extent that looses the limiting role, which passes to the stage of supply of mercury ions to the electrode, as with solutions containing thallium, bismuth, and lead. Comparing these results with earlier data on the adsorption of cyanide ions on gold shows that the discharge of the Hg(CN)^2- ₄ anions stops limiting the formation of a layer of mercury atoms when the adsorption of cyanide ions turns insignificant.     

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).     

酸性AlCl3-BMIC离子液体中的铝阳极溶解

采用线性扫描伏安法研究了Lewis 酸性AlCl₃-BMIC (BMIC: 1-butyl-3-methylimidazolium chloride)离子液体中铝电极的溶解. 铝电极在阳极极化时出现了钝化现象, 钝化是由于在铝电极表面形成了固体AlCl₃钝化膜造成的. 铝的电化学溶解过程可以依次分为三个区: 电化学控制区、过渡区和钝化区. 在电化学控制区, 铝的电化学溶解速率随着电位的正移而逐渐增加; 在过渡区, 由于电极表面AlCl₄^-和Al₂Cl₇^-浓度发生改变而析出固体AlCl₃使得铝电化学溶解速率随着电位的正移而逐渐减小; 当钝化膜形成之后, 铝的电化学溶解速率不再随着电位的正移而发生改变, 铝溶解进入钝化区. 增加搅拌、升高温度、降低离子液体AlCl₃摩尔分数都可以增加铝溶解阳极极限电流密度.     

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.     

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.     

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.     

The CuII−CuI−Cu0(Hg) system in the presence of benzotriazole: Part II. A chronocoulometric investigation

Both the oxidation of Cu⁰ at dropping amalgam electrodes immersed in solutions of benzotriazole (BTA) and the reduction of Cu^II at a dropping mercury electrode from BTA solutions have been investigated by the single potential-step chronocoulometric technique. The dependence of the charge Q(t) flowing as a consequence of a given potential jump E_i→E_f upon the initial and final potentials E_i and E_f, as well as upon the time t elapsed from the instant of the potential jump provides direct evidence for the presence of a single adsorbed monolayer of a Cu^I compound on a mercury electrode immersed in a Cu^II solution containing BTA, at applied potentials positive to ≈?0.4 V/SCE. Analogous measurements carried out at dropping amalgam electrodes reveal the presence of a single adsorbed monolayer of a Cu^I compound, or else of an adsorbed multilayer, depending on the potential range investigated. The results of the chronocoulometric measurements are in agreement with those of the polarographic measurements of Part I.