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.     

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.     

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.     

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.     

Influence of the degree of surface oxidation of polycrystalline Rh electrodes on the underpotential deposition of Cu

The underpotential deposition of copper onto polycrystalline rhodium was studied as a function of the degree of oxidation of the electrode surface in acidic media using potentiodynamic techniques. Surface oxidation of the rhodium electrode was carried out using a triangular sweep potential between E _L (lower limit) and E _U (upper limit: 0.94≤E _U≤1.4 V). Cu electrodeposition was performed at the same time as the total or partial reduction of the oxidized species. The surface oxides produced at E _U≤1.09 V were completely reduced during Cu electrodeposition. In this case, the potentiodynamic I-E patterns for oxidative dissolution of Cu were characterized by three anodic peaks located at 0.41 V (peak I), 0.47 V (peak II) and 0.59 V (peak III) and the coverage degree by Cu, θ_Cu, was on the order of a monolayer. Surface oxides produced at E _U>1.09 V were partially reduced during the copper electrodeposition. In this case, the I-E profiles exhibited only two anodic peaks (II and III) and θ_Cu was <1. The Rh-oxygen species that remain on the electrode surface block the active sites of lower energy and modify the binding energy of strongly adsorbed Cu. Electronic Publication     

Direct electrochemistry of myoglobin in a layer-by-layer film on an ionic liquid modified electrode containing CeO2 nanoparticles and hyaluronic acid

We describe an ionic liquid modified electrode (CPE-IL) for sensing hydrogen peroxide (HP) that was modified by the layer-by-layer technique with myoglobin (Mb). In addition, the surface of the electrode was modified with CeO₂ nanoparticles (nano-CeO₂) and hyaluronic acid. UV-vis and FTIR spectroscopy confirmed that Mb retains its native structure in the composite film. Scanning electron microscopy showed that the nano-CeO₂ closely interact with Mb to form an inhomogeneously distributed film. Cyclic voltammetry reveals a pair of quasi-reversible redox peaks of Mb, with the cathodic peak at ?0.357?V and the anodic peak at ?0.269?V. The peak separation (??E _p) and the formal potential (E ^0??) are 88?mV and ?0.313?V (vs. Ag/AgCl), respectively. The Mb immobilized in the modified electrode displays an excellent electrocatalytic activity towards HP in the 0.6 to 78.0???M concentration range. The limit of detection is 50?nM (S/N?=?3), and then the Michaelis-Menten constant is 71.8???M. We believe that such a composite film has potential to further investigate other redox proteins and in the fabrication of third-generation biosensors.

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 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.     

The effects of isotopic substitution and competitive adsorption on the oxidation of aldehydes and alcohols at gold electrodes

The nature of the mechanism of the anodic oxidation of aldehydes in aqueous base on gold electrodes has been probed using isotopic substitution and competitive adsorption studies. A primary kinetic isotope effect of k_H/k_D=3?4 was observed upon substitution of deuterium for protium on the formyl group of benzaldehyde and cyclohexanecarboxaldehyde on gold in aqueous base using the techniques of cyclic voltammetry, rotating disc electrode voltammetry and chronoamperometry. Similar results are reported for the same aldehydes on a silver electrode, and also for the anodic oxidation of 2-propanol and 2-propanol-2-d on gold under similar conditions. Inhibition of the anodic cyclic voltammetric peak for benzaldehyde on gold by a variety of adsorbed species including CN^?, I^?, Br^?, (C₂H₅)₄N⁺ and diethylenetriamine is also described. These observations are used to propose a mechanism for the low potential oxidations of aldehydes and alcohols on gold involving a rate limiting dissociative adsorption step with cleavage of the α-carbon-hydrogen bond.     

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.     

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.     

On Gold(I) Complexes and Anodic Gold Dissolution in Sulfite–Thiourea Solutions

Processes involving gold(I) complexes were studied in sulfite–thiourea (TU) solutions. It is shown that at pH >5 the complex [\( {\text{AuTU}}_{2}^{ + } \)] undergoes irreversible decomposition followed by deprotonation and formation of a solid phase. From the data of pH in mixed solutions, the equilibrium constants were evaluated: \( {\text{Au}}({\text{SO}}_{3} )_{2}^{3 - } + i{\text{TU}} \rightleftharpoons {\text{Au}}({\text{SO}}_{3} )_{2 - i} {\text{TU}}_{i}^{2i - 3} + i{\text{SO}}_{3}^{2 - } \), log₁₀ β ₁ = ?1.2, log₁₀ β ₂ = ?3.6. Some aspects of the anodic dissolution of gold in mixed sulfite–thiourea solutions are considered. With the help of the carbonate buffer system the change of the anodic current density j _a was studied at high pH; j _a (pH) has a maximum at pH 11.6–11.9 for E _a = 0.3–0.6 V (vs. NHE). At pH > 12.0, the j _a values decrease sharply. Possible mechanisms of anodic gold dissolution, as well as the role of sulfite, are discussed.     

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.     

Electrochemical investigation of a glassy carbon electrode modified with carbon nanotubes decorated with (poly)crystalline gold

Multiwalled carbon nanotubes with nanosized sputtered gold were used to modify a glassy carbon electrode (GCE). The substrate was characterized by scanning electron microscopy (SEM), X-ray diffraction, cyclic voltammetry and amperometry. SEM micrographs indicated an uniform coverage of the carbon nanotubes with nanosized (poly)crystalline gold. Cyclic voltammetry reveals that peak separation of the unmodified GCE in the presence of 1?mM ferricyanide is 131?mV, but 60?mV only for the modified GCE. In addition, the oxidation of NADH (1?mmol?L^?1 solution) begins at negative potentials (around ?100?mV vs. Ag/AgCl), and the anodic peak potential (corresponding to the irreversible oxidation of NADH) is found at +94?mV. The effect of pH on the electrocatalytic activity was studied in the range from 5.4 to 8.0. The relationship between the anodic peak potential and the pH indicated a variation of ?33.5?mV/pH which is in agreement with a two-electron and one-proton reaction mechanism. Amperometry, performed at either ?50 or +50?mV vs. an Ag/AgCl reference electrode, indicates that the modified electrode is a viable amperometric sensor for NADH. At a working potential of +50?mV, the response to NADH is linear in the concentration range from 1 to 100???mol?L^?1, with an RSD of 6% (n?=?4).

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.     

Potential dependence of anodic current transients on the renewed gold surface in thallium-containing thiosulfate solutions

The effect of potential on the anodic current transient times τ_p, which were measured on the gold electrode surface renewed by cutting off a thin surface metal layer immediately in the thallium-containing thiosulfate solutions of compositions (M): 0.05 Na₂S₂O₃, 10^?5 to 10^?4 TlNO₃, and 0.25 K₂SO₄, is studied. It is shown that the logarithm of inverse transient times 1/τ_p linearly depends on the electrode potential. Using the microgravimetrical method, it is found that, in the studied potential range, the amount of chemisorbed thallium ions only slightly depends on the potential, and at E = 0.3 V, it is approximately 0.12 μg/cm². It is evidenced that the transients reaches a plateau, when an equilibrium surface concentration of catalyst is reached. The value 1/τ_p reflects the rate of electrochemical oxidation of preliminarily adsorbed thallium(I) ions with the formation of catalytically active thallium(III) ions, and the first electron transfer is the limiting stage of the process.     

Anodic behavior of tin, indium, and tin–indium alloys in oxalic acid solution

The anodic behavior of tin, indium, and tin–indium alloys was studied in oxalic acid solution using potentiodynamic technique and characterized by X-ray diffraction and scanning electron microscopy. The E/I curves showed that the anodic behavior of all investigated electrodes exhibits active/passive transition. In the case of tin, the active dissolution region involves two anodic peaks (I and II) prior to permanent passive region. On the other hand, the active dissolution of indium involves four peaks (I–IV) prior to permanent passive region. The first (I) can be associated with the active dissolution of indium to InOOH, the second peak (II) to the formation of In(OH)₃, the third peak (III) to partially dehydration of In(OH)₃, and the peak (IV) to complete dehydration of In(OH)₃ to In₂O₃. When the surface is entirely covered with In₂O₃ film, the anodic current falls to a small value (I _pass) indicating the onset of passivation. The active dissolution potential region of the first three tin–indium alloys involves a net anodic contribution peak, and this is followed by a passive region. It is expected that the investigated peak is related to the formation of In₂O₃ and SnO (mixed oxides). When the formation of oxides (the oxides of In and Sn) exceeds its dissolution rate, the current drops, indicating the onset of passivation precipitation of In₂O₃/SnO and SnO₂ on the surface which blocks the dissolution of active sites. The alloys IV and V showed small second peak at about −620 mV which may be related to oxidation of In to In₂O₃ due to high In content in the two examined alloys. The active dissolution and passive current are increase with increasing temperature for all investigated metals and their alloys.     

Electrochemical properties of sulfur adsorbed on gold electrodes

The electrochemical properties of sulfur adsorbed on gold electrodes were studied in 10^?5M solutions of S^2? in 1 M NaOH. In general, ∵_S is less than a monolayer. At E=0.05 V only, a monolayer will be formed after long times. The sulfur layer is stable in the potential range between ?0.6 and +0.4 V. At lower potentials, sulfur can be desorbed cathodically (charge Q_red), but at higher potentials, where layers of gold oxide are formed, the sulfur is oxidized anodically (charge Q_ox). From the ratio Q_red·6/Q_ox=γ, the electrosorption valency γ=?2 is obtained. This means, that the sulfide ions are almost completely discharged during adsorption. The same layer can be formed by adsorption from polysulfide solutions, which can be explained by a break of the sulfur bond and adsorption of single sulfur atoms. The double layer capacity decreases during adsorption of sulfur indicating the formation of an insulating sulfur layer with a dielectric constant of about 2. The anodic adsorption of sulfide ions is limited by diffusion only. For longer polarisation times, the coverage is independent of time, i.e. place exchange reactions between Au and S can be excluded. The cathodic desorption as well as the anodic oxidation of the adsorbed sulfur are potential dependent charge transfer processes, as can be concluded from potentiodynamic measurements with various sweep rates.     

Influence of sulfide and cyanide ions on the electrochemical behavior of the Fe(II)/Fe(Hg) system in thiocyanate solutions at mercury electrodes

The reduction and reoxidation processes of the Fe(II)/Fe(Hg) system in thiocyanate solutions at stationary mercury electrodes have been investigated by cyclic voltammetric, anodic stripping and controlled potential electrolysis methods. In 0.1 M NaSCN and 0.4 M NaClO₄ solution containing 1×10^?3M Fe(II), the voltammogram on the first cycle at. 0.05 V s^?1 gives two consecutive cathodic peaks near ?1.2 and ?1.39 V with a hysteresis on the reversal, and an anodic wave with two large peaks near ?0.58 and ?0.05 V and two small peaks near ?0.52 and ?0.43 V, respectively. The multicyclic voltammogram under the same conditions in the potential region between 0.00 and ?1.50 V gives a cathodic wave with a principal peak near ?1.02 V and two small peaks near ?0.02 and ?0.53 V, respectively, and an anodic wave with a principal peak near ?0.72 V, three small peaks near ?0.64, ?0.52 and ?0.40 V, and with a shoulder near ?0.05 V, respectively. The variation of the shape of the voltammogram on the second and subsequent runs is due to the formation of S^2? and CN^? during the process of electroreduction of Fe(II). A mechanism is proposed which involves an initial reduction of Fe(II)?SCN^? produced in an activation step at a mercury electrode, followed by the chemical redox reaction of a part of Fe(0)?SCN^? in the species giving FeS and CN^?, and takes into account the influence of FeS and CN^? on the further reduction and reoxidation of iron. Both FeS and CN^? stimulate further reduction, and reoxidation of iron. The hysteresis of the cathodic wave on the first cycle arises from the fact that Fe(II) is reduced more easily at the mercury electrode covered with FeS than at a pure mercury electrode.     

The anodic oxidation of gold + palladium alloys

The anodic oxidation of Au+Pd alloys has been studied in solutions of 1 M H₂SO₄ and 0.1 M K₂SO₄ by voltammetric methods. A linear relationship between oxide reduction maximum and bulk alloy composition, often used to determine the surface composition of homogeneous alloys, could be shown to hold only for alloys up to 60 at% gold. At higher gold content the Au oxide peak must be additionally evaluated. With continuous cycling in acid solution the anodic dissolution of Pd, especially from gold-rich places, leads to a rather heterogeneous surface. The O--chemisorption is not governed by a transfer mechanism from Pd to Au surface atoms. The alloys are able to absorb the oxygen species generated in the positive potential region; however, this ability decreases with increase of the gold content.