Electrochemical behavior of pyrite in sulfuric acid solutions containing silver ions

A systematic electrochemical study of pyrite in H₂SO₄ solutions containing dissolved silver was undertaken to gain more information about the transfer of silver ions to pyrite and their role in enhancing the direct oxidation of pyrite. The results of cyclic voltammetry experiments provide additional evidence of the formation of metallic silver on the FeS₂ surface under open-circuit conditions. A pyrite electrode held at the open-circuit potential for 2 h in the presence of 10^–3 m Ag⁺ exhibits a large and sharp anodic peak at about 0.7V. The current associated with this peak is the result of the dissolution of metallic silver deposited during the initial conditioning period. There is no evidence of silver deposition without preconditioning until the potential drops below about 0.6V for Ag⁺ concentrations ranging from 10^–4 to 10^–2 m. However, subsequent silver deposition appears to be very sensitive to the dissolved silver concentration in this range. There is also evidence that the state of the pyrite surface has a pronounced influence on its interaction with silver ions. Agitation has also been found to have a significant effect on the electrochemistry of the Ag–FeS₂ system.     

Using probe beam deflection (PBD) to investigate the electrochemical oxidation of silver in perchlorate media in the presence and absence of chloride ions

The electrochemical oxidation of silver in 0.1 M KClO₄ solutions containing KCl were investigated by cyclic voltammetry (CV) and electrochemical probe beam deflection (PBD). Ag⁺_(aq) ions were the main product of the silver oxidation in the absence of the halide. The formation of Ag⁺_(aq) provoked a beam deflection towards the electrode surface. A beam deflection away from the electrode surface was then observed during the reduction of the Ag⁺_(aq) ions. A convolution analysis yielded a diffusion coefficient of 1.2×10⁻⁹ m² s⁻¹ for Ag⁺_(aq) in this medium. An anodic peak due to the formation of AgCl_(s) film was observed for the oxidation of silver in solutions containing Cl⁻_(aq). As the applied potentials were made more positive in media containing chloride (after the peak due to the AgCl_(s) formation), a flux of ions away from the electrode surface was clearly detected by PBD. This was assigned to the formation of Ag⁺_(aq) ions through the porous AgCl_(s) film structure. Oscillations on the position of the laser beam were present during the oxidation at high chloride concentrations, due to the precipitation of AgCl_(s) from the solution phase. The electrochemical and PBD data were consistent with a dissolution-precipitation mechanism for the AgCl_(s) film formation.     

Electrochemical window of molten LiCl-KCl-CaCl2 and the Ag/Ag reference electrode

The electrochemical window of an LiCl-KCl-CaCl₂ eutectic melt (52.3:11.6:36.1 mol%) was determined by cyclic voltammetry and open-circuit potentiometry at 723-873 K. The reaction at the anodic limit was confirmed to be Cl₂ gas evolution. The reaction at the cathodic limit was found to be a liquid Ca-Li alloy formation on the basis of ICP analysis of the deposits. An Ag⁺/Ag reference electrode separated with a Pyrex membrane showed good stability for more than 1 week. The standard electrode potential of Ag⁺/Ag was determined in the temperature range of 723-823 K by measuring the potential of a silver electrode in different concentrations of Ag⁺ ions.     

Ag transfer across the water/1,2-dichloroethane interface facilitated by complex formation with tetraphenylborate derivatives

The assisted transfer of silver ion, Ag⁺, by complexation of Ag⁺ with the tetrakis(4-chlorophenyl)borate (TPBCl⁻) anion was studied at the interface between two immiscible electrolyte solutions (ITIES).The elucidation of the interfacial mechanisms employed the cyclic voltammetry technique applied to macro and micro liquid/liquid interfaces. This procedure allowed to identify Ag⁺ as the species which is transferred at the liquid/liquid interface and also the formation of an 1:2 metal:ligand complex when TPBCl⁻ is in excess.The value for the formal transfer potential of silver(I) from aqueous to 1,2-dichloroethane was evaluated and the association constant for the [Ag(TPBCl)₂]⁻ complex was determined.     

Anodic oxidation of aluminium in sulphuric acid containing aluminium sulphate or magnesium sulphate

The effect of Al₂(SO₄)₃ or MgSO₄ in the sulphuric acid electrolyte on the formation and dissolution of anodic films on aluminium has been investigated. When the sulphate concentration was increased, kinetic viscosity of the electrolyte became higher and the anode surface temperature during anodizing rose gradually. Increase in the concentration of the sulphate suppressed chemical dissolution of the anodic film. As the sulphate concentration was increased, hardness of the film generally increased at first and then decreased, while the coefficient of static friction of the film decreased. From measurement of microroughness and electro-microscopic observations, it was found that by the addition of the sulphate, the surface became more microscopically even and the porosity near the outer surface was reduced. The uniformity of the film was decreased with increased in sulphate concentration.     

Electrochemical behaviour of the solid electrolyte Ag6I4WO4

Electrochemical investigation of the solid electrolyte Ag₆I₄WO₄ (0.8 AgI + 0.2 Ag₂WO₄ mixture) was performed. By means of cyclic voltammetry, normal pulse polarography and ac polarography methods the electrode processes and the capacity of electric double layer on electrode/electrolyte interface were investigated at several temperature levels (room temperature, 373 K, 386 K and 423 K). It was shown that on the electrode, either a Pt or Ag one, at direct and reverse cathodic polarization, Ag⁺ ? Ag redox process occurs, and for it a substantial exchange current value was found. Hence it is concluded that the redox process at all the observed temperatures is very fast and that the rate increases with a rise in temperature. Also the passivation of the electrode at anodic polarization affecting the electrolyte decomposition voltage level considerably was pointed out. The electric double-layer capacity values found show that the structure of the layer changes with temperature and dc potential, which is manifested on the capacity.     

The electrochemical formation of Ag2O in KOH electrolyte

In very recent years there has been some conflicting information in the literature regarding the initial stages of the formation of a multilayer of Ag₂O by anodic oxidation of Ag in aqueous alkaline solutions. We have re-examined the formation of Ag₂O and the pre-Ag₂O oxidation stages by means of cyclic voltammetry, rotating disk and rotating ring disk electrode techniques to help resolve the controversy. Our results support the conclusion that the initial oxidation of Ag is a one electron dissolution step controlled by diffusion of a soluble species, Ag(OH)^?₂, away from the electrode surface. Additionally our electrochemical findings throw some light on the formation of surface phases on the electrode during the process of Ag₂O growth.     

Dissociative adsorption of thiourea at a polycrystalline silver electrode in alkaline media

In alkaline solutions, when the potential of the silver electrode is more than −450 mV vs sce, thiourea undergoes decomposition and a sparingly soluble silver sulfide film forms as a product of the accompanying anodic reaction. These processes are controlled by convective diffusion and depend upon the thiourea concentration. During the reverse cathodic scan, two cathodic stripping peaks are observed. The first one corresponds to the stripping of Ag₂S (bulk) and the second one to the stripping of an adsorbed layer of Ag₂S. Hydrosulfide ion (HS⁻) are produced by these two cathodic reactions.The formation of an adsorbed layer of Ag₂S has been confirmed in rotating ring-disk electrode experiments.     

Application de la microscopie electronique,des mesures d'impedance et des: Radiotraceurs a l'etude du comportement de l'interface Ag+/Ag monocristallin

The interface Ag/Ag⁺ has been studied using several techniques: radiotracers, current—voltage curves and impedance measurements over a wide frequency range. Surfaces of the specimen are observed after the electrodeposition or anodic dissolution by means of transmission and scanning electron microscopy. The silver (100), (110) and (111) surfaces were previously chemically polished. The silver—silver ion exchange has been shown to be dependent on anions adsorption. The behaviour of silver electrodes in AgNO₃ solutions is described by an appropriate model.     

Ag nanowires and its application as electrode materials in electrochemical capacitor

Silver nanowires were synthesized on a large scale by using anodic aluminum oxide (AAO) film as templates and serving ethylene glycol as reductant. Their morphological and structural characterizations were characterized with field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and selected area electron diffraction (SAED). The electrochemical properties of silver nanowires as electrode materials for electrochemical capacitors were investigated by cyclic voltammetry (CV) and galvanostatic charge/discharge technique in 6 M KOH aqueous electrolyte. The Ag₂O/Ag coaxial nanowires were formed by the incomplete electrochemical oxidation during the charge step. The maximum specific capacitance of 987 F g^?1 was obtained at a charge–discharge current density of 5 mA cm^?2.     

Comportement electrochimique et reactivite du systeme Ag(II)-Ag(I)-bipyridine dans le carbonate de propylene

The electrochemical behaviour of silver(I)- and silver(II)-bipyridyl complexes has been investigated using voltammetry, cyclic voltammetry and coulometry in propylene carbonate. In unbuffered non-acidic medium, when the stoichiometry of the solutions is C_L/C_Ag = 2, the Ag^I(bipy)⁺ concentration is low; the preponderant species is Ag^I(bipy)₂⁺. Under such conditions, the Ag^II(bipy)₂²⁺/Ag^I(bipy)₂⁺ couple hs a reversible behaviour and is one of the must powerful oxidants used in the solvent (E^′° = 0.790 V vsE_Fc^′° + _Fc). As a function of the acidity level of the solutions, one observes the oxidation of either Ag^I(bipy)⁺ or Ag⁺ in the presence of the protonated ligand to complexed Ag(II). The silver(II) species produced oxidizes water and the excess of ligand (C_L/C_Ag > 2).     

Properties of fused polysulphides—V. voltammetric studies on sulphur and polysulphides in fused KSCN and LiCl-KCl eutectic

The oxidation of alkali metal polysulphides in fused KSCN and LiCl/KCl eutectic has been studied by cyclic voltammetry and potential step techniques. Sulphur was found to be the final oxidation product. For solutions in KSCN at 210°C only one oxidation peak was observed, involving a common, one-electron electrochemical process for all the polysulphides in the composition range M₂S₃-M₂S₆. It is proposed that the common electroactive species leading to the formation of the sulphur deposit is S^?₂ or S^?₃. The anodic formation of sulphur was found to be diffusion controlled, and no inhibition due to the formation of a poorly-conducting liquid phase was apparent. The cathodic dissolution of sulphur was studied in both solvents. It resembles the anodic dissolution of a metal, giving rise to characteristics stripping peaks in the cyclic voltammograms.     

Effect of sulphuric acid and copper sulphate concentrations on the morphology of silver deposit in the cementation process

The silver ion cementation on copper was investigated in the presence or absence of oxygen in solutions containing 1.85 × 10⁻⁴ M Ag⁺ at 25 °C. The influence of sulphuric acid and copper sulphate concentration (0.005-0.5 M) on the silver cement morphology was studied in details and results were linked with the previously determined kinetics data of the process. The morphology of silver deposit was found to be independent of the presence of oxygen in the system as well as the sulphuric acid concentration. Contrary, the concentration of copper sulphate strongly influenced the morphology of silver deposit. At the beginning of the cementation process silver covers uniformly the copper surface. Afterwards, a growth of dendrites is initiated on preferential parts of the surface. The growing dendrite behaves as cathodic sites, with relatively huge surface area and promotes the creation of anodic sites in a close neighbourhood. Finally, the anodic site encloses the dendrite island and develops its area inward the copper material. Copper ions at low concentration modified slightly silver dendrites but the increase in concentration up to 0.5 M Cu²⁺ leads to completely disappearance of dendrites from the surface. The lack of dendrites on the surface is a result of the competitive process that consumes additional silver ions, occurring in the bulk of the solution. The morphology of silver deposit cemented in the deoxygenated solution containing 0.5 M H₂SO₄ + 0.5 M CuSO₄ depends strongly on the mechanism of the process.     

Reaction of Ozone with Ag(I)—Mechanistic Considerations

Ozone reacts slowly with Ag⁺ (circumneutral pH, k = (11 ± 3) × 10^?2 M^?1 s^?1). After some time, ozone decay kinetics may suddenly become faster with the concomitant formation of silver sol. As primary process, an O-transfer from O₃ to Ag(I) is suggested, whereby Ag(III) is formed [Ag⁺ + O₃ + 2 H₂O → Ag(OH)₃ + O₂ + H⁺]. This conproportionates with Ag(I), which is in large excess, leading to Ag(II) [Ag⁺ + Ag(OH)₃ ? 2 Ag(OH)⁺ + HO^?]. Further, Ag(II) reacts with ozone in a high exergonic reaction [Ag(OH)⁺ + O₃ → Ag + 2 O₂ + H⁺], where ozone acts as a reducing agent. Thereby, a single silver atom, Ag, is formed that can be oxidized by O₂ and O₃ or can aggregate to a silver sol. Aggregation slows down the rate of oxidation. When Ag⁺ is complexed by acetate ions, ozone decay and silver sol formation are speeded up by enhancing Ag(II) formation [Ag(I)acetate + O₃ → Ag(III)acetate → Ag(II) + CO₂ + ?CH₃]. In the presence of oxalate, the formed complex reacts faster with ozone than Ag⁺, and Ag(III)oxalate decarboxylates rapidly [Ag(I)oxalate + O₃ → Ag(III)oxalate → Ag⁺ + 2 CO₂]. This enhances ozone decay but prevents silver sol formation. Quantum chemical calculations have been carried out for substantiating mechanistic suggestions.     

Influence of anode material on the electrochemical oxidation of 2-naphthol: Part 1. Cyclic voltammetry and potential step experiments

The anodic oxidation of 2-naphthol has been studied by cyclic voltammetry and chronoamperometry, using a range of electrode materials such as Ti-Ru-Sn ternary oxide, lead dioxide and boron-doped diamond (BDD) anodes. The results show that polymeric films, which cause electrode fouling, are formed during oxidation in the potential region of supporting electrolyte stability. IR spectroscopy verified the formation of this organic film. While the Ti-Ru-Sn ternary oxide surface cannot be reactivated, PbO₂ and BDD can be restored to their initial activity by simple anodic treatment in the potential region of electrolyte decomposition. In fact, during the polarization in this region, complex oxidation reactions leading to the complete incineration of polymeric materials can take place on these electrodes due to electrogenerated hydroxyl radicals. Moreover, it was found that BDD deactivation was less pronounced and its reactivation was faster than that of the other electrodes.     

Atomic emission spectroelectrochemistry applied to dealloying phenomena: I. The formation and dissolution of residual copper films on stainless steel

The rates of elemental dissolution were measured for a 304 stainless steel containing 0.19% Cu in real time during linear sweep voltammetry experiments using the atomic emission spectroelectrochemistry (AESEC) method. The results demonstrate that Fe, Cr, Ni, Mn, and Mo dissolve simultaneously leaving a residual copper film that inhibits the steel dissolution. The formation and dissolution of the copper film leads to the appearance of two peaks in the anodic dissolution transient, one due to inhibition of steel by residual copper and the other due to the formation of the passive film. The addition of NaCl to the electrolyte results in a marked increase in the intensity of the second dissolution peak, while hardly affecting the first peak. This is interpreted in terms of the lowering of copper dissolution potential by chloride ions due to the stabilization of CuCl₂⁻. A simple phenomenological kinetic model is used to simulate the variation of dissolution rate with potential.     

Anodic oxide on silver in alkaline solution

Ag₂O and AgO formed by potentiostatic oxidation of silver in 0·1N KOH have been chemically analysed for the mean valency of silver ion. The anodic oxide formation proceeds in two stages and the silver ion in the oxide changes from Ag⁺ to Ag²⁺ at a transition potential. The reduction of AgO at constant cathodic current occurs via two potential arrests. The transition from the first to the second potential arrest, however, does not correspond to the complete reduction of AgO to Ag₂O.     

Electrochimie du polynitrure de soufre dans les solvants non aqueux—I. Comportement dans l'acetonitrile

The electrochemical behaviour of polysulphur nitride was studied by cyclic voltametry in alkaline and silver salt acetonitrile solutions. In an alkaline salt solution during the cathodic polarization, superficial degradation of (SN)_x probably gives S₇N^? ions. In a silver perchlorate solution silver deposition was observed. It can be dissolved anodically. During the first anodic polarization, oxidation pic occurs. The quantity of electricity involved during this reaction is a function of the association constant of the electrolyte.     

Pitting corrosion of lead in sodium carbonate solutions containing NO3 ions

Pitting corrosion of Pb in Na₂CO₃ solutions (pH=10.8) containing NaNO₃ as a pitting corrosion agent has been studied using potentiodynamic anodic polarization, cyclic voltammetry and chronoamperometry techniques, complemented with scanning electron microscopy (SEM) examinations of the electrode surface. In the absence of NO₃⁻, the anodic voltammetric response exhibits three anodic peaks prior to oxygen evolution. The first anodic peak A₁ corresponds to the formation of PbCO₃ layer and soluble Pb²⁺ species in solution. The second anodic peak A₂ is due to the formation of PbO beneath the carbonate layer. Peak A₂ is followed by a wide passive region which extends up to the appearance of the third anodic peak A₃. The later is related to the formation of PbO₂. Addition of NO₃⁻ to the carbonate solution stimulates the anodic dissolution through peaks A₁ and A₂ and breaks down the dual passive layer prior to peak A₃. The breakdown potential decreases with an increase in nitrate concentration, temperature and electrode rotation rate, but increases with an increase in carbonate concentration and potential scan rate. Successive cycling leads to a progressive increase in breakdown potential. The current/time transients show that the incubation time for passivity breakdown decreases with increasing the applied anodic potential, nitrate concentration and temperature.