I.M. Ritchie

A cyclic voltammetric study of the aqueous electrochemistry of some quinones

Abstract The aqueous electrochemistry of a number of quinones and two hydroxyquinones was studied using cyclic voltammetry. The quinones were chosen to represent a variety of structural types, and several were sulphonated in order to make them sufficiently water soluble. From measurements of the half-wave potential as a function of pH, potentialpH diagrams were constructed for each of the compounds. In some cases, the data were supplemented by pKa values which were determined spectrophotometrically. In the particular case of 1,4-benzoquinone, it was shown that the results obtained for the potentialpH diagram agreed very closely with equilibrium measurements which had been reported in the literature for the same system. The general shape of the potentialpH diagrams for the other quinones, but not hydroxyquinones, was similar to that of 1,4-benzoquione. For each compound, values of the potentialpH slopes and pKa s are reported. Half-wave potential measurements were not only made in buffered solutions, but also unbuffered solutions. Differences between the two type of measurements are reported. Most of the quinones decomposed at high pH. The course of the decomposition is discussed.

Mixed potential measurements in the elucidation of corrosion mechanisms— 1. Introductory theory

Abstract Corrosion reactions are classified according to the nature of the rate-determining step and its position in the reaction mechanism. The theoretical basis of a method, based on potential measurements, of experimentally determining the position of a given reaction within this classification is presented. Information useful to the practice of corrosion control is obtained in this way.

Study of gold leaching in oxygenated solutions containing cyanide-copper-ammonia using a rotating quartz crystal microbalance

Gold and copper often occur together in ores, an occurence which creates processing difficulties since cyanide solutions are not selective for gold. Although it has been shown that ammoniacal cyanide solutions can leach gold selectively, little practical use has been made of this finding since the mechanism is not yet understood and strategies for optimisation have not been fully developed. This paper describes a kinetic study of one part of this system, namely the dissolution of gold in oxygenated solutions containing mixtures of cyanide-ammonia-copper. The rates were measured using a rotating quartz crystal microbalance. The results confirm that the dissolution of gold in alkaline cyanide solutions proceeds at a lower rate than expected as a result of a blocking film on the gold surface. The addition of copper(I) to the cyanide solutions retards the leaching rate by an amount which increases as the copper content increases. However, this is not just because there is less free cyanide present. A speciation calculation shows that the copper is in some way involved in the dissolution reaction. In contrast, the addition of ammonia to the cyanide leaching solution causes the rate to progressively decrease, ultimately reaching an ammonia concentration above which there is no further decrease in leaching rate. Electrochemical measurements showed that the ammonia has greatly suppressed the anodic half-reaction. The final gold leach system to be investigated was one containing oxygen, cyanide, copper(I) and ammonia, the latter two components being maintained at a fixed ratio of 1:4 while varying the copper to cyanide ratio from approximately 1:6 to l:2. However, for a total cyanide concentration of 0.032 M and ammonia concentration of about 0.04 M, the rate starts to increase. It is presumably in this region that selective leaching of gold from copper occurs.

The reaction between reduced ilmenite and oxygen in ammonium chloride solutions

Abstract A mechanistic study of the aeration reaction used for removing iron from a reduced ilmenite matrix is described. Using polarization and mixed potential measurements, it was shown that the rate of the aeration reaction in ammonium chloride solution is largely determined by the speed at which oxygen diffuses to the reduced ilmenite surface, and that, within the limit of experimental error, the reaction rate was independent of the Ti2O3 content in the reduced ilmenite. Diffusion control was confirmed by rate measurements. Aeration rate measurements were also carried out in other electrolytes in addition to ammonium chloride. It was concluded from these and other studies that the ammonium chloride plays three distinct and important roles in the aeration reaction. Firstly, the ammonium ion acts as a buffer for hydroxyl ions and prevents excessively high local pH values, which might otherwise cause precipitation of iron (II) hydroxide before the iron (II) ions could diffuse from the rutile matrix. Secondly, the ammonia formed as a result of this buffering reaction complexes iron (II) ions until they also have moved away from the regions of high pH, thus preventing the precipitation of iron (II) hydroxide in the pores of the rutile. Finally, the chloride ion helps to break down any passive films which might form during aeration.

The Leaching and Electrochemistry of Gold in High Purity Cyanide Solutions

For the last 100 years, cyanidation has been the most important process in the extraction of gold from its ores. However, despite the large number of papers published on the reaction between gold and cyanide, there is little agreement about the kinetics and mechanism of this important reaction. In the present paper, the leaching reaction was studied as a function of solution purity using a rotating electrochemical quartz crystal microbalance. Surprisingly, the experimental results show that pure gold does not leach in highly pure aerated cyanide solutions. On the other hand, gold leaches at a measurable rate in solutions prepared using AR grade reagents. Therefore, it is not surprising that many of the past investigations have resulted in connecting data. A model based on the formation and dissolution of a chain-like film of AuCN has been proposed to account for these results. It is believed that the dissolution of AuCN only occurs at the chain ends, and the presence of impurities in the cyanide solution accelerates this process by depositing on the gold surface and increasing the number of chain ends.

Electrochemical Oxidation of Gold and Thiourea in Acidic Thiourea Solutions

The dissolution of gold in acidic thiourea solutions, together with the oxidation of thioureu on gold, platinum, and glassy carbon (GC) electrodes, has been investigated using voltammetric methods. The cyclic voltammograms on platinum and GC electrodes showed a single pair of peaks for the oxidation of thiourea to formamidine disulfide and its subsequent reduction. The mechanism of the reaction is discussed. When a gold electrode was used, gold dissolution took place at a more negative potential than thiourea oxidation, with the latter reaction being shifted to a potential about 300 mV more positive than that on the platinum electrode. It was found that, at 25°C and without formamidine disulfide, gold did not dissolve when the potential was scanned rapidly. This result shows that the process of gold dissolution might include a slow surface step which can be accelerated by formamidine disulfide. It is possible that the slow surface step results from the strong adsorption of thiourea on the gold surface. The adsorption of thiourea on gold was demonstrated using the electrochemical quartz crystal microbalance.

The metal–solution interface

This article reviews our present understanding of the metal-solution interface, drawing comparisons between it and the metal-gas interface. The chief difference between these two systems is the solvent, which has a profound effect on the physics and chemistry of the interface. The solvent is never entirely inert, and if polar, as water is, will adsorb on the metal surface and stabilize ions in solution. However, the presence of ions in solution allows a current to be passed between two electrodes inserted in the solution, and one of these electrodes can be the metal under investigation. This is the field of electrochemistry, and much of our knowledge of the metal-solution interface is derived from electrochemical measurements. Complementary information is gained from many of the techniques used to study the metal-gas interface. The first type of electrochemical system to be considered is one in which the application of a potential across the interface does not lead to charge transfer. Such a system behaves as a capacitance due to the formation of a complex double layer. The effects of various types of adsorption is then described. The deposition of metals on the electrode surface is next considered, and this is followed by a discussion of the reverse metal dissolution reaction. The final section concerns the formation of films on the metal surface, a reaction which has its parallel in metal-gas reactions.

Mixed potential measurements in the elucidation of corrosion mechanisms—II. Some measurements

Abstract The behaviour of the mixed potential as a function of oxidant concentration and agitation is examined for a variety of corrosion reactions. Using the theory developed in a previous paper, this information is used to elucidate the mechanism of each reaction. The dissolution of mercury, copper and silver in the presence of ferric ions was studied. For the Fe(III)-CuSO 4 2− system, it was shown that the anodic dissolution half-reaction was activation controlled and the cathodic half-reaction was diffusion controlled. Changing the anion to Cl − made both half-reactions diffusion controlled. For the Fe(III)AgNO 3 − system, the anodic half-reaction is diffusion controlled the ferric reduction being activation controlled. In the attack of ferric ions on mercury in nitrate solution, both half-reactions are activation controlled. The metal oxidation reaction in which silver is oxidized to silver iodide by the triiodide ion was considered. It was shown that the ir drop in the AgI layers had a negligible effect on the corrosion potential. An examination of the behaviour of the corrosion potential in the Cu(II)Zn metal displacement reaction enabled estimates to be made of the anodic area at various times during the reaction. It was shown that in the first stage of the reaction the anodic area is fairly constant and equal to the geometric area of the zinc. As the second stage is entered, the anodic area suddenly drops to about 3 per cent of the geometric area, at which level it remains constant.

The kinetics of dissolution of slaked lime

The rates of dissolution of Ca(OH)2 in the form of rotating discs have been studied in water and in aqueous calcium nitrate and sodium hydroxide solutions, and have been found to be consistent with the rates of combined slaking and dissolution of CaO discs. A similar consistency has been found between rates of dissolution of powdered Ca(OH)2 and of powdered CaO in water. Our results support a mechanism for CaO slaking in which under moderate agitation the overall rate is controlled by diffusion of Ca2+ and OH− ions from the reacting surface, and under more vigorous agitation the dissolution of Ca(OH)2 at the surface controls the rate.

The adsorption of gold thiourea complex onto activated carbon

The equilibrium and kinetics for the adsorption of gold onto activated carbon from acidic thiourea solutions have been studied. The equilibrium gold loading decreases with increasing thiourea concentration, pH and temperature. The gold loading is not affected by small amounts of Fe2+ or Fe 3+, but is greatly lowered by Cu2+ presumably through competitive adsorption. The rate of gold adsorption at the initial stage, in terms of the decrease of gold concentration in solution, can well be described with first-order kinetics. The rate increases with increased initial gold concentration, agitation and temperature, but is not greatly affected by thiourea concentration or the presence of Fe2+. Ag+ and Cu2+ ions significantly reduce the rate since they strongly compete for the surface sites. A high concentration of Fe3+ (5 g/L) also substantially reduces the rate, probably due to a change in solution chemistry. Gold is normally adsorbed on activated carbon as the gold thiourea complex. However, partial decomposition of the complex to metallic gold has been observed under certain conditions. The surface of the activated carbon loaded with gold has been studied using SEM, EDS and XPS techniques.