E. Arce

A comparative study of electrochemical behavior of chalcopyrite, chalcocite and bornite in sulfuric acid solution

Abstract A comparative study of the electrochemical behavior of different copper sulfide minerals (chalcopyrite, bornite and chalcocite) in sulfuric acid solutions was performed. Carbon paste electrodes (CPE) containing mineral particles with nonconducting binder were used. The comparative study shows that the chalcopyrite oxidation process does not produce covellite as claimed by some authors. Chalcopyrite oxidation has been found to produce a nonstoichiometric sulfide, Cu n −1 Fe n −1 S 2 n . On the other hand, it is apparent that bornite is not an intermediary species formed in the chalcopyrite reduction; the reduction of chalcopyrite directly produces chalcocite.

Electrochemical nucleation of cobalt onto glassy carbon electrode from ammonium chloride solutions

Abstract In the usual aqueous electrolytic baths, cobalt electrodeposition is accompanied by the hydrogen evolution reaction (HER), and therefore, in such circumstances, a quantitative evaluation of the kinetic parameters of the cobalt electrocrystallization process, is almost impossible. Here, we show a possibility to separate these two processes. The existence of a potential range where the cobalt deposition on glassy carbon electrode (GCE) occurs independently of HER, from an aqueous solution of 10 −2 M CoCl 2 , in 1 M NH 4 C1 ( pH = 4.66), was found, and the nucleation kinetic parameters, using the potentiostatic method and existing theoretical formalism, were determined. It was concluded that the mechanism of the cobalt deposition in 1 M NH 4 C1 occurs by multiple, progressive three-dimensional nucleation with growth controlled by mass transfer. Regarding the potential dependence of the steady-state nucleation rate, and atomistic theory of the electrolytic phase formation, it was shown that an active site plays the major role in critical nucleus formation over the entire potential range.

Influence of the coordination sphere on the mechanism of cobalt nucleation onto glassy carbon

Abstract Electrolytic phase formation of cobalt onto a glassy carbon electrode (GCE) was investigated using linear sweep voltammetry and the potential step technique in aqueous 10 −2 M CoCl 2 +1 M NH 4 Cl at pH 4.6 and 9.5. Thermodynamic, voltammetric and spectrophotometric analysis of the solutions showed that the predominant chemical species of Co(II) in solution were the Co(H 2 O) 6 2+ ion at pH 4.6 and the Co(NH 3 ) 5 2+ complex at pH 9.5. Voltammetric analysis showed that the experimental equilibrium potential of the Co(NH 3 ) 5 2+ /Co(0) system was more negative than the Co(H 2 O) 6 2+ /Co(0) couple. However the electrocrystallization overpotential for cobalt deposition on GCE from the aquo complex was higher than for the amine complex. Analysis of the current–time transients obtained at each pH, indicated that distinct mechanisms of nucleation are involved during the early stages of cobalt deposition. In the case of Co(H 2 O) 6 2+ reduction, the transients were described theoretically in terms of 3D nucleation with diffusion controlled growth. For cobalt deposition from the Co(NH 3 ) 5 2+ species, the transients were explained by a combination of three different kinds of parallel nucleation processes, 2D progressive nucleation, 2D instantaneous nucleation and 3D progressive nucleation, each of which was limited by lattice incorporation of cobalt ad-atoms.

The use of carbon paste electrodes with non-conducting binder for the study of minerals: Chalcopyrite

A pulverized chalcopyrite mixture with graphite and silicon was subjected to electrochemical studies and found to have similar behaviour to that of chalcopyrite massive electrodes, with the advantage of excellent reproducibility, since the problems of polishing deficiency, fracture and lack of homogeneity that solid electrodes present, are avoided. From voltammetric studies of these carbon paste-chalcopyrite electrodes in H2S04, HN03 and HCl media, it has been established that the reduction process is limited by the diffusion of protons to the interface. This study also shows that the increase of chloride ion concentration energetically favours the chalcopyrite reduction process, allowing for the separation of the processes of formation of chalcocite and bomite. On the other hand, this study also shows a reversible oxidation of the bomite and chalcocite obtained in the previous chalcopyrite reduction.

Electrochemical Study of Passive Layer Formation on Lead-Base Alloys Immersed in 5.31 M H 2 SO 4 Solution

The process for the anodic formation of PbSO 4 films was studied over lead and two tin-containing lead-alloys electrodes with high- and low-tin contents, in 5.31 M sulfuric acid aqueous solution, using the potential step technique. It was found that the mechanism governing the formation of the lead sulfate anodic film was common for the overall electrodic process and that it takes place comprising three different contributions to the total current [J(t)]. The individual contributions defined were J g the current density contribution due to 2D nucleation and growth of an insoluble PhSO 4 conducting adlayer, J d the current density involved during electrodissolution of the lead electrode in the zones which were not covered yet by the lead sulfite film, and J f was the current due to the growth of the passive layer induced by electrodissolution of lead metal and transport of the resulting Pb(II) ions through the PbSO 4 film. However, each stage contributed to the overall process in a different manner depending upon the Pb:Sn ratio. The results from voltammetry indicated that it is possible to establish the potential at which the lead sulfate film begins to form and that this was also a function of the ratio. The oxidation potential resulted most positive for the low-tin-containing alloy electrode. For all cases, the applied potential needed to form the passive layer influenced the relative importance of each contribution to the overall process.

Interpretation of rate profiles of the pH-dependent trypsin- and α-chymotrypsin-catalyzed hydrolysis of esters with a free α-amino group

Abstract An hypothesis is postulated to interpret the anomalous pH dependence of the rate of hydrolysis of esters with a free α-amino group catalyzed by trypsin (EC 3.4.4.4) and α-chymotrypsin (EC 3.4.4.5). This involves the supposition of a hybrid system composed of E·Sα NH 3 + (I) + E·Sα NH 2 (II) , each of these having distinct constants for the three steps of the reaction. The optimum pH of System I, about 6–6.5, maybe accounted for in terms of an effect upon the enzyme of the α-ammonium group of the substrate species prevailing at low pH. It is possible that when near the enzyme this cationic group perturbs ionization equilibria of groups within the “charge relay system” of the active center. This effect appears as an acid shift of the rate profiles and apparently reinforces the catalytic activity: the rates observed in the acid region are faster in spite of high K m values. Consequently the fall of rate around pH 7 observed in reactions of α-aminoacyl esters with trypsin and α-chymotrypsin, seems to be due to the conversion of substrate from the more reactive α-ammonium species to the less reactive α-amino species. The rate dependence for System II is apparently like that observed with “normal” substrates without a free α-amino group.