The steady-state voltammetric behavior for the oxidation of aqueous solutions containing the strong bases sodium and barium hydroxide was studied with gold microelectrodes in the absence and in the presence of different concentrations of supporting electrolyte. A well-defined oxidation wave attributed to the oxidation of hydroxide ions to oxygen was observed in all the solutions investigated, regardless of both the nature of the base and the supporting electrolyte employed. However, in solutions with excess electrolyte, the steady-state limiting current was found to depend on the actual concentration of the supporting electrolyte, as the diffusion coefficient of the electroactive species varies with both the ionic strength and viscosity of the medium. Since the hydroxide ion is a negatively charged species, solutions with low or without supporting electrolyte yielded currents enhanced by migration contributions. Theoretical equations for the dependence of steady-state limiting currents with ionic strength were derived; theoretical and experimental data compared satisfactorily. The usefulness of the oxidation wave of hydroxide ions for analytical applications was shown by examining the dependence of steady-state limiting currents on concentration and pH. At a given ionic strength, the steady-state limiting current is proportional to the concentration of hydroxide ion over the range 0.5-5 mM. Moreover, the pH of the basic solutions can be determined by amperometry in place of potentiometry, with the use of an absolute equation.In aqueous solutions, the cathodic and anodic potential limits at noble metals such as platinum and gold are defined by the hydrogen and oxygen evolution processes. 1 These, according to several reviews, 1-5 occur through quite complex reaction pathways. It is also well established that dilute aqueous solutions of acids show a wave for hydronium ion reduction which is separated from the background discharge to an extent that depends, for a given electrode material, on the nature of the acid. 6-16 By analogy, an oxidation wave would be expected from dilute solutions of bases, where free hydroxide ions are available to a considerable extent. Although there are many papers dealing with the anodic voltammetric behavior of noble metals in aqueous solutions of strong bases, we have not found a single article reporting a clear oxidation wave due to hydroxide ion and separated from the oxygen evolution due to the background. This, in part, can be because the anodization of noble metals involves oxide film formation. 1 Recently, 17 during explorative measurements aimed at characterizing, under steady-state conditions, the anodic behavior of gold microelectrodes in aqueous solutions made basic with NaOH, a well-defined wave was observed at very positive potentials (∼1.3 V versus a saturated calomel electrode), just before the background discharge. In excess sodium sulfate or sodium perchlorate,

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An oxidation process observed in dilute aqueous solutions of ammonia was investigated under steady-state conditions with gold microelectrodes with radii in the range 2.5-30 microm. Over the ammonia concentration range 0.1-10 mM, a well-defined voltammetric wave was observed at approximately 1.4 V versus Ag/AgCl. It was attributed to the oxidation of hydroxide ions that arise from the dissociation of the weak base. The steady-state limiting current was found to depend on the concentration of supporting electrolyte, and in solution with low electrolyte, it was enhanced by migration contribution, as expected for a negatively charged species that oxidizes on a positively charged electrode. In addition, the steady-state limiting current was proportional to both the ammonia concentration and the electrode radius. The overall electrode process was analyzed in terms of a CE mechanism (homogeneous chemical reaction preceding the heterogeneous electron transfer) with a fast chemical reaction when measurements were carried out in solutions containing NH3 at < or = 5 mM and with electrodes having a radius of > or = 5 microm. This was ascertained by comparing experimental and theoretical data obtained by simulation. The formation of the soluble complex species Au(NH3)2+ was also considered as a possible alternative to explain the presence of the oxidation wave. This process however was ruled out, as the experimental data did not fit theoretical predictions in any of the conditions employed in the investigation. Instead, the direct oxidation of NH3, probably to N2O, was invoked to explain the anomalous currents found when the CE process was strongly kinetically hindered. Throughout this study, a parallel was made between the CE mechanism investigated here and that known to occur during the hydrogen evolution reaction from weak acids.

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This review surveys the main aspects of the combination of microelectrodes and stripping analysis, including recent developments in working microelectrodes, analytical methodologies, instrumentation, and applications in both synthetic and real matrices. Emphasis is given to papers published since 2000.

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