Steady-State Voltammetry of Hydroxide Ion Oxidation in Aqueous Solutions Containing Ammonia

Abstract

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.

Oxidation of hydroxide ions from dilute unbuffered solutions of ammonia at platinum microdiscs surrounded by thick and thin insulating sheaths

“…Ru(NH 3 ) 6 Cl 3 in aqueous solutions was used to characterise the Pt microdiscs employed here. Carbonate-free 0.1 M ammonia solutions were prepared and stored in polythene bottles and maintained under a nitrogen atmosphere [4]. The absence of carbonate in the solutions was tested daily as described elsewhere [2].…”

“…In recent papers [1][2][3][4] we have reported on the anodic behaviour of solid microdisc electrodes in dilute aqueous solutions of either strong (NaOH, Ba(OH) 2 ) or weak (NH 3 ) bases. Gold, platinum and nickel microdiscs have been investigated in solutions containing strong bases [1][2][3].…”

“…With the aim of ascertaining the occurrence of reaction (1) also in aqueous solutions containing weak bases, gold microdiscs have been investigated in dilute solutions containing the environmentally and biologically important compound, ammonia [4]. Also in this case a sigmoidal wave due to the oxidation of hydroxide ions has been recorded.…”

“…However, because of the chemical nature of the base, reaction (1) was coupled with the preceding homogeneous chemical reaction (CE mechanism) in Eq. (2) [4]:…”

Electrochemical characterization of carbon paste electrodes modified with MgZnGa and ZnGaAl hydrotalcite-like compounds

“…It is worth mentioning that the hydroxide oxidation process has been commonly reported on noble metal electrodes as gold electrodes [30][31][32], however in our knowledge there are surprisingly few reports for hydroxide oxidation on HT [33,34].…”