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

Dilute aqueous solutions of ammonia were investigated by cyclic voltammetry at low scan rates at Pt microdiscs surrounded either by a rather thick (IS) or thin (FS) insulating sheath. The latter type of electrode was prepared by an electrophoretic deposition of paint. Over the ammonia concentration range 0.01–1 mM, a wave was observed at both types of microelectrodes near the positive end of the potential window available on Pt. At the FS electrode, the wave appeared much better defined, higher and less affected by hysteresis than that recorded at the IS electrode. These differences were accounted for by the larger diffusional flux and faster achievement of steady-state conditions that characterise the FS electrode. The observed waves were attributed to the oxidation process of hydroxide ions that arise from the dissociation of ammonia. The overall electrode process was analysed in terms of a CE mechanism (homogeneous chemical reaction preceding the heterogeneous electron transfer), with a fast chemical reaction, and the experimental currents fit well those predicted by digital simulation. Because of the latter mechanism and the difference between the diffusion coefficient values of OH− and NH3, the steady state limiting current against the analytical concentration of ammonia is not theoretically linear. However, over the relatively narrow concentration range investigated here, an apparent linearity useful for analytical purposes, could be observed.