Potentiometric response characteristics of oxide-coated gallium electrodes in aqueous solutions

Abstract

This paper reports on the potentiometric response characteristics of gallium electrodes coated with native or anodic oxide layers in aqueous solutions. Solid metallic gallium electrodes were prepared by cooling liquid gallium monoliths supported either on planar silicon substrates or incorporated within glass capillaries. These metallic gallium electrodes showed potentiometric pH responses with a slope of ca. -38 mV/pH in the pH range between 3 and 9. The potentials measured could originate from redox processes involving surface oxide that was formed as a result of reactions between the metallic gallium and water (and/or OH-), as observed using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). Similar potentiometric measurements were examined for planar and capillary-incorporated gallium electrodes that were anodized in ice-cooled aqueous H2SO4 at 10 V. Although the resulting nanoporous anodic oxide-coated electrodes did not initially show potentiometric pH responses, they exhibited pH responses with a smaller response slope (ca. -30 mV/pH; pH 3∼9) after potentiometric measurements in a solution of pH ≤ 3. This observation could be explained by the acid-induced dissolution of an oxide-based barrier layer that passivated the underlying gallium electrode. This explanation was compatible with the observation of an additional gallium oxide layer over the nanoporous anodic oxide after the potentiometric pH measurements. Interestingly, in contrast to the metallic gallium electrodes, the anodic oxide-coated electrodes exhibited potentiometric Nernstian responses to SO 42- at ≤ 10-4 M, reflecting the presence of sulfate doped into the oxide during the anodization. These results provide fundamental knowledge required to design miniaturized electroanalytical probes based on gallium electrodes. © 2014 Elsevier Ltd.