The modern understanding of the glass electrode response

Abstract

Electrochemical experiments in combination with infrared spectroscopy and subsurface concentration profiling yielded information on ionic processes at the membrane surface of glass electrodes and resulted in a detailed mechanism of the glass electrode response. This “dissociation mechanism” is based on phase boundary equilibria between anionic groups at the glass surface and hydrogen or alkali ions in the solution, which control the potential of the glass and the concentrations of ions attached to the surface groups. The resulting concentration gradients between glass surface and bulk glass cause an interdiffusion of the different ions, if they are not hindered sterically to penetrate into the glass, and a diffusion potential beneath the glass surface. The interdiffusion determines the course of the glass corrosion, which thus depends on whether protons or alkali ions are attached to the surface and interdiffuse with the alkali ions of the glass, leading, respectively, to a steady-state leached layer or a continuously growing alkali-exchanged layer. pH electrodes with La₂O₃-containing membrane glasses show delayed interfering potential drifts in neutral and alkali phosphate and fluoride solutions caused by LaPO₄ and LaF₃ deposits, respectively, at the membrane surface (“phosphate” and “fluoride errors”). Equations for the phase boundary potential and phase boundary concentrations are derived on a thermodynamic and kinetic basis and are verified experimentally. Glass leaching and alkali ion interdiffusion are studied, and the diffusion potential within a leached layer is determined. The formation periods of pH and of pM glass electrodes are explained, and an indirect part of the sodium error and an irreversible sodium error are detected. Leached and alkali-exchanged layers consist of modified glass and are not “gel” layers as generally assumed. Large exchange current densities of the phase boundary equilibria explain the stability of glass electrode potentials. Migration experiments with subsequent concentration profiling showed that glasses can be alkali ion, proton, and mixed alkali ion-proton conductors and yielded concentration-dependent mobilities of lithium ions and replacing protons in a lithium silicate glass. The unchanged pH response of membranes before and after field-driven protonation, in alkali-free solutions, excludes the exchange of different ions as the origin of the glass electrode response, as assumed by the ion exchange theory, and verifies the dissociation mechanism.