Surface-layer properties of glass electrodes responsive to sodium and hydrogen ions

Cation-sensitive glasses exhibit an alkali-metal ion concentration profile and an electrical conductivity profile which increase smoothly with the distance in towards the bulk glass, as a result of interaction with aqueous solutions. In a pH-sensitive glass, however, a stepwise alkali-metal ion-distance profile is obtained and a low electrical-conductivity region is restricted to a thin film near the gel-layer/bulk-glass transition region. The replacement of silicon by aluminium causes the degree of H(+)-M(+) ion-exchange to decrease, and consequently favours alkali-metal ion selectivity of the glass. Lower conductivity is attributed to a low proton-interdiffusion coefficient. The conductivity is lowest where the SiOH concentration is high, i.e., where the water concentration and the SiO(-) concentrations are low. High conductivity of the gel-layer of pH-sensitive glasses is caused by the presence of water which depolymerizes the glass structure. The network of alkali-sensitive glasses is less affected by water owing to the aluminium decreasing the concentration of terminal oxygen atoms in the structure. A low-conductivity surface layer on the latter glasses contributes to the sluggishness in the response of the electrodes.