Chemical and electrical properties of the surface layers of some glass electrodes

Some sodium— and lithium—silicate pH responsive glasses exhibited a stepwise alkali ion profile with distance inwards towards the glass as a result of attack by aqueous solutions, ie, the average alkali ion concentration of the gel layer was low but increased rapidly within a transition region between the gel layer and the glass bulk. The electrical resistivity of the gel layer was a factor at least five times lower than that of the original glass. An extremely high electrical resistance in the transition region was found to be proportional to the chemical durability of the glass independent of hydration time. The smaller size of the lithium ion which favours stronger binding, compared with the sodium ion, as well as the action of the calcium ion in the sodium silicate network to block the sodium ion/hydrogen ion exchange were among the factors that increased the chemical durability by orders of magnitude. Equilibrium conditions were not reached for the deeper parts of the gel layers. The chemical durability of the glass increased in the same order as found for the ideal function of the glass electrode. The size of the electrical resistance of the transition region was thus useful for deducing glass compositions suitable for electrode purposes.