Glass electrodes: Why and how they function

Subsurface concentration profiling by Ion Bombardment for Spectrochemical Analysis (IBSCA) and Nuclear Reaction Analysis (NRA) combined with other methods yielded information on the ionic processes at and in glass electrode membranes and results in the mechanism (''dissociation mechanism'') of glass electrodes. The primary step is the formation of a phase boundary equilibrium between functional groups, e.g. siloxy, at the glass surface and hydronium or alkali ions in the solution. II determines the potential of the glass due to the charge density established al the glass surface by the dissociated form of the surface groups, e.g. =SiO-, and the surface activities of the acidic or salt forms of the surface groups, e.g. =SiOH and =SiOM, respectively. This inseparable connection of solution-dependent potential and ''surface coverage'' was verified by electrolytic migration experiments. The concentration gradients thus caused below the glass surface lead to an ionic interdiffusion in the subsurface glass range (if the ions attached to the surface groups are not hindered sterically to enter the glass), which is the first step of glass corrosion, and to a diffusion voltage, which is part of the overall membrane potential. Quantitative expressions are given, and it is shown that, in principle, the response of glass electrodes is below ''ideal'', i.e. smaller than R T (In 10)/F; Which explains the well known sub-Nernstian response of pH glass electrodes. Two new sodium errors were found. Construction and operation of glass electrodes are briefly sketched as an introductory basis. The reason for the long existence of the purely thermodynamic ''ion exchange theory'' is analysed.