THERMODYNAMIC ORIGIN OF THE SUB-NERNSTIAN RESPONSE OF GLASS ELECTRODES

A thermodynamic explanation of the sub-Nernstian response of pH glass electrodes is presented on the basis of the dissociation mechanism, which attributes the functioning of glass electrodes to the phase boundary equilibrium of functional groups at the glass surface and ions in the solution. It is found that the activity change of the functional surface groups caused by pH variations, which basically reduces the electrode slope, cannot be neglected and results in a measurable electromotive efficiency below 1. All membrane glasses, independent of composition and response, have a comparable electromotive efficiency, which is confirmed by the equal magnitude of measured and reported data. This adds a thermodynamic meaning to this quantity and yields detailed quantitative information on the phase boundary equilibrium, i.e., the pH dependence of the activity of negative surface groups (which;actually determine the membrane potential) and the dependence of the membrane potential on the activity of the negative groups (''internal slope''), which amounts to several tens of volts per decade. The sub-Nernstian response is characteristic of glass electrodes and is not expected for redox, metal-metal ion, and crystal membrane electrodes.