Temperature compensation in potentiometry: isopotentials of pH glass electrodes and reference electrodes. Part I. Theory

Abstract

The reasons for the non-linearity of the temperature response of glass pH electrodes are explored theoretically and the relationship between pH_iso, the isopotential pH (at which the e.m.f. is invariant with temperature), and the different components of the potentiometric cell is demonstrated. In particular, the temperature coefficient of pH of the glass electrode's filling solution is shown to be paramount in designing electrodes with characteristics convenient for instrumental temperature compensation. The pH of most filling solutions varies parabolically with temperature and the linear correction applied by the temperature compensation circuits of pH meters will have a limited useful range. Filling solutions with linear temperature characteristics are proposed, but the resultant electrodes would be incompatible with the many pH meters having a fixed isopotential setting of 7. Liquid junction potentials are shown to make a small and essentially linear contribution to the temperature dependence of the cell.