ACTIVITY OF PHOSPHATE IONS MEASURED WITH PHOSPHATE ELECTRODES OF SECOND KIND

Phosphate electrodes of the second kind were employed to measure the activity of phosphate ions in aqueous solutions of orthophosphoric acid. The equation relating the emf with the activity of the various phosphate ions, which has been previously assumed to apply only under limited experimental conditions, is developed rigorously, and shown to be of general validity. The calculated results obtained by the use of the approximate equations developed agree well with the experimental values in the range of concentrations of phosphoric acid of 10-2 to 10-1 M. In the range of concentrations of phosphoric acid of 10-4 to 10 M the activity of HPO42- appears to be constant, and the change of emf between the phosphate electrodes and the H+ electrodes is due only to the change in the activity of the hydrogen ions. The value of the phosphate electrodes of the second kind described in the literature appears to be limited, and considerable additional evaluation is necessary before they can be employed as reliable ion-specific reference electrodes. © 1969.