Performance testing of pH electrodes suitable for low ionic strength solutions
Performance tests using dilute acid were carried out on 16 glass-reference electrode pairs. Recently marketed electrodes designed for use in dilute solutions and having high leakage rates at the reference junction performed well, but were no better than some others with low leakage rates. The determination of the second dissociation constant of phosphoric acid by potentiometric titration was a more demanding test of electrode performance: satisfactory results were only obtained using a renewable free diffusion junction. There are various indications that the Henderson equation may be inappropriate for calculating liquid junction potentials for dilute acids, but there is a lack of precise experimental work in dilute solutions. Consequently there is uncertainty regarding the errors involved in equating the measured pH of dilute acids to the calculated activity of the hydrogen ion.