Conventional residual liquid junction potentials in dilute solutions

Abstract

Conventional residual liquid junction potentials were measured between NBS 1:1 phosphate buffer and various dilute solutions with ionic strengths of 1-100 mmol kg −1, using 3.5 mol kg −1 potassium chloride as the bridge electrolyte. All junctions were of the free-diffusion type, formed within a 1-mm capillary. The conventional residual junction potential was indistinguishable from zero (± 0.5 mV), for dilute solutions of primary reference standards, for which calculations based on the Henderson equation predict values of 0.8–1.3 mV. Solutions of Tris-HCl (20–100 mmol kg −1) and dilute HCl (1–50 mmol kg −1) had appreciable conventional residual liquid junction potentials (1.1–3.3 mV). The experimental values for dilute HCl solutions compare favourably with those calculated with the Henderson equation. These results suggest that the pH of hard waters, measured using the NBS pH scale, will be a good approximation (within 0.01 pH) to p a H, so that pH can be interpreted in terms of the activity of the hydrogen ions. Further measurements are required to ascertain whether this approximation is true for soft and acidic waters.