The standard potential for the hydrogen-silver, silver chloride electrode in synthetic seawater

The characterization of acid-base equilibria in seawater requires the measurement of the pH using buffers that have been calibrated using the H₂, Pt Ag, AgCl electrode system. The calculation of the pH of these buffers and the dissociation constants of acids in seawater requires a knowledge of the standard potential for seawater with added HCl. In this study we present measurements of the standard potential for the AgCl(s) + 1 2 H₂(g) {negation of mutual implies} Ag(s) + HCl(aq) cell reaction in synthetic seawater from 0 to 55°C and at salinities from 5 to 45. The measurements were made at various concentrations of HCl and the results extrapolated to the pure medium. The extrapolations were made by using the total proton scale, m^*(H⁺) = m(H⁺) + m(HSO₄^-). The measured emf values and the extrapolated standard cell potentials in seawater were in excellent agreement (±0.04 mV) with measurements of Khoo et al. and Dickson. The combined data can be used to evaluate the dissociation constants of acids and bases in seawater and to characterize the pH of seawater buffers for measurements at sea. Pitzer equations for seawater have been formulated from 0 to 55°C and I = 0 to 4 m that can be used to estimate the emf values for these solutions. The calculated emf values agree with the measured values in seawater without sulfate to ±0.08 mV. To make the estimates in seawater with sulfate it was necessary to use the experimental emf values to derive Pitzer parameters for Mg-HSO₄^- interactions. These parameters yield emf values that agree with the measured values on the average to ±0.06 mV from 0 to 45°C, S = 5 to 35, and added H⁺ to 0.04 mol/kg.