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 th e H-2, Pt\Ag, AgCl electrode system. The calculation of the pH of thes e 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 H-2(g) double left right arrow Ag(s) + HCl(aq) cell reac tion in synthetic seawater from 0 to 55 degrees C and at salinities fr om 5 to 45. The measurements were made at various concentrations of HC l 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 char acterize the pH of seawater buffers for measurements at sea. Fitter eq uations for seawater have been formulated from 0 to 55 degrees C and I = 0 to 4 m that can be used to estimate the emf values for these solu tions. The calculated emf values agree with the measured values in sea water without sulfate to +/-0.08 mV. To make the estimates in seawater with sulfate it was necessary to use the experimental emf values to d erive Fitter parameters for Mg-HSO4- interactions. These parameters yi eld emf values that agree with the measured values on the average to /-0.06 mV from 0 to 45 degrees C, S = 5 to 35, and added H+ to 0.04 mo l/kg.