The chemical potentials of seawater components

An equation for the chemical potential of “seasalt” (R T ln a_s) as a function of salinity is derived from isopiestic and potentiometric data, via the Gibbs-Duhem equation and a simple ion-interaction theory. Estimates of the activity coefficients of seasalts and the major ions are listed for salinities 25‰ and 36‰. The direct measurement of ln a_s is considered, and a related quantity, that could be measured in situ, is suggested as a future oceanographic parameter. The results of laboratory measurements on an average seawater sample, using four ion-selective electrodes, serve to illustrate the concepts involved.