On the physical chemistry of seawater with deviating ion composition

The salt composition in natural seawaters is not strictly conservative. Physico-chemical properties of the mixed electrolyte "standard seawater" and their variations with ionic composition are briefly reviewed. It is shown that a "same absolute salinity" rule, known from seawater densities, may lead to good results for sound speeds, too. Refractive index measurements have now become sufficiently precise to detect local deviations of ion abundances along with routine ocean profiling. The question is discussed which quantities/formulas still need to be quantitatively determined for this purpose, and which theoretical, empirical or experimental aids can be applied.