The vapour pressure and osmotic equivalence of sea water

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Abstract
Sea water is a complex solution in which the principal ions are sodium, potassium, calcium, magnesium, chloride and sulphate. The vapour pressure (V.P.) of such a solution can be calculated approximately by making the assumption that each salt contributes to the vapour pressure lowering in amount proportional to its concentration, but such a calculation would ignore the interactions between the various ions. The theory of these interactions has been worked out only for very dilute solutions and it is, therefore, better to rely on direct experimental determinations. Measurements have now been made by the isopiestic vapour-pressure method (Robinson & Sinclair, 1934), in which samples of sea water are equilibrated with sodium chloride solutions until they have the same vapour pressure. The results are expressed in terms of chlorinities of sea water and molalities (moles per kilogram of H2O) of sodium chloride solution which have the same vapour pressure. It is hoped that the results will be of use to physiologists who have occasion to make up salt solutions equivalent to sea water.