Ion association of chloride and sulphate with sodium,potassium, magnesium and calcium in seawater at 25°C

Stoichiometric association constants, which have been measured for the ion pairs of Cl^? and SO₄^?2 with Na⁺, K⁺, Mg²⁺, and Ca²⁺, were used to determine the speciation in an artificial sea water containing only these ions. The resulting distribution is quite different to that found in earlier models in which chloride ion association was ignored. The concentrations of chloride ion pairs with the cations are 4 to 5 times larger than the concentrations of the sulphate ion pairs with the same cations. The total activity coefficients of the neutral salts in sea water calculated from the model are in good agreement with the experimentally measured values. The concentration of MgSO₄⁰ calculated to be present agrees with the amount determined from ultrasonic absorption data. The calculated solubility of gypsum is also in good agreement with the measured values.