Sodium, Magnesium, and Calcium Sulfate Ion‐pairs in Seawater at 25c1

The stoichiometric association constant of CaSO 4 0 ion‐pairs, K* CaSO4 = [CaSO 4 0 ]/[Ca 2+ ]× [SO 4 2− ], was found to be 10.8 ± 0.7 at 25C at an effective ionic strength of 0.67 and at 1‐atm pressure. The percentages of free and ion‐paired species in seawater were calculated from the experimentally determined association constants of NaSO 4 − , MgSO 4 0 , and CaSO 4 0 ion‐pairs. The results were used to calculate the effective ionic strength of seawater and to account for the difference in activity coefficients in seawater and in single‐salt solutions.