Electrical conductivities of very dilute aqueous solutions of magnesium sulfate and nickel sulfate (∼ 10−3 mol·kg−1) were measured at temperatures from (398.15 to 548.15) K and from (398.15 to 498.15) K, respectively, with an alternating current (AC) conductivity flow cell at a pressure of 18.62 MPa. The resulting conductivity data were modeled using the Fuoss−Hsia−Fernández−Prini (FHFP) conductance equations together with the appropriate activity coefficient expressions, to obtain equilibrium constants for the ion association reactions: Mg2+(aq) + SO4 2−(aq) ⇌ MgSO4 o(aq) and Ni2+(aq) + SO4 2−(aq) ⇌ NiSO4 o(aq). The equilibrium constants K A for the two systems, which correspond to the formation of all types of ion pairs (solvent−solvent-separated, solvent-separated, and contact ion pairs), agree remarkably well with results calculated from solubility measurements at much higher concentrations. The results were described by the temperature-dependent equilibrium constant model to a precision of ± 0.03 in log K A.

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