Ion Association in Dilute Aqueous Magnesium Sulfate and Nickel Sulfate Solutions Under Hydrothermal Conditions by Flow Conductivity Measurements

Electrical conductivities of very dilute aqueous solutions of magnesium sulfate and nickel sulfate (similar to 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-Fernandez-Prini (FHFP) conductance equations together with the appropriate activity coefficient expressions, to obtain equilibrium constants for the ion association reactions: Mg2+(aq) + SO42- (aq) reversible arrow MgS4 degrees(aq) and Ni2+(aq) SO42- (aq) reversible arrow NiSO4 degrees(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.