NaSO 4 − ion pairs in aqueous solutions at pressures up to 2000 atm

Electrical-conductance measurements have been made at 25° C up to concentrations of 0.06M in aqueous solutions of Na₂SO₄ at atmospheric pressure and as a function of pressure up to 2000 atm. Calculations of the change of the dissociation constant for the NaSO ₄ ^? ion pair with pressure indicate that the difference in partial molal volumes between products and reactants at infinite dilution is (Delta bar V^ circ tilde = - 8.25{text{ }}cm^3 /mole) . Using the equation of Davies, Otter, and Prue, we found the average dissociation constants for five concentrations between 0.005 and 0.06 moles/liter to be 0.097, 0.131, and 0.165 at 1, 1000, and 2000 atm, respectively, with a standard deviation of ±0.003. The atmospheric-pressure value is 0.080±0.016 over the entire concentration range from 0.00005 to 0.06 moles/liter, about half that obtained by Jenkins and Monk. This is consistent with a value ofK=0.077±0.006 recalculated from the limited-concentration-conductance work of Jenkins and Monk, with a value ofK=0.073 obtained by Fisher from data of Kurtze and Tamm on ultrasonic absorption in MgSO₄?NaCl solutions, and withK=0.067 calculated by Fisher from the potentiometric data of Pytkowicz and Kester at high ionic strength. The relationship of this work to sound absorption and ion pairing in seawater is discussed. The predicted pressure dependence of the NaSO ₄ ^? ion pair in seawater from this work differs substantially from earlier work by Kester and Pytkowicz and by Millero.