Determination of the association constant of sulfuric acid

The activities of HC1 and H₂SO₄ in aqueous solutions of HC1-H₂SO₄ and H₂SO₄-HC1O₄, respectively, were measured at 298 K by an electromotive force (emf) method. Using the activity data, the association constant,K*_{H 2SO 4} was calculated as a function of the effective ionic strength of the solution by a method similar to that proposed by Johnson and Pytkowicz. The association constant of H₂SO₄, which corresponds to the reciprocal value of the first ionization constant of H₂SO₄, decreased with an increase in the effective ionic strength and was correlated to the effective ionic strength,I _e (mol dm⁻³), as follows: logK*_{H 2SO 4} = 0.42 - 0.91I_e, 0 I_e1 The mean free activity coefficients of H₂SO₄ were calculated from the association constant of H₂SO₄ and were found to decrease with an increase in the effective ionic strength. Formerly Graduate Student, Department of Metallurgy, Kyoto University.