The second ionization constant of aqueous sulfuric acid at 298.15 K from the electromotive force of the unbuffered cell : H2(g)|H2SO4(aq)|Hg2SO4(s)|Hg

The second ionization const. K20 of H2SO4(aq) at 298.15 K was detd. through nonlinear multiregression anal. of a combined function of K20 and of the emf. of the unbuffered cell: Pt|H2|H2SO4(aq)|Hg2SO4|Hg|Pt, with resulting optimization of K20 and of the ion-size parameter a0 for H2SO4(aq). For this anal., a value of E0 (the std. emf) independent of the indeterminacies involved when one tried to assign the correct a0 value to H2SO4 in the above cell to derive E0 therefrom, is indispensable. In compliance with this requirement, the value E0 = 0.61257 V at 298.15 K, recently detd. critically by Rondinini et al. from the emf of the independent cell: Pt|NaxHg1-x|Na2SO4(aq)Hg2SO4|Hg|Pt, was used. The calcn. yields unambiguous convergence on the value K20 = (0.01039 ± 0.00018). This method proves superior to that of the classical buffered cell (which keeps its well known indeterminacies when K20 is around 10-3 or more) and lends itself best to K20 detns. in nonaq. solvents or (aq. + org. solvent) mixts. Also, the available emfs for the unbuffered twin-acid cell: Pt|H2|HCi(m1), H2SO4(m2)|AgCl|Ag|Pt were analyzed to check consistency, using the same ion-size parameter a0 = 0.43 nm optimized with the above single-H2SO4 cell, and the result K20 = (0.01043 ± 0.00020) is obtained, in very good agreement.