Abstract
The second ionization constant K2o of H2SO4(aq) at 298.15 K has been determined through non-linear multi-regression analysis of a combined function of K2o and of the e.m.f. of the unbuffered cell: Pt/H2/H2SO4(aq)/Hg2SO4/Hg/Pt, with resulting optimization of K2o and of the ion-size parameter a0 for H2SO4(aq). For this analysis, a value of Eo (the standard e.m.f.) independent of the indeterminacies involved when one tried to assign the correct a0 value to H2SO4 in the above cell to derive Eo therefrom, is indispensable. In compliance with this requirement, the value Eo = 0.61257 V at 298.15 K, recently determined critically by Rondinini et al. from the e.m.f. of the independent cell: Pt/NaxHg1−x/Na2SO4(aq)/Hg2SO4/Hg/Pt, has been used. The calculation yields unambiguous convergence on the value K2o = (0.01039 ± 0.00018). This method proves superior to that of the classical buffered cell (which keeps its well known indeterminacies when K2o is around 10−3 or more) and lends itself best to K2o determinations in non-aqueous solvents or (aqueous + organic solvent) mixtures. Also, the available e.m.f.s. for the unbuffered twin-acid cell: Pt/H2/HCl(m1), H2SO4(m2)/AgCl/Ag/Pt have been analysed to check consistency, using the same ion-size parameter a0 = 0.43 nm optimized with the above single-H2SO4 cell, and the result K2o = (0.01043 ± 0.00020) is obtained, in very good agreement.
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