The second ionization constant of aqueous sulphuric acid at 298.15 K from the electromotive force of the unbuffered cell: H 2(g)/H 2SO 4(aq)/Hg 2SO 4(s)/Hg

Abstract

The second ionization constant K 2 o of H 2SO 4(aq) at 298.15 K has been determined through non-linear multi-regression analysis of a combined function of K 2 o and of the e.m.f. of the unbuffered cell: Pt/H 2/H 2SO 4(aq)/Hg 2SO 4/Hg/Pt, with resulting optimization of K 2 o and of the ion-size parameter a 0 for H 2SO 4(aq). For this analysis, a value of E o (the standard e.m.f.) independent of the indeterminacies involved when one tried to assign the correct a 0 value to H 2SO 4 in the above cell to derive E o therefrom, is indispensable. In compliance with this requirement, the value E o = 0.61257 V at 298.15 K, recently determined critically by Rondinini et al. from the e.m.f. of the independent cell: Pt/Na x Hg 1−x /Na 2SO 4(aq)/Hg 2SO 4/Hg/Pt, has been used. The calculation yields unambiguous convergence on the value K 2 o = (0.01039 ± 0.00018). This method proves superior to that of the classical buffered cell (which keeps its well known indeterminacies when K 2 o is around 10 −3 or more) and lends itself best to K 2 o determinations in non-aqueous solvents or (aqueous + organic solvent) mixtures. Also, the available e.m.f.s. for the unbuffered twin-acid cell: Pt/H 2/HCl( m 1), H 2SO 4( m 2)/AgCl/Ag/Pt have been analysed to check consistency, using the same ion-size parameter a 0 = 0.43 nm optimized with the above single-H 2SO 4 cell, and the result K 2 o = (0.01043 ± 0.00020) is obtained, in very good agreement.