The problem of acidity in concentrated aqueous solutions of sulfuric acid

Abstract

Concentrated aqueous sulfuric acid mixtures, which ionize according to the equilibria (a) and (b) [(a) HSO₄^– ⇄ SO₄^2– + H⁺; (b) H₂SO₄ ⇄ HSO₄^– + H⁺], have been analysed and the corresponding activity coefficient functions [Mc(s)] of the solvent [Mc(HSO₄^–) = –log (f_SO4^2– f_H⁺/f_HSO4^–); Mc(H₂SO₄) = –log (f_HSO4^– f_H⁺/f_H2SO4)] have been determined by using the concentrations of the species SO₄^2–, HSO₄^–, H₂SO₄. The Mc(s) have been compared with the Mc(i) function [Mc(i) = –log (f_B f_H⁺/f_BH⁺)], the latter one being determined by the protonation equilibria of weak bases (B) (B + H⁺ ⇄ BH⁺). The results show that the interactions of solute and solvent can be linearly related [i.e.Mc(i) = n_isMc*(s) and Mc*(s) = Mc(HSO₄^–) + Mc(H₂SO₄)] in the range 0.1–100 wt% H₂SO₄ provided that both equilibria of sulfuric acid, correctly normalized, are taken into account.