Dissociation of the bisulphate ion at moderate concentrations

Abstract

A Raman study has been made of the dissociation of the HSO^–₄ ion at 25°C in H₂SO₄, NH₄HSO₄ and LiHSO₄ over a wide concentration range. Evidence for the presence of an ion pair H₃O⁺·SO^2–₄ was obtained and its formation constant found to be in the region of 16 to 25 mol^–1 kg. It is suggested that electrochemical and spectrophotometric methods of measuring the dissociation constant of HSO^–₄ include such ion pairs as undissociated HSO^–₄. If allowance is made for their presence, the Raman data yield a conventional dissociation constant of 0.0112 mol kg^–1 in agreement with recent e.m.f. results. At ionic strengths where specific ion effects become important (µ >0.2 mol kg^–1) it is assumed that the activity coefficient quotient, y(SO^2–₄)y(H⁺)/y(HSO^–₄), is the same in H₂SO₄ and LiHSO₄. This leads to the conclusion that in these solutions the ion-pair to covalent HSO^–₄ concentration ratio is relatively independent of ionic strength up to µ′= 4 mol kg^–1 and that the conventional dissociation quotient Q′_c can be expressed, log Q^′_c=–1.95 +2.04 √µ′//_{1 + 0.85 √µ′}, up to an ionic strength 2.5 mol kg^–1. In ammonium bisulphate the expression, log Q^′_c=–1.95 +2.04√µ′//_{1 + 1.0 √µ′}, holds up to an ionic strength of 4 mol kg^–1.