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+·SO2–4 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(SO2–4)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.