The thermodynamic dissociation constant of the bisulphate ion HSO-₄ and of the cereus sulphate ion-pair CeSO+₄ in water and aqueous methanol at 25°C

Newton and Arcand have reported that in aqueous solution a marked change in the ultra-violet absorbance of the cerous ion occurs on addition of sulphate ions. This they interpret as due to complex formation between cerous and sulphate ions, enabling them to calculate a dissociation constant for the reaction CeSO²⁺4 ⇌ Ce³⁺ + SO4²- at several temperatures. Association of cerous and sulphate ions has also been characterised by several other methods. The agreement between the corresponding dissociation constants derived from various determinations is, however, not good. Since the work of Newton and Arcand, several investigators, using spectrophotometric techniques,²,³ have obtained quantitative evidence for the existence of a complex between cerous and perchlorate ions in aqueous solution. As relatively high perchlorate concentrations were used by Newton and Arcand, their value for the dissociation constant of cerous sulphate might be in error due to appreciable CeClO²⁺4 icon formation. Accordingly, it was decided to redetermine the dissociation constant of the cerous sulphate ion in water at 25°C using the method of Newton and Arcand, but by using much lower ionic strengths and corresponding low perchlorate concentrations. In view of current interest relating to ionic association in mixed solvents, measurements were extended to methanol-water mixtures. As complex formation in cerous perchlorate has been questioned, this system has also been- reinvestigated in water, and additional measurements made in aqueous methanol. For comparison with the similar cerous sulphate ion-pair, and in an attempt to resolve the difference in derived dissociation constants obtained from conflicting sets of spectrophotometric data,⁵,⁶ the cobaltic hexammine sulphate ion-pair has also been investigated in aqueous solution at 25°C by a similar method. To minimise hydrolysis in solutions containing cerous ion, acid should be present. As the bisulphate ion is a weak acid, its dissociation constant must be available for use in the cerous sulphate investigation. Previous work on the acidity of the bisulphate ion covers only the relevant solvent range 0-20 per cent methanol. Consequently, the dissociation constant of the bisulphate ion has been determined at 25°C in water and over a wider range of methanol-water mixtures. A spectrophotometric method, employing a suitable nitrophenol as indicator, was used. Before proceeding with this work, the dissociation constants of several nitrophenols, which it appeared might be suitable for use as indicators, were spectrophotometrically determined at 25°C in water and in aqueous methanol. In reporting on the present investigation, consideration will be given in sequence to the work on nitrophenols, to that on sulphuric acid, and lastly to that on cerous sulphate