Activity coefficients in aqueous mixtures of phosphates with sodium chloride, sodium bromide, and sodium iodide, and the pH of phosphate buffer solutions

Abstract

The electromotive force of cells with hydrogen electrodes and silver-silver bromide or sil ve r-sih'er iodide electrodes was measured at 25° C, Th e electrolyte i n each cell consisted of an aqueous mixture of (a) potassiu m dihydrogen phosphate, (b) di sodium hydroge n phosphate, and (c) sodium bromide or sod iu m iodide. The molalities or (a) and (b) were always equal, whereas the ratio or the molality of (c) to that of (a) or (b) was 1, 0. 5, or 0.2. The ionic strengths ranged from 0.04 to 0.25. The data are compared \vi th similar measu re-ments for phosphate-chloride m ixtures reported in an earli er paper. At a given ion ic strength , the activity-coefficient term, jH2POJxlfHP04 wh ere X represe nts halide, in creases when b rom id e at a given conce ntration is s ubstituted for chl orid e at t he same conce ntration , or when iod ide r eplaces bromide. Accurate valu es for the pH of phosphate buffer solu tions with and without added salt ca n be obtain ed o nl y at low ion ic s trengths.