Isopiestic determination of the osmotic and activity coefficients of {(1 − y)H 2SO 4 + yNa 2SO 4}(aq) at 298.15 K I. Results for y = 0.5 (NaHSO 4) and y = 0.55595, 0.70189, and 0.84920

Abstract

Isopiestic vapor-pressure measurements were made for {(1 − y)H 2SO 4 + yNa 2SO 4}(aq) with y = 0.5 (NaHSO 4) from 0.23 mol · kg −1 and with y = 0.55595, 0.70189, and 0.84920 from 0.16 mol · kg −1 up to the crystallization limits of the mixtures (4.6057 to 10.681 mol · kg −1) at 298.15 K. Experimental water activities are much higher than predicted by Zdanovskii's empirical rule; this difference is consistent with extensive HSO 4 −(aq) formation. If osmotic coefficients are calculated by using the stoichiometric molalities of Na 2SO 4 and H 2SO 4 and assuming that they behave as 1–2 electrolytes, then osmotic coefficients for the mixtures fall considerably below values predicted from the Scatchard neutral-electrolyte binary-solution mixing approximation. In fact, for ionic-strength fractions of Na 2SO 4 greater than 0.7, the experimental osmotic coefficients fall below those of both limiting binary solutions over most of the molality range. However, if these values are recalculated by treating NaHSO 4 as a 1-1 electrolyte that does not dissociate with changes in molality, then deviations of experimental results from the Scatchard neutral-electrolyte binary-mixing approximation are much smaller and usually negative for osmotic coefficients. The limited number of previous isopiestic values for {(1 − y)H 2SO 4 + yNa 2SO 4} are critically compared with our new experimental results.