Isopiestic determination of the osmotic and activity coefficients of {zH2SO4+(1−z) MgSO4}(aq) at the temperatureT=298.15 K. I. Results forz=(0.85811, 0.71539, and 0.57353)

Abstract

Isopiestic vapor-pressure measurements were made for {zH2SO4+(1−z])MgSO4}(aq) with H2SO4molality fractions ofz=(0.85811, 0.71539, and 0.57353) at temperature 298.15 K. Measurements extend from total molalitiesmTof (0.12548, 0.13563, and 0.14741) mol·kg−1to the supersaturated molalitiesmT=(12.0496, 11.0112, and 7.20595) mol·kg−1, respectively. Forz=(0.71539 and 0.57353) these highest molalities correspond to the greatest degrees of supersaturation that could be achieved during isothermal removal of the solvent. The corresponding water, activity ranges are 0.9958≥aw≥0.2758, 0.9958≥aw≥0.3312, and 0.9958≥aw≥0.5581 forz=(0.85811, 0.71539, and 0.57353), respectively. These values ofzwere chosen to be separated approximately equally by 0.142 (Δz≈1/7). The water activity curves were also found to be nearly equally spaced from each other, as was thez=0.85811 curve from that for H2SO4(aq) (z=1). Given that both H2SO4(aq) and MgSO4(aq) are extensively associated electrolytes, this regularity in the mixing behaviour was not anticipated. A well-defined crossover point was found at which the sums of the stoichiometric ionic molalities were equal at isopiestic equilibrium forz=(1, 0.85811, 0.71539, and 0.57353), as were the water activities and stoichiometric osmotic coefficients of these solutions. This occurred at Σivimi=(5.9632±0.0036) mol·kg−1, whereviis the stoichiometric ionization number of electrolyte i.