Isopiestic studies of aqueous solutions at elevated temperatures VII. MgSO 4 and NiSO 4

Abstract

Isopiestic measurements have been made on MgSO 4(aq) and NiSO 4(aq) at 383.14 K with NaCl(aq) serving as the reference electrolyte. Irreversible solubility effects prevented successful measurements at higher temperatures. Osmotic coefficients at 383.14 K are smaller and less dependent on molality than at 298.15 K. The ion interaction model, as modified for 2-2 electrolytes, was fitted to the osmotic coefficients. Only three of the four parameters of the model could be evaluated, thereby precluding the direct calculation of activity coefficients from the isopiestic results. Combination of the present results with published freezing temperatures and enthalpies does permit the computation of activity coefficients in the case of MgSO 4(aq). At 383.15 K the activity coefficient of MgSO 4(aq) becomes quite small (<0.05) at very moderate molalities (0.5 mol·kg −1). Because of uncertainties in the calculations only general observations concerning trends in the activity coefficient at elevated temperatures and high molalities are warranted.