THERMODYNAMIC PROPERTIES OF 0-6 MOL KG-1 AQUEOUS SULFURIC-ACID FROM 273.15 TO 328.15-K

Generalised equations are presented for an extended form of the Pitzer molality-based thermodynamic model, involving an ionic strength-dependent third virial coefficient. Compatibility with the established formulation is retained. Osmotic coefficients, emf measurements, degrees of dissociation of the HSO4- ion, differential enthalpies of dilution and heat capacities for aqueous H2SO4 from 273.15 to 328.15 K, 0-6.1 mol kg-1 and at 1 atm pressure have been critically evaluated. Treating this solution as the mixture H+-HSO4--SO42--H2O, and using hydrogen sulfate dissociation constants from the literature, the model parameters were fitted to the data yielding a self-consistent representation of activities, speciation and thermal properties together with the standard potentials of four electrochemical cells and standard-state heat capacities of the SO42- ion as functions of temperature. The model equations represent the experimental data accurately (without the use of mixture parameters theta(HSO4,SO4) and psi(HSO4,SO4,H)), and should yield values of the osmotic coefficient that are suitable for use as an isopiestic standard over this temperature and molality range. The new model will also enable improved prediction of the properties of mixed acidic sulfate systems.