The Debye-Huckel treatment of the properties of salt solutions is based on a linearization of the Botzmann Equation leading to an equation for the radial distribution function, \(\mathrm{g}_{i j} (\mathrm{r})\). If a further term is taken into the expansion, the equation for \(\mathrm{g}_{i j} (\mathrm{r})\) takes the following form [1]. \[g_{i j}(r)=1-q_{i j}+\left(q_{i j}^{2} / 2\right) \nonumber \] When equation (a) was tested against the results of a careful Monte Carlo calculation the conclusion was drawn that the three-term equation is good approximation [2]. The result is a set of equations for both the practical osmotic coefficient \(\phi\) and mean ionic activity coefficient for the salt in a solution having ionic strength \(\mathrm{I}\) [3,4]. The theory has been extended to consider the properties of salt solutions at high \(\mathrm{T}\) and \(\mathrm{p}\) [5,6]. In fact key parameters in Pitzer equations covering extensive ranges of \(\mathrm{T}\) and \(\mathrm{p}\) have been extensively documented [7]. The Pitzer treatment has been extended to a consideration of the properties of mixed salt solutions [8]. Footnotes [1] K. S. Pitzer, Acc. Chem.Res.,1977, 10 ,371. [2] D. N. Card and J. P. Valleau, J. Chem. Phys.,1970, 52 ,6232. [3] K. S. Pitzer, J.Phys.Chem.,1973, 77 ,268. [4] Activity and Osmotic Coefficients for 1:1 salts: K. S. Pitzer and G. Mayorga, J.Phys.Chem.,1973, 77 ,2300. For 2:2 salts: K. S. Pitzer and G. Mayorga, J. Solution Chem., 1974, 3 ,539. For 3:2 salts etc; K. S. Pitzer and L. V. Silvester, J.Phys.Chem.,1978, 82 ,1239. L. F. Silvester and K. S. Pitzer, J. Solution Chem.,1978, 7 ,327. K. S. Pitzer, J. R. Peterson and L. E. Silvester, J. Solution Chem.,1978, 7 ,45. [5] R. C. Phutela, K. S. Pitzer and P. P. S. Saluja, J. Chem. Eng. Data, 1987, 32 ,76. H. F. Holmes and R. E. Mesmer, J. Phys.Chem.,1983, 87 ,1242. [6] R. C. Phutela and K. S. Pitzer, J. Phys Chem.,1986, 90 ,895. [7] J. Ananthaswamy and G. Atkinson, J. Chem. Eng. Data,1984, 29 ,81. R. P. Beyer and B. R. Staples, J. Solution Chem.,1986, 15 ,749. P. P. S. Salija, K. S. Pitzer and R. C. Phutela, Can J.Chem.,1986, 64 ,1328. R. T. Pabalan and K. S. Pitzer, J. Chem. Eng. Data, 1988, 33 ,354. [8] R. C. Phutela and K. S. Pitzer, J. Solution Chem.,1986, 15 ,649. A. Kumar, J. Chem. Eng. Data,1987, 32 ,106. K. S. Pitzer and J. J. Kim, J. Am. Chem.Soc.,1974, 96 ,5701. K. S. Pitzer and J. M. Simonson, J. Phys.Chem,.,1986, 90 ,3005. C. J. Downes and K.S. Pitzer, J. Solution Chem.,1976, 5 ,389. J. C. Peiper and K. S. Pitzer, J.Chem.Thermodyn.,1982, 14 ,613.