A new approach in the use of SIT in determining the dependence on ionic strength of activity coefficients. Application to some chloride salts of interest in the speciation of natural fluids

This paper describes a modified version of the SIT (Specific ion Interaction Theory) method and its use in determining the dependence on ionic strength of activity coefficients. In the new approach the interaction coefficients (epsilon) are not constant but depend on ionic strength (l/mol kg(-1)) according to the simple relationship: epsilon = epsilon(infinity) + (epsilon(0) - epsilon(infinity)) / (l + 1) where epsilon(0) and epsilon(infinity) are true constants for l --> 0 and l --> infinity, respectively. To check the two parameter SIT equation, we calculated epsilon(0) and epsilon(infinity) for the activity coefficients of HCl, LiCl, NaCl, KCl, MgCl2, CaCl2 and SrCl2, in a wide ionic strength range (0.1 less than or equal to l/mol kg(-1) less than or equal to 4.5, for KCl; 0.1 less than or equal to l/mol kg(-1) less than or equal to 6, for HCl, LiCl, NaCl; 0.3 less than or equal to l/mol kg(-1) less than or equal to 12, for SrCl2; 0.3 less than or equal to l/mol kg(-1) less than or equal to 15, for MgCl2; 0.3 less than or equal to l/mol kg(-1) less than or equal to 18, for CaCl2). Results show that the gamma values calculated using this approach fit quite well overthe whole l-range for all the electrolytes considered. Comparison is made with the analogous one parameter SIT equation. The temperature coefficients of interaction coefficients were also calculated using gamma(infinity) (HCl) in the range 0 less than or equal to t/degreesC less than or equal to 60.