Comparison of Pitzer and HKF Models

These two models present a fascinating contrast in their approach to a complex problem. Pitzer stands back, as it were, from the details of ionic interactions, and builds up an empirical model of complex solutions from data on the simpler binary systems of whieh it is composed. No data as to individual ionic processes are required 

It will be of great interest to follow the progress of these and other models in advancing our understanding of geochemical phenomena in the years to come. 

Calculate the activity coefficients of Ca and SO using the Pitzer equations for the example in 17.8.4. The only data not given in the text are 

The text gives the final results various parameter values are given in Appendix F. 

Calculate the dissociation constant for NaCl(ag) at 25°C, 1 bar. Calculate this constant at 500°C, 1 and 2 kbar, obtaining the necessary values of e and Y from Tanger and Helgeson, (1988). A graph of A NaCi versus T is shown by Sverjensky (1987). 

There are advantages and disadvantages to both approaches. The achievements of Harvie and Weare and others in modeling evaporitic brines with the Pitzer model is at the moment well beyond the range of the HKF model, but in providing standard thermodynamic parameters for individual ions to high temperatures and pressures the HKF model is reaction-oriented, and hence fits the way most geochemists think. It has therefore been more widely used at present. 

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