water activity in SIT

[pzhang117]

Hi,

Can someone help me find the equation used in SIT model to estimate activity of water? The water activities estimated by SIT and PhreeqC database for my system seem very different. I found an equation in the manual of PhreeqC version 2, suggesting calculation is based on Raoult's Law, but I didn't find any information about this in version3 manual (where SIT is included).

Also, if there's any good reference materials about SIT calculation in PhreeqC, please recommend to me. I'd very much appreciate that!

Kind regards,

[dlparkhurst]

"The implementation of the SIT aqueous model has been taken from Grenthe and others (1997). "

Grenthe, Ingmar, Plyasunov, A.V., and Spahiu, Kastriot, 1997, Estimations of medium effects on thermodynamic data, in Grenthe, Ingmar, and Puigdomenech, Ignasi, eds., Modelling in aquatic chemistry, chap. IX: Paris, OECD Nuclear Energy Agency, p. 325-426.

Code: [Select]

COSMOT = 1.0e0 + OSMOT*LOG_10 / OSUM;
/*
   C
   C     CALCULATE THE ACTIVITY OF WATER
   C
*/
AW = exp(-OSUM * COSMOT / 55.50837e0);

AW is the activity of water OSMOT is the osmotic  coefficient, and OSUM is the sum of molalities of aqueous species.

[pzhang117]

Thanks so much for the timely reply. These answered my questions.