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Pitzer coefficients

Tom Meuzelaar

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[admin notice: the below is from the former GWB users group email distribution list. This message was originally posted 11/29/2005]


Posted by: Johannes Luetzenkirchen




we will be starting to use GWB extensively with the Pitzer formalism in

the near future. From previous experience with the EQ3/6 packages we

know that under certain circumstances it is necessarry to put in ternary

interaction parameters (with parameters put to zero, i.e. EQ3/6 does not

do that itself) if reasonable results are to be obtained. This means

that for those cases a lot of entries in the data base are required and

the number of those entries goes up quite rapidly if species are added

to the database. We would like to know how GWB works in this respect. We

would be very grateful for any hint.



With best regards




Posted by: Craig Bethke


Hi Johannes,


I don't think you'll encounter any problems like that; the GWB is designed to handle zero-valued virial coefficients correctly, whether or not they are explicitly included in the thermo dataset.


Some years ago we adopted the HMW formalism for evaluating the Pitzer equations. The HMW method is better defined than some other variants and I believe that it is implemented correctly in the GWB. You should make sure the line "activity model: h-m-w" is at the top of your thermo dataset. If it is, you will be executing the proper part of the code.


The steps in the HMW calculation procedure are given in Chapter 7 of the "Geochemical Reaction Modeling" text and there is a worked example in Appendix 2. You can check the GWB calculations in this fashion, or by running the PHRQPITZ code, which was coded independently by Neil Plummer and David Parkhurst and should give the same answer.


Hope this helps,


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