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Pitzer equations

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 6/15/2006]


Posted by: Ross McCartney


Dear all:


In a recent paper, Maureen Alai and co-authors

(Geochemical Transactions, v 6, No 2, p 31-45)

presented a table of Pitzer interaction parameters

which were used with EQ3/6. I would be interested in

using this data set with Geochemist's Workbench (GWB)

but have noticed that they make use of a higher order

term when determining activity coefficients of neutral

species. This term was recommended by He and Morse

(1993) to better model neutral species activity

coefficients in higher salinity solutions (e.g. >3M)

but it is missing in Table 7.2 (p. 118) in

'Geochemical Reaction Modelling' and there does not

appear to be an option to include Pitzer parameters

for neutral-cation-anion triplets in GWB Pitzer

databases (p. 211, reference manual).


Does anyone know whether GWB does actually allow the

use of these parameters and incorporates the higher

order term? If so, does anyone know the format for

entering the Pitzer parameters for

neutral-cation-anion triplets into the GWB Pitzer

database? Otherwise, would it be possible to modify

GWB so that it includes the higher order term and

allows for entry of the associated Pitzer parameters

in the database? Any advice would be gratefully



Ross McCartney


Posted by: Craig Bethke


Hi Ross,


First of all, make sure you are using a dataset with the line "activity model: h-m-w" in the header. This invokes the Harvie-Moller-Weare formalism, which is the only variant of the Pitzer equations the GWB supports; all other virial methods are obsolete and have known issues.


Now, move to the end of the database. You will see a block of data identified as the "psi set". The block contains species triplets and the associated ternary virial coefficients.


You can include neutral species-cation-anion triplets in this block. Even though such terms are not formally a part of the H-M-W model, the GWB should pick them up and process them correctly.


Hope this helps,


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