Conceptual Models > Database selection and modification

Adding Thermodynamic Data

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HamatBris:
Hi!

Im constructing a model for Dolomitization but the input for the model has activity coefficients for CO3 and Mg (3.16E-09 and 1.20E-01 respectively).

The calculation used to derive these values utilized thermodyamic data from Bowers et al 1984, specifically
Keq(dolomite),100deg.C = 6.76E-21

My question is; how do I incorporate this thermodynamic data for a non-standard (25deg.C) temperature? Will this affect the internal consistency of the database?

Title Variable Input test

SOLUTION
Units mol/l
temp 100    %
Mg   0.4     % temp,Mg,Ca Values used to calculate the activity coefficients.
Ca   1.00   %

END

dlparkhurst:
The activity coefficients have temperature dependence in the formulation. Debye Hueckel A and B parameters are functions of temperature. You can plot them as a function of temperature using the DH_A and DH_B Basic functions.

Pitzer parameters are a function of temperature. The next release of PHREEQC (3.1.8) will have some updates for better temperature dependence.

Note that the ionic strength is also important for the calculation of activity coefficients, so make sure you are using the correct medium.

Finally, equilibrium constants for aqueous species and minerals are calculated with either an analytical expression or the van't Hoff expression. You will have to look at the database to see which is used for the relevant aqueous species. 100 C is probably a stretch for the van't Hoff extrapolated from 25 C. You can always specify a new expression or a 100 C value for log K by definitions in the input file.

Finally, you have the issue of the consistency between the PHREEQC aqueous model and the manner by which the literature value of log K was calculated. If the literature used an aqueous model to arrive at log K, it very well may be different than the aqueous model(s) of PHREEQC. Perhaps the original data should be reinterpreted with a PHREEQC aqueous model if you want to  be completely consistent.

HamatBris:
Hi,
Thank you very much for the reply.

'You can always specify a new expression or a 100 C value for log K by definitions in the input file.'
This is precisely what I wish to do yet I cannot see how exactly I can specify log K at 100 deg.C within a PHASES blocks.

for example how would I specify that this log K represents a value of 100 deg.C.
Dolomite
   CaMg(CO3)2 = Ca+2 + Mg+2 + 2 CO3-2
   log_k      -17.083

dlparkhurst:
If you enter just logK in the PHASES definition then it will be used no matter what the temperature. Just enter the log K if all of your calculations are at 100 C.

HamatBris:
Cheers thanks for this!! I thought it would extrapolate the logK to 100deg.C.

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