Harvie-Moeller-Weare database

[Tom Meuzelaar]

[admin notice: the below is from the former GWB users group email distribution list. This message was originally posted 12/8/2005]

 

Posted by: Johannes Luetzenkirchen

 

hello,

 

in the thermo_hmw.dat file I found an inconsistency compared to the

EQ3/6 hmw data set concerning the log K for CO2(gas). the log K given

with GWB is -7.8051, whereas in the EQ3/6 package it is -7.8193, which

corresponds to what I calculate from table 4 in the original hmw paper.

could you comment on that

 

regards

 

johannes

 

Posted by: Craig Bethke

 

Hi Johannes,

 

It looks like the difference in the log K for CO2(g) between the GWB and EQ3/6 databases stems rounding and curve-fitting. Thermo_hmw.dat is limited to 25°C, but I believe (this was a long time ago) the people working on it wanted it to be consistent with thermo_phrqpitz.dat, which is an extension of the HMW database that includes the possibility of working over a range of temperatures. In turn, they wanted thermo_phrqpitz.dat to be consistent with PHRQPITZ itself.

 

Instead of figuring log K's from Table 4 directly, they calculated them from the Maier-Kelly equations used in Phrqpitz. In the case of CO2(g), the value figured this way differs from the direct calculation from the free energies in HMW Table 4 in the third decimal point, which of course is well within the value's range of uncertainty.

 

This issue doesn't affect speciation in solution, only the predicted CO2 fugacity. If you'd prefer to use the log K calculated directly from HMW Table 4, you can edit the database, or simply use the "alter" command.

 

Hope this helps,

Craig