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Unit conversions and activity coefficients highly inaccurate using Pitzer thermo_hmw

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I have been using GSS and SpecE8 to calculate activity coefficients and perform concentration unit conversions in brine solutions with extremely high ionic strength. I have noticed that when converting between g/kg (ppt) to mol/l, the converted values are off by 1-2 moles compared to values obtained by doing 'long-hand' calculations using density measurements. Also, I am interested in activity of H2O, and this parameter is drastically lower when calculated with SpecE8 than when measured using a calibrated relative humidity meter.


I have loaded the thermo dataset thermo_hmw, which should employ Pitzer equations for calculations at high ionic strength, and have been applied previously to solutions analogous to mine. I was wondering whether I have made some error or whether I am outside the usability of the hmw dataset in the GWB Student edition.


An example dataset:


raw data in g/kg:


Mg++ 75.44

Na+ 0.3047

K+ 1.097

SO4-- 102.9

Cl- 119.3


density: 1.227 g/cm^3




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Hello Mark,


Can you please explain how you are calculating your g/kg to mol/l unit conversion? What values do you think you should get and what values are calculated using GSS? Please attach your GSS and SpecE8 files so we can take a better look at your problem.


In any modeling exercise, there is often some difference between values calculated from a model and those determined directly. You can find information on calculating the activity of water using the B-dot model and virial methods in Chapter 8 of the Geochemical and Biogeochemical Reaction Modeling text.


Kind regards,


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