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Speciation problem with CO2 charged water

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Hi there,

I am trying to speciate co2 charged water related to my study. When I am doing the speciation without a charge balance, the solution has converged and is showing speciation with <1% of charge balance error. However, when I am trying to speciate the solution with Na-charge balance on, it is throwing an error, indicating:

Residuals too large, 693-th interations.

Largest residual

                       Resid     Resid/Totmol   Cbasis  

Na+                    0.0731    1.14e+200   6.399e-202


Please suggest me how to overcome this problem. I have attached both the files for your reference. The database that I have used is thermo.dat

CO2 charged water with charge balance.sp8 CO2 charged water without charge balance.sp8

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Hello Anith,

I apologize for the delayed response. I have looked at the file in detail and I believe there are a couple of contributing factors. As you have observed, the program converges with no issue if you turn off charge balance. When you do that and run the calculation, a small but positive charge imbalance was reported. In this case, you would ideally choose an anion component for charge balance.

Another complication here is that the carbonate component can dissociate into many things and form complexes with many of the cations in your fluid. This adds further difficulty when chosen as the charge balancing ion. I also noticed that at the given pH, a majority of the carbonate component will exist in fluid as CO2(aq) and a very small amount remains as HCO3-. When you have such a drastic difference in species concentration, it will help the program by swapping the more abundant species, CO2(aq) for HCO3-, into the Basis. However, your fluid contains only one negative component and swapping in CO2(aq) means that I won't be able to charge balance on this species.

In general, the charge balancing ion is best to no affect how other species behave. I tried  adding F- as charge balancing and SpecE8 was able to converge.

Hope this helps,

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