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[Vixenbergen]

I am trying to create a thermodynamic model for CO2 and H2O equilibrium. The experimental setup consists of a water tank with co2 being slowly pumped in and keeping a constant temperature of 25 Celcius (the pressure increases from 1 bar to 10 bars).

I first modeled it using Carroll's solubility model to find the Henry's constants. However, I am now including the fact that carbon dioxide is a weak electrolyte, so I would like to find the values of certain gamma's.

However, I need to find the ionic strength. The formulas I am using now are

H2O + CO2 --> H2CO3
H₂CO₃ HCO₃⁻ + H


and I am ignoring the dissociation of HCO3. My question is, how do I obtain the electrolyte gammas (that is, how do I find the ionic strength?)

[kkala]

You can go to http://en.wikipedia..../Ionic_strength; then also click "Debye-Huckel theory" inside the wikipedia article.
Only dissociated ions have to be considered in formulas.
I remember these from student years, but never had to apply in practice. In your case you have an overhead CO2 pressure up to 10 Bar; so effect of ionic strength in solution had better be looked into (also fugacity of CO2 in gas phase?) to see whether it is significant.
Water is assumed desalinated, so no more ions have to be considered.
I suppose that CO2 diluted in water (presented as H2CO3 in the equations) does not need some (activity) coefficient, but this may be clear to you.