the effect of salinity on solubility

[Mr.white]

I would like to understand the effect of salinity on the solubility of non-charged substances (such as methane and ethane) in PHREEQC. Based on the Setchenow-type equation: ln𝜆= k*μ，I have already determined the k values. How can I add this to the database? Alternatively, what is the relationship between a and b in the WATEQ equation (gamma) in the PHREEQC database?

[dlparkhurst]

Both a and b are basically fitting parameters. a is called the ion-size parameter based on the Debye-Huckel theory, but it is often adjusted to fit empirical data. Similarly, b is adjusted to match experimental data of activity coefficient at increasing ionic strength.

For uncharged species, the term containing "a" is zero, so b is the only parameter that affects the activity coefficient for an uncharged species.

a and b are defined in the SOLUTION_SPECIES definitions for a species with the -gamma identifier.