Saturation Index

The saturation index is a measure of the saturation of a solution relative to a particular solid or gas. For all of the TCE packages, it can either be directly specified or the relevant solid or gas can be suppressed from forming.

Consider NaCl(s) as an example. The formula for the Saturation Index is:

[math]\displaystyle{ SI = \frac{a_{Na^+}a_{Cl^-}}{K_{sp,NaCl}} }[/math]

The default value of SI is 1.0, meaning that the product of [math]\displaystyle{ Na^+ }[/math] and [math]\displaystyle{ Cl^- }[/math] activity at saturation will equal [math]\displaystyle{ K_{sp,NaCl} }[/math]. There are cases where, due to chemical kinetics, the saturation is not achieved. It could either be undersaturated, due to kinetics of dissolution, or supersaturated, due to kinetics of precipitation. In this case, adjusting the value of SI will incorporate this effect.

The effective solubility product used by the TCE will then be:

[math]\displaystyle{ K_{sp,NaCl}^{eff} = K_{sp,NaCl} * SI }[/math]