## Wednesday, January 23, 2008

### Salting-in and Salting-out

Since activity is a measure of the effective concentration of solute in a solution, it can be used to describe solubilities of slightly soluble solutes in ionic solutions. In other words, we can investigate the effects of ionic strength on solubility.

For example, since copper (II) sulfide is only somewhat soluble in water, we can write its dissolution as an equilibrium:

CuS(s) ↔ Cu2+(aq) + S2-(aq)

The thermodynamically correct equilibrium constant is then

Ksp=aCuaSCuγSmCumS±2Kspobs

The parameter Kobs is the observed (or apparent) equilibrium constant and will be the one that is measured by standard measurements like titration. Since this is a 1:1 ionic compound, Ksp is related to S, the molal solubility, by:

Ksp = S2 and Kspobs = (Sobs)2

Combining the two equations above, we can solve for the ratio of solubilities:

S/Sobs = γ±

Therefore, for mean activity coefficients less than one, we see an increase in solubility with respect to water [salting-in] and a corresponding solubility decrease when γ± is greater than one [salting-out].