From the Debye-Huckel Theory, increasing the concentration of counterions reduces electrostatic interactions between macroions by shielding them from each other.

Up to a point, adding more counterions (i.e., increasing ionic strength, I) helps to dissolve a protein. This technique is referred to as salting in.

At extremely high salt concentrations, however, much of the water that would solvate the protein is taken up in the hydration shells of the counterions. Thus, the protein precipitates out of solution in a process called salting out.

Because different proteins respond differently to the same salt concentrations, both salting in and salting out can be used to purify proteins.