Ammonium sulfate precipitation is a method used to purify proteins by altering their solubility.It is a specific case of a more general technique known as salting out. Ammonium sulfate is commonly used as its solubility is so high that salt solutions with high ionic strength are allowed.his is usually done by adding solid ammonium sulfate, but calculating how much ammonium sulfate to add to a solution at one concentration to achieve a desired higher concentration is tricky, since addition of ammonium sulfate significantly increases the volume of the solution.