Why is ammonium sulphate used in protein precipitation?

Ammonium sulfate is commonly used in protein precipitation because it can selectively precipitate proteins based on their size and charge. By adjusting the concentration of ammonium sulfate, different proteins can be selectively precipitated, allowing for the purification and concentration of specific proteins from complex mixtures. Additionally, ammonium sulfate is relatively inexpensive, soluble in water, and widely available for laboratory use.