As proteins are amphoteric in nature, they can form salts with both cations and anions. Many ions form insoluble salts with proteins and therefore they are used as precipitating agents for protein.

Example: Anions of some acids like phosphotungstic, trichloroacetic acid, picric acid, and so on form insoluble acids with proteins. Heavy metals are used for precipitating proteins on the alkaline side of pI, the proteins behaving as anions. Ions of Hg, Cu, Ag, Zn, and so on are frequently used for this purpose