The sulfite process produces wood pulp which is almost pure cellulose fibers by using various salts of sulfurous acid to extract the lignin from wood chips in large pressure vessels called digesters. The salts used in the pulping process are either sulfites (SO₃^2-), or bisulfites (HSO₃^-), depending on the pH. The counter ion can be sodium (Na⁺), calcium (Ca²⁺), potassium (K⁺), magnesium (Mg²⁺) or ammonium (NH₄⁺).