Examples of the counter ion represented by Xn??? include halogen ions such as chlorine, bromine, and iodine ions; carboxylate ions such as acetate, oxalate, fumarate, and benzoate ions; sulfonate ions such as p-toluenesulfonate, methanesulfonate, butanesulfonate, and benzenesulfonate ions; a sulphate ion; a perchlorate ion; a carbonate ion; and a nitrate ion.