Acid salt is a term for a class of salts formed by the partial neutralization of diprotic or polyprotic acids. Because the parent acid is only partially neutralized, one or more replaceable hydrogen atoms remain. Typical acid salts have one or more alkali (alkaline) metal ions as well as one or more hydrogen atoms. Well known examples are sodium bicarbonate (NaHCO₃), sodium hydrosulfide (NaHS), sodium bisulfate (NaHSO₄), monosodium phosphate (NaH₂PO₄), and disodium phosphate (Na₂HPO₄). Often acid salts are used as buffers.