Acid salt

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Acid salts are a class of chemical compounds that are formed when a dibasic or tribasic acid has been neutralized to some degree. Because the acid is only partially neutralized, one or more replaceable protons remain. Typically this will lead to a formula with one or more cation(s), one or more protons, and an anion, such as NaHCO₃, NaHS, NaHSO₄, NaH₂PO₄, Na₂HPO₄, etc. For example, in preparing sodium sulfate (Na₂SO₄) a certain amount of sodium hydroxide (NaOH) is needed to neutralize the sulfuric acid (H₂SO₄) to produce this salt. When preparing sodium hydrogensulfate (NaHSO₄, an acid salt), half of the required amount of sodium hydroxide for neutralization is used.

Such compounds can act as an acid or a base: Addition of an acid will restore protons, and addition of a base will consume protons. The actual pH of a solution of an acid salt will depend on the equilibrium constants involved, and whether the anion is a better proton donor or proton acceptor. A comparison between the K_b and K_a will indicate this: if K_b > K_a, the solution will be basic, whereas if K_b < K_a, the solution will be acidic.

Examples of acid salts include: sodium hydrogensulfate (NaHSO₄), sodium hydrogen carbonate (NaHCO₃), sodium hydrogen sulfide (NaHS), and phosphatase (a class of enzymes).

Common low-temperature acid salts include cream of tartar, calcium phosphate, and citrate. High-temperature acid salts are usually aluminium salts, such as calcium aluminum phosphate. They can be found in many baking powders, also in many non-dairy coffee creamers.