Chemical structure of sodium bicarbonate

Acid salts are a class of salts formed when a dibasic or tribasic acid has been neutralized to some degree. Because the acid is only partially neutrallized, one or more replaceable protons remain. Typically this will lead to a formula with one or more metal ions, one or more protons, and an anion, such as sodium bicarbonate (NaHCO₃), sodium hydrosulfide (NaHS), sodium bisulfate (NaHSO₄), monosodium phosphate (NaH₂PO₄), and disodium phosphate (Na₂HPO₄).

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 hydrogen sulfate (NaHSO₄), half of the required amount of sodium hydroxide for neutralization is used.

Such compounds can act either 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.

Use in food

Main article: baking powder

Some acid salts are used in baking. They are found in baking powders and are typically divided into low-temperature (or single-acting) and high-temperature (or double-acting) acid salts. Common low-temperature acid salts react at room temperature to produce a leavening effect. They include cream of tartar, calcium phosphate, and citrates. High-temperature acid salts produce a leavening effect during baking and are usually aluminium salts such as calcium aluminium phosphate. Some acid salts may also be found in non-dairy coffee creamers.

See also

• alkali salt