Acid salt

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.

For example, the acid salt sodium bisulfate is the main species formed upon the half neutralization of sulfuric acid with sodium hydroxide:

An acid salt can act either as an acid or as a base: addition of a suitably strong acid will protonate anions, and addition of a suitably strong base will split off H⁺. The pH of a solution of an acid salt will depend on the relevant equilibrium constants and the amounts of any additional base or acid. 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.

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.

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