Acid salt

Acid salts are a class of salts that produce an acidic solution after being dissolved in a solvent. Their formation typically results from the partial neutralization of diprotic or polyprotic acids, where some replaceable hydrogen atoms remain unreacted with hydroxide ions. This half-neutralization leaves the salt capable of releasing protons into solution. A key characteristic of an acid salt is that its solution exhibits greater electrical conductivity than the pure solvent alone.

Formation

The acid–base nature of the resulting solution depends on the salt’s constituent ions. Reactive cations within the salt can undergo hydrolysis, reacting with water molecules and causing deprotonation of their conjugate acids. For instance, ammonium chloride, an acid salt, forms upon the half-neutralization of ammonia with hydrogen chloride in aqueous solution. In such a case, the ammonium ion’s acid dissociation constant (Ka) is the primary influence on acidity; its Ka value is significantly higher than that of water, ensuring deprotonation when reacting with water and yielding a pH below 7 at room temperature.

Intensity of acid

Conversely, the chloride ion has very low affinity for protons and does not hydrolyze. The overall acidity or alkalinity of an acid salt solution is determined by comparing the Ka of the cation and the Kb of the anion: the solution is acidic if the cation hydrolyzes more extensively than the anion, alkaline if the opposite occurs, and neutral only when the two constants are equal. Additional factors such as relevant equilibrium constants and the presence of extra base or acid can further alter the pH.

Use in food

In food applications, acid salts serve as leavening acids, often mixed with base salts like sodium bicarbonate to create baking powders that release carbon dioxide. Common examples include cream of tartar and monocalcium phosphate; some leavening agents react immediately at low temperatures, while others, like sodium aluminum phosphate, react when heated. Double-acting baking powders combine both types for even rising. Disodium phosphate is used in foods, while monosodium phosphate appears in animal feed, toothpaste, and evaporated milk.