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 metal ions, 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 Kb and Ka will indicate this: if Kb > Ka, the solution will be basic, whereas if Kb < Ka, 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.
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