Conjugate acid

Conjugate acid

Within the Brønsted–Lowry acid-base theory (protonic), a conjugate acid is the acid member, HX, of a pair of two compounds that transform into each other by gain or loss of a proton. A conjugate acid can also be seen as the chemical substance that releases, or donates, a proton in the forward chemical reaction, hence, the term acid. The base produced, X, is called the conjugate base, and it absorbs, or gains, a proton in the backward chemical reaction. In aqueous solution, the chemical reaction involved is of the form

HX + H2O is in equilibrium with X + H3O+

This principle is discussed in detail in the article on acid-base reaction theories. The position of the equilibrium is measured as an acid dissociation constant (Ka)

Tabulated below are several examples of conjugate acid-base pairs. Acid strength decreases and base strength increases down the table.

Acid Base
HFSbF5 Fluoroantimonic acid SbF6 Hexafluoroantimonate ion
HCl Hydrochloric acid Cl Chloride ion
H2SO4 Sulfuric acid HSO4 Hydrogen sulfate ion
HNO3 Nitric acid NO3 Nitrate ion
H3O+ Hydronium ion H2O Water
HSO4 Hydrogen sulfate ion SO42− Sulfate ion
H3PO4 Phosphoric acid H2PO4 Dihydrogen phosphate ion
CH3COOH Acetic acid CH3COO Acetate ion
H2CO3 Carbonic acid HCO3 Hydrogen carbonate ion
H2S Hydrosulfuric acid HS Hydrogen sulfide ion
H2PO4 Dihydrogen phosphate ion HPO42− Hydrogen phosphate ion
NH4+ Ammonium ion NH3 Ammonia
HCO3 Hydrogencarbonate (bicarbonate) ion CO32− Carbonate ion
HPO42− Hydrogen phosphate ion PO43− Phosphate ion
H2O Water (neutral, pH 7) OH Hydroxide ion

See also

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