Conjugate acid

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Within the Brønsted-Lowry (protonic) theory of acids and bases, 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 a proton in the backward chemical reaction. Thus, the term acid. The base produced, X−, is called the conjugate base and it absorbs a proton in the backward chemical reaction. In aqueous solution, the chemical reaction involved is of the form

HX + H2O \rightleftarrows X− + H3O+

This principle is discussed in detail in the article on acid-base reaction theories.

The conjugate base of a weak acid is a strong base, and the conjugate base of a strong acid is a weak base, and vice versa.

Tabulated below are several examples of conjugate acid-base pairs. Acid strength decreases and base strength increases down the table. (The dissociation reaction reaches equilibrium further to the right, with more X− produced.)

Acid Base
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
HC2H3O2 Acetic acid C2H3O2− 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
HFSbF5 Fluoroantimonic acid SbF5− Antimony pentafluoride ion

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