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 (hydrogen ion). A conjugate acid can also be seen as the chemical substance that releases, or donates, a proton(hydrogen ion) 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 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)
Strong acids tend to have a weak conjugate base, and vice versa.
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 |