A conjugate acid, within the Brønsted–Lowry acid–base theory, is a chemical compound formed when an acid gives a proton (H+) to a base—in other words, it is a base with a hydrogen ion added to it, as it loses a hydrogen ion in the reverse reaction.
On the other hand, a conjugate base is what remains after an acid has donated a proton during a chemical reaction.
Hence, a conjugate base is a substance formed by the removal of a proton from an acid, as it can gain a hydrogen ion in the reverse reaction.
A proton is a subatomic particle in the nucleus with a unit positive electrical charge.
Also, OH− can be considered as the conjugate base of H2O, since the water molecule donates a proton to give NH+4 in the reverse reaction.
A stronger conjugate acid will split more easily into its products, "push" hydrogen protons away and have a higher equilibrium constant.
If a conjugate base is classified as strong, it will "hold on" to the hydrogen proton when dissolved and its acid will not split.
Since HCl is a strong acid (it splits up to a large extent), its conjugate base (Cl−) will be weak.
Therefore, in this system, most H+ will be hydronium ions H3O+ instead of attached to Cl− anions and the conjugate bases will be weaker than water molecules.
On the other hand, if a chemical is a weak acid its conjugate base will not necessarily be strong.
In order for a species to have a strong conjugate base it has to be a very weak acid, like water.
The water molecule acts as a base because it receives the hydrogen cation (proton) and its conjugate acid is the hydronium ion (H3O+).
A second common application with an organic compound would be the production of a buffer with acetic acid.
Ringer's lactate solution is an example where the conjugate base of an organic acid, lactic acid, CH3CH(OH)CO−2 is combined with sodium, calcium and potassium cations and chloride anions in distilled water[4] which together form a fluid which is isotonic in relation to human blood and is used for fluid resuscitation after blood loss due to trauma, surgery, or a burn injury.
[5] Below are several examples of acids and their corresponding conjugate bases; note how they differ by just one proton (H+ ion).