Looking back at the equilibrium set up when acetic acid dissolves in water, but drawing the mechanism of the back reaction, we see acetate ion acting as a base and H₃O⁺ acting as an acid. In all equilibria involving just proton transfer a species acting as a base on one side acts as an acid on the other. We describe H₃O⁺ as the conjugate acid of water and water as the conjugate base of H₃O⁺. In the same way, acetic acid is the conjugate acid of acetate ion and acetate ion is the conjugate base of acetic acid.

●For any acid and any base:

AH is an acid and A⁻ is its conjugate base and B is a base and BH⁺ is its conjugate acid. That is, every acid has a conjugate base associated with it and every base has a conjugate acid associated with it.

Water doesn’t have to be one of the participants—if we replace water in the reaction we have been discussing with ammonia, we now have ammonia as the conjugate base of NH₄ ⁺ (the ammo-nium cation) and the ammonium cation as the conjugate acid of ammonia. What is different is the position of equilibrium: ammonia is more basic than water and now the equilibrium will be well over to the right. As you will see, pKa will help us assess where equilibria like these lie.

The amino acids you met in Chapter 2 have carboxylic acid and amine functional groups within the same molecule. When dissolved in water, they transfer a proton from the CO₂H group to the NH₂ group and form a zwitterion. This German term describes a double ion having positive and negative charges in the same molecule.