In contrast, the cyanide ion, CN^-, is a strong B-L base. It has a great attraction for protons, and the reaction

is heavily shifted to the right. As a result, HCN is a very weak acid.

In the examples discussed so far, the acidic species has been electrically neutral and the basic species charged. This need not always be so. Ammonia is a B-L base, and the ammonium ion is a B-L acid, capable of releasing a proton:

We can write an acid-dissociation equilibrium espression in the usual way:

This is related to the base-dissociation constant seen previously, K_b, by

This is the value of K_a for the ammonium ion that is given in the table on Page 8. You can either think of NH₃ as a moderately strong base with a K_b of 1.76 x 10^-5, or NH₄⁺ as a very weak acid with a K_a of 5.68 x 10^-10. The results of any calculation involving this acid-base equilibrium always will be the same whether you use K_a or K_b, as long as you know what you are doing.

Water can act as both a B-L acid and a B-L base.

We can regard the hydrating water molecules around a proton in solution as a weak B-L base

The Bronsted-Lowry theory has so influenced chemistry that two generations of chemists have written H₃O⁺ for the hydrated form of H⁺ and called it the "hydronium ion", even though the true hydration state of the proton is not known, but it is probably more like H⁺(H₂O)₄ or H₉O₄⁺.