What is the definition of buffering capacity?

Buffering capacity is defined as the number of moles of strong base or acid needed to change the pH of a liter of buffer solution by one unit. A general buffer capacity estimate is 40 percent of the total sum of the molarites of the conjugate base and acid.

For a buffer solution, an acid with a pKa value close to the buffer solution itself should be chosen, which ensures that each part of the conjugate base-acid pair is close to equal. It also helps make the buffer more effective in neutralizing any addition of strong acid or strong base. The pKa acid solution is defined as the negative of the logarithm of the acid-dissociation equilibrium constant Ka.