A buffer is mixture containing a weak acid and its conjugate base or a mixture containing a weak base and its conjugate acid. For example, CH₃COOH, NH₄⁺ and HPO₄^2- are weak acids. CH₃COO^-, NH₃ and PO₄^3- are their corresponding weak bases. Therefore, CH₃COOH and CH₃COO^-, NH₄⁺ and NH₃, and HPO₄^2- and PO₄^3- are buffers.

Notice, that in the above examples spectator ions are omitted. CH₃COOH and CH₃COONa, NH₄Cl and NH₃, and Li₂HPO₄ and Na₃PO₄ are buffers with possible spectator ions shown.