The buffering action of amino acids such as alanine, CH₃CH(NH₂)COOH, is sometimes referred to in A level syllabuses. The nature of this action is not usually made clear, however, and is often confused with the fact that these compounds are amphiprotic, that is can accept or donate protons.

An aqueous solution of alanine has a pH not far off neutrality. It is certainly true that it can accept protons:

                                                     ₊
CH₃CH(NH₂)COOH + H₃O⁺—> CH₃CH(NH₃)COOH + H₂O

and it can donate them:

CH₃CH(NH₂)COOH + H₂O —> CH₃CH(NH₂)COO ^- + H₃O⁺

In both cases, though, the pH changes quite considerably; it is only when enough acid has been added to give a mixture of CH₃CH(NH₂)COOH and CH₃CH(N⁺H₃)COOH that the buffering action can occur by the usual means, that is the pH is on the relatively flat portion of the curve. The amino-acid becomes a buffer once some acid has been added.

A similar argument can be made for the addition of base; in this case the buffer mixture is CH₃CH(NH₂)COOH and CH₃CH(NH₂)COO ^- . As with all buffer mixtures, there must be an acid and its conjugate base present for the buffering action to happen.

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Dr Rod Beavon   17 Dean's Yard  London SW1P 3PB

e-mail:  rod.beavon@westminster.org.uk