Buffer action of amino acids

Amino acids have a natural buffer action, and so, on adding small quantities of acid or alkali the pH changes gradually. If larger quantities of acid or alkali are added the buffer will become exhausted and the pH will change rapidly. This buffer action is explained by the presence of the two groups in amino acids, the weakly basic amino group and the weakly acidic carboxylic acid group. Added acid will react with the amino group, removing some of it, and so reducing the pH change:

RCH(NH₂)COOH + HCl ⇒ RCH(NH₃Cl)COOH

Added alkali will react with the carboxylic acid group, and in the same way the pH change is limited:

RCH(NH₂)COOH + NaOH ⇒ RCH(NH₂)COONa + H₂O

The strong acid, hydrochloric acid, showed little change in pH until nearly all the acid had been used up during the final addition of alkali. At this point there was a sudden jump in the pH. In theory it should have been neutral after the addition of 2.0 cm³ of alkali, but the volumes were not precisely measured. A similar rapid change is seen on addition of the hydrochloric acid to the sodium hydroxide solution.

The glycine solution changed much more gradually in pH because of the buffer action explained above. You should also have noted that the presence of the amino group makes the original glycine (aminoethanoic acid) solution less acidic. It is around pH 5.