Finding the pH in Diprotic Systems

Consider the amino acid leucine, designated HL. The neutral molecule shown in color has one acidic proton on the amino group. It can accept a proton on the carboxyl group to form H2L or it can lose the proton from the ammonium group to 

The side chain in leucine is an isobutyl group R = —CH2CH(CH3)2. 

The acid dissociation constants begin with the fully protonated form H2L  

A salt such as leucine hydrochloride contains the protonated species H2L , which can dissociate twice, as indicated in Reactions 11-3 and 11-4. Because Ki = 4.70 X 10, H2L is a weak acid. HL is an even weaker acid, with K 2 — 1.80 X 10 ° It appears that H2L will dissociate only partly, and the resulting HL will hardly dis-sociate at all. For this reason, we make the (superb) approximation that a solution of H2L behaves as a monoprotic acid, with K = K.  

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