This exercise introduces the acid/base properties of weak acids and bases in water using two specific examples.  Titration of glycine illustrates the acid-base behavior of carboxylic acids and amines, and of buffers in general.  Titration of 2,4-pentanedione illustrates the stabilizing effect of resonance delocalization on carbanion stability, an issue that is critical for understanding most mechanisms of C-C bond formation and breakage in metabolism.

Students titrate three samples. The first two are 40-mL samples of water and of 0.40 M glycine, both adjusted to a pH of about 1.5 with HCl. The third is 40 mL of water to which has been added one drop of 1 M HCl and 2 mL of 2,4-pentanedione (dissolves, but slowly). The titrant is 1.0 M KOH, which I prefer to NaOH to avoid sodium ion interference seen with less expensive electrodes at higher pH. Students add KOH until the pH passes 12.

They are then to tabulate and graph their data and to answer several questions.  Regarding the first two titrations, they are asked to (1) account for the significant differences between the first two titration curves, (2) draw the four possible ionic forms of glycine, (3) identify which of these predominates at pH 1, pH 6, and pH 11, and which doesn't exist in water, (4) identify on their graph and estimate the two pKa's of glycine, (5) write the equations showing how glycine buffers solutions at either pKa by consuming small amounts of added hydrogen or hydroxide ions. Regarding the third titration, they are asked to (1) estimate the pKa of 2,4-pentanedione and (2) to show three resonance structures that account for the fact that 2,4-pentanedione is about 100 billion times stronger acid than acetone.

This exercise is simple, and reviews material students have already been exposed to in general and organic chemistry and (for most of them) quantitative analysis. It requires enough pH meters for students to finish in a lab period.

Revised 2/21/08