pH and Buffers

The purpose of this exercise is to illustrate buffering properties of phosphate, acetate, and an amine and to give students a sense of how buffers work. I develop the subject very briefly from equilibrium constants and the Henderson-Hasselbalch equation, but I want them to appreciate how buffers work from the more illuminating perspective of the acid-base properties of the buffer components and the neutralization reactions of the conjugate acid and base.

Students have 1 M solutions of monobasic and dibasic potassium phosphate; acetic acid and sodium acetate; and ethanolammonium chloride and ethanolamine. In the first part of the exercise, they determine the pH of 1 mL of each solution separately to recognize that one is a base and one an acid, and the pH of the three respective buffers (0.5 mL each of acid and conjugate base). They measure pH with pH meters. A tip about pH electrodes: most commercial electrodes have a diameter of a half inch, which makes them too fat to fit into a 13 x 100-mm test tube and too short to fit into a 16 x 150-mm tube. I have acquired two Accu-Met combination electrodes which have a 3/8-in. diameter and work perfectly.

In the second part of the exercise, students determine the pH of water and each of the three buffers to which has been added one drop of either 1 M HCl or 1 M NaOH.

They are asked to identify or devise equations that explain the pH of the various weak acid and conjugate base solutions. They are asked (having been given phosphate's pKa of 7) how they could have predicted that the phosphate buffer's pH would be around 7, and to estimate from their data the pKa of acetic acid and of ethanolammonium ion. Finally, they are asked to identify or devise equations that explain why the pH's of the various buffers didn't change with added HCl or NaOH.