Overview

pH indicators are compounds, typically water-soluble organic acids, that change their ionization state and color with pH. Bromocresol green (BCG) is representative. It is moderately acidic (easily deprotonated) so it can be used to detect pH changes between approximately pH 3 (yellow) and 6 (blue-green).

You will investigate BCG's colorful behavior by measuring its UV-vis spectrum at different pH and then use these data to estimate its pK_a.

In other parts of the experiment, you will use molecular modeling to investigate the electronic changes that occur in BCG when it is deprotonated, and you will investigate the pH behavior of the acetic acid-sodium acetate buffer system.

Time required: 2 laboratory sessions

Credit

This experiment was adapted from "Chemistry 110 Laboratory Experiments", T.G. Dunne, D.P. Gerrity, R.W. McClard, A. Mega, Reed College, 1989.