Passage VI

A typical acid-base indicator is a compound that will be one color over a certain lower pH range but will be a different color over a certain higher pH range. In the small range between these pH ranges—the transition range—the indicator's color will be an intermediate of its other 2 colors.

Students studied 5 acid-base indicators using colorless aqueous solutions of different pH and a well plate (a plate containing a matrix of round depressions—wells—that can hold small volumes of liquid).

Experiment 1

The students added a pH=0 solution to 5 wells in the first column of the well plate, then added a pH=1 solution to the 5 wells in the next column, and so on, up to pH=7. Next, they added a drop of a given indicator (in solution) to each of the wells in a row, and then repeated this process, adding a different indicator to each row. The color of the resulting solution in each well was then recorded in Table 1 (B=blue, G= green, O=orange, P=purple, R=red, Y=yellow).

Experiment 2

Experiment 1 was repeated with solutions that had a pH of 8 or greater (see Table 2).

Experiment 3

Students were given 4 solutions (Solutions I-IV) of unknown pH. The well plate was used to test samples of each solution with 4 of the 5 indicators (see Table 3).