Passage IV

Ozone (O₃), like household bleach, can break down molecules by a process called oxidation. Students did 3 experiments to study the effects of O₃ on different food colorings.

Each experiment was done using a colorimeter. In a colorimeter, light of a certain wavelength is directed through a sample solution onto a detector. If a compound in the solution absorbs some of the light, that light will not reach the detector (see diagram) .

The solution's absorbance is related to the amount of light absorbed, and thus, to the concentration of the compound in the solution.

Experiment 1

Students added 0. 010 mL of blue food coloring to 500.0 mL of H₂O in a large flask. A sample of this solution was added to a transparent vial. The vial was placed in a colorimeter and its absorbance was measured at 2 wavelengths, 427 nm and 603 nm. The sample was returned to the flask. The following procedure was then repeated several times in succession:

1. The solution in the flask was stirred continuously while O₃ was bubbled through the solution at a constant rate for 1 min; then the flow of O₃ was stopped.

2. A sample of the solution was placed in the transparent vial and its absorbance was measured in the colorimeter at 427 nm and at 603 nm.

3. The sample was returned to the flask.

The students then plotted absorbance versus exposure time (total time that O₃ was bubbled through the solution) as shown in Figure 1.

Experiment 2

Experiment 1 was repeated using yellow food coloring instead of blue food coloring (see Figure 2).

Experiment 3

Experiment 1 was repeated using green food coloring instead of blue food coloring (see Figure 3).