Passage III

When the ozone (O₃) in air is mixed with an acidic solution of iodide ion (I^-), the O₃ reacts to form triiodide ion (I₃^-), O₂, and H₂O.

O₃ + 3I^-+ 2H⁺ → I₃^- + O₂ + H₂O

Students performed an experiment to determine the concentration of O₃ in samples of air. Figure 1 shows the relationship between the concentration of I₃^-, in μmol/L, and the concentration of O₃, in parts per billion (ppb), under the conditions of the experiment.

Experiment

At each of 4 outdoor sites, the students assembled the apparatus shown in Figure 2. First, they attached a long piece of tubing to a stand so that one end of the tubing was 1.5 m above the ground. Then, they placed the other end of the tubing into one hole of a 2-holed stopper. Next, they placed one end of a shorter piece of tubing into the second hole of the 2-holed stopper and attached the other end of the shorter tubing to a vacuum pump.

At each site, the students performed the following 2 steps every half hour from 12:30 p.m. to 5:30 p.m. on a particular day:

1. A 10.0 mL volume of an aqueous acidic solution having a 10 mg/mL concentration of I^- was placed into a clean empty test tube that was then sealed with the 2-holed stopper.

2. The vacuum pump was turned on for 25 min to collect an air sample. Then, the pump was turned off and the concentration of I₃^-, in micromoles/liter (μmol/L), in the solution was measured.

The results are shown in Figure 3.

Figures adapted from J. V. seeley et al., "A Simple Method for Measuring Ground-Level Ozone in the Atmosphere." @2005 by the Division of Chemical Education, Inc., American Chemical Society.