High altitude balloons used by environment Canada to measure O3 levels in the atmosphere also use a similar idiometric titration where the iodine liberated is measured by back titration with sodium thiosulfate. In this case the iodine is generated by oxidation of iodide by ozone:
03 + 2I + 2H? I2 + O2 + H2O
If 10 000 L of air were bubbled through an acidic iodide solution (at stp) and was then back titrated with 19.4mL of a 1.2 x 10-4 M Na2S2O3 solution to reach equivalence, what is the concentration of ozone in that sample of air (in ppm)