Question

Explain the process to estimate ${\mathrm{O}}_3$ quantitatively.

Answer 1

Ozone (${\mathrm{O}}_3$):-

1. Ozone is an inorganic molecule with formula ${\mathrm{O}}_3$ which is also often known as Trioxygen.
2. It has a strong unpleasant odour and is also light blue in colour.
3. Comparing with diatomic allotrope ${\mathrm{O}}_2$, oxygen allotrope ${\mathrm{O}}_3$ is very much less stable than it and thus gets decomposed to ${\mathrm{O}}_2$ in the lower atmosphere.

Process to estimate ${\mathrm{O}}_3$ quantitatively:-

1. Ozone can be measured quantitatively using Potassium iodide $\left(\mathrm{KI}\right)$.
2. When ozone is reacted with Potassium iodide solution which is buffered with a borate buffer, Iodine is released.
3. Using starch as an indicator, this released iodine may be titrated against a standard solution of Sodium thiosulphate.
4. The reaction is: $\underset{\mathrm{Iodine}\mathrm{ion}}{2{\mathrm{I}}^{-}\left(\mathrm{aq}\right)}+\underset{\mathrm{Water}}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}+\underset{\mathrm{Ozone}}{{\mathrm{O}}_3\left(\mathrm{g}\right)}\to \underset{\mathrm{Hydroxide}\mathrm{ion}}{2\mathrm{O}{\mathrm{H}}^{-}\left(\mathrm{aq}\right)}+\underset{\mathrm{Iodine}}{{\mathrm{I}}_2\left(\mathrm{s}\right)}+\underset{\mathrm{Oxygen}}{{\mathrm{O}}_2\left(\mathrm{g}\right)}$
5. This ${\mathrm{I}}_2$which is liberated is titrated against a standard solution of starch and Sodium thiosulphate which is used as an indicator here.
6. The reaction is: $\underset{\mathrm{Sodium}\mathrm{thiosulphate}}{2\mathrm{N}{\mathrm{a}}_2{\mathrm{S}}_2{\mathrm{O}}_3\left(\mathrm{aq}\right)}+\underset{\mathrm{Iodine}}{{\mathrm{I}}_2\left(\mathrm{s}\right)}\to \underset{\mathrm{Sodium}\mathrm{tetrathionate}}{\mathrm{N}{\mathrm{a}}_2{\mathrm{S}}_4{\mathrm{O}}_6\left(\mathrm{aq}\right)}+\underset{\mathrm{Sodium}\mathrm{iodide}}{2\mathrm{NaI}\left(\mathrm{aq}\right)}$
7. Here this Sodium thiosulphate gives Sodium tetrathionate when reacted with Iodine which is quantitatively estimated for ozone estimation.