Q.How is O3 estimated quantitatively?

Ozone, $O_3$, is a bent, resonance-stabilised triatomic allotrope of oxygen in which the central oxygen is linked to the two terminal oxygens by bonds that are, by resonance, exactly equivalent and intermediate between a formal single and double bond. It is an extremely powerful oxidising agent because it readily decomposes to release reactive atomic (nascent) oxygen, $O_3 \rightarrow O_2 + [O]$, and this nascent oxygen oxidises substrates far more vigorously than ordinary $O_2$, for example turning moist starch-iodide paper blue-black by liberating iodine from potassium iodide. Ozone is estimated quantitatively by passing a measured volume of it through excess potassium iodide solution, $O_3 + 2KI + H_2O \rightarrow 2KOH + I_2 + O_2$, and titrating the liberated iodine against a standard sodium th …