Titration of KI with H₂O₂ in presence of acid is a:

The reaction involves :
$\begin{array}{l}{\mathrm{H}}_2{\mathrm{O}}_2+2{\mathrm{I}}^{-}+2{\mathrm{H}}^{+}⟶{\mathrm{I}}_2+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\\ 2{\mathrm{Na}}_2{\mathrm{S}}_2{\mathrm{O}}_3+{\mathrm{I}}_2⟶{\mathrm{Na}}_2{\mathrm{S}}_4{\mathrm{O}}_6+2\mathrm{NaI}\end{array}$
The reaction gives blue colour only after all the Na₂S₂O₃ is used. The reaction is carried out with adjusted amount of Na₂S₂O₃ so that only a fraction of H₂O₂ and KI reaction occurs before the blue colour of starch-l₂ appears, however the slow redox reaction of H₂O z-lzcontinues. The appearance of blue colour is like clock alarm and in  such reactions time for.the appearance of blue colour is noticed. The phenomenon is used in studying rate of reaction. If time taken fbr blue.colour appearance is longer, the reaction is slow and vice - versa.