In the titration of ${\mathrm{I}}_2 \left(\mathrm{aq}\right)$ by ${\mathrm{S}}_2{\mathrm{O}}_3^{2-}$ $\left(\mathrm{aq}\right)$ using the starch indicator, the end point is indicated by:

Titration of iodine $\left({\mathrm{I}}_2\right)$ by thiosulphate solution $\left({\mathrm{S}}_2{\mathrm{O}}_3^{2-}\right)$ follows the reaction:

$2{\mathrm{S}}_2{\mathrm{O}}_3^{2-}+{\mathrm{I}}_2\to 2{\mathrm{I}}^{-}+{\mathrm{S}}_4{\mathrm{O}}_6^{2-}$

In the presence of starch, free iodine present in the solution imparts blue colour to the solution.

End point marks the completion of reaction, that is, at this point all the iodine present in the solution is reduced to iodide ions by thiosulphate solution.

So, due to the absence of free iodine in the solution, the solution turns colourless.

Thus, the end point is indicated by colour change in the solution from blue to colourless.