Question

In which technique, iodimetry or iodometry, is starch indicator not added until just before the end point? Why?

Short answer

Starch is added just before the end point in iodometry technique when the ${{\mathrm{I}}_3}^{-}$starts to fade. It is not added at the beginning of the technique because some of ${\mathrm{I}}_2$ would stay bounded to starch and the end point would not be detected right.

Step-by-step solution

Definition of Iodometry.

• Iodometry, regularly called iodometric titration, is a redox titration approach wherein the emergence or disappearance of essential iodine determines the cease point.
• In iodometry iodide is added to a solution of an oxidizing agent. The iodide is oxidized to iodine. The iodine is titrated with sodium thiosulfate for quantitative analysis.
• In iodimetry, the titration is done with iodine solution.

Determine the technique that used starch indicator not added until the end point.

• Starch is added just before the end point in iodometry when the ${{\mathrm{I}}_3}^{-}$starts to fade.
• It is not added at the beginning of the technique because some of would stay bounded to starch and the end point would not be detected right.
• A starch solution is used as an indicator in an iodometric titration because it can absorb the ${\mathrm{I}}_3$that is emitted.
• When titrated with standardised thiosulfate solution, this absorption causes the solution's colour to change from deep blue to pale yellow.
• This represents the titration's termination point.
• Iodometry is a technique for determining the quantity of oxidising chemicals in water samples, such as oxygen saturation in ecological research or active chlorine in pool water analysis.

Hence, starch is added before the end point in iodometry technique.