Question

Vitamin C (ascorbic acid) is an example of iodimetric determination, so based on the rules that

“in iodimetry (titration with I3), starch can be added at the beginning of the titration. The first drop of excess I3 after the equivalence point causes the solution to turn dark blue. In iodometry (titration of I3), I3 is present throughout the reaction up to the equivalence point. Starch should not be added until immediately before the equivalence point.”

However, why starch is still not added until just before the end point when we were doing Vitamin C experiment?

Answer 1

Iodine in water solutions is usually colored strong enough so that its presence can be detected visually. However, close to the end point, when the iodine concentration is very low, its yellowish color is very pale and can be easily overlooked. Thus for the end point detection starch solutions are used.

Iodine gets adsorbed on the starch molecule surface and product of adsorption has strong, blue color.

Iodine gets adsorbed on the starch molecule surface and product of adsorption has strong, blue color. Exact mechanism behind adsorption and color change is not known, see for example this explanation of starch as an indicator usage.

In the presence of small amounts of iodine adsorption and desorption are fast and reversible. However, when the concentration of iodine is high, it gets bonded with starch relatively strong, and desorption becomes slow, which makes detection of the end point relatively difficult. Luckily high concentrations of iodine are easily visible, so if we are using thiosulfate to titrate solution that initially contains high iodine concentration, we can titrate till the solution gets pale and add starch close to the end point. In the case of titration with iodine solution we can add starch at the very beginning, as high iodine concentrations are not possible before we are long past the end point.That's why starch is still not added until just before the end point when we were doing Vitamin C experiment