Why is starch added in vitamin C titration?

This method determines the vitamin C concentration in a solution by a redox titration using iodine. Once all the ascorbic acid has been oxidised, the excess iodine is free to react with the starch indicator, forming the blue-black starch-iodine complex. This is the endpoint of the titration.

What is the principle for using starch in the iodine titration?

In an iodometric titration, a starch solution is used as an indicator since it can absorb the I 2 that is released. This absorption will cause the solution to change its colour from deep blue to light yellow when titrated with standardised thiosulfate solution. This indicates the end point of the titration.

Why is acetic acid used in iodometric titration?

The role of glacial acetic acid is to maintain the pH around 3-4 so that starch can do it job perfectly in the titration .

What happens when iodine is added to a starch solution that also contains vitamin C?

The color is the result of the formation of the starch-iodine complex. If vitamin C is present, the brownish color of the iodine solution will become colorless — the vitamin C serves as a reducing agent and reduces iodine to iodide ions (colorless in solution).

Why is the starch added almost at the end of the titration?

Starch is added in a iodometric titration to make clear the presence of the last bit of iodine. The color of iodine in low concentration is light yellow and difficult to see. The iodine – starch complex is dark blue to black and makes it easy to observe.

Why starch is used in iodometric titration?

The Starch-Iodide complex is not very soluble in water, so the starch is added near the endpoint of an Iodine titration, when the Iodine concentration is low. This eliminates errors due to the fact that some Iodine may remain adsorbed on the complex and go undetected.

Why freshly prepared starch is used?

Starch made complex with iodide when heated with water decomposition occure ,beta-amylose formed which combined with iodine to form starch/iodine complex of highly intense color. Freshly prepared starch is used because it is biodegraded.

Why does iodine react with ascorbic acid?

When ascorbic acid reacts with iodine, the ascorbic acid is oxidized (looses electrons) and the iodine is reduced (gains electrons). As long as the solution contains ascorbic acid, the iodine is used up in a rapid. reaction with ascorbic acid, during which dehydroascorbic acid and iodide ion are formed.

Why is starch not added at the beginning of iodometric titration?

Adding the starch earlier on in the titration would cause the formation of an iodine-starch complex that yields a reddish-brown colored endpoint. In addition to this, iodometric titration is done under highly acidic conditions. Starch hydrolyzes in highly acidic media and may interfere with the reaction. 6.