Question

Assertion:Starch is used as an adsorption indicator in iodometric and iodimetric titrations.
Reason:Starch forms iodo starch complex with iodine, which is blue coloured.
(A) Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
(B) Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
(C) Assertion is correct but Reason is incorrect.
(D) Reason is correct but Assertion is incorrect.

Answer 1

Hint: Iodometry, known as iodometric titration, is a popular method of volumetric analysis of a redox titration in which the appearance or disappearance of elementary iodine dictates the endpoint.


Complete solution:
In iodometry, the titration of iodine is indirect while in iodometry the titration is done directly. The titration is done with the help of sodium thiosulphate. Its chemical formula is \[N{a_2}{S_2}{O_3}\]. It is used as a reducing agent. The titration is called iodometric titration because it is used specifically to titrate iodine. It is a common method that is used to find the concentration of an oxidizing agent in a solution. In an iodometric titration, we use the starch solution as an indicator. Starch forms an iodo starch complex with iodine, which is blue coloured as it can absorb the ${I_2}$ which is released in the process. The absorption of iodine results in the colour change of the solution from deep blue to light yellow when it is titrated with thiosulfate solution. This is the end point. At the initial stage of the titration, when there is a high concentration of ${I_2}$ in the solution, the solution turns out to be deep brown or reddish-brown. It is due to the presence of $I_3^ - $ ions in the solution. As the reaction proceeds further, the concentration of ${I_2}$ in solution decreases and its colour turns to a light brown, then yellow, and then very pale yellow. After that, we add starch as an indicator which turns the solution blue-black in colour and then colourless which is the endpoint.
Hence, both assertion and reason are correct and the reason is the correct explanation for Assertion.

Thus, the correct option is (A) .


Note:Redox titrations have importance in industries.The application of Iodometry is in the analysis of the concentration of oxidizing agents present in water samples like oxygen saturation and in ecological studies or active chlorine in swimming pools.