Question

The end point of iodometric titrations is detected by adding starch just near the end point and not at the beginning of titration. Which statement is not valid for this fact?
A.Starch forms a complex with $I_2$.
B.Due to slow decomposition of complex, a diffuse endpoint is obtained.
C.Iodometric titrations are carried out in a strong alkaline medium where starch is either hydrolysed or decomposed.
D.Iodometric titrations are made in a weak acidic medium where starch is neither hydrolysed nor decomposed.

Answer 1

Hint: The procedure of adding the known amount of a solution of known concentration to another solution of the unknown concentration slowly in order to know the neutralisation point of the reaction which is often indicated by the change in the colour of the solution.

Complete answer:
Iodometry is the method of the volumetric chemical analysis, where the appearance and disappearance of the iodine in the solution indicates the end point. The procedure of the Iodometry is based on the oxidation of the iodide into iodine. The amount of the oxidising agents is determined by the Iodometric titrations. There are four types of titrations commonly used these are:
1.Acid- base titrations.
2.Complexometric titrations
3.Redox titrations
4.Precipitation titrations.
In order to determine the end point of the reaction starch is used as an indicator. The formation of the Blue black complex indicates the appearance of the end point of the reaction. This can be chemically represented as:
${I_2} + N{a_2}{S_2}{O_3} \to 2NaI + N{a_2}{S_2}{O_4}$

Thus the option C. Iodometric titrations are carried out in strong alkaline medium where starch is either hydrolysed or decomposed, is the correct answer.

Note:
Iodometric titrations are commonly used to analyse the concentration of the oxidizing agents in water samples, to study the oxygen saturation in the ecological studies or to analyse the active chlorine content in the swimming pool.