Question

# The indicator used in iodometric titrations isA. PhenolphthaleinB. LitmusC. Potassium iodideD. Starch

Hint: In an iodometric titration, the solution changes its colour from deep blue to light yellow when titrated with standardized thiosulfate solution. This marks the end point of the titration. The reaction involved is $\text{I}_{3}^{-}+\text{2}{{\text{S}}_{2}}\text{O}_{3}^{2-}\to {{\text{S}}_{4}}\text{O}_{6}^{2-}+3{{\text{I}}^{-}}$.

Let us first understand what is iodometric titration; Iodometric titration is also known as Iodometry is a method of volumetric analysis and a redox titration where the appearance or disappearance of elementary iodine indicates the end point. This titration involves indirect titration of iodine liberated by the reaction.
Let us now discuss the options to know the correct answer-
A. Phenolphthalein: Phenolphthalein is a chemical compound with the formula ${{\text{C}}_{20}}{{\text{H}}_{14}}{{\text{O}}_{4}}$, which is represented as $\text{HIn}$. It is used as an indicator in acid–base titrations. The completion of reaction is indicated by colour change as it turns colourless in acidic solutions and pink in basic solutions.
B. Litmus: A commonly used and natural indicator is litmus. It is extracted from lichens. It turns purple in neutral solutions. Litmus indicator turns red in acidic solutions and blue in alkaline solutions.
C. Potassium Iodide: Aqueous potassium iodide (KI) has a light orange-brown colour. It is used for testing the presence of oxidising agents; It is also used in iodometric solutions.
D. Starch: In iodometric titration, sodium thiosulphate, $\text{N}{{\text{a}}_{2}}{{\text{S}}_{2}}{{\text{O}}_{3}}$ (a reducing agent) is used specifically to titrate iodine. It is a general method to determine the concentration of an oxidising agent in solution. In an iodometric titration, a starch solution is used as an indicator as it can absorb the ${{\text{I}}_{2}}$ that is released.
The correct answer is the indicator used in iodometric titrations is starch
So, the correct answer is “Option D”.

Applications of iodometry:
(1)- When an excess amount of standard iodine solution is added to the known volume of sample. Sulphites reduce iodine readily in acidic medium to iodide, the sulphites present reduces iodine quantitatively: $\text{SO}_{3}^{2-}+{{\text{I}}_{2}}+{{\text{H}}_{2}}\text{O}\to \text{SO}_{4}^{2-}+\text{2}{{\text{H}}^{+}}+2{{\text{I}}^{-}}$
(2)- Iodometry is useful in volumetric analysis. Like, the determination of copper (II) and chlorate. The reactions include $\text{2C}{{\text{u}}^{+2}}+\text{4}{{\text{I}}^{-}}\to 2\text{CuI}+{{\text{I}}_{2}}$ and $6{{\text{H}}^{+}}+\text{ClO}_{3}^{-}+6{{\text{I}}^{-}}\to \text{3}{{\text{I}}_{2}}+\text{C}{{\text{l}}^{-}}+3{{\text{H}}_{2}}\text{O}$.
(3)- This method is used to analyze the concentration of oxidising agents in water samples such as active chlorine in swimming pools.

Note:
Potassium iodide is added to the titration as molecular iodine (${{\text{I}}_{2}}$) is not easily soluble in water. Sometimes, we get confused between iodometric and iodimetric titrations,

 IODOMETRY IODIMETRY Quantitative analysis of an oxidising agent by iodide that on reaction forms iodine, which is then titrated. Volumetric analysis which involves the titration with a standardized solution of iodine. Iodide reacts with another oxidising agent in an acidic medium. Uses free iodine that reacts with reducing agent in an acidic medium. It is a direct method. It is an indirect method. It quantifies oxidising agents. It quantifies reducing agents.