Question

Which indicator we prefer to use in iodometric titration?
A. Starch
B. Phenolphthalein
C. 1, 10 phenolphthalein
D. All of the above

Answer 1

Hint: During Iodometric titration which involves the determination of the concentration of oxidising agent, elemental Iodine is released which complexes with the indicator resulting in the formation of blue color.

Complete answer:
Iodometry is a type of redox titration where we determine the concentration of a oxidizing agent in a given sample solution
Determination of concentration of oxidising agent
Suppose if the given sample solution contains Potassium dichromate which is a oxidising agent whose concentration should be known
${{\text{K}}_{\text{2}}}{\text{C}}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{7}}}{\text{ + 7}}{{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}{\text{ + KI}}\xrightarrow{{}}{\text{C}}{{\text{r}}_{\text{2}}}{\left( {{\text{S}}{{\text{O}}_{\text{4}}}} \right)_{\text{3}}}{\text{ + }}{{\text{K}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}{\text{ + 7}}{{\text{H}}_{\text{2}}}{\text{O + 3}}{{\text{I}}_{\text{2}}}$
An excess amount of Potassium Iodide is taken for iodometric titration compared to other solutions
Potassium dichromate reacts with Potassium iodide by oxidizing it to Iodine
i.e, ${\text{C}}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{7}}}^{{\text{2 - }}}{\text{ + }}{{\text{I}}^{\text{ - }}}\xrightarrow{{}}{\text{C}}{{\text{r}}^{{\text{ + 3}}}}{\text{ + }}{{\text{I}}_{\text{2}}}$
On oxidation of Iodide to Iodine the colour of the solution changes to yellowish brown
There is still some amount of Iodide in the sample which then combines with Iodine forming ${{\text{I}}_{\text{3}}}^{\text{ - }}$
In the next step starch indicator is added to the sample solution which combines with ${{\text{I}}_{\text{3}}}^{\text{ - }}$ forming starch iodine complex which is deep blue in colour
When sodium thiosulphate is added to the solution the thiosulphate ion will oxidise the iodine from starch iodine complex to Iodide ion
${\text{2}}{{\text{S}}_{\text{2}}}{{\text{O}}_{\text{3}}}^{{\text{2 - }}}{\text{ + }}{{\text{I}}_{\text{2}}}\xrightarrow{{}}{{\text{S}}_{\text{4}}}{{\text{O}}_{\text{6}}}^{{\text{2 - }}}{\text{ + 2}}{{\text{I}}^{\text{ - }}}$
Thus the indicator used is starch as it can form the complex with the Iodine released.

Hence, the answer to the above question is option A.

Additional Information:
Iodometric titration is a redox titration which involves volumetric chemical analysis where the endpoint is determined by the appearance and disappearance of elementary iodine. Iodometry mainly involves indirect titration of liberated iodine with the analyte.

Note: Iodometric titrations are usually helpful to determine the concentration of oxidizing agent that is present in the
solution. The concentration of oxidizing agents present in the water samples can also be determined by Iodometry.