Question

Sodium thiosulphate is the laboratory reagent used in the iodometric titration. This also finds use in photography. Based on the above passage answer the following questions
If \[{I_2}\] is the titre in a titration A and ${I_2}$ is formed in situ in a titration B then these are respectively:
A.Iodometric, iodimetric
B.Iodimetric, iodometric
C.Both iodimetric
D.Both iodometric

Answer 1

Hint: The term "iodometry" describes the type of titration that uses a standard sodium thiosulfate solution such as titrant, one of the few stable reducing agents where oxidation of air is concerned. Iodometry is used to determine the concentration of oxidizing agents through an indirect process involving iodine as a mediator. In the presence of iodine, thiosulphate ions are quantitatively oxidized to tetrathionate ions.

Complete step by step answer:
 Iodometry, known as iodometry titration, is a method of volumetric chemical analysis, a redox titration where the presence or disappearance of primary iodine indicates the endpoint. Note that iodometry involves indirect titration of free iodine by reaction with analysis, whereas iodometry involves direct titration using iodine.
 Redox titration using sodium thiosulfate, $N{a_2}{S_2}{O_3}$ (usually) as a reducing agent, is known as iodometric titration because it is used exclusively to use iodine. Iodometric titration is a common method for determining the concentration of oxidizing agents in solution.
 In an iodometric titration, a starch solution is used as an indicator because it can absorb the ${I_2}$ that is released. This absorption will cause the solution to change from title to dark blue to light yellow with standardized thiosulfate solution.
Iodometric titrations:
\[
  2CuS{O_4} + 4KI \to C{u_2}{I_2} + 2{K_2}S{O_4} + {I_2} \\
  10KI + 5{H_2}S{O_4} + 5O \to 5{K_2}S{O_4} + 5{H_2}O + 5{I_2} \\
\]
 Iodimetric titrations:
\[
  2N{a_2}{S_2}{O_3} + {I_2} \to N{a_2}{S_4}{O_6} + 2NaI \\
  N{a_2}{S_2}{O_3} + {I_2} + {H_2}O \to N{a_2}S{O_4} + 2HI \\
  N{a_3}As{O_3} + {H_2}O \to N{a_3}As{O_4} + 2HI \\
\]
The produced iodine is estimated with hypo solution
\[\;2N{a_2}{S_2}{O_3} + {I_2} \to N{a_2}{S_4}{O_6} + 2NaI\]

So, the correct answer is “Option A”.


Note:
In this clock reaction sodium, potassium or ammonium sulfate is used to oxidize iodide ions to iodine. Sodium thiosulphate is used to reduce iodine before iodine can complex with starch to form blue-black