8.2 Iodometric Titrations

  Stoichiometry

Iodometric titrations are defined as those iodine titrations in which some oxidizing agent liberates iodine from an iodine solution and then lilberated iodine is titrated with a standard solution of a reducing agent added from a Burette. In such titrations, a neutral or an acidic solution of oxidizing agent is employed. The amount of iodine liberated from an iodide, $(i.e. KI)$ is equivalent to the quantity of the oxidizing agent present. Iodometric titrations are used for the determination of $CuS{O_4},$ ${K_2}C{r_2}{O_7},{\text{ }}KMn{O_4},$ ferric ions, antimonite ions, ${H_{2}}{O_2},$ $Mn{O_2},$ bromine and chlorine etc. The equations for some of the reactions are as follows:

(i) $2CuS{O_4} + 4KI \to C{u_2}{I_2} + {\text{ }}2{K_2}S{O_4} + {\text{ }}{I_2}$

(ii) $2KMn{O_4} + {\text{ }}3{H_{2}}S{O_4} \to {K_2}S{O_4} + {\text{ }}2MnS{O_4} + {\text{ }}3{H_{2}}O{\text{ }} + {\text{ }}5O$

(iii) ${K_2}C{r_{2}}{O_7} + {\text{ }}4{H_2}S{O_4} \to {K_2}S{O_4} + {\text{ }}C{r_2}{\left( {S{O_4}} \right)_3} + {\text{ }}4{H_2}O{\text{ }} + {\text{ }}3O$

$6KI{\text{ }} + {\text{ }}3{H_2}S{O_4} + {\text{ }}3O \to 3{K_2}S{O_4} + {\text{ }}3{H_2}O{\text{ }} + {\text{ }}3{I_2}$

In the above reactions, the liberated iodine is titrated with a standard sodium thiosulphate.

$$2N{a_2}{S_2}{O_3} + {\text{ }}{I_2} \to N{a_2}{S_4}{O_6} + {\text{ }}2NaI$$