Iodine Hydrolysis Equilibrium

Abstract

The equilibrium constant for the hydrolytic disproportionation of I₂

$$\begin{gathered} 3{\text{I}}_{\text{2}} + 3{\text{H}}_{\text{2}} {\text{O}} \rightleftharpoons {\text{IO}}_{\text{3}}^ - + 5{\text{I}}^ - + 6{\text{H}}^{\text{ + }} \hfill \\ {\text{ }}K_1 = \frac{{[{\text{IO}}_{\text{3}}^ - ][{\text{I}}^ - ]^5 [{\text{H}}^{\text{ + }} ]^6 }}{{[{\text{I}}_2 ]^3 }} \hfill \\ \end{gathered}$$

has been determined at 25°C and at ionic strength 0.2 M(NaClO₄) in buffered solution. The reaction was followed in the pH range where the equilibrium concentration of I₂, I⁻, and IO₃ ⁻are commensurable, i.e., the fast equilibrium

$$\begin{gathered} {\text{I}}_{\text{2}} {\text{ + I}}^ - \rightleftharpoons {\text{I}}_{\text{3}}^ - \hfill \\ {\text{ }}K_2 = \frac{{[{\text{I}}_{\text{3}}^ - ]}}{{[{\text{I}}_2 ][{\text{I}}^ - ]}} \hfill \\ \end{gathered}$$

is also established. The equilibrium concentrations of I₂and I₃ ⁻were determined spectrophotometrically, and the concentrations of all the other species participating in process (1) were calculated from the stoichiometric constraints. The constants determined are \log K_1 = -47.61\pm 0.07 and \log K_2 = 2.86 \pm 0.01.