Kinetics of the reversible reaction between arsenious acid and aqueous iodine

The kinetics of the reversible reaction\documentclass{article}\pagestyle{empty}\begin{document}$$ {\rm H}_3 {\rm AsO}_3 + {\rm I}_{3^-} + {\rm H}_2 {\rm O}\rightleftharpoons{\rm H}_3 {\rm AsO}_4 + 2{\rm H}^+ + 3{\rm I}^- $$\end{document} have been studied spectrophotometrically in acid solution under conditions in which both the forward and reverse reactions go to virtual completionand in which the reaction comes to a practical equlibrium. The rates of theforward ( R f ) and reverse ( R r ) reactions are given by\documentclass{article}\pagestyle{empty}\begin{document}$$R_f = \{f + g/[\rm H^+] + h/[\rm H^+]^2\} [\rm H_3 {\rm AsO}_3][I_{3^-}]/[I^-]^2 $$ $$ R_r = \{u[\rm H^+] + \upsilon [\rm H^+]^2\} [{\rm H}_3 {AsO}_4][I^-] $$\end{document} where f, g, h, u , and v have the values (4 ± 1) × 10 −5 mole/1.·s, (4.2 ± 0.2) × 10 −5 mole 2 /1. 2 ·s, (5.0 · 0.3) × 10 −7 mole 3 /1. 3 ·s, (1.1 ± 0.1) × 10 −3 1. 2 /mole 2 ·s, and (3.7 ± 0.2) × 10 −3 1. 3 /mole 3 ·s at 298.2°K and at an ionic strength of 2.00 M maintained by adding sodium chloride. The stoichiometric equilibrium constant under similar conditions is 0.022 ± 0.003. Differentvalues of these parameters were obtained when sodium perchlorate and sodiumnitrate were used to control ionic strength. The results are compared with those from previous reports and a mechanism is proposed based upon an initial rapid equilibrium\documentclass{article}\pagestyle{empty}\begin{document}$${\rm H}_3 {\rm AsO}_3 + {\rm I}_2 \rightleftharpoons{\rm H}_3 {\rm AsO}_3 {\rm I}^ + + {\rm I}^ -$$\end{document} followed by a rate‐determining attack of water upon H 3 AsO 3 I + , H 2 AsO 3 I, and HAsO 3 I − .