Radiation-induced redox reactions of iodine species in aqueous solution. Formation and characterisation of III, IIV, IVI and IVIII, the stability of hypoiodous acid and the chemistry of the interconversion of iodide and iodate

Abstract

The unstable oxidation states of iodine have been investigated using pulse radiolysis.

I^IV is generated by one-electron reduction of IO^–₃ with e^–_eq, CO^–₂ and (CH₃)₂CO^–; in the pH range 3–14 it exists as HIO^–₃ and IO^2–₃ with pK_a= 13.3. I^IV reacts rapidly with itself, (CH₃)₂ĊOH and CH₂(CH₃)₂COH.

I^VI is generated by one-electron oxidation of IO₃^– or one-electron reduction of I^VII. At pH 13 both methods produce species having similar spectra (λ_max= 350 nm) where the oxidant is O^–. At pH < 9 the spectrum of I^VI formed by reduction is little changed, but that produced by oxidation of IO^–₃ by OH is extremely weak, showing that these species are dissimilar. In the presence of I^VII, I^VI decays by first-order kinetics at pH 6.3–9 and oxidises I^VII to I^VIII; at pH 13 it decays by second-order kinetics and does not oxidise I^VII. At pH < 9 I^VI also decays to form OH in an acid-catalysed reaction. A mechanism is proposed which accounts for these observations.

I^VIII results from oxidation of I^VII by OH and I^VI. Optical data show that the absorbing species is the dinuclear iodine species I^VIII^VIII with λ_max= 525 nm. The kinetics of formation of I^VIII are pseudo-first order but show a complex dependence on [I^VII]. A mechanism is presented which describes the data.

The spectrum of IO is obtained by oxidation of IO^– with O^– at pH 13. It absorbs with λ_max= 490 nm and ε_max= 2.1 × 10³ dm³ mol^–1 cm^–1; it decays by reaction with itself and I₂^– at diffusion-controlled rates.

Hypoiodous acid is shown to be a significant product of the radiolysis of unbuffered dilute solutions of I^– saturated with N₂O. Under these conditions HOI decays slowly by second-order kinetics with k= 5.5 ± 1 dm³ mol^–1 s^–1. It is much less stable in solutions containing OH^– or borate, where k=(5.6±1.2)+ 10⁴[OH^–] dm³ mol^–1 s^–1 and (7.2±3.3)+ 2.2 × 10³[borate] dm³ mol^–1 s^–1, respectively.