γ Radiolysis of Liquids at High Pressures. VII. Oxidation of Iodide Ion by Hydrogen Atoms in Aqueous Solutions

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Abstract
The H-atom oxidation of iodide, H + H + + I − – → H 2 + I , was studied in γ radiolysis of deaerated acidic iodide solutions at 20° and applied pressures in the range 0–6.38 kbar. Without added iodine, slopes of dose plots for [H2] and total iodine, [ I 2 ] t ≡[ I 2 ] + [ I 3 − ] , begin to decrease appreciably at relatively low doses and approach limiting values at high doses. Such behavior is a consequence of the reaction H + I 3 − ( or I 2 ) → H + + I − + I 2 − ( or I ), for which k 2 = 3.0 × 10 10 M −1 · sec −1 was measured in solutions containing glucose and iodine with [I3 −] ≥ 0.83 [ I 2 ] t . In solutions without added iodine, results are consistent with the established model for water radiolysis and with the previously established pressure independence of primary yields (g) : at all doses [H2]=[I2]t; from initial slopes at both 1 atm and 6.38 kbar, G( I 2 ) t = 4.1 = g( H 2 ) + g( H ) ; from the high-dose limiting slope at 1 atm, G( I 2 ) t = 0.45 ≃ g( H 2 ) . Competition between Reactions [1] and [2] was studied by measurement of initial H2 yields from solutions with added iodine. Reaction [2] was assumed diffusion controlled with ΔV 2 ‡ = 1.6 ml mole −1 . For [I−] up to at least 0.6M [H2SO4 up to at least 0.8N , rate of Reaction [2] at all pressures is given by k 1 [H][I − ][H + ] t , in which [ H + ] t ≡[ H + ] + [ HSO 4 − ] ; i.e., HSO4 − is indistinguishable from H+. Reaction [1] occurs in termolecular encounters via H + I−→(HI−)‡ followed by proton transfer from the acid ion present. At 1 atm, k 1 = 5.3 × 10 6 M −2 · sec −1 and ΔV 1 ‡≃−11 ml mole −1 ; ΔV 1 ‡ = − 6.3 ml mole −1 for P in the range 2–6.4 kbar.