In aqueous solution OH radicals react with chloride ions to form initially ClOH-, the rate constant being 4.3 k 0 . 4 ~ lo9 1. mol-' s-'. The rate constant for the dissociation of ClOH-back to OH radicals and chloride ions is 6.1 k0.8 x lo9 s-I. ClOH-is converted to chlorine atoms via the reaction, C10H-+H++C1+H20 (k = 2.1 k 0.7 x 10'O 1. mol-' s-' at an ionic strength of unity), the rate constant for the reverse reaction being 1.3 x lo3 1. mol-' s-' (0.3-3.0 x lo3 1. mol-' s-').Chlorine atoms combine with chloride ions to form Cl, (k = 2.1 x 1O'O 1. mo1-' s-I), the rate constant for the dissociation of Cl, back to chlorine atoms and chloride ions being 1.1 k 0.4 x lo5 s-'.Cl, absorption has a maximum at 340 nm where the extinction coefficient is 8.8 t-0.5 x lo3 1. mol-' cm-', whereas ClOH-has a maximum at 350 nm with an extinction coefficient of 3.7k0.4 x lo3 1. mol-' cm-l. The reactions of chlorine atoms and Cl, with ferrous ions have also been investigated and the constants are 5.9k0.6 x lo9 and 1.4k0.2 x lo7 1. mol-' s-' (ionic strength = 0.1 moll.-') respectively. The effect of chloride ions on the mechanism of the Fricke dosimeter is discussed.The absorption spectra of ClOH-and Cl, have been measured in the range 230-450 nm.

The rate of the reaction between H02 radicals and Fe2+ is dependent on temperature, the rate constant at 20°C being 9.1 f0.4 x lo5 M-' s-I (1.2 2 0.05 x lo6 M-' s-I at 25°C) and the activation energy 10.0+ 1.0 kcal mol-' (42 kJ mol-I). The initial reaction product is considered to be an outer-sphere complex between Fe3+ and HO;. This hydroperoxide complex appears to react with Fe2+ in a fast reversible reaction to give a bridge compound. The absorption spectra of the two complexes have been measured. The molar extinction coefficients at 450 nm are 140f: 50 M-* cm-' and 1240rf: 100 M-I cm-l, respectively. The equilibrium constant for the formation of the bridge compound varies from 22 k 2 M-' at 20°C to 37.5 rf: 0.5 M-I at 40°C. The hydroperoxide complex dissociates unimolecularly with a rate constant at 20 or 25°C of 1.8 & 0.1 x lo3 s-l, activation energy 2.1 k 0.2 kcal mol-l. The bridge compound dissociates unimolecularly with a rate constant at 20°C of 1.8k0.1 x lo4 s-I (2.5rf:O.l x lo4 s-I at 25"C), activation energy 11.4f 1.1 kcal mo1-'. The final product of the whole reaction is the hydrated ferric ion.

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Hydroxyl radicals have been found by pulse radiolysis to oxidize hydrated ferrous to ferric ions with a rate constant of (2.3 & 0.2) x los M-' s-l . The corresponding reaction of OD radicals in D20 solution has a rate constant of (9.4k0.8) x lo7 M-I s-l. OD radicals oxidize ferrocyanide in D20 solution with (9.7f 1.0) x lo9 M-l s-I. The mechanism of the oxidation of ferrous ions is thought to consist of simple electron transfer, and the difference in rate between H 2 0 and D 2 0 solutions is attributed to differences between free energies of hydration in the two solvents.

Pulse radiolysis of oxygen-containing solutions of ferrous perchlorate in perchloric acid (1 M) enables the formation and disappearanceof a ferrichydroperoxidecomplex, F e 3 + H 0 ~, to be observed. The rate constant for its formation by reaction of Hot radicals with free ferrous ions is 2.1 f0.4 x lo6 M-ls-l in the pH range 0-2.1. The complex disappears in a first-order reaction with a rate constant independent of pH in the range 0-2.1. It exhibits a broad absorption in the region 280-600nm, with &a30 nm = 280f30 M-l cm-l. In the absence of oxygen, a different absorbing species can be seen, attributed to a ferric hydride complex, Fe3+H-. This is formed by the reaction of hydrogen

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