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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