The thiocyanate dosimeter (lo-, rnol dm-3 SCN-in 0,-saturated water) has been standardised against the super-Fricke dosimeter mot dm-3 Fe" in 0,-saturated 0.4 mot dm-3 H2S04) using the hexacyanoferrate(1i) dosimeter [5 x rnol dm-3 Fe(CN)64-in 0,-saturated water] as a secondary standard. On the basis that G(Fe"') = 1.67 x = 220.4 m2 mol-' at 304 nm and 25°C in the super-Fricke dosimeter, we obtain G&[Fe(CN)63-] = (3.47 2 0.06) x m2 J-' at 420 nm and Gc(SCN),'-= (2.59 &-0.05) x m2 J-' at 475 nm. These values remain unchanged when the solutions are saturated with air instead of 0, and are doubled in N20-saturated solution.rnol J-' and Aqueous solutions of thiocyanate are commonly used as dosimeters for pulse radiolysis because (SCN),' -formed in reaction (1) has a strong absorption band' with Amax at ca. 475 nm.

The rates of reaction of OH with benzene, chlorobenzene, nitrobenzene, benzoate ion and benzoic acid have been measured in aqueous solution up to 200 "C using pulse radiolysis to generate OH. The temperature dependence of the observed rate constant, kobs , is essentially the same for each compound and kobs changes by less than three-fold between 20°C and 200°C. The kinetic data are consistent with a mechanism whereby OH reversibly forms a n-complex with the aromatic compound, irrespective of the substituent on the ring, which then transforms to a a-bonded hydroxycyclohexadienyl radical. The values of kobs were determined from the rate of formation of this radical. There is no evidence for dissociation of the a-bonded radical nor for H atom abstraction from the ring which have been reported for the gas phase. The apparent mechanistic differences between the two phases may be due to the different timescales over which the kinetics measurements were made.

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Radiolysis kinetics in NO(3)(-) and NO(2)(-) solutions during γ-irradiation were studied at an absorbed dose rate of 2.1 Gy·s(-1) at room temperature. Air- or argon-saturated nitrate or nitrite solutions at pH 6.0 and 10.6 were irradiated, and the aqueous concentrations of molecular water decomposition products, H(2) and H(2)O(2), and the variation in the concentrations of NO(3)(-) and NO(2)(-) were measured as a function of irradiation time. The experimental data were compared with computer simulations using a comprehensive radiolysis kinetic model to aid in interpretation of the experimental results. The effect of nitrate and nitrite, present at concentrations below 10(-3) M, on water radiolysis processes occurs through reactions with the radical species generated by water radiolysis, (•)e(aq)(-), (•)O(2)(-), and (•)OH. The changes in H(2) and H(2)O(2) concentrations observed in the presence of nitrate and nitrite under a variety of conditions can be explained by a reduction in the radical concentrations. The kinetic analysis shows that the main loss pathway for H(2) is the reaction with (•)OH and the main loss pathways for H(2)O(2) are reactions with (•)e(aq)(-) and (•)OH. Nitrate and nitrite compete for the radicals leading to an increase in the concentrations of H(2) and H(2)O(2). Post-irradiation measurements of H(2), H(2)O(2), NO(2)(-) and NO(3)(-) concentrations can be used to calculate the radical concentrations and provide information on the redox conditions of the irradiated aqueous solutions.