Investigation of the •NH2 radical in aqueous solution up to 200°C by pulse radiolysis

The absorption spectrum of (NH2)-N-. has been measured in aqueous solutions of ammonia at pH 10.3 and ambient temperature It exhibits two bands with lambda(max) at 530 nm, epsilon(530) = 8.6 +/- 0.1 m(2) mol(-1), and ca. 230 nm, epsilon(230) = 23.2 +/- 1.2 m(2) mol(-1). (NH2)-N-. is produced in the reaction (OH)-O-. + NH3(aq) --> (NH2)-N-. + H2O with k = (8.0 +/- 1.3) x 10(7) dm(3) mol(-1) s(-1) at 25 degrees C and E-act = 11.4 +/- 0.5 kJ mol(-1). The self-reaction of (NH2)-N-. occurs with 2k = (3.6 +/- 0.6) x 10(9) dm(3) mol(-1) s(-1) at 25 degrees C and E-act = 11.6 +/- 0.5 kJ mol(-1). This temperature dependence indicates that the reaction rate is influenced by the rate of diffusion, and the relatively low value of 2k (25 degrees C) is rationalised by including a spin statistical factor of 0.25 in the expression for the rate constant. The reaction (NH2)-N-. + N2H4 has k = (1.18 +/- 0.03) x 10(7) dm(3) mol(-1) s(-1) at 18.5 degrees C and E-act = 8.2 +/- 0.7 kJ mol(-1). Some implications of these data for the coolant chemistry of water-cooled nuclear power reactors are noted.