Radiation chemistry of dilute aqueous solutions of thallous ion

In the absence of O2 relatively stable thallium metal colloids are formed when dilute aqueous solutions of thallous ion are irradiated under reducing conditions in the pH range 6 to 12 in the presence of 10-3 mol dm-3 surfactant. The colloid is also stable at pH 3, but it cannot be formed at this pH because H30+ competes with Tl+ for e-sub(aq). Once nucleation has occurred, Tl+ is reduced by (CH3)2COH and CH3CHOH at the particle surface. Electrophoresis measurements showed that the particles are negatively charged, and kinetic analysis indicated that their mean diameter ranges from 30 nm at pH 3.4 to 24 nm at pH 11.8 under the experimental conditions specified above. Increasing the dose rate or [Tl+] resulted in smaller particles being formed. Colloidal thallium catalyses the reduction of water by (CH3)2COH and (CH3)CHOH, and also catalyses the disproportionation of these radicals. A mechanism is proposed for these processes in which the rate-determining step for hydrogen production is the discharge of H3O+ or H20 at the metal surface, and radical disproportionation involving electron transfer to and from the particle. The rate constant for the discharge of H2O is estimated to be 820 s-1. (author)