Radiation chemistry of dilute aqueous solutions of thallous ion. Formation of colloidal thallium and its catalysis of the reduction of water by (CH3)2ĊOH and CH3ĊHOH radicals

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Abstract
In the absence of O2 relatively stable (several hours to several days) thallium metal colloids are formed when dilute aqueous solutions of thallous ion ([Tl+]0≈ 1.2 × 10–4 mol dm–3) are irradiated (dose rate ≈ 10 Gy min–1) under reducing conditions (10–1 mol dm–3 propan-2-ol or ethanol) in the pH range 6–12 in the presence of 10–3 mol dm–3 surfactant (sodium dodecyl sulphate or Triton-X-100). The colloid is also stable at pH 3, but it cannot be formed at this pH because H3O+ competes with Tl+ for e– aq. Once nucleation has occurred, Tl+ is reduced by (CH3)2ĊOH and CH3ĊHOH 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)2ĊOH and (CH3)ĊHOH, 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+(low pH) or H2O (pH 7) 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.

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