Specific features of reactions of Am(V) with reductants in weakly acidic solutions

Abstract

In EDTA solutions with pH ∼5 at 25°C, Am(V) in a concentration of 5 × 10⁻⁴−3 × 10⁻³ M slowly transforms into Am(III). The Am(V) reduction and Am(III) accumulation follow the zero-order rate law. In the range 60–80°C, the reaction is faster. In some cases, an induction period is observed, disappearing in acetate buffer solutions. In the range pH 3–7, the rate somewhat increases with pH. In an acetate buffer solution, an increase in [EDTA] accelerates the process. The activation energy is 47 kJ mol⁻¹. Zero reaction order with respect to [Am(V)] is observed in solutions of ascorbic and tartaric acids, of Li₂SO₃ (pH > 3), and of hydrazine. The process starts with the reaction of Am(V) with the reductant. The Am(III) ion formed in the reaction is in the excited state, *Am(III). On collision of *Am(III) with Am(V), the excitation is transferred to Am(V), and it reacts with the reductant: *Am(V) + reductant → Am(IV) + R₁ and then Am(IV) + reductant → *Am(III) + R₁, Am(V) + R₁ → Am(IV) + R₂. A branched chain reaction arises. The decay of radicals in side reactions keeps the system in the steady state; therefore, zero reaction order is observed.