An unusually intense effect of the medium acidity on the rate of oxidation of U⁴⁺ and the yield of chemiluminescence in aqueous solutions of HClO₄ containing K₂S₂O₈

Abstract

Light emission from aqueous solutions of HClO₄ containing U⁴⁺ and S₂O₈ ²⁻ has been observed. The emitter of chemiluminescence (CL) is the electron-excited uranyl ion (^*UO ²⁺₂ ), the product of U⁴⁺ oxidation. A hundredfold decrease in the HClO₄ concentration (from 1 to 0.01 mol L⁻¹) results in a 250-fold increase in the reaction rate constant, a 5000-fold increase in the initial CL (from 4·10⁵ to 2·10⁹ photon s⁻¹), a more than tenfold increase in the CL yield (from 5.6·10⁻⁸ to 1.6·10⁻⁶), and a 200-fold increase in the excitation yield of UO ²⁺₂ (from 6·10⁻⁶ to 1.3·10⁻³). The kinetic isotope effect of the reaction has been studied. The value for the ratio of the rate constantsk _H/k _D=2.1 has been determined by extrapolation to the 100% degree of deuteration of 0.1M perchloric acid. The peculiarities of the chemiluminescence stage in the reaction of U⁴⁺ oxidation in solutions of potassium persulfate were explained by the participation of the products of hydrolysis of the U⁴⁺ aqua ion (UOH³⁺ and U(OH) ²⁺₂ ), whose relative fraction increases as the HClO₄ concentration decreases.