Excitation of the uranyl ion in U(IV) oxidation with xenon difluoride in frozen aqueous sulfuric acid solutions: II. Effect of solvent and H₂SO₄ concentration

Abstract

The effect of solvent (aqueous solutions of HClO₄ and H₂SO₄) on the low-temperature (T < 273 K) reaction of U(IV) with XeF₂, accompanied by formation of uranyl ion in the electronically excited state *(UO ²⁺₂ ), was examined. In the course of heating of 0.2–8.31 M HClO₄ solutions after their quick cooling to 77 K, the chemiluminescent reaction occurs at a detectable rate only at T > 220 K. In the similar experiments performed with 0.05–0.5 M H₂SO₄ as solvent, the emission appears at a considerably lower temperature, 165–175 K. The chemiluminescence (CL) intensity increases in the course of exothermic phase transitions occurring when the temperature of H₂SO₄ solutions (at [H₂SO₄] > 0.05 M) is changed. The increase in the CL intensity in the course of a phase transition is associated with the appearance in a multicomponent frozen system of a juvenile surface of H₂SO₄ crystal hydrates. The lack of CL at T < 220 K in HClO₄ solutions is associated with the fact that heating of the samples after their quick cooling to 77 K is not accompanied by formation of crystalline phases exhibiting catalytic properties toward the oxidation of U(IV) with XeF₂.