L. A. Khamidullina

Autoacceleration of the free-radical chain luminescent oxidation of U(IV) with xenon trioxide in aqueous HClO4

Autoacceleration is observed in the chemiluminescent radical chain oxidation of U(IV) with xenon trioxide in aqueous perchloric acid in a Teflon (not conventional glass) reactor. The decay of chemiluminescence accompanying the reaction between U(IV) and XeO3 in the case of excess oxidizer obeys a first-order kinetic equation only at low U(IV) concentrations of 10−5–10−6 mol/l. Autoacceleration takes place at comparatively high reactant concentrations, when the contribution from the heterogeneous loss of radicals on the reactor walls into chain termination is comparatively small and the role of degenerate chain branching reactions is significant. It is inferred that a critical phenomenon rare for liquid-phase radical chain processes takes place in U(IV) oxidation with xenon trioxide: a comparative small increase in the reactant concentrations causes this chemiluminescent redox process to pass from the quasi-steady-state regime to autoacceleration.

Chemiluminescence in the hydrolytic reduction of XeF2

The authors found that aqueous solutions of xenon difluoride give an emission with maximum in the green spectral region. The introduction of UO/sub 2/SO/sub 4/ was found to enhance the luminescence brightness. Sensitization is especially effective in acid aqueous solutions in which U(VI) exists in the uranyl form. Thus, in the presence of 0.01 M UO/sub 2//sup 2 +/ in a 0.01 M solution of XeF/sub 2/ in 0.2 M H/sub 2/SO/sub 4/, the chemiluminescence brightness is enhanced by three orders of magnitude. The participation of uranyl in the chemiluminescence step of the hydrolytic reduction of Xef/sub 2/ is not limited to the role of a reaction energy acceptor. Processes involving the adsorption of active species on the surface of the reaction vessel and the complexation of UO/sub 2//sup 2 +/ with XeF/sub 2/ and HF are also important.

Formation of Excited Uranyl in Oxidation of U(IV) with Oxygen in HClO4 Aqueous Solutions: I. Effect of pH on Kinetic and Chemiluminescence Parameters of the Reaction

Chemiluminescence accompanying U(IV) oxidation with atmospheric oxygen in 0.1-0.0004 M HClO4 was studied. It was found that the electronically excited uranyl ion, (UO22+)*, is the luminescence emitter. The maximum on the kinetic curve is due to accumulation in the solution of UO2+ and H2O2, which are intermediates of U(IV) oxidation. The kinetic scheme of U4+ reaction with O2 suggests that uranyl excitation proceeds in the elementary stage of electron transfer from UO2+ to the oxidant, which is OH radical. With increasing pH from 1 to 3.4, the rate constant of the chemiluminescent stage (k) of the reaction increases almost 50 times and the luminescence efficiency (ηcl), 3 times. The effect of pH on the oxidation rate is due to the high reactivity of U4+ hydrolysis products UOH3+ and U(OH)22+ with respect to O2. The rate constant k of the reaction between U4+ and O2 and the chemiluminescence efficiency are the least among the other U(IV) chemiluminescence reactions: at 298 K and 0.1 M HClO4, k (l mol-1 s-1) is equal to 3, 36, 40, and 108 and ηcl, to 7.4 × 10-8, 8.1 × 10-5, 1.6 × 10-6, and 3 × 10-7 for O2, XeO3, H2O2, and HSO5- as oxidants, respec- tively. The activation energy of the chemiluminescent stage of U(IV) oxidation with oxygen in 0.001 M HClO4Ea is 90.5 kJ mol-1 within the 285-310 K range.

Involvement of ·OH radicals in the mechanism of excitation of the uranyl ion in the chemiluminescent oxidation of U(IV) by atmospheric oxygen in aqueous solutions of perchloric acid

The chemiluminescence (CL) kinetics in U(IV) oxidation by atmospheric oxygen in aqueous HClO4 has been investigated. The CL quantum yield (ηCL, E/(mol U(IV))) in this reaction is 1.4 × 10−8. The elementary event generating the CL emitter, which is the electronically excited uranyl ion *(UO22+), is electron transfer from the uranyl ion UO2+ to the oxidizer (·OH radical). The Ag+ ion quenches CL, and the Cu2+ ion enhances CL.

Formation of excited uranyl in oxidation of tetravalent uranium with oxygen in HClO4 aqueous solutions: IV. Enhancement of the chemiluminescent reaction in the presence of Fe2+

Experimental data that support the hypothesis on the determining role of •OH radicals in the emergence of luminescence during the oxidation of U(IV) with atmospheric oxygen in aqueous HClO4 solutions have been obtained using the H2O2-FeSO4 system as a source of OH radicals. It has been found that brighter chemiluminescence (CL) is observed in the presence of 10−5 mol/l Fe2+ in a 5 × 10−4 mol/l U(IV) solution in 0.1 mol/l HClO4 compared with the FeSO4-free solution. The CL yield in the presence of Fe2+ (ηCL = 3.9 × 10−8) is 2.8 times that in the solution without iron (ηCL = 1.4 × 10−8). These results can be regarded as a further piece of evidence for the idea that the elementary event of the formation of a CL emitter—electronically excited uranyl ion *(UO22+)—in radical chain U(IV) oxidation reactions is electron transfer from the uranoyl ion (UO2+) to the oxidant, the •OH radical. Thus, one of the main prerequisites for light emission during U(IV) oxidation reactions is a high generation efficiency of •OH radicals and their easy access to the uranoyl UO2+ ion.

Influence of Temperature on the Lifetime of Electronically Excited Uranyl Ion: II. Frozen Aqueous Solutions of H2SO4. Effect of Phase Transitions

The temperature dependences of the lifetime (τ) of electronically excited uranyl ion (UO22+)*, measured both on heating from 77 K and on cooling from 300 to 77 K of UO2SO4 solutions in 0.2-5.0 M of H2SO4, pass through extrema. The peaks of phase transitions in DTA curve and peaks in the temperature dependence of τ appear at the same temperature. The influence of the phase transitions on the temperature dependence of τ is due not only to a change in the molecular mobility upon a phase transition (this factor causes the temperature quenching) but also to the concentration quenching of excited states of (UO22+)*.

Formation of Excited Uranyl in Oxidation of U(IV) with Oxygen in HClO4 Aqueous Solutions: II. Catalytic Effect of UO22+

Strong effect of uranyl ion on the kinetic and chemiluminescence characteristics of U(IV) oxidation with oxygen in weakly acidic HClO4 solutions is found. For instance, in the presence of 8 × 10-4 M UO22+ the reaction rate constant, maximal intensity of the chemiluminescence, and chemiluminescence efficiency increase by factors of 50, 100, and 8, respectively. The catalytic effect of uranyl ion is probably caused by both formation of the complex UO22+·UO2+ (which decreases the rate of disproportionation of UO2+ participating in the chain propagation) and generation of UO2+ in the reaction of UO22+ with U(IV).

Uranyl-catalyzed chemiluminescent reaction of U4+ oxidation by dioxygen in aqueous HClO4 solution

Chemiluminescence (CL) accompanying the reaction of U4+ with O2 in 0.0004–0.1M HClO4 was studied. It was found that the electron-excited uranyl ion (UO22+)* is the CL emitter. The fact that the reaction rate and the CL yield increase as the solution acidity decreases was explained by different reactivities of the Uaq4+ aquation and the products of its stepwise hydrolysis, UOH3+ and U(OH)22+, toward O2. Based on the results of analysis of the chain-radical mechanism of the reaction between U4+ and O2, it was concluded that transfer of an electron from the UO2+ ion to the oxidizing agent (a ·OH radical) is the most plausible elementary step of the reaction of (UO22+)* formation. It was found that the reaction rate, as well as the CL yield, increase substantially in the presence of uranyl ion. Catalytic action of UO22+ was explained by the formation of a UO22+·UO2+ complex, which reduces the rate of the UO2+ disproportionation reaction (UO2+ is an intermediate of the reaction and is involved in chain propagation), and by regeneration of the active center, UO2+, in the reaction of UO22+ with U4+.

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

Light emission from aqueous solutions of HClO4 containing U4+ and S2O82− has been observed. The emitter of chemiluminescence (CL) is the electron-excited uranyl ion (*UO22+), 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 UO22+ (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 constantskH/kD=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)22+), whose relative fraction increases as the HClO4 concentration decreases.

Influence of temperature on the lifetime of electronically excited uranyl ion: IV. Effect of isotope enrichment in low-temperature crystallization of partially deuterated aqueous solutions of sulfuric acid

The isotope effect manifested in phase transitions in deuterium-substituted aqueous solutions of sulfuric acid was found. The phase transitions occurring with variation of temperature of 0.5 M H2SO4 solution in H2O, 0.5 M D2SO4 solution in D2O, and their mixtures were studied by differential thermal analysis. Additional data on the phase transitions in these samples were obtained using chemiluminescence [low-temperature reaction of U(IV) with XeF2] and τ-metry (measurement of the lifetime τ of uranyl ion UO22+ in the electronically excited state). The uranyl ion was used as a luminescence probe. The whole set of data obtained can be interpreted on the basis of the concept that a mixture of 0.5 M H2SO4 and 0.5 M D2SO4 solutions (H/D = 1), subjected to fast (10–15 K s−1) cooling to 77 K followed by warming, contains a liquid phase in which the deuterium content in the temperature range 202–210 K is higher than that in the initial solution.