LAUSR

In the experiments, uranium(IV) was used, which was obtained by reducing uranium(VI) in HCl solution on a mercury cathode in a cell with a cathodic and anodic space separated by a cation-exchange membrane, in an argon fl ow. The complex K10P2W17O61•nH2O prepared according to the known technique2 was added to the U(IV) solution in a molar ratio of UIV : L = = 1 : 2.1 and then a certain amount of a solution of 2.5 mol L–1 of KNO3. The mixture was kept in the refrigerator. The precipitated K16U(P2W17O61)2•nH2O crystals were separated by centrifugation, washed with a minimal amount of cold water, and dried in air. We used H2SO4, and AcOH of the chemically pure qualifi cation. Sodium acetate (the qualifi cation pure) was twice recrystallized. All solutions were prepared in double-distilled water. A weighed portion of K16U(P2W17O61)2•nH2O was dissolved in a bidistillate. The stock solution was stored in the dark. An aliquot of the solution was mixed with 3 mL of H2O, and the concentration of UIV was determined spectrophotometrically using the maxima of the absorption band at 510 nm (molar extinction coeffi cient ε510 = 1000 L mol–1 cm–1),3 as well as at 695 nm using Shimadzu UV-3100 PC (Shimadzu, Japan) and SF-46 (LOMO, Russia) instruments. The experiments were performed according to the following technique. A water or H2SO4 solution either HAc + NaAc solution was placed in a quartz cuvette with a stopper 1 or 5 cm length, background absorption was measured in the range of 250—700 nm, and then a capillary was lowered into the cuvette and an ozoneoxygen mixture was bubbled. After 10—15 min, the capillary was removed, the cuvette was closed with a stopper, and the optical density at 260 nm was recorded, corresponding to an absorption maximum of O3. To calculate [O3], a molar absorption coeffi cient of ε260 = 3300 L mol–1 cm–1 was used.4,5 An aliquot of the UIV standby solution was added to the cuvette and the absorption decrease at 510 nm was monitored. At the end of the experiment, the pH of the solution was measured with a digital pH meter with a glass electrode OP-210/1 (Radelkis, Hungary).