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The Potentiometric and Laser RAMAN Study of the Hydrolysis of Uranyl

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The Potentiometric and Laser RAMAN Study of the Hydrolysis of Uranyl AECL-9266 ATOMIC ENERGY [/ \ fl L'ENERGIEATOMIQUE OF CANADA LIMITED \ #^4 '/ DU CANADA LIMITEE THE POTENTIOMETRIC AND LASER RAMAN STUDY OF THE HYDROLYSIS OF URANYL CHLORIDE UNDER PHYSIOLOGICAL CONDITIONS AND THE EFFECT OF SYSTEMATIC AND RANDOM ERRORS ON THE HYDROLYSIS CONSTANTS Etude de raman au potentiometre et au laser de I'hydrolyse du chlorure d'uranyle dans des conditions physiologiques et de I'effet des erreurs systematiques et aleatoires de constantes d 'hydrolyse L.L. DESCHENES, G.H. KRAMER, K.J. MONSERRAT and P.A. ROBINSON Chalk River Nuclear Laboratories Laboratoires nucleates de Chalk River Chalk River, Ontario December 1986 decembre ATOMIC ENERGY OF CANADA LIMITED THE POTENTIOMETRIC AND LASER RAMAN STUDY OF THE HYDROLYSIS OF URANYL CHLORIDE UNDER PHYSIOLOGICAL CONDITIONS AND THE EFFECT OF SYSTEMATIC AND RANDOM ERRORS ON THE HYDROLYSIS CONSTANTS by L.L. Deschenes*, G-H. Kramer, K.J. Monserrat* and P.A. Robinson *Systems Materials Branch Dosimetric Research Branch Chalk River Nuclear Laboratories Chalk River, Ontario KOJ 1JO 1986 December AECL-9266 L'ENERGIE ATOMIQUE DU CANADA, LIMITEE ETUDE DE RAMAN AU POTENTIOMETRE ET AU LASER DE L'HYDROLYSE DU CHLORURE D'URANYLE DANS DES CONDITIONS PHYSIOLOGIQUES ET DE L'EFFET DES ERREURS SYSTÉMATIQUES ET ALEATOIRES DE CONSTANTES D'HYDROLYSE par L.L. Deschenes*, G.H. Kramer, K.J. Monserrat* et P.A. Robinson RÉSUMÉ On a étudié l'hydrolyse des ions d'uranyle dans une solution de 0,15 mol/L de (Na)C1 à 37°C par titrage potentiométrique. Les résultats ont correspondu à la formation de (U02)2(OH)2, U03(0H)„, (U02)3(0H)5 et (U02)„(0H)7. On a constaté que les constantes de stabilité, évaluées à l'aide d'une version de MINIQUAD, étaient: log ß22 = -5,693 ± 0,007, log Ê3.» = -11,^99 + 0,024, log ß35 = -16,001 ± 0,050, log ß„7= -21,027 ± 0,051. On s'est servi de la spectroscopie de Raman au laser pour identifier les produits dont les espèces (U02 ),, (0H)7. En outre, on a étudié les difficultés d'identification des espèces chimiques en solution ainsi que l'effet des petites erreurs sur cette sélection par simulation sur modèle de calcul. Les résultats indiquent nettement que les petites erreurs peuvent conduire à la sélection d'espèces qui pourraient ne pas exister. *Service des Matériaux de systèmes Service de Recherche en dosimétrie Laboratoires de Recherches de Chalk River Chalk River, Ontario KOJ 1J0 1986 décembre AECL-9266 ATOMIC ENERGY OF CANADA LIMITED THE POTENTIOMETRIC AND LASER RAMAN STUDY OF THE HYDROLYSIS OF URANYL CHLORIDE UNDER PHYSIOLOGICAL CONDITIONS AND THE EFFECT OF SYSTEMATIC AND RANDOM ERRORS ON THE HYDROLYSIS CONSTANTS by L.L. Deschenes*, G.H. Kramer, K.J. Monserrat* and P.A. Robinson ABSTRACT The hydrolysis of uranyl ions in 0.15 mol/L (Na)Cl solution at 37°C has been studied by potentiometric titration. The results were consistent with the formation of (UO2>2(OH)2> (UO2)3(OH)4» (UO2)3(OH)5 and (UO2>4(OH)7. The stability constants, which were evaluated using a version of MINIQUAD, were found to be: log ".•: = -5.693 + 0.007, log 35,4 =-11.499 + 0.024, log r-35 = -16.001 j 0.050, log -7 = -21.027 +_ 0.051. Laser Raman spectroscopy has been used to identify the products including (UO2)4(OH)7 species. The difficulties in identifying the chemical species in solution and the effect of small errors on this selection has also been investigated by computer simulation. The results clearly indicate that small errors can lead to the selection of species that may not exist. *Systems Materials Branch Dosimetric Research Branch Chalk River Nuclear Laboratories Chalk River, Ontario KOJ 1JO 1986 December AECL-9266 INTRODUCTION The hydrolysis of the uranyl ion has been studied over 30 years (e.g. references within reference 1). The bulk of these studies have been performed by potentiometric titration at 25°C and fixed ionic strength (0.1 to 3.0 mol/L). Other techniques for studying the uranyl hydrolysis include cryoscopy (2, 3), conductivity measurements (4), Raman spectroscopy (5, 6), dilution methods (7, 8) and kinetic methods (9). Other experimental conditions include a study of uranyl hydrolysis in heavy water (10) and temperatures other than 25°C (11-13). The continued study of the uranyl-water system is probably explained by its complexity. The earlier studies agree in few things except that polynuclear hydrolytic ions are formed. Indeed, one can find postulates for species ranging from U02(0H)+ to (UO2)5(OH)g (14) with many differing intermediates. The popular theory of "core + link" (15) in which the hydrolysis is explained as an unlimited series UO2((OH)2 UO2) has been both criticized (12, 16) and later supported (2); however, in contrast to most of the earlier work, some recent Raman spectroscopic studies (5, 6) 2+ indicate that only U0 , (UO2)2(OH) t and (U02)3(0H)5 exist in nitrate solution. There was a minor contribution of (1102)3(011)3 also reported (5). Despite this evidence for only trimeric species being present there is still much evidence in the recent literature (17-19) for the inclusion of higher polynucleated species such as (U0)(0H) The hydrolysis of the uranyl ion is undoubtedly complicated and sensitive to other components as is evidenced by the well documented medium effects (12, 16, 20) of chloride ion which give rise to the additional complex (1102)3(011)7. Milic (21) has attempted to explain this phenomenon in terms of "the factor of the medium" derived from the hydration energy, charge and concentration of the ions of the medium. Clearly then, this is an area yet to be resolved and each study performed under different conditions must stand alone. This work has attempted to elucidate the hydrolysis of the uranyl ion under physiological conditions (37°C, 0.15 mol/L NaCl) as the precursor to further studies of uranium complexes in a physiological environment by both potentiometric titration and Laser Raman spectroscopy. The utilization of Laser Raman spectroscopy has proved to be an excellent method of experimentally verifying the chemical model suggested by the potentiometric titration. We report here for the first time the identification of (UO2)4(OH)J species. Additionally, we have performed a computer simulation to test the effect of small systematic and/or random errors on the chemical models selected. These results further substantiate the need for experimenters to attempt to verify their conclusions based on potentiometric titrations by an alternate physical method. - 2 - EXPERIMENTAL Reagents Uranyl nitrate (depleted) was obtained from the Fisher Scientific Company and purified and converted to uranyl chloride as described elsewhere (18). Sodium chloride was purchased from Anachemia Limited as ACS reagent grade and was dried at 120°C for two hours and kept in a dessicator prior to use. Sodium hydroxide (Anachemia, reagent grade) was purified by preparing a 50% w/v aqueous solution maintained under a CO2 free atmosphere. Distilled deionized water ( >13 Mf2) was used for all reagents. Sodium hydroxide was standardized against potassium biphthalate (Baker analyzed reagent, dried before use). The standardized hydroxide was in turn used to standardize diluted doubly distilled hydrochloric acid. Uranium concentrations were estimated by standard gravimetric procedures (22). Sodium chloride solutions were prepared by weight. Air buoyancy corrections were applied to all weighings. Titration Procedure Titrations were performed using a Metrohm E655 piston buret controlled by a Hewlett-Packard HP-85 microcomputer. Hydrogen ion concentrations were measured as [HT*"] using an Orion Ross electrode and a Ross double-junction electrode, and a Metrohm pH-104 meter also controlled by the HP-85. The solution to be studied was titrated in a vessel similar to that described by Perrin and Sayce (23) which was thermostatted at 37°C (+ 0.1) by a Haake FK circulating water bath. Temperature within the cell was monitored using a model 2100 digital thermometer obtained from the H-B Instrument Company. Purified nitrogen gas was continually passed through the titration vessel to exclude CO2. Each titration was preceded by an electrode calibration performed by titrating NaOH against HC1 with the same ionic background as the uranium hydrolysis determinations. Preliminary values of the standard electrode potential and ionization constant of water were obtained using a BASIC program, KW-IT, on the HP-85. These values verified the apparatus was performing correctly and that the hydrolysis determination could proceed. Further refinements of the data were performed on the Chalk River Nuclear Laboratories' (CRNL) main frame computers using FORTRAN77 programs. Laser Raman Studies Raman spectra were recorded on a conventional spectrometer. An Ar+ ion laser (Coherent) was used as the excitation source. A Spex 1403 double monochromator, equipped with photon counting electronics, was used to analyse the scattered Raman light. An RCA C31034, housed in a Peltier cooling jacket, was used. Spectra were recorded using the 514.5 nm line of the Ar+ laser. The beam was passed through a Spex Lasermate and focused onto the sample. To minimize sample degradation by laser heating incident beam power was limited to 200 mW. Scattered light was collected at 90°. The double monochromator was electronically tuned using the lines of a standard low pressure Hg lamp. The calibration and resolution of the instrument were checked - 3 - using CCI4. Samples examined by LR spectroscopy were taken directly from the titration cell, described above, and stored under nitrogen in sealed tubes. RESULTS Experimental Data Preliminary Data Reduction The reactions studied by this investigation can be summarized as follows: 2p q)+ + i (U02) (OH)J " + q H The various species will be referred to by their formulae or by (p,q) pairs.
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