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STUDIES on FRACTIONATION of YTTERBIUM ISOTOPES in Yb(III)-ACETATE/Yb-AMALGAM SYSTEM EVEN-ODD EFFECT

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STUDIES on FRACTIONATION of YTTERBIUM ISOTOPES in Yb(III)-ACETATE/Yb-AMALGAM SYSTEM EVEN-ODD EFFECT ISSN 1425-7351 I'll! PL9800491 fNCT — RAPORTY IChTJ. SERIA B nr 7/97 STUDIES ON FRACTIONATION OF YTTERBIUM ISOTOPES IN Yb(III)-ACETATE/Yb-AMALGAM SYSTEM EVEN-ODD EFFECT Wojciech Dembiriski, Marek Poninski, Rudolf Fiedler INSTYTUT CHEMII ITECHNIKI JADROWEJ INSTITUTE OF NUCLEAR CHEMISTRY AND TECHNOLOGY WARSZAWA RAPORTY IChTJ. SERIA B nr 7/97 STUDIES ON FRACTIONATION OF YTTERBIUM ISOTOPES IN Yb(III)-ACETATE/Yb-AMALGAM SYSTEM. EVEN-ODD EFFECT Wojciech Dembinski, Marek Poninski, Rudolf Fiedler Warszawa 1997 EDITORIAL BOARD Wiktor Smulek, Ph.D., Ewa Godlewska, Sylwester Wojtas ADDRESS OF THE EDITORIAL OFFICE Institute of Nuclear Chemistry and Technology Dorodna 16, 03-195 Warszawa, POLAND phone: (+4822) 11 06 56; tlx: 813027 ichtj pi: fax: (+4822) 11 15 32; e-mail: [email protected] UKD: 541.1, 541.2, 546 INIS:B12.20 KEY WORDS: YTTERBIUM, ISOTOPE EXCHANGE, ISOTOPE SEPARATION, ISOTOPE EFFECTS, EVEN-ODD EFFECT, YTTERBIUM AMALGAM Papers are published in the form as received from the Authors Studies on fractionation of ytterbium isotopes in Yb(lll)-acetate / Yb-amalgam system. Even-odd effect The fractionation of ytterbium isotopes with the even and odd numbers of neutrons was investigated in a Yb(lll)-acetate / Yb-amalgam exchange system. The light isotope was preferentially fractionated to the amalgam phase. The values of the unit separation gain per mass difference, e, were found to be -0.00054 for 176/171Yb and -0.00069 for i76/i74yb fne difference which amounted to 0.00015 is an evidence for the occurrence of the so-called "even-odd" effect. It was also found that the chemical isotope shift of ytterbium was mirrored by the optical isotope shift in its atomic spectra. Badanie podziatu izotopdw iterbu w ukladzie Yb(lll)-octan / Yb-amalgamat. Efekt parzysto-nieparzysty Badano proces podziatu izotop6w iterbu o parzystej i nieparzystej ilosci neutron6w w ukladzie Yb(lll)-octan / Yb-amalgamat. Stwierdzono, ze w uWadzie tym faza amalgamatu wzbogaca si? w izotop Izejszy. Znaleziono nast^pujqce wartosci jednostkowych stopni podziaki, e : -0.00054 dla pary 176/171Yb oraz -0.00069 dla pary i7e/i74Yb R6znica wynosza.ca 0.00015 Swiadczy o wyst^powaniu w tym uWadzie tzw. efektu parzysto-nieparzystego. Pokazano, ze chemiczny efekt izotopowy jest podobny do optycznego efektu izotopowego w widmach atomowych. NEXT PAGE(S) left BLANK CONTENTS 1. INTRODUCTION 9 2. EXPERIMENTAL 9 2.1. REAGENTS g 2.2. ANALYSIS 2.3. SEPARATION PROCESS 2.4. OCCURRENCE AND STABILITY OF Yb2+ 2.5. CASCADE PROCEDURE 12 3. DISCUSSION 14 4. REFERENCES 16 TABLES 75 FIGURES NEXT PAGE(S) left BLANK 1. INTRODUCTION The phenomenon called "even-odd effect" or "even-odd staggering of the isotopes" was first announced by Fuji et al. (1989) [1]. They found that in the red-ox exchange system of U(IV)-U(VI), the unit separation factor for the 238/235u isotope pair did not fit the linear interpolation between 238/234u and 238/236u. This meant that the experimental results did not follow the relation: ln(q) « Am/(mi-m2) derived by Bigeleisen and Mayer [2] on the basis of quantum effects in the molecular vibration of isotopic species. Nishisawa et al. observed the same difference in the behavior of even and odd isotopes of Sr and 8a [3], Zn [4] and Mg [5] in liquid-liquid extraction systems with dicyclohexano-18-crown-6 (DC18C6). The even-odd staggering of Gd isotopes was also announced by Nomura et al. [6] in cation exchange displacement chromatography with EDTA and malonic acid as eluents, however with the reservation that the measurement errors were too large to conclude about the deviation from the theoretical relation of the mass dependence. Parallel unsuccessful attempts to find the even-odd effect in fractionation of Sr [7] and Ba [8] by the cation displacement chromatography have been reported. It should be stressed here that studies on the even-odd isotope staggering in the chemical exchange reaction represent a challenge for the analysts who measure the isotope ratio by mass spectrometry. Nowadays it is not sufficient to measure the difference between isotope ratios in enriched and depleted fractions but it is necessary to measure much smaller differences between isotope ratios of the chosen isotope pairs in both enriched and depleted fractions. For these reasons it is also advisable to study the phenomena rather in red-ox (electron) exchange reactions than in univalent (ligand) exchange reactions. The former offer a higher absolute value of the isotope effect expressed by natural logarithm of the unit separation factor (Inq). More experimental data are needed to offer reasonable interpretation of the phenomenon. This is because the energy value of hyper fine interactions connected with isotope nuclear spin (I), nuclear magnetic moment (n), as well as electric quadruple moment (Q) are considered to be too small to change the orbital coupling at the molecular level, the more so, as values of I, n and Q vary strongly with Z and A. However, the theory of the phenomenon is being developed. Recently Bigeleisen [9,10] has announced that in addition to classic mass shift the nuclear field shift must be taken into consideration whereas the nuclear spin effect can be neglected. The direction of the nuclear field term can be opposite to that of the mass term and its absolute value can be higher. The nuclear field shift is presumably responsible for the fractionation of the heavy isotope into the U(IV) phase in U(IV)<->U(VI) the red-ox exchange system. It was also observed that chemical isotope shifts of uranium isotopes were mirrored by isotope shifts in their atomic spectra. The latter are related to charge, size and shape of the nuclei and electron density at the nuclei. For our studies we have selected ytterbium from the rare earth region because of its interesting nuclear and chemical properties. The natural isotope composition of ytterbium is shown in Table 1 along with its isobars. Ytterbium has five and two isotopes with even and odd mass numbers, respectively. The latter have different spin numbers (l171 = 1/2; Ii73= 5/2) and different sign of nuclear magnetic moment expressed in nuclear magnetons (^171 = + 0.49188; Hi7i = - 0.67755). Such a composition is rather unique for the periodic system and after successful chemical fractionation may offer more information on the even-odd effect than for example in the case of barium with two odd isotopes (133 and 135) with the same spin number (3/2) and nearly the same value of the magnetic moment (|*133 = + 0.8329; n135 = + 0.9311). From the chemical point of view ytterbium belongs to a characteristic group of 14 rare earth elements with the general electronic configuration of free atoms: [Xe]4fn5d°6s2. In the solution chemistry of Ln the valence 3+ with the configuration [Xe]4f1+U is typical, however a few exceptions occur. Cerium forms stable Ce4+ ions, europium forms relatively stable Eu2+ ions, ytterbium and samarium form much less stable Yb2+ and Sm2+. The latter two, because of the very low value of the standard red-ox potential (Table 2, uranium is included for comparison) are reoxidized by water itself or according to [11,12] by H+ ions with evolution of hydrogen. The instability of Yb does not permit to study the fractionation of its isotopes in red-ox exchange reactions utilizing counter-current extraction or displacement chromatography as it was done for U(IV/VI) [1], Ce(lll/IV) [13,14] , Eu(ll/lll) [15]. Ytterbium, however, forms amalgams. This inspired us to elaborate a new separation process based on the reduction of Yb(lll) by means of the sodium amalgam to produce the ytterbium amalgam, Yb(Hg). The isotope effect in such system should reflect the three electron exchange reaction; Yb(lll)oYb(0). 2. EXPERIMENTAL 2.1. Reagents Yb2C3 of purity grade 99.99 was supplied by Aldrich Chemical Co. The mean content of the isobars amounted to: Er - 20 ppm, Lu - 3 ppm. Acetate solutions of Yb+3 were obtained by dissolving the oxide in an excess of acetic acid: 3.5M HAc/1M Yb. The final pH of 0.1M Yb(Ac)3 solution in 0.05M HAc was about 4.5. Sodium amalgam of concentration 2-^3.5M Na was prepared by dissolving sodium in mercury purified by digestion with a 5% Hg(NO3) solution in 8% HNO3 followed by washing with bi-distilled water and drying under vacuum. After dissolution the Na amalgam was washed with xylene and absolute ethanol. 2.2. Analysis Ytterbium was determined gravimetrically by oxalate precipitation and calcination at 850°C for 2 h or by a spectrographic method using a Carl Zeiss (Jena) spectrograph PGS-2. The concentration of Yb(ll) was determined by titration with 0.05 N K2Cr2O7 using barium diphenyloamine sulphonate as indicator. Titration was performed in presence of ferric ammonium sulphate, phosphoric acid and sulphuric acid. The reagents solution was deaerated by a stream of argon. Yb(ll) was also determined spectrophotometrically using a Beckman DU 68 spectrophotometer and 0.25 cm quartz cells. The isotope ratios, 176/17lYb and 176/ 174Yb, were measured by means of a MAT 262 (Finnigan MAT) mass spectrometer. This instrument is equipped with 7 Faraday cup detectors. Six of them can be moved for an optimal adjustment to select masses. The method of total evaporation of the sample was applied, thus under normal conditions there should be no mass fractionation effect [16]. 2.3. Separation process The separation was performed in a 100 ml separation cell shown in Figure 1. At the beginning of the experiment the cell contained 50 ml of 0.1M Yb(lll) acetate solution and 260 g Hg.
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