Ⅲ-3. Radiochemistry & Nuclear Chemistry

RIKEN Accel. Prog. Rep. 56 (2023)
RIKEN
Accel. Prog. Rep. 56 (2023)

Coprecipitation of 133 Ba, 226 Ra, and 152 Eu with calcium oxalate forthe chemical study of nobelium
R. Nakanishi,∗1,∗2 E. Watanabe,∗1,∗2 S. Otaka,∗1,∗2 R. Masuda,∗1,∗2 R. Wang,∗1,∗2 Y. Itakura,∗2,∗3T. Yokokita,∗2,∗4 Y. Shigekawa,∗2 A. Nambu,∗2 X. Yin,∗2 H. Haba,∗2 A. Shinohara,∗5 and Y. Kasamatsu∗1,∗2Superheavy elements (SHEs) with Z ≥ 101 are synthesized by heavy-ion-induced nuclear reactions. The
production rates of these elements are very low, andtheir half-lives are usually short.1) Therefore, it is difﬁcult to investigate their chemical properties. Element102, nobelium (No), is one of the actinide elements. No
has unique property that the element exists stably as
No2+ ion in aqueous solutions,2) whereas all lanthanide
and actinide elements have the most stable valency of+3 or higher. Previous chemical studies on No havereported that No exhibits similar chemical behavior tothose of the group 2 elements in the periodic table.3) In
this study, we focused on oxalic acid, which precipitates
the group 2 elements by bonding with metal ions, andmalonic acid, which has a slightly longer carbon chain
length than oxalic acid. We expect that the complexation behavior of these dicarboxylic acids with No can beinvestigated by the coprecipitation method. Studies forcomplexation with organic ligands are scarce in SHE
chemistry, and through the present study, the dependence of the No behavior on the ligand size and shapeis expected to be discussed. In this report, we present
the results of the precipitation and coprecipitation experiments of Ca, Sr, Ba, Ra, and Eu with oxalic acid.
We determined the suitable experimental conditions forinvestigating the properties of No-oxalic complexes.
Oxalic acid precipitation experiments for Ca, Sr, andBa were performed to determine the carrier metal elements that rapidly and fully precipitate by bonding
with oxalic acid. The precipitation experiments wereperformed at pH = 2, 3, 4, and 5 with reaction timesof 3 min. 5 mL of 2 mM Ca2+ , Sr2+ , or Ba2+ hydrochloric acid (pH = 2, 3, 4, or 5) was mixed with5 mL of 4, 20, and 200 mM oxalic acid (pH = 2, 3, 4,or 5). The amounts of Ca and Sr in the precipitate andﬁltrate samples were analyzed by EDTA titration andthat of Ba was determined by γ-ray measurement for133
Ba tracer with a Ge semiconductor detector. The
precipitation yields were estimated from the amounts.
Based on the precipitation results, coprecipitation experiments of 133 Ba, 226 Ra, and 152 Eu with calcium oxalate were performed using Ca as the carrier at pH = 3.
20 µL of hydrochloric acid (pH = 3) containing 133 Ba,
226
Ra, and 152 Eu was added to 230 µL of 2.17 mM Ca2+hydrochloric acid (pH = 3), which was followed by addition of 250 µL of 20 and 200 mM oxalic acid (pH =∗1∗2∗3∗4∗53). The mixture was shaken for 5 min. The coprecipitation yield of each element was determined by γ-ray
measurement for 133 Ba and 152 Eu with the Ge semiconductor detector, and by α-particle measurement with a
Si detector for 226 Ra.
The results of the oxalic acid precipitation experiments of Ca, Sr, and Ba showed that the precipitation
yield of Ca rapidly reached about 100%, and a clear difference in the precipitation yields of Ca (92.6 ± 4.9%),Sr (5.8%), and Ba (0.6 ± 0.3%) was observed under
the condition of 10 mM oxalic acid at pH = 3. Consequently, we determined suitable experimental conditions, as Ca carrier will be used in coprecipitation at pH= 3, to observe the similarity of No behavior to whichgroup 2 element.
Figure 1 shows the coprecipitation yields of 133 Ba,
226
Ra, and 152 Eu with calcium oxalate. The yields for133
Ba and 226 Ra were low and increased with increasing
oxalic acid concentration, in the range 10–40% for 133 Ba
and 5–20% for 226 Ra. The coprecipitation yields for152
Eu were over ∼90% at all oxalic acid concentrations
studied. The coprecipitation yields were in the order ofCa ≈ 152 Eu > 133 Ba > 226 Ra, which is inverse orderof the ionic radius4) : Ca2+ (1.12 Å) ≈ Eu3+ (1.066Å) < Ba2+ (1.42 Å) < Ra2+ (1.48 Å). This suggeststhat smaller metal ions tend to match the size of oxalic
acid and stably form complexes. We plan to investigate
the coprecipitation behavior of No with oxalic acid todiscuss the ionic radius of No.

Fig. 1. Coprecipitation yield of 133 Ba, 226 Ra, and 152 Eu.

References
1) J. Maly et al., Science 160, 1114 (1963).
2) A. Toyoshima et al., J. Am. Chem. Soc. 131, 9180
(2009).
3) R. J. Silva et al., Inorg. Chem. 13, 2233 (1974).
4) R. D. Shannon, Acta Cryst. A 32, 751 (1976).

Graduate School of Science, Osaka University
RIKEN Nishina Center
Department of Chemistry, Osaka University
Salesian Polytechnic
Osaka Aoyama University- 144 -