III-3. Radiochemistry & Nuclear Chemistry

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Solvent extraction of Fr and Cs with calix[4]arene-bis(benzocrown-6)
Y. Komori∗1 and H. Haba∗1
Francium (Fr) is the heaviest alkali metal, with theatomic number 87. It is one of the least-studied elements among the naturally occurring elements because
all its isotopes are short-lived. The half-life of its longestlived isotope, 223 Fr, is only T1/2 = 21.8 min. Owing toexperimental diﬃculties, the chemical properties of Fr
have not been studied in detail so far. We aim to clarify
the chemical bonding nature of Fr, which is inﬂuenced
by relativistic eﬀects, through complex formation studies of Fr. For that purpose, we performed solvent extraction experiments on 212 Fr (T1/2 = 20 min) with several
crown ethers and compared the results with those on137
Cs (T1/2 = 30.1 y).1) We found that the distribution
ratios (D) of Fr and Cs are almost the same, although theD values of Cs are slightly higher than those of Fr in theextraction with dibenzo-21 crown-7 (DB21C7). Haverlock et al. examined the complex formation of Fr+ withcalix[4]arene-bis(benzocrown-6) (BC6B) in 2003.2) The
D values of Fr are almost one order of magnitude higher
than those of Cs. Such a large diﬀerence is surprisingbecause the ionic radius of Fr (173 pm3) ) is close to thatof Cs (167 pm4) ). However, the mechanism for the large
diﬀerence has not been clariﬁed yet. The higher aﬃnityof BC6B for Fr than for Cs is interesting and important to understand the chemical properties and chemicalbonding of Fr. In this work, we reinvestigated the solvent
extraction behavior of Fr and Cs with BC6B to conﬁrm
the large diﬀerence between Fr and Cs.2)
We used 221 Fr (T1/2 = 4.8 min) produced as the αdecay daughter of 225 Ac (T1/2 = 9.9 d). First, we prepared an 225 Ac/221 Fr generator. 4 MBq of 225 Ac wasdissolved in 1 mL of 0.01 M HNO3 . It was loaded onto
an LN resin column (100–150 µm; φ5 mm × H10 mm).
225Ac was adsorbed on the LN resin, and its daughter nuclide 221 Fr was generated inside the column. 221 Fr wasrepetitively eluted with 0.5 mL of 0.01 M HNO3 after its
growth. The yield of 221 Fr was ≥98% with one-time elution. 500 Bq of 137 Cs dissolved in 5 µL of H2 O was added
into the 221 Fr solution. The solution was dried up, following which 221 Fr and 137 Cs were dissolved in 1.4 mL of10−4 –3 M NaNO3 aqueous solutions to achieve 221 Fr and137
Cs concentrations of ∼10−13 –10−12 M and ∼10−10 M,respectively. 700 µL of this aqueous solution was shakenfor 5 min together with an equal volume of an organic
phase, 1.5 × 10−3 M BC6B in 1,2-dichloroethane. Aftercentrifugation, 500 µL of each phase was subjected to γray spectrometry with Ge detectors to determine the Dvalues as D = [M+ ]organic /[M+ ]aqueous , where M denotes
221
Fr or 137 Cs. We also investigated the D values of Cs
under almost the same experimental condition as Haverlock et al.2) by dissolving 221 Fr and 137 Cs with 10−4 –3 M
NaNO3 /10−4 M CsNO3 aqueous solutions.
The time required for Fr and Cs to reach extraction equilibrium was conﬁrmed to be less than 1 min,which agrees with the results of Haverlock et al.2) Fig∗1RIKEN Nishina CenterFig. 1. Variation of D values of Fr and Cs as a function of NaNO3 concentration [NaNO3 ] in their extraction
from 10−4 –3 M NaNO3 and 10−4 –3 M NaNO3 /10−4 M
CsNO3 aqueous solutions into 1.5 × 10−3 M BC6B in 1,2dichloroethane.

ure 1 shows the D values of Fr and Cs as a functionof NaNO3 concentration. Closed symbols indicate the Dvalues from the 10−4 –3 M NaNO3 aqueous solutions, andopen ones indicate those from 10−4 –3 M NaNO3 /10−4 M
CsNO3 aqueous solutions. The D values of Fr and Cs arecomparable in both the solutions, although the D valuesof Cs are slightly higher than those of Fr. These results
are reasonable when considering the similar ionic radii ofFr and Cs and our previous results obtained using crownethers.1) In the 10−4 –3 M NaNO3 /104 CsNO3 aqueoussolutions, however, the D values of both Fr and Cs aresmaller by factors of 2 to 14 than those in the 10−4 –3 M NaNO3 aqueous solutions. This might be because
macro amounts of Cs (10−4 M) inhibit the extractionof Fr and Cs, although an excess amount of BC6B exists in the extraction system (the concentration ratio of[Cs]:[BC6B] = 1:15 Note that our D values of Fr from10−4 –3 M NaNO3 aqueous solutions and those of Cs from10−4 –3 M NaNO3 /10−4 M CsNO3 aqueous solutions areconsistent with those reported by Haverlock et al.2) Because Haverlock et al.2) independently obtained the Dvalues of Fr and Cs using the 221 Fr tracer and 10−4 M
CsNO3 , respectively, the large diﬀerence between the Dvalues of Fr and Cs was observed.
The slightly higher D values of Cs than those of Fr
using BC6B are similar to those using DB21C7.1) The
reason of the order of the D values of Cs > Fr in theextraction with BC6B and DB21C7 is unclear at thismoment. Quantum chemistry calculations for Fr as wellas Cs are needed to interpret the experimental results
and to discuss the electronic state and chemical bonding
nature of Fr.
References

1) Y. Komori et al., RIKEN Accel. Prog. Rep. 52, 185 (2019).
2) T. J. Haverlock et al., J. Am. Chem. Soc. 125, 1126 (2003).
3) L. H. Delmau et al., J. Phys. Chem. B 117, 9258 (2013).
4) L. H. Ahrens, Geochim. Cosmochim. Acta 2, 155 (1952).

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