Journal of Nuclear and Radiochemical Sciences, Vol. 12, No.1, pp. A1-A6, 2012

Accounts

Chemical Studies of Rutherfordium (Rf) and Nobelium (No) on an Atom-at-a-time Scale
Atsushi Toyoshima*
Advanced Science Research Center, Japan Atomic Energy Agency (JAEA), Tokai, Ibaraki 319-1195, Japan
Received: November 4, 2011; In Final Form: June 1, 2012
Hexafluoro comlexation of rutherfordium (Rf) and successful oxidation of the divalent state of nobelium (No)recently carried out at JAEA are reviewed. The 261Rf and 255No isotopes were produced in the 248Cm(18O, 5n) and248
Cm(12C, 5n) reactions, respectively, at the JAEA tandem accelerator. Anion-exchange behavior of Rf in HF and
HNO3 (HF/HNO3) mixed solution was studied together with short-lived Zr and Hf isotopes using an automatedchemistry apparatus to characterize anionic fluoride species of Rf and to evaluate its formation constants. It was
found that the hexafluoro complex of Rf is present in the HF/HNO3 solutions under the given conditions where thesame complexes of Zr and Hf are existing. The formation constant of the Rf complex was evaluated to be at least
one order of magnitude smaller than those of Zr and Hf, which suggests a larger ionic radius of Rf 4+ than those of
Zr4+ and Hf 4+. Oxidation experiments of No2+ were conducted in 0.1 M α-hydroxyisobutyric (α-HIB) acid using a
newly developed electrolytic column chromatography apparatus on a single atom scale. The result clearly
demonstrated that stable No2+ is successfully oxidized to the trivalent state, No3+, which is held in the α-HIB
solution.

8 and 12 in the newly appearing 6d transition series, respectively. Very recently, chemical behavior of element 114 wasreported to deviate from the systematic trend in that of lightergroup-14 elements although further confirmation is necessary.6
It is of special interest to study chemical properties of thetransactinide elements in detail to elucidate the influence ofincreasingly strong relativistic effects. Relativistic mass
increase of electrons due to its velocity approaching to the
speed of light enforces the contraction and stabilization of sand p1/2 orbitals. The increased screening by these contracted
orbitals for a positive nuclear charge makes outer p3/2, d, and forbitals expanded and destabilized. Relativistic contribution
also comes from the spin-orbit splitting of electron orbitals

1. Introduction
Chemistry of the transactinide elements with atomic numbers (Z) ≥ 104 is one of the most fascinating and challenging
subjects in the research field of nuclear and radiochemistry. At
present, the elements up to element 118 are reported to be produced in heavy-ion-induced nuclear reactions.1, 2 The positionof these elements at the Periodic Table is of fundamental
importance. Pioneering chemical works performed with a few
of atoms have thus far experimentally verified to place thetransactinide elements on respective positions at the Periodic
Table (see Figure 1). Rutherfordium (Rf) through hassium(Hs) 3, 4 and copernicium (Cn)5 are arranged on the groups 4 to
1
1

H

18
2

3

2
4

13
5

Li

Be

B

11

12

Na Mg

14
6

F

13

14

Al

Si

15
7

N

15

P

16
8

O

17
9

He
10

16

17

F

Ne
18

33

34

S

Cl

Ar

Zn

Ga Ge As
49

50

51

Se

Br

Kr

Ag

Cd

In

Sn

Sb

Te
84

I

85

Xe

Au

Hg

Tl

Pb

Bi

Po

At

Rn

19

20

3
21

4
22

5
23

6
24

7
25

8
26

9
27

10
28

11
29

12
30

K

Ca

Sc

Ti
40

V

41

Cr

Mn

Fe

Co

Ni

Cu

Rb

Sr
56

Y

57

Zr

Nb Mo
73

74

Tc

Ru Rh
76

77

Pd

Cs

Ba

La
89

Hf

104

Ta
105

W

106

Re Os
107

108

Ir

109

Pt

110

Fr

Ra

Ac

Rf

Db

Sg

Bh

Hs

Mt

Ds Rg Cn 113 114 115 116 117 118

57

58

59

60

61

62

63

64

65

66

67

La

Ce

Pr

Nd Pm Sm Eu Gd
92

93

94

Tb

Dy

Ho

Ac

Th

Pa

U

Np

Pu Am Cm Bk

Cf

37

55

87

38

88

lanthanide
actinide

39

89

72

90

91

42

43

75

44

45

95

46

78

96

47

79

111

97

48

80

112

98

31

81

113

32

82

114

83

115

52

116

35

53

117

68

69

70

71

Er

Tm Yb

Lu

Es Fm Md No

Lr

99

100

101

Figure 1. Periodic Table of the elements.
*Corresponding author. E-mail; toyoshima.atsushi@jaea.go.jp,
Fax; +81-29-282-5939© 2012 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 07/30/2012

102

103

36

54

86

118

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J. Nucl. Radiochem. Sci., Vol. 12, No. 1, 2012

with angular moment > 0. These three effects change approximately as Z 2 for the valence shells down a column of the
Periodic Table. The ground state electronic configuration of
the transactinide elements has drastic rearrangement due tostrong relativistic effects,7-9 and accordingly, their chemical
properties cannot be anticipated simply from systematic trends
in those of lighter homologues. Detailed chemical characterization of these elements is, therefore, necessary to allow thorough comparison of their properties with those of lighter
homologues, which provides a unique opportunity to examine
modern relativistic quantum-chemical treatments.10-12
Because of the short half-lives (T1/2) and the extremely low
production rates of the transactinide nuclides, chemical experiments must be carried out with single atoms. One needs to
conduct chemical experiments giving the same results between
a macro amount and single atoms. For single atoms, the classical law of mass action is no longer valid because the single
atom cannot exist in different chemical forms taking part inchemical equilibrium at the same time. It is suggested that theconcept of the chemical equilibrium is replaced by the equivalent expression of finding probabilities of the single atoms in
one phase or the other measured in repetitive experiments.13, 14
Partition methods such as ion exchange, solvent extraction, andgas chromatography are often employed for single atoms.
In the last decade, great progress was made in aqueous
chemistry of the transactinide elements. Precisely measured
chemical quantities such as a distribution coefficient (Kd) of theheaviest elements enabled quantitative interpretation for complex formation and redox (oxidation – reduction) reactions,which are the most fundamental chemical properties of ions inaqueous solution. At JAEA, chemical properties of Rf was
systematically explored in acidic solutions using AIDA (theAutomated Ion-exchange separation apparatus coupled with
the Detection system for Alpha spectroscopy).15 Chloride,
nitrate, fluoride, and sulfate complexation of Rf was thoroughly
investigated.16-22 Redox studies of the heaviest elements by anovel electrochemical technique were also successfully carriedout using a flow electrolytic chromatographic column apparatus.23, 24
In this account, hexafluoro complex formation of Rf andoxidation of No2+ conducted at JAEA are presented as examples to elucidate the influence of strong relativistic effects
through the determination of an ionic radius and a redox potential. First of all, production of 261Rf and 255No with a 248Cm target at the JAEA tandem accelerator is presented. Then,fluoride complexation of Rf with anion-exchange chromatography using AIDA is showed.19 The formation constants of theanionic fluoride-complexes of Rf are experimentally obtained
from the measurement of its Kd values. An ionic radius of Rf 4+
is evaluated from the formation constants and is compared withtheoretical predictions. Our newly developed electrochemistry
technique for single atom chemistry using a flow electrolytic
chromatography apparatus23 is, thereafter, introduced. This
unique technique is successfully applied to the oxidation of
No2+ to No3+ on a single atom scale,24 which demonstrates thecapability of the technique to perform redox experiments of thetransactinide elements. Finally, prospects of aqueous chemistry of the heaviest element at JAEA are briefly considered.
2. Production of 261Rf and 255No
The heaviest elements are produced at accelerators using
nuclear reactions of heavy-ion beams with heavy target materials. A 248Cm target was used to produce neutron-rich and relatively long-lived isotopes in hot-fusion reactions at the JAEA
tandem accelerator. Rutherfordium-261 (T1/2 = 78 s)25 and 255No(T1/ 2 = 3.1 min) 25 were produced in the 248 Cm(18 O, 5n) and248
Cm(12C, 5n) reactions with production rates of approximately
2 and 30 atoms per minute, respectively, under typical experi-mental conditions.
Gadolinium isotopes were mixed into the 248 Cm target tosimultaneously produce short-lived isotopes of respective
lighter homologues Hf and Yb which were used for monitoring
their chemical behavior and yields during repetitive chromatographic experiments of Rf and No, respectively. This is an
important experimental manner to guarantee reproducibility ofthe experimental cycles. We also produced short-lived isotope
pairs of Zr/Hf and Yb/Sr for the Rf and No experiments,
respectively, to establish chromatographic behavior of theselighter elements. These pairs were simultaneously producedby irradiating mixed Ge/Gd targets with the same beams as
those for the Rf and No production.
These reaction products recoiling out of the target wereattached to KCl aerosols seeded in a helium gas stream and arecontinuously transported to the chemistry laboratory through athin Teflon capillary at a typical gas flow rate of 2.0 L min-1.
The transport efficiency of the He/KCl gas-jet system was estimated to be 35%.26
3. Hexafluoro complex formation of Rf
Fluoride complexation of Rf has been reported to be quite
different from that of its lighter homologues Zr and Hf.27-30
Strub et al. systematically studied the adsorption of Rf on the
cation- and anion-exchange resin in the 0.0001 - 1 M HF/0.1 M
HNO3 solution and reported that the adsorption behavior of Rf
is remarkably different from that of Zr and Hf.29, 30 Haba et al.
then examined the anion-exchange behavior of Rf, Zr, and Hf
in 1.9 - 13.9 M HF using AIDA.17 The result showed that theadsorption probabilities of Rf are much smaller than those ofZr and Hf and suggested that the different fluoride species
between Rf and the homologues are present in the HF solution.17 To further understand the characteristic behavior of Rf
and to assess its ionic radius from the behavior, it is essentially
important to study the formation of anionic fluoride complexesof Rf quantitatively. In the present study, we systematicallymeasured Kd values of Rf as well as those of its homologues Zr
and Hf as a function of the concentrations of F- and NO3- which
acted as a counter ion competing with anionic complexes on
the binding site.31 Anionic fluoride species of Rf in HF/HNO3
was clearly identified by a slope analysis with the NO3- counter-ion on anion-exchange resin. Then formation constants ofanionic fluoride-complexes of Rf was evaluated by analyzing
variation of the Kd value against the concentration of the F- ion.
An ionic radius of Rf 4+ was discussed based on the formation
constants.
In the experiment, two different-sized chromatographic columns of 1.6 mm i.d. × 7.0 mm long and of 1.0 mm i.d. × 3.5
mm long were employed to study chromatographic behavior of
Rf in a wide range of HF/HNO3 concentrations. The transported products by the gas-jet method were collected on thedeposition site of AIDA for 2 min. After collection, the products were dissolved with HF/HNO3 and were subsequently fed
onto an anion-exchange column at a flow rate of 740 µL min-1.
Remaining products on the resin were stripped with 4.0 M HCl
at a flow rate of 1000 µL min-1. The effluents were collected
on Ta discs as fractions 1 and 2, respectively. These samples
were separately evaporated to dryness with hot He gas and a
halogen heat lamp to prepare dried samples for α-spectrometry.
The two discs were then transferred to the α-spectrometry station of AIDA equipped with 600 mm 2 passivated implanted
planar silicon (PIPS) detectors. The detection efficiency andenergy resolution was 35% and approximately 70 keV FWHM,
respectively. After the α-particle measurement, γ-ray of 169Hf
was monitored using Ge detectors for every third or fourth pairof the Ta discs to determine its adsorption probability and thechemical yield. These experiments were repeated in a 2-mincycle. The elution behavior of Zr and Hf was also examined toChemical studies of rutherfordium (Rf) and nobelium (No) on an

Ac- + cRL ⇄ RcA + cL- .

(1)

The consecutive formation of the fluoride complexes of Zr and
Hf is represented31-35 as[MFn-1]4-(n-1)+ F ⇄ [MFn]-4-n(n = 1 - 6)(2)

where M indicates Zr or Hf and n denotes the coordination
number in the products. From these chemical equilibria, theKd value is derived as
logKd = logD2 – 2log

[L-]
+ log
[RL]

β6 [F-] 6
1+

.

(3)

β[F ]

- n

The symbols βn (n = 1 - 6) show the stability constants repren
sented as β n = Π i=1 Ki and D 2 is the equilibrium constant forEq. (1); see ref. 17 for details. The -2 slope shown in Figure3(b) demonstrates that a hexafluoro complex [MF6 ] 2- is thedominant species for these elements. In Figuer 3(a), calculated
Kd values for Zr and Hf with Eq. (3) are shown by the dottedand broken curves, respectively, where the logK n (n = 1 - 6)
values from refs. 34 and 35 are used. The calculation with Eq.
(3) well reproduces the experimental data of these elements.
Assuming that Rf forms fluoro complexes according to Eq.
(2), its hexafluoro complex [RfF6] 2- is identified from the -2
slope shown in Figure 2. Increasing formation of [RfF6] 2- inthe HF/HNO3 solution with an increase of [F-] in Figure 3 (a)is also verified. Formation constants of the anionic fluoridecomplexes of Rf, K 5 and K6 , in addition to its D 2 value were

106

88Zr

(static conditions)(static conditions)
85Zr (1.6 mm i.d. x 7.0 mm)
169Hf (1.6 mm i.d. x 7.0 mm)
85Zr (1.0 mm i.d. x 3.5 mm)
169Hf (1.0 mm i.d. x 3.5 mm)
261Rf (1.6 mm i.d. x 7.0 mm)
261Rf (1.0 mm i.d. x 3.5 mm)
175Hf

105
Kd / mL g-1

104
103
102
101
100

10-2

10-1
[NO3-] / M

100

Figure 2. Variation of the Kd values of 88Zr and 175Hf under static
conditions and those of 85Zr, 169Hf and 261Rf from column chromatography as a function of [NO3-] at the constant [F-] of 3.0 × 10 -3 M. The
Kd values of 88Zr and 175Hf obtained in the batch experiments are plotted by open squares and open triangles, respectively. The values of85
Zr, 169Hf, and 261Rf with the column chromatography are shown by
closed symbols. Linear relationships of the logKd vs. log[NO3-] for Rf
and Zr/Hf are indicated by the solid and dashed lines, respectively.
Adapted from ref. 19.

106

(a)

88Zr (0.010 M [NO -])3
175Hf (0.010 M [NO -])3

105

88Zr (0.030 M [NO -])3
175Hf (0.030 M [NO -])3
88Zr (0.10 M [NO -])3

Kd / mL g-1

104
103

175Hf (0.10 M [NO -])3
88Zr (0.30 M [NO -])3
175Hf (0.30 M [NO -])3
261Rf (0.010 M [NO -])3
261Rf (0.015 M [NO -])
3

102
101

Slope

investigate their chromatographic behavior under the identicalconditions. The elution behavior was established by assaying
effluent samples in plastic tubes with γ-ray spectrometry.
From a total of 3788 separation cycles of the anion-exchange
experiments, 334 α-particles of 261Rf and 257 No including 46
time-correlated α-particle pairs from 261 Rf and 257 No wereobserved. In Figure 2, variation of Kd values for Zr, Hf, and Rf
in the constant [F- ] of 3.0 × 10 -3 M is shown as a function of[NO3-]. Concentrations of dissociated ions in HF/HNO3 were
evaluated applying the law of mass action to chemical equilibria of H + + F- ⇄ HF and HF + F- ⇄ HF 2 -.32 The Kd values of85
Zr and 169 Hf by the column chromatography are in good
agreement with those of 88Zr and 175Hf measured in separate
batch experiments under static conditions. It indicates that thechemical reactions in the rapid chromatography reach equilibrium. The Kd values of Zr and Hf are identical with each other,and the slopes in the logKd vs. log[NO3-] plot are -2.0 ± 0.1 as
indicated by the dashed line. We deduced Kd values of Rf fromits adsorption probabilities by assuming that the chemical reactions of Rf in the anion-exchange chromatography are as fast
as those of Zr and Hf.17-19 As shown in Figure 2, the Kd valuesof Rf are much smaller than those of Zr and Hf, and the slope
is -2.2 ± 0.2 depicted by the solid line. In Figure 3(a), variationof the Kd values for 261Rf, 88Zr and 175Hf is shown as a functionof [F-]. Curves are results of the theoretical calculation discussed later. The Kd values of 88Zr and 175Hf start to increase ataround [F-] = 1 × 10 -5 M, gradually reaching constant values
between 10 -4 and 10 -3 M, and decrease beyond [F-] = 5 × 10 -3
M. In contrast to this behavior, the Kd values of Rf begin to
increase at [F-] = 5.0 × 10 -4 M. In Figure 3(b), variation of theslopes for Rf, Zr and Hf in the logKd - log[NO3-] plot is shown
as a function of [F-]. The slope for 261Rf is taken from Figure
2. The slopes for Zr and Hf are approximately -2 in the [F-]range of 10 -5 M to 10 -2 M.
The variation of the Kd values is calculated based on anionexchange reactions and formation reactions for fluoride complexes of the group-4 elements. An exchange reaction between
an anionic complex A c- and a counter ion L - 31 on the anionexchange resin R is described as

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J. Nucl. Radiochem. Sci., Vol. 12, No. 1, 2012

100
-1
-2
-3
-4
10-6

(b)

88Zr
175Hf
261Rf

10-5

10-4
[F-] / M

10-3

10-2

Figure 3. (a) Variation of the Kd values of 88Zr and 175Hf in static
conditions and of 261Rf in the column chromatography as a functionof the F- concentration. The Kd values of 88Zr under static conditions
at [NO3-] of 0.010 M, 0.030 M, 0.10 M and 0.30 M are represented byopen squares, open diamonds, open squares with diagonal line, andopen squares with cross, respectively, and those of 175Hf are shown by
open triangles, open inverted triangles, open left-facing triangles, andopen right-facing triangles, respectively. Kd values for 261Rf at [NO3-]of 0.01 M and 0.015 M are depicted by closed circles and closed
squares, respectively. The solid, broken, and dotted curves are resultsof the theoretical calculation for the Kd values of 261Rf, 88Zr, and 175Hf,
respectively. (b) Variation of the slopes for Zr, Hf and Rf in the logKd- log[NO3-] plot as a function of [F-]. Adapted from ref. 19.

evaluated on the basis of Eq. (3) to be > 1 × 103, < 1 × 102, > 1
× 104, respectively, with the assumptions of Kn > 104 (n = 1 - 4).
The K5,34 K6,34 and D 2 value of Zr are 3.2 × 10 4, 1.9 × 103, 5 ×105, respectively, and those of Hf 35 are 5.0 × 104, 3.2 × 103, 5 ×105, respectively. It is found that the maximum possible valueof the K6 value of Rf is at least one order of magnitude smaller
than K6 of the homologues. As an example, the results of thecalculation for the Kd of Rf using the values of K5 = 4 × 103, K6

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Toyoshima

J. Nucl. Radiochem. Sci., Vol. 12, No. 1, 2012

= 2, and D2 = 5 × 10 are shown as solid lines in Figure 3(a).
An ionic radius of Rf 4+ is evaluated to be larger than thoseof Zr4+ and Hf 4+ based on the inverse trend between stabilitiesof the fluoride complexes and crystallographic ionic radii dueto the predominant electrostatic interaction between the metallic ion and fluoride ligands. This was also confirmed by our
cation-exchange experiment of Rf with comparison with the
behavior of Zr, Hf, and Th.20, 21 The larger ionic radius of Rf 4+
is consistent with the early theoretical calculation.36 This calculation predicts that the ionic radius of Rf 4+ is 79 pm with thecoordination number of 6 and is larger than those of Zr4+ (72pm37) and Hf 4+ (71 pm37) with the same coordination number
by the comparison of the maxima of the charge density in theoutermost p orbitals for Ti, Zr, Hf, and Rf to the measured
ionic radii of Ti4+, Zr4+, and Hf 4+. This is explained due to theorbital expansion of the 6p orbital.11, 12
The modern prediction with the fully relativistic densityfunctional theory showed the trend of Zr > Hf > Rf 38 which isconsistent with the observed sequence in the K 6 values,
although the theory is assuming hydrolysis to happen in the
pH-range where the experiment was performed. In this calculation, free energy changes of the complex formation reactions
for fluoride-complexes and hydrolyzed complex are considered.
The bond lengths of Rf complexes were taken to be 5 pm larger
than those of Hf.38

tion behavior of Sr (T1/2 = 22.1 min) and Yb with 10 6
atoms was investigated to establish a clear separation between
the divalent (Sr2+) and trivalent (Yb3+) ions under the identicalconditions as those with No and Yb using the present system,
the Sr2+ expressing the anticipated behavior of No2+. The effluents from the column electrode were collected in plastic tubes
and were assayed with γ-ray spectrometry.
Figures 5(a) and 5(b) show the elution behavior of 255No and162
Yb under the applied potentials of (a) 0.2 V and (b) 1.2 V,
respectively, while the elution of 81Sr and 162Yb in the separate
experiment is depicted in Figure 5(c). From the reference runwith 81Sr and 162Yb, the clear separation between the trivalentand divalent ions is obvious; 162Yb3+ is eluted with 0.1 M αHIB, while 81Sr2+ is present in the subsequent strip fraction of
3.0 M HCl. The elution positions of 81Sr and 162Yb are almost

4. Electrolytic oxidation of No

Glassy-carbon rod(for the lead of theworking electrode)

It is quite difficult to investigate the redox properties of theheaviest elements by ordinary electrochemical methods utilized on a macro-amount scale because these elements must betreated with on a single atom scale. Redox reactions of theheaviest elements in aqueous solution have been so far studied
using column chromatography with some kinds of oxidizing or
reducing agents. Hulet et al.39 and Maly et al.40 determined theredox potential of Md in HCl with V2+ -V3+ and Ti-TiO2 couples,
respectively. Silva et al. showed the oxidation of No2+ to No3+
and their separation by column chromatography in oxidizing
solutions such as H5IO4.41 These methods are, however, timeconsuming and complicated, and in some cases have slow
kinetics. In this study, therefore, we have developed a rapid
and efficient electrochemical approach to determine redox
potentials with controlling oxidation states in a chromatographic process.23 This electrochemistry method was successfully applied to the oxidation of No2 + to No 3 + in 0.1 M
α-hydroxyisobutyric (α-HIB) acid solution.24
In the experiment, transported products were deposited on acollection site of AIDA15 for 10 min. After collection, theproducts were dissolved with 0.1 M α-hydroxyisobutiric acid
(α-HIB) (pH 3.9) and were subsequently fed through a thin
Teflon tube into a column-type working electrode which wasmade of glassy carbon fibers modified by Nafion perfluorinated ion-exchange resin.23 A cross sectional view of the flow
electrolytic column apparatus that is based on the ones by
Kihara et al.42 is shown in Figure 4. The potential applied to
the working electrode was adjusted to 0.2 to 1.2 V referred to a
Ag/AgCl electrode in 1.0 M LiCl. The effluent from the column electrode was consecutively collected with a volume of
180 µL on 6 separate Ta discs. The remaining products in thecolumn were stripped with 360 µL of 3.0 M HCl and collected
on another 2 Ta discs. The 8 samples were evaporated to dryness using hot helium gas and halogen heat lamps and werethen transferred to an α-spectrometry station of AIDA.
Counting efficiencies and energy resolution of the detectors
were 35% and 100 keV FWHM, respectively. After the α-particle measurement of No, the Ta discs were assayed by γ-ray
spectrometry with a Ge detector to monitor elution behavior of162
Yb (T1/2 = 18.9 min) 25 as the representative trivalent ion andits chemical yields. Prior to the experiment of No and Yb, elu-

8125

Referevce Electrode(Ag-AgCl)

Working Electrode(glassy-carbon fibers
modified with Nation)

Solution
inlet

162

Counter Electrode(Pt mesh)

SolutionoutletElectrolyte pool

Figure 4. Schematic view of the electrochemistry apparatus.
Adapted from ref. 23.

0.1 M α-HIB (pH 3.9) 3.0 M HCl
80
60

255No2+(a) 0.2 V

40

162Yb3+20

Eluted radioactivity / %

5

0
60
40

(b) 1.2 V
255No3+

162Yb3+

20
0
80
60
40

(c) Reference
81Sr2+
162Yb3+

20
0
0

500

1000

Eluted Volume / µL

1500

Figure 5. Elution behavior of 255No and 162Yb at the applied potentials of (a) 0.2 V and (b) 1.2 V. (c) Elution of the typical trivalent
162
Yb3+ and the divalent 81Sr2+ in the reference experiment: close symbols at 0.2 V, open symbols at 1.2 V. Adapted from ref. 24.

Chemical studies of rutherfordium (Rf) and nobelium (No) on an

A5

120

Oxidation probability / %independent of the applied potentials in the studied range of
0.2 - 1.2 V. At 0.2 V, 255 No is detected in the 3.0 M HCl
fraction while the monitored 162Yb3+ is eluted earlier with αHIB as shown in Figure 5(a). The adsorption of 255No is thesame as that of 81Sr2+, clearly indicating that No is bound in its
stable divalent state, which is consistent with the early cationexchange study.43 On the other hand, the elution of 255No in the
0.1 M α-HIB fraction is unambiguously observed at 1.2 V at
the position of 162Yb3+. This behavior demonstrates that No2+ is
successfully oxidized to No 3 + and the trivalent state iscomplexed in the α-HIB solution with the present apparatus.
Figure 6 shows the oxidation probability of No as a functionof the applied potential, that probability defined as 100 ×[No3+] / ([No2+] + [No3+]), where [No2+] and [No3+] represent
the radioactivities of 255No measured in the 3.0 M HCl and 0.1
M α-HIB fractions, respectively. The oxidation reaction begins
at around 0.7 V and is complete by 1.0 V. The formal redox
potential of the No3+ + e- ⇄ No2+ reaction corresponding to ahalf of the oxidation probability is evaluated to be approximately 0.75 V under the present conditions. This novel
technique has a great potential for investigating redox reactionsof transactinide elements in queous solutions.

J. Nucl. Radiochem. Sci., Vol. 12, No. 1, 2012

100
80
60
40
20
0

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Applied potential / V
Figure 6. Oxidation probability of 255 No vs. applied potentials.
Adapted from ref. 24.

in Research and Development in the Field of Ion Beam
Application.

5. Summary and perspectives

References

The hexafluoro complex of Rf [RfF6] 2- was clearly identified
by the slope analysis with the anion-exchange in the HF/HNO3
solutions. The formation constant of the hexafluoro complexof Rf was evaluated to be at least one order of magnitude
smaller than those of Zr and Hf. An ionic radius of Rf 4+ wasevaluated to be larger than those of Zr4+ and Hf 4+, which is ingood agreement with the theoretical predictions. The electrolytic column chromatography apparatus was newly developed
for the redox studies on a single atom scale. Oxidation experiments of No2+ were conducted in 0.1 M α-HIB acid solution
using the apparatus. The stable No2+ was successfully oxidized
to No3+ which is held in the α-HIB solution.
In application with a developed technique of the electrolytic
column chromatography, an ionic radius of No3+ which is a key
to understand the contraction of the ionic radii at the end of theactinide series44 will be determined. Elution position of No3+
from a column electrode in α-HIB will be measured by comparison with those of trivalent lanthanides and actinides.
For the elucidation of relativistic effects on stabilities ofvalence electron orbitals of the transactinide elements, a reduction of Sg will be investigated with the isotope 265Sg producedin the 248Cm(22 Ne, 5n) reaction. A flow electrolytic-column
apparatus combined with a continuous extraction apparatus of
SISAK 45, 46 will be utilized to overcome experimental difficulty due to the short half-life (~15 s) and the extremely small
production yield (~1 atom/h).

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Acknowledgement
These works were carried out by members of ResearchGroup for Superheavy Elements of JAEA. We would like tothank all our colleagues from JAEA, Osaka University,
RIKEN, Tokyo Metropolitan University, Niigata University,
Kanazawa University, University of Tokushima, University ofTsukuba, GSI Helmholtzzentrum für SchwerionenforschungGmbH, and Universität Mainz for their constant support. The
author would like to especially acknowledge Dr. Yuichiro
Nagame and Prof. Atsushi Shinohara for their kind encouragement and fruitful discussion throughout the course of the present works. Thanks also go to the crew of the JAEA tandem
accelerator for their invaluable assistance in the course of theseexperiments. These works were supported in part by the
JAEA-University Collaboration Research Project and theProgram on the Scientific Cooperation between JAEA and GSI

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