Journal of Nuclear and Radiochemical Sciences, Vol. 8, No. 2, pp. 69-72, 2007

Anion-exchange Behavior of Nb, Ta, and Pa as Homologues of Db in HF/HNO3 Solutions
Y. Kasamatsu,*,a A. Toyoshima,a H. Toume,a K. Tsukada,a H. Haba,b and Y. NagameaabAdvanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan
Nishina Center for Accelerator Based Science, RIKEN, Wako, Saitama 351-0198, Japan

Received: January 31, 2007; In Final Form: June 7, 2007
To find suitable conditions for the experimental study of anion-exchange behavior of element 105, dubnium (Db),
we have investigated anion-exchange behavior of the group-5 elements Nb, Ta, and their pseudo homologue Pa in
HF/HNO3 media by a batch method using radiotracers of 95 Nb, 179 Ta, and 233Pa. Clearly different adsorption
behavior was observed among those homologues in the fluoride ion concentrations of 1 × 10 −6 M < [F−] < 2 × 10 −2
M, suggesting that each element forms different anionic complexes. Based on the observed distribution coefficients of these elements, we propose appropriate experimental conditions to investigate fluoride complexation ofDb.

1. Introduction
The transactinide element with atomic number Z ≥ 104 is
produced at accelerators using heavy ion-induced nuclear reactions as only one atom at a time because of the considerably
low production rate and short half-life. Thus, the chemical
properties of the transactinide element must be evaluated bysingle-atom base experiments and the chemical reaction system
for such experiments should reach an equilibrium state fastenough within the lifetime of an objective transactinide nuclide.
Partition methods as chromatographic systems with fast kinetics are applied to the chemical studies of the transactinide elements.1,2
Several studies on the solution chemistry of Db were conducted by applying rapid partition methods.3–10 From the resultsof the experiments, it was found that the chemical behavior of
Db is distinctively different from that of the closest homologue
Ta and is similar to those of Nb and Pa.4,5,7 As the fluoride
anion is a very strong complexing agent for the group-5 elements, several chemical experiments of Db were conducted in
HF solutions or in mixed solutions of HF with other strongmineral acids.4,5,7–10 The results of the experiments showed thatDb forms anionic fluoro complexes which are strongly retainedon an anion-exchange resin.10 The chemical properties of Db,
however, have not yet been well clarified due to ambiguities ofthe data caused by insufficient statistics. In our previous
works, distribution coefficients (Kd) of element 104 (Rf) were
determined by anion-exchange experiments in HF and HF/
HNO3 solutions with AIDA (Automated Ion-exchange separation apparatus coupled with the Detection system for Alphaspectroscopy), and the fluoride complexation of Rf was studied
in detail by comparing the results with those of its homologues.11–13 We, in this work, have investigated anion-exchange
behavior of Nb, Ta, and Pa in HF/HNO3 media by a batch
method to study the formation of their anionic fluoro complexes and to find suitable experimental conditions for thechemical study of Db.
2. Experimental
2.1. Preparation of radiotracers.
2.1.1. Preparation of mixed solution of 95Nb and 179 Ta.
Niobium-95 (T1/2 = 34.97 d) and 179Ta (T1/2 = 665 d) were pro*Corresponding author. E-mail: kasamatsu.yoshitaka@jaea.go.jp.
Fax: +81-29-282-5939.

duced in proton bombardments on Zr and Hf metallic foil targets with natural isotopic abundance, respectively, using the
RIKEN K70 AVF cyclotron. The incident energy of the proton
beam was 14 MeV with the beam current of approximately 5µA for the production of both 95Nb and 179Ta. Several hundreds
to thousands kBq of the radiotracers were produced.
Purifications of Nb and Ta were individually carried out asfollows. Each target foil was dissolved in 0.2 mL of concentrated HF solution in a polytetrafluoroethylene (PTFE) beaker.
After evaporation to dryness, the residue was dissolved with
0.2 mL of 1 M HF and the solution was fed onto a chromatographic column (2.5 mm i.d. × 25 mm long for Nb, 2.5 mm i.d.
× 10 mm long for Ta) filled with the anion-exchange resin(Dowex 1×8, 200–400 mesh, F− form). By washing the resin
column with 1–2 mL of 1 M HF, most of by-products of thenuclear reaction such as lanthanide elements were eluted fromthe column, while the group 4 and 5 elements were retained on
the resin. Three to five mL of the mixed solution of 9 M
HCl/0.004 M HF was subsequently fed on the column to elutethe target materials of Zr and Hf. Niobium was then eluted
with 3 mL of 5 M HNO3 /0.2 M HF, while Ta was eluted withadditional 30 mL of concentrated HF solution. The Nb and Ta
effluents were separately collected in PTFE beakers and were
evaporated to dryness. The above anion-exchange procedure
was repeated until little residue remains in the beaker after theevaporation. Thereafter, Nb and Ta in the PTFE beakers weredissolved with a small amount of 1 M HF. After adding several
drops of hydrogen peroxide (H2O2) into the solution, the mixture was heated to eliminate the decomposition products of theanion-exchange resin. The purified Nb and Ta radiotracers
were dissolved with 1 M HF solution and mixed each other.
The solution was stocked in a perfluoro alkoxy (PFA) vessel.
X and γ rays of 179Ta and 95Nb were measured with a Ge semiconductor detector to ascertain the isolation of the elements.
2.1.2. Preparation of 233Pa solutions. Protactinium-233 (T1/2= 27.0 d) was chemically separated from its α -decay parent
237
Np (T1/2 = 2.144 × 10 6 y). Neptinium-237 with 69 kBq was
dissolved with 0.2 mL of 9 M HCl, and then the solution was
fed onto a column (2.5 mm i.d. × 25 mm long) filled with theanion-exchange resin (Dowex 1×8, 200–400 mesh, Cl− form).
After washing the resin with 2 mL of 9 M HCl, Pa was eluted
with 3 mL of 9 M HCl/0.025 M HF. The eluate was collected
in a PTFE beaker. Remaining Np on the resin was eluted with
distilled water and 0.2 M HNO3. The effluent of the Pa fraction obtained was evaporated to dryness and underwent thesame anion-exchange procedure to remove Np thoroughly.

© 2007 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 10/19/2007

Kasamatsu

J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007
233

About 40 kBq of Pa was obtained and stocked in 1 M HF
solution in a PFA vessel.
2.2. Adsorption on PFA and PP surfaces. Adsorption of
the group-5 elements Nb and Ta on glass, Pt, and even Teflonin concentrated HCl and HNO3 was reported in Reference 3.
As the adsorption interferes with precise batch-wise determination of distribution coefficients on the anion-exchange resin,
we investigated the adsorption behavior of 95Nb and 179 Ta on
the surfaces of PFA and polypropylene (PP) test tubes used inthis experiment. HF solutions of three different concentrations(10 −4, 10 −2, and 1 M) and mixed solutions of two different compositions (10 − 4 M HF/0.1 M HNO3 and 10 −2 M HF/0.1 M
HNO3) were used in the present adsorption experiment. After
determination of the “initial reference radioactivity” of eachsolution, several mL of the radiotracer solutions were fed into
the PFA and PP test tubes (12 mm i.d. × 140 mm long for PFA,
17 mm i.d. × 119 mm long for PP). Then, the solutions were
well shaken and left. After some standing time, 30 µL of thesolution was pipetted into a small PP container and was subjected to X- and γ-ray spectrometry. The ratio, R, of the radioactivity in each solution to the initial reference radioactivity ofthe solution was obtained as a function of the standing time toobserve the time-dependent adsorption behavior of the radiotracers on the PFA and PP surfaces. The adsorption behaviorof 233Pa was also investigated in the same manner.
We confirmed that the adsorption of the radioisotopes on thepipettes made of PP is negligible. Only statistical errors weretaken into account in the estimation of errors of the radioactivity ratios R. The concentrations of the HF and HNO3 solutions
were determined by titrations with standardized NaOH andNaHCO3, respectively, before the experiments.
2.3. Ion-exchange kinetics. In order to determine the distribution coefficient in equilibrium, kinetics of the chemical
reaction has to be examined. The distribution coefficient of anion-exchange system is defined using a ratio of the radioactivity in the resin to that in the solution as
Kd =errors of the Kd values.
2.4. Determination of distribution coefficients. The Kdvalues of Nb, Ta, and Pa were determined in various concentrations of HF/HNO3, where the concentration of HNO3 was kept
constant at 0.1 M. The experimental procedure was almost thesame as that described in sec. 2.3. For the solutions with the
HF concentration [HF] ≥ 10 −2 M, the shaking time was 15 min,while the time was at least 5 h for the solutions with 10 −3 M ≤[HF] < 10 −2 M, and 1 day for those with [HF] < 10 −3 M. The
shaking time was determined based on the results of the kinetic
studies.
3. Results and Discussion
3.1. Adsorption on PFA and PP surfaces. Variations of
the radioactivity ratios, R, which represent the adsorption of
the radiotracers on the surfaces of the PFA and PP test tubes in
HF and HF/HNO3 solutions are shown as a function of elapsed
time in Figure 1. It is found that the decrease of the ratio
resulting from the adsorption on the surface of the tube is notobserved for every element in the solutions with the fluoride
ion concentration [F−] ≥ 10 −4 M. Here, the fluoride ion concentration was calculated using the dissociation constants of HF
and HF2− reported in Reference 14. For the solutions with [F−]< 10 −4 M, contrarily, the adsorption on the PFA and PP surfaces is observed for Ta after about 1 week. In such low F −concentrations, hydrolytic species would be dominant for Ta;15
various kinds of species including neutral complexes coexist in
the solutions. To avoid the adsorption, we stocked the radiotracers in HF solutions with the concentrations of 10 −2 M or
higher and those mixed with 0.1 M HNO3.
As a result, it is evident that influence of the adsorption of
the group-5 elements on both the PP and PFA surfaces is negligible within 1 week for the determination of the Kd values inthe solutions with [F−] > 10 −6 M.
3.2. Ion-exchange kinetics. Variations of the Kd values of

ArVs
,
AsWr

where Ar and As are the radioactivities (Bq) in the resin and thesolution, respectively, Vs is the volume of the solution (mL),and Wr is the mass of the dry resin (g). By observing the timedependence of the Kd value, we examined the kinetics of theion-exchange reaction which includes the coordination reactionof the metal element with the fluoride ions.
The resin used was the strongly basic anion exchanger MCI®
GEL CA08Y supplied by Mitsubishi Chemical Corporation.
The resin was washed with distilled water, and then stirred in 2
M HNO3 and 2 M NaOH solutions alternately. After washing
the resin with 2 M HNO3 twice, the resin was washed with thedistilled water several times and dried in a vacuum oven at 70°C for 24 h.
Kinetics of the ion-exchange reaction was investigated for95
Nb, 179 Ta, and 233Pa in HF/0.1 M HNO3 solutions with HF
concentrations of 10 −4, 10 −3, 10 −2 , 1, and 10 M. In a PP test
tube, 8–200 mg of the dried resin and 3 mL of HF/HNO3 solution containing 30–50 µL of the tracer solution were mixed. In
each sample, 108 –1012 atoms of the tracer nuclides were contained. Then, the tube was shaken for a certain time (shakingtime). After centrifugation, 1 mL of the solution was pipetted
in a small PP container and subjected to X- and γ-ray spectrometry with a Ge detector. The same treatment was conductedwithout the resin to determine the reference radioactivity of thesolution. The radioactivity of each isotope on the resin wasdetermined by subtracting the radioactivity in the supernatantsolution from the reference radioactivity of the solution. We
investigated a dependence of the Kd value on the shaking timethat includes the time consumed for the centrifugation.
Statistical errors were taken into account in the estimation of

1.5

(a)

(b)

(c)

1.0
0.5

one week[F-] = 1.6

0

1.5

Ra t i o70100[F-] = 2.7

10-2 M102

104 100(d)102

10-3 M

104 100[F-] = 9.2

10-5 M

102

104

(e)

1.0

: Nb-95: Ta-179: Pa-233

0.5[F-] = 1.1

0 0
10
1.5

102

10-4 M[F-] = 1.1

104 100(f)

102

10-6 M

104

(g)

1.0
0.5
0 0
10
1.5

[F-] = 2.7[F-] = 9.2

10-3 M102

104 100(h)102

10-5 M

104

(i)

1.0
0.5
[F-] = 1.1

0 0
10

102

10-4 M[F-] = 1.1

104 100

10-6 M

102
104
Time / minFigure 1. Adsorptions of Nb, Ta, and Pa on the surfaces of the (a)–(e) PFA and (f)–(i) PP test tubes as a function of elapsed time in (a) 1
M HF ([F−] = 1.6 × 10 −2 M), (b)(f) 10 −2 M HF ([F−] = 2.7 × 10 −3 M),(c)(g) 10 −4 M HF ([F − ] = 9.2 × 10 −5 M), (d)(h) 10 −2 M HF/0.1 M
HNO3 ([F−] = 1.1 × 10 −4 M), and (e)(i) 10 −4 M HF/0.1 M HNO3 ([F−]= 1.1 × 10 −6 M). The vertical axis denotes the radioactivity ratio, R,
which is defined in the text. R = 1 (horizontal line) corresponds to noadsorption, while R = 0 means 100% adsorption.

Anion-exchange Behavior of Nb, Ta, and Pa as Homologues of Db
30

J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007

10 4

2000

10-2 M(a) [F-] = 1.7(b) [F-] = 7.8

10-3 M

1500

20

10

0

-

(c) [F ] = 1.1

800
-1

100

200

300

400

0
200

-4

10 M

600

100

100

10

50

5

0

0

30

100

(e) [F-] = 1.1

2003001400

1500

0

100

200

-

300

400

-5

(d) [F ] = 1.1

10 M: Nb-95: Ta-179: Pa-233

10 2
10 1
10 0

0

100

200

300

1400 1500

10 -1 -7
1010 -6

10 -5- 10-4
[F ] / M

10-3

10-2

10-6 M20: Nb-95: Ta-179: Pa-233

1000

Kd / mL g

5000

Kd / mL g

-1

1000
10

3

71

500
1000
Time / min

1500

Figure 2. Variations of the distribution coefficients of Nb, Ta, andPa on the anion-exchange resin as a function of the shaking time in
the HF/HNO3 solutions. The concentration of HNO3 was constant(0.1 M). The concentrations of HF were (a) 10 M ([F−] = 1.7 × 10 −2
M), (b) 1 M HF ([F−] = 7.8 × 10 −3 M), (c) 10 −2 M ([F−] = 1.1 × 10 −4M), (d) 10 −3 M ([F−] = 1.1 × 10 −5 M), and (e) 10 −4 M ([F−] = 1.1 × 10 −6
M).

Nb, Ta, and Pa as a function of the shaking time are depicted
in Figure 2. It is found that the ion-exchange kinetics depends
on the fluoride ion concentration of the solution. The distribution coefficients do not change significantly over the shaking
time ranging from 3 to 360 min, indicating that equilibrium
kinetics is accomplished within 3 min in the solutions with [F−]≥ 7.8 × 10 −3 M. In the solutions with [F−] < 10 −4 M, especiallyfor Ta, it takes several hundreds min at most to achieve the ionexchange equilibrium. This means that [F−] should be higher
than 10 −4 M in rapid ion-exchange experiments for short-lived
isotopes of the group-5 elements to obtain chemical constantsin equilibrium.
It is evident from the results of the kinetic studies that the Kdvalues in chemical equilibrium can be determined by the present batch experiments in the wide range of fluoride ion concentration, [F − ] > 10 − 6 M, without the interference from theadsorption on the surfaces of the test tubes.
3.3. Distribution coefficients. Variations of the Kd valuesof Nb, Ta, and Pa as a function of [F−] are shown in Figure 3.
The Kd values of the elements increase with increasing [F − ],which reflects the successive formation of anionic fluoro complexes. The decreasing feature of the Kd values in the concentrated [F − ] region would be due to the displacement of theanionic fluoro complexes of the group-5 elements on the resin
through the HF2− anions.16
Clear differences in the adsorption behavior among thegroup-5 elements on the anion-exchange resin are observed.
The large difference in the Kd values of the homologues Nb
and Ta observed in the present study would arise from the factthat Ta forms fluoro complexes TaFn5−n while Nb is present as
oxo-fluoro complexes NbOFn3−n dominantly.15–19 It is reported
that Pa forms fluoro complexes PaFn5−n like Ta.15–17 However,
the observed Kd values of Pa are considerably smaller thanthose of Ta in the studied conditions. As an average charge ofanionic complexes influences on the Kd value, the number ofthe fluoride anions bound with Pa possibly differs from that of
Ta in the present conditions. The average charge of the metal

Figure 3. Distribution coefficients of Nb, Ta, and Pa in HF/HNO3
solutions as a function of the fluoride ion concentration.

complexes can be estimated from the relationship between theKd values and the counter-anion (NO3−) concentration.20,21 For
more detailed discussion on the anionic fluoride complexationof the group-5 elements, therefore, further experiments toobtain Kd values as a function of the NO3 − ion concentration
are needed.
The differences in the anion-exchange behavior among thegroup-5 elements seem to originate from the differences intheir chemical species. Fluoro complex formation of Db isexpected to be discussed from experimental results on Db performed in the present chemical system. Taking into accountthe experimental restriction that chemical equilibrium shouldbe achieved within the lifetime of 262Db (T1/2 = 34 s), the anionexchange experiments must be carried out in the solutions with
[F−] ≥ 10 −4 M. Furthermore, the distribution coefficient should
be 10–500 mL/g to be determined using the microcolumn of
AIDA.11 From these standpoints, the experimental conditions at[NO3 −] = 0.1 M would be appropriate for the anion-exchange
experiments of Db on the assumption that Db behaves like Nb
or Pa as reported in References 4, 5, and 7.
4. Conclusion
We found out the experimental conditions to obtain the distribution coefficients of the group-5 elements in anionexchange chromatography with HF/HNO3 solutions using theradiotracers of 95Nb, 179Ta, and 233Pa. Anion-exchange behaviorof Nb, Ta, and Pa was minutely examined in the wide range ofthe fluoride ion concentration. Clear differences in the behavior among Nb, Ta, and Pa were observed in the studied conditions, suggesting that each element forms different anionic
fluoro complexes. The present result gives the reality to therapid chemical experiment of Db in HF/HNO3 media for the
study of fluoride complexation.
Acknowledgement. We express our gratitude to the staff ofthe RIKEN Accelerator Research Facility for the productionsof 95Nb and 179Ta.
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