Ligand-sensitized fluorescence of Tb3+ in Tb3+-dibutylphosphate complexes: Application for the estimation of DBP

Article abstract of DOI:10.1016/j.saa.2005.09.006

The fluorescence of Tb³⁺ is sensitized by complexation with dibutylphosphate (DBP) and tri-n-butylphosphate (TBP). The excitation maximum for the Tb³⁺-DBP complex occurs at 218.5 nm, while that for the Tb³⁺-TBP complex is observed at 228.0 nm. Both complexes yield Tb³⁺ fluorescence at 548 nm. The difference in the excitation maxima for the two complexes has been used to advantage for the estimation of DBP in the presence of TBP. DBP is the main degradation product of TBP in the PUREX process and the method described in this work can thus serve as a useful analytical tool in monitoring the quality of the TBP in the process. This method has been shown to be applicable for the estimation of DBP when present to an extent of 0.1-10% of TBP, in TBP/dodecane solutions.

Full text of DOI:10.1016/j.saa.2005.09.006

Spectrochimica Acta Part A 64 (2006) 972–976
3
+
3+
Ligand-sensitized fluorescence of Tb in Tb –dibutylphosphate
complexes: Application for the estimation of DBP
∗
S. Maji, K.S. Viswanathan
Materials Chemistry Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India
Received 11 May 2005; accepted 7 September 2005
Abstract
The fluorescence of Tb is sensitized by complexation with dibutylphosphate (DBP) and tri-n-butylphosphate (TBP). The excitation maximum
3
+
3
+
3+
3+
for the Tb –DBP complex occurs at 218.5 nm, while that for the Tb –TBP complex is observed at 228.0 nm. Both complexes yield Tb
fluorescence at 548 nm. The difference in the excitation maxima for the two complexes has been used to advantage for the estimation of DBP in
the presence of TBP. DBP is the main degradation product of TBP in the PUREX process and the method described in this work can thus serve
as a useful analytical tool in monitoring the quality of the TBP in the process. This method has been shown to be applicable for the estimation of
DBP when present to an extent of 0.1–10% of TBP, in TBP/dodecane solutions.
©
2005 Elsevier B.V. All rights reserved.
Keywords: Dibutylphosphate; Tri-n-butylphosphate; PUREX; Terbium; Fluorescence
1
. Introduction
tration, which can therefore be exploited for the determination
of DBP concentrations to monitor the quality of the extractant
in the PUREX process.
Terbium is often found to exhibit fluorescence enhancement
when complexed with suitable organic ligands [1–6]. In earlier
works from this laboratory, we have shown that the fluorescence
of Tb can be enhanced by over three orders of magnitude
by complexing it with a number of aromatic carboxylic acids
In the PUREX process, tri-n-butylphosphate diluted with
a hydrocarbon is used as an extractant for uranium and plu-
tonium. This extraction mixture operates in a strongly acid
medium and in a radiation environment; the TBP, conse-
quently, undergoes degradation to yield mainly dibutylphos-
phate along with monobutylphosphate and phosphoric acid as
minor products. The presence of DBP deteriorates the pro-
cess performance [11,12] by forming strong organic soluble
actinide–dibutylphosphate complexes. TBP is, therefore, given
an alkali wash to remove these degradation products, prior to
recycling. To monitor the extent of degradation and the quality
of the extraction reagent, it is necessary to determine DBP in the
reprocessing solvent.
Various analytical methods have been used for the determina-
tion of the concentration of DBP. They are gas chromatography,
several types of liquid chromatography, ion-pair chromatogra-
phy, nuclear magnetic resonance (NMR) spectrometry, titrime-
try, spectrophotometric and electrospray ionization mass spec-
trometry (ESI-MS) methods [13–19]. Chromatographic tech-
niques are time-consuming methods, for example, in the gas
chromatography method [13,14], DBP is first methylated, while
pre-concentration is required in case of ion-pair chromatogra-
phy [15]. DBP estimation using NMR spectrometry [16] is also
3
+
[
7,8]. The fluorescence enhancement is due to efficient energy
transfer from the ligand to the terbium. The fluorescence of this
complex was further enhanced by a factor of 10 when treated
with TOPO/Triton X 100. These methods lead to the use of
terbium as probes and labels for direct determination of organic
analytes, nucleic acids, etc. [9,10].
In this study, we have examined the use of ligand-sensitized
fluorescence spectroscopy, not for the determination of the
metal ion, but for the determination of the ligand. Specifi-
cally, we have examined the possibility of determining the con-
centration of dibutylphosphate (DBP) in tri-n-butylphosphate
(
TBP)/dodecane solutions, with Tb3+ as the fluorescence probe.
As with aromatic carboxylic acids, DBP sensitizes the fluores-
3+
cence of Tb , through an efficient energy transfer to the metal
ion. The fluorescence of Tb3 is proportional to the DBP concen-
+
∗
Corresponding author. Tel.: +91 44 2748 0098; fax: +91 44 2748 0065.
E-mail address: vish@igcar.ernet.in (K.S. Viswanathan).
1
386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2005.09.006

S. Maji, K.S. Viswanathan / Spectrochimica Acta Part A 64 (2006) 972–976
973
time intensive, particularly for low DBP levels. In tritrimetry
17], DBP is estimated in the two-component TBP–nitric acid
medium after extraction with benzene. Spectrophotometry [18]
is used for the DBP estimation based on the ability of DBP to
reduce the colour intensity of a thorium–thoron complex, which
method involves elaborate sample preparation.
From the mixtures of TBP/DBP in dodecane prepared above,
solutions for fluorimetric analysis were prepared as follows. The
25 ␮l of DBP–TBP mixtures in dodecane was placed in a 5 ml
calibrated flask and the volume was made up with HPLC grade
cyclohexane (the rationale behind the choice of cyclohexane will
be presented later; in short, cyclohexane was the best solvent for
[
3+
Fluorescence spectroscopy, described here, provides a simple
method for the estimation of DBP. To the best of our knowledge,
fluorimetric estimation of DBP using sensitized Tb3 fluores-
cence has not been reported earlier.
observing Tb –DBP fluorescence). To this solution, 30 ␮l of
aqueous Tb3 (6.5 × 10 M), prepared using either the nitrate
or chloride salt, was added and was allowed to mix thoroughly
by shaking it manually for 5 min. The fluorescence intensity of
+
−2
+
3+
Tb was measured with excitation and emission wavelengths
of 218.5 nm and 548 nm, respectively.
2
. Experimental
2
.1. Apparatus
3. Results and discussion
All fluorescence spectra were recorded using a Shimadzu
3.1. Spectrofluorimetric results
RF 5000 spectrofluorimeter, with a 150 W xenon lamp source.
Solutions were taken in a 1 cm path length fused silica cell. The
band pass for the excitation and emission monochromators was
set at 5 nm each. A long-wavelength pass filter (UV-39, Shi-
madzu), with a maximum and uniform transmittance (>85%)
above 400 nm, was placed in front of the emission monochro-
mator, in order to reduce the scatter of the incident beam into
the emission monochromator.
At the outset, we show in Fig. 1, the excitation spectra of
3+
3+
3+
Tb , Tb –TBP and Tb –DBP in cyclohexane. The excita-
3+
tion spectra were recorded by monitoring the Tb emission
3+
at 544/548 nm. For Tb , the emission maximum is at 544 nm,
3+
3+
while for the Tb –TBP and Tb –DBP complexes, the maxima
occur at 548 nm.
It can be seen that no signal due to Tb3+ was seen (Fig. 1,
trace ‘a’) when terbium was taken without the phosphates, as
2
.2. Purification and preparation of DBP and MBP
3+
Tb has poor solubility in cyclohexane. Similarly, no signal was
solutions
3+
observed with Tb –MBP (Fig. 1, trace ‘b’). Even though MBP
3+
3+
is known to complex strongly with Tb , the absence of any Tb
Commercially, monobutylphosphate (MBP) and dibutyl-
fluorescence may be either because of poor sensitization of the
lanthanide fluorescence by MBP or possible quenching of the
phosphate are available as mixtures (Fluka, AG) and were there-
fore separated using the following procedure. The DBP–MBP
mixture was treated with CCl4 and washed with water. As MBP
is more soluble in water than in CCl4, it is extracted into the
aqueous phase. The aqueous layer was washed thoroughly with
CCl4 and dried to obtain pure MBP. Similarly, the organic layer
containing DBP was washed repeatedly with water to remove
any traces of MBP and then dried to obtain pure DBP. The purity
of the MBP and DBP, thus obtained, was estimated tritimetri-
cally and found to be 98.5%. Stock solutions of DBP and MBP
were prepared by dissolving appropriate amounts of these phos-
phates in dodecane.
3+
3+
Tb fluorescence by the O H groups in MBP. With Tb –TBP,
a weak feature was observed near 228 nm (Fig. 1, trace ‘c’).
3+
However, a strong signal due to Tb fluorescence was observed
2
.3. Preparation of solutions for fluorimetric studies
A 1.5 ml of commercial TBP (98%) (BDH, GPR) was taken
in a 5 ml test-tube along with different amounts of DBP and
the resulting mixture was diluted with AR grade dodecane.
These solutions contained DBP over the concentration range
−
3
−1
from 1.1 × 10 M to 1.1 × 10 M, while the TBP concentra-
tion was 1.1 M. The concentration of DBP in these solutions was
0
.1–10% of TBP and represented typical solutions encountered
3+
in degraded TBP solutions. Stock solutions of Tb were pre-
3+
pared by dissolving nitrate and chloride salts of Tb . TbCl3 was
obtained by treating Tb4O7 with concentrated hydrochloride
acid and evaporating to dryness. The residue was then dissolved
in distilled water. Tb(NO3)3 available commercially was used
as such.
Fig. 1. Excitation spectrum of: (a) Tb3+; (b) Tb3+–MBP (1 × 10 M); (c)
−4
3+
−2
3+
−5
3+
Tb –TBP (5.5 × 10 M); (d) Tb –DBP (5.5 × 10 M); (e) Tb –DBP
−4
(
6.6 × 10 M). All experiments were done using cyclohexane as solvent except
(e) where the solvent was dodecane.

9
74
S. Maji, K.S. Viswanathan / Spectrochimica Acta Part A 64 (2006) 972–976
3+
at 218.5 nm, with Tb –DBP complex (Fig. 1, trace ‘d’). As this
fluorescence is obtained only in the presence of DBP, it clearly
indicates the sensitization of terbium fluorescence by DBP.
3
+
3+
Both Tb –DBP and Tb –TBP complexes, when excited at
18.5 nm and 228 nm, respectively, yield the emission spectra
2
3+
characteristic of Tb .
3
.2. Choice of solvent
Various solvents, such as cyclohexane, dodecane, hexane,
acetone, carbon tetrachloride, chloroform were tried out in these
experiments; among them, cyclohexane was found to be the best.
As an example, we show in Fig. 1 (trace ‘e’) the excitation spec-
3+
trum of Tb –DBP in dodecane. It can be seen that fluorescence
intensity of Tb3 in the Tb –DBP complex is clearly stronger
+
3+
in cyclohexane than in dodecane.
3
.3. Effect of Tb3+ concentration
The effect of terbium concentration on the fluorescence inten-
3+
sity of Tb –DBP complex was studied at DBP concentrations
3+
of 0.25%, 6% and 10%. Tb concentration was varied from
−
4
−4
1
× 10 M to 6 × 10 M. In the complex with 0.25% DBP,
3+
the fluorescence intensity does not increase with Tb con-
Fig. 2. A plot of excitation intensity of Tb3+ in the Tb3+–DBP complex as a
function of DBP concentration. Excitation wavelength was 218.5 nm and the
emission wavelength monitored was 548 nm. Plots obtained using both TbCl3
and Tb(NO3)3 have been shown.
centration, whereas at 6% and 10% DBP concentration, the
fluorescence intensity increases with Tb3+ concentration. The
3+
maximum intensity was observed for Tb concentration of
−
4
−4
∼
4 × 10 M. In the following experiments, 3.9 × 10 M of
3+
imental procedure outlined in this work. The results are shown
in Table 1. The estimated values were within 5% of the actual
values.
Tb was chosen as the optimum concentration.
3
.4. Analytical calibration
3
.6. Effect of MBP
Fig. 2 plots the excitation intensity of Tb3+ as a function of the
3+
DBP concentration in a solution containing Tb , DBP and TBP.
Both DBP and TBP were used in these experiments, as in real
life samples, DBP would be required to be estimated in the pres-
Very often, DBP may have to be estimated in the presence of
MBP, as both MBP and DBP are degradation products of TBP.
It has already been shown earlier that MBP does not sensitize
Tb fluorescence as does DBP. However, it is still important
to determine the influence that MBP exerts on the fluorescence
of the Tb –DBP complex. The effect of MBP on the fluores-
cence of Tb –DBP was examined and shown in Fig. 3. As long
as the MBP concentration is less than 10% of the DBP con-
3+
ence of large amounts of TBP. The concentration of Tb was
3+
−
4
−3
maintained at 3.9 × 10 M and that of TBP at 5.5 × 10 M.
−
6
The range of the DBP concentration was from 5.5 × 10 M to
3+
−4
5
.5 × 10 M, whichamountstoDBPbeingatlevelsof0.1–10%
of TBP.
Experiments were carried out to obtain calibration graphs,
3+
3
+
centration, no perceptible change in the intensity of Tb –DBP
using both TbCl3 and Tb(NO3)3. It can be seen that the slopes
of the analytical curves are comparable for both the salts. The
−
6
Table 1
detection limit is around 2.5 × 10 M with a S/N of ∼3 at this
Estimates of DBP in TBP/dodecane solutions
concentration.
Sample
[DBP] as prepared (M)
[DBP] estimated (M)
Deviation (%)
1
2
3
4
5
6
4.95 × 10 (0.45)a
−
−
3
5.21 × 10 (0.47)
−3
5.2
1.5
1.3
2.8
3.7
3.7
3
.5. Application of the method
2
−3
1.00 × 10 (0.91)
9.85 × 10 (0.89)
−
2
−2
1.50 × 10 (1.36)
1.52 × 10 (1.38)
In order to check the performance of the method, a set of six
−
2
−2
1.80 × 10 (1.64)
1.75 × 10 (1.59)
solutions containing known amounts of DBP in TBP/dodecane
were prepared to mimic solutions that would be encountered in
PUREX reprocessing applications. These six synthetic samples
contained DBP ranging in concentration from 0.45% to 3.45% of
TBP in TBP/dodecane solutions. The concentrations of DBP in
these synthetic samples were estimated according to the exper-
−2
−2
1.90 × 10 (1.73)
1.83 × 10 (1.66)
−2
−2
3.80 × 10 (3.45)
3.94 × 10 (3.57)
Concentration of TBP in all the synthetic solutions was maintained at 1.1 M.
Values in parentheses are in percent.
NumbersinparenthesisgivethepercentagecompositionofDBPwithrespect
to TBP in the different solutions.
a

S. Maji, K.S. Viswanathan / Spectrochimica Acta Part A 64 (2006) 972–976
975
Table 2
Estimated values of DBP by the present method and GC method
Sample No.
Concentration of DBP (M) estimated by
a
Fluorimetric method (described in this work)
GC method
−
−
2
2
−2
−2
1
2
1.35 × 10
2.32 × 10
1.17 × 10
2.00 × 10
a
Errors in the estimates by fluorimetric method is about 7%.
4. Conclusion
Fluorescence of Tb3+ has been studied in the presence of
MBP, DBP and TBP. DBP showed the strongest fluorescence
sensitization of the Tb3 fluorescence in cyclohexane solutions.
The results of this study were used to establish a method for the
determination of DBP in degraded TBP solutions in the PUREX
process. The reproducibility and linearity of the analytical curve
suggests that the method is suitable for the determination of
DBP over a concentration range of 0.1–10% DBP in TBP. This
concentration range is well suited for applications in real life
for the estimation of DBP in recycled reprocessing solvents.
This method is simple, accurate and quick compared with the
currently used chromatographic methods. The method is also
applicable in the presence of MBP, another common degrada-
tion product, as it has been shown that at the concentrations of
MBP typically found in the degraded reprocessing solutions, the
+
3+
Fig. 3. Excitation spectra of Tb3+ in Tb3+/DBP (1.2 × 10 M)/TBP
−4
fluorescence of the Tb –DBP complex is only hardly affected.
−
2
(
1.1 × 10 M)/MBP as a function of MBP concentration in the mixture. The
−5
−4
concentrations of MBP were: (a) 0 M; (b) 1.0 × 10 M; (c) 1 × 10 M; (d)
Acknowledgements
−
4
5
× 10 M.
The authors gratefully acknowledge the help of Ms. Jaya
Mohandas, KARP, BARC Facilities, Kalpakkam, in determin-
ing the concentration of DBP using the GC method and Dr. D.
Ponraju, Health and Safety Division, IGCAR, Kalpakkam for
assistance in carrying out the irradiation of the TBP/dodecane
samples.
was observed. When the concentration of MBP was ∼80% that
3+
of the DBP concentration, the fluorescence of the Tb –DBP
decreased by only 10%. When the MBP concentration was
increased to about 400% that of the DBP concentration, the
decrease in the fluorescence of the complex was about 30%.
In typical TBP degraded solutions, the concentration of MBP
is likely to be far less than that of DBP and hence the present
method would not involve significant errors due to the presence
of MBP in real life samples.
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.7. Validity of the method
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