Short communication
Benzo-15-Crown-5 as Synergistic Agent in the Extraction of Pr(III) from Perchlorate Medium with Thenoyltrifloroacetone in 1,2-Dichloroethane, Carbon Tetrachloride and Chloroform
Valeri Jordanov
Department of General and Inorganic Chemistry, University of Chemical Technology and Metallurgy, 8 Kliment Okhridski Blvd., 1756 Sofia, Bulgaria
* Corresponding author: E-mail: val_bgm@abv.bg
Received: 15-05-2007
Abstract
The synergistic solvent extraction of Praseodymium (Pr) from Perchlorate medium by mixtures of thenoyltrifluoroace-tone (HTTA) and a crown ether benzo-15-crown-5 (B15C5) is studied. The composition of the extracted species has been determined as Pr(TTA)3.2B15C5. The values of the equilibrium constant KTS are calculated. Furthermore, the influence of the polarity of organic diluents on the extraction process is investigated.
Keywords: synergistic extraction, lanthanides, thenoyltrifluoroacetone, benzo-15-crown-5.
1. Introduction
The chelating extractant thenoyltrifluoroacetone has been extensively studied in the extraction of lanthanides alone1'2 or in mixtures with various synergistic agents.3-8
In the last 15-20 years, crown ethers (CE) have been often used as synergistic agents. Extraction of trivalent lanthanides,9-20 divalent transition and alkali metals21'22 with mixtures of HTTA or other chelating extractants and different CE have been investigated. It was found out that both the crown ether type and its concentration influence the complexes' stoichiometry.10-12 Using synergistic agents having high basicity and concentration results in formation of complexes-M(TTA)3.2CE, while M(TTA)3.CE has been obtained in low basicity and concentration of CE (M=Ln).
The ionic medium and the ionic strength in the aqueous phase as well as the polarity of the organic diluents influence the extracted complex species, too.13-16 Extraction of metals from NO3- -medium in organic phase containing polar diluents usually results in the formation of ionic type complex -[M(TTA)2CE]+(NO3)-. On the other hand, throughout the extraction from Cl- medium in low polar diluents, metals form adducts - M(TTA)3.nCE (n = 1 or 2). The two types of complexes have been established
during the extraction of /-elements from ClO4- -medium.
In the present work, the extraction of Praseodymium (Pr) from perchlorate medium with mixtures of HTTA and B15C5 is studied.
2. Results and Discussion
The solvent extraction of lanthanides with HTTA alone in C6H6 is studied by Poskanzer and Foreman.1 It has been found out that metal extraction can be expressed by
L<q)+3HTTA(0)
Ln{TTA)3(0)+3H(;q) (1)
K,=
DT =
[Ln(TTA)3 ](0).[H+
J lap
[Ln ](«,)• [HTTA]J0) [Ln(TTA)3](0)

(2)
(3)
where DT denotes the distribution coefficient, Ln - lanthanides, and "aq" and "o" denote the aqueous and organic phase, respectively.
Figure 1. Log DT vs. pH for Pr extraction with HTTA.1- CCl4; 2- C2H4Cl2; 3- CHCl3
Later the same type of complexes have been obtained in metals extraction with HTTA in other organic diluents, too.2'3
Table 1. Values of the equilibrium constant for Pr extraction with HTTA alone and with mixtures of HTTA and B15C5
Diluent	log#T	log^T,S	logPT,S
CCl4	-9.65 ± 0.03	-1.89 ± 0,03	7.76 ± 0.06
CHCl3	-9.45 ± 0.04	-2.89 ± 0.04	6.56 ± 0.08
C2H4Cl2	-9.16 ± 0.03	-3.47 ± 0.04	5.69 ± 0.07
Figure 2. Log DT vs. [HTTA] for Pr extraction at pH = 4.40
In the present study, the extraction of Pr with HTTA from perchlorate medium in 1,2-dichloroethane (C2H4Cl2), carbon tetrachloride (CCl4) and chloroform (CHCl3) is investigated. The experimental data about the distribution coefficient DT vs. pH and [HTTA] are shown in Fig.1 and 2. The values of equilibrium constant KT for metal chelates formation are given in Table 1.
Alone, the Pr extraction with B15C5, is negligible under the experimental conditions of the present research.
The synergistic solvent extraction of Pr is studied by an examination of the variation of DT,S (the distribution coefficient caused by the synergistic effect) with relevant experimental variables. If the extractants' concentration is
Figure 3. Log DT,S vs. pH for Pr extraction with HTTA and B15C5 at [B15C5] = 5 • 10-3 mol dm-3. 1- C2H4Cl2; 2- CCl4; 3- CHCl3
constant, and if the hydrolysis and polymerization in aqueous and organic phases, respectively, are negligible, then the plots will give the number of ligands included in the extraction complexes. Double logarithmic plots of DTS vs. one of the variables pH, [HTTA] or [B15C5], retaining the other two constant are shown in Figs.3-6.
0.8
0.6
0.4
0.2 « 0.0
S -0-2
-0.4
-0.6
-0.8
Figure 4. Log DT,S vs. log.[HTTA] for Pr extraction at B15C5] = 5 • 10-3 mol dm-3.
The plots of log DT,S vs. pH and [HTTA] are linear, with slopes close to 3, and the plots of log DTS vs. [B15C5] show slopes close to 2.
On the basis of slope analysis data, the synergistic extraction of Pr can be described by the equation
Pr^ + 3MTTA + 2S(0)^Pr(TTA),. 2S(0)+3H^ (4)
where S denotes B15C5.
The equilibrium constant KT,S for the synergistic complex formation can be shown as follows:
logKTS = logDTS - 3log[HTTA] - 2log[S] - 3pH (5)
The formation of adducts in the organic phase can be expressed by the equation
Pr<TTA)3{0)+2S(0)i? Pr(TTA), ■ 2S(0)	(6)
The equilibrium constant /¡TS for the synergistic reaction in the organic phase can be determined as
log^TS = logKTS - logKj.	(7)
tog[HTT«
0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0
3.6 3,7 3.8 3.9 4.0 4,1 4.2 4.3 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3,7
Figure 5. Log DT,S vs. pH for Pr extraction at [HTTA] = 5 • 10-2 mol dm-3. 1- C2H4Cl2; 2- CCl4; 3- CHCl
3.5 3.6 3,7 3.8 3.9 4.0 4.1 pH
-2.6
-2.4	-22
log[B15C5]
-2.0
Figure 6. Log DT,S vs. log.[B15C5] for Pr extraction at [HTTA] : 5 • 10-2 mol dm-3.
The values of log K%S and /iYS are given in Table 1. The equilibrium constants are based on the assumption that the activity coefficients of the species do not change significantly under experimental conditions, i.e. they are concentration constants.
The data in Table 1 shows that the values of log KTS and /¡T,s decrease with increasing of the dielectric constant of organic diluents. That can be explained by the possibility of hydrogen-bonding formation due to interaction between extractants and polar diluents.
In the extraction of trivalent lanthanides and actini-des from ClO4- -medium with HTTA and DCH18C6 into 1,2-dichloroethane by Katatsuji et al. ionic complexes [M(TTA)2 DCH18C6]+ClO4- are obtained.13 Complexes in which the anion of the aqueous phase is a part of the extracted species-M(X)(L2).CE have also been formed in the
Figure 7. Log DT,S vs. pH for Pr extraction at [HTTA] mol dm-3 and [B15C5] = 5 • 10-3 mol dm-3.
5 • 10-
extraction of light lanthanides with others P-diketones into polar diluents.14-16 On the contrary, adducts M(L3).CE have been registered in non-polar medium (M=Ln, L=P-diketones and CE=18C6, DB18C6 or DCH18C6).9-11 In our case, the change of diluents' polarity results in considerable variation of the value of the distribution coefficient DT,S (fig.7). No possibility for changes in the composition of the synergistic complexes is confirmed.20
Lakkis et al. have obtained ionic type complex (KCE)+.M(PMBP)3- in the extraction of Co(II) and Ni(II) with mixture of 1-phenyl-3- methyl-4- benzoylpyrazol-5-one and CE (18C6 or DCH18C6) from aqueous phase containing 1 mol dm-3 KClO4.21 On the other hand, ad-ducts M(PMBP)2.CE from 1 mol dm-3 LiCl or (CH3)4NCl medium are obtained. In our investigation, ionic type complex formation is not established in the extraction of Pr from aqueous phase containing KClO4. In all experimental conditions the adduct-Pr(TTA)3 2B15C5 is obtained. In similar complexes, Mathur and Choppin admit that metal ion is bonded with only three oxygen atoms, from each B15C5.19 There are two crown molecules, placed above and under the metal chelate plane, respectively.
3. Experimental
3. 1. Reagents
HTTA (Merck, p.a.) and B15C5 (Fluka, puriss.) were used as supplied. The stock solutions of the Pr were prepared from its oxide Pr6O11 (Fluka,puriss). Arsenazo III (Fluka) was of analytical grade purity as were the other reagents used.
3. 2. Procedure
The experiments were carried out using 10 cm3 volumes of aqueous and organic phases. The samples were shaken mechanically for 60 min at room temperature
which was sufficient to reach equilibrium. After phase separation, the concentration of Pr in the aqueous phase was determined photometrically using Arsenazo III.23 The acidity of the aqueous phase was measured by a pH-meter with an accuracy of 0,01 pH unit. The ionic strength was maintained at 0,1 mol dm-3 with (K,H)ClO4. The initial concentration of Pr was 2,5.10-4 mol dm-3 in all experiments.
4. Conclusions
Pr is synergistically extracted with HTTA-B15C5 mixtures as an adduct -Pr(TTA)3 2B15C5. Polarity of organic diluents causes a considerable influence on the metal extraction but not to the extracted complexes' stoichio-metry. The values of distribution ratio DTS decrease in the order CCl4 > CHCl3 > C2H4Cl2, i.e. the synergistic extraction is greatest in the least polar diluent. Extraction from aqueous phase containing KClO4, does not result in ionic type complex formation.
5. References
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2.	J. Alstad, L.H. Augustson and L. Farbu, J. Inorg. Nucl. Chem. 1974, 36, 889-903.
3.	L. Newman and P. Klotz, Inorg. Chem. 1972, 11, 2150-2154.
4.	M. S. Bhatti and G. Duyckaerts, Anal. Chim. Acta 1983, 149, 369-373.
5.	S. Nakamura and N. Suzuki, Polyhedron 1988, 7, 155-159.
6.	I. L. Dukov and V. M. Jordanov, Solvent Extr. Ion.Exch. 1995, 13, 997-1008.
7.	I. L. Dukov and V. M. Jordanov, Solvent Extr. Ion.Exch. 1998, 16, 1151-1160.
8.	M. Atanassova, V. M. Jordanov and I. L. Dukov, Hydrome-tallurgy 2002, 63, 41-47.
9.	G. M. Nair and D. R. Prabhu, J. Radioanal. Nucl. Chem. 1988, 121, 83-90.
10.	Y. Meguro, H. Muto and Z. Yoshida, Anal. Sci. 1991, 7, 51-54.
11.	D. D. Ensor and A. H. Shah, J. Radioanal. Nucl. Chem. Lett. 1988, 127, 235-239.
12.	I. L. Dukov, Monatsh.Chem. 1993, 124, 689-693.
13.	Y. Kitatsuji, Y. Meguro, Z. Yoshida, T. Yamamoto and K. Nis-hizawa, Solvent Extr. Ion. Exch. 1995, 13, 289-300.
14.	Y. Meguro, Y. Kitatsuji, T. Kimura and Z. Yoshida, J. Alloys Compd. 1998, 271-273, 790-793.
15.	M. L. P. Reddy, R. L. Varma and T. R. Ramamohan, Radioc-him. Acta, 1999, 80, 151-154.
16.	F. A. Shehata, N. El-Hefny and H. F. Aly, Solvent Extraction for the 21st Century, Proceedings of ISEC'99, Barcelona, Spain, 1999 July 11-16 (publ. 2001), 2, 1387-1393.
17.	R. Pavithran and M. L. P. Reddy, Radiochim. Acta 2003, 91, 163-168.
18.	N. Yamamoto, N. Hirayama and T. Honjo, Solvent Extr. Res. Dev. Jpn. 1999, 6, 182-188.
19.	J. N. Mathur and G. R. Choppin, Solvent Extr. Ion. Exch. 1993, 11, 1-9.
20.	I. L. Dukov, Solvent Extr. Ion. Exch. 1986, 10, 637-653.
21.	M. Lakkis, J. P. Brunette and M. J. F. Leroy, Solvent Extr. Ion. Exch. 1986, 4, 287-299
22.	T. Shimosato and N. Hirayama Solvent Extraction for the 21st Century, Proceedings of ISEC'99, Barcelona, Spain, 1999 July 11-16 (publ. 2001), 1, 585-590.
23.	S. V. Savvin: Arsenazo III, Atomizdat, Moskva, 1966, 177178.
Povzetek
Raziskovali smo sinergijo topil pri ekstrakciji praseodimija (Pr) iz perkloratnega medija z mešanicami tenoiltrifluoroa-cetona (HTTA) in crown etra benzo-15-crown-5 (B15C5). Določili smo strukturo ekstrahiranih delcev (Pr(TTA)3.2B15C5), vrednosti ravnotežnih konstant KTS ter vpliv polarnosti organskih topil na proces ekstrakcije.