Anal. Chem. 1999, 71, 5060-5063
Hydration Number of Tris[1-(2-thienyl)-4,4,4trifluoro-1,3-butanedionato]lanthanoids in Chloroform across the Lanthanoid Series Yuko Hasegawa,* Eiichi Ishiwata, and Tadahiro Ohnishi
Department of Chemistry, Faculty of Science, Science University of Tokyo, Tokyo 162-8601, Japan G. R. Choppin
Department of Chemistry, The Florida State University, Tallahassee, FL 32306-4390
The hydration numbers of lanthanoid(III) chelates with 2-thenoyltrifluoroacetone prepared by extracting into chloroform have been determined by analyzing the relation between lanthanoid(III) concentration and water content. The water contents were measured by coulometric Karl Fischer titration, and the concentration of lanthanoid(III) extracted was determined by ICP/AES. The hydration number of europium(III) chelate was also checked by measuring the luminescence lifetime. The hydration number is about 3 from lanthanum to holmium and then decreases to about 2.4. The change reflects the variation in the lanthanoid(III) ionic size. The hydration number of Eu(TTA)3 obtained by the Karl Fischer titration agrees with that from measurement of the luminescence lifetime, within experimental accuracy. The synergistic extraction of various metal ions (including trivalent lanthanoids) with β-diketones and neutral ligands has been investigated extensively. In the synergistic extraction of lanthanoids(III) with 1-(2-thienyl)-4,4,4-trifluoro-1,3-butanedione (2thenoyltrifluoroacetone or TTA) and carboxylic acids1 or 1,10phenanthroline(phen),2 the trend of the magnitude of the synergistic effects across the lanthanoid series (i.e., the formation constants of the adducts with Ln(TTA)3) is a decrease in them with increasing lanthanoid atomic number when carboxylic acids are used as the synergist, while an increase is observed when phen is the adduct ligand. Such different trends may be associated with the variation of the difference in residual hydration of LnIII in Ln(TTA)3 and LnA3‚E across the series, because Lewis bases of different basicity may replace the different numbers of water molecules hydrated to LnIII of the complex, resulting in an effect on the enthalpy and the entropy changes.2 The result would be a change in the magnitude of the synergistic effect across the lanthanoid series even when a certain adduct-forming ligand is used. Accordingly, if the hydration number of the parent chelates, e. g., Ln(TTA)3, is known across the lanthanoid series, it could be useful in expaining the trend of varying adduct formation * Corresponding author: (fax) 81-3-3235-2214; (e-mail) yhasegaw@ ch.kagu.sut.ac.jp. (1) Hasegawa, Y.; Yamada, T.; Nagata, K. Solvent Extr. Ion Exch. 1996, 14, 89. (2) Yajima, S.; Hasegawa, Y. Bull. Chem. Soc. Jpn. 1998, 71, 2825.
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constants as well as the magnitude of synergistic extraction. Consequently, we have studied the hydration of the extracted species. There are few reports of lanthanoid(III) hydration in organic solvents;3 the hydration numbers of the LnIII complexes in aqueous solutions have been reported to be nine in the lighter series and eight in the heavier ones.4-8 EXPERIMENTAL SECTION Reagents. All reagents were analytical-grade. Chloroform obtained from Kanto Chemical Co., Inc. was washed three times with deionized water while care was taken to have proper ventilation. (Precaution: Since chloroform is harmful, particular attention is drawn to substances which are suspected to have a carcinogenic, mutagenic, or toxic reproductive effect).9 2-Thenoyltrifluoroacetone(TTA) was obtained from Dojindo Laboratories (purity, >99%). Lanthanoid(III) oxides, with a purity of each oxide higher than 99.9%, were obtained from the following companies: Shin-Etsu Chemicals, Mitsuwa Chemicals, Nacalai Tesque, and Soekawa Chemicals. For Karl Fischer coulometric titration, HYDRANAL Coulomat AK as the anolyte, serving as a generator of I2, and HYDRANAL Coulomat CG-K as the catholyte were obtained from Riedel-de Hae¨n, Germany. Heavy water (D2O) and CDCl3 were obtained from Merck, Japan, for use in measurement of the luminescence lifetime of europium(III). Since perchloric acid is oxidizing and corrosive, a small portion of purchased 60% perchloric acid was carefully diluted with water and standardized by titration with sodium hydroxide solution. Procedure. Most procedures were carried out in a thermostated room at 298 K. (i) Karl Fischer Coulometric Titration. A weighted lanthanoid oxide was dissolved in a small excess of perchloric acid aqueous solution for the stoichiometric concentration of the lanthanoid(III), and the concentration was determined by EDTA (3) Lis, S.; Mathur, J. N.; Choppin, G. R. Solvent Extr. Ion Exch. 1991, 9, 637. (4) Rizkalla, E. N.; Choppin, G. R. Handbook on the Physics and Chemistry of Rare Earths; Elsevier: Amsterdam, 1991; Vol. 15, Chapter 103. (5) Habenschuss, A.; Spedding, F. H. J. Chem. Phys. 1979, 70, 3758. (6) Yamaguchi, T.; Nomura, M.; Wakita, H.; Ohtaki, H. J. Chem. Phys. 1989, 89, 5153. (7) Cossy, C.; Barnes, A. C.; Enderby, J. E.; Merbach, A. E. J. Chem. Phys. 1989, 90, 3254. (8) Horrocks, W. D., Jr.; Sudnick, D. R. J. Am. Chem. Soc. 1979, 101, 334. (9) MERCK. Chemicals Reagents 1999/2000; Germany. 10.1021/ac990485d CCC: $18.00
© 1999 American Chemical Society Published on Web 10/16/1999
RESULTS AND DISCUSSION The water in the organic phase when lanthanoids(III) are extracted with a β-diketone may reflect hydration of the β-diketone as well as that of the Ln(TTA)3 chelates and the solubility of water in the organic solvent. Prior to the determination of the hydration number of LnIII complexed with TTA in chloroform, the concentration of water in the β-diketone/chloroform solution was measured. (i) The Water Transferred into CHCl3 Accompanied with TTA. When TTA distributes between chloroform and 0.1 M sodium perchlorate aqueous solution, the total TTA concentration, AT, can be represented as follows
AT ) [HA] + [A-] + [HA]o
Figure 1. The correlation between the concentrations of TTA and H2O in CHCl3. The solid line is calculated from y ) 0.102x + 0.0728 with r ) 0.994.
titration using Xylenol Orange as an indicator. A weighted TTA (equivalent to 0.1 M (1 M ) 1mol‚dm-3)) was dissolved in chloroform. The stock solution was kept overnight prior to use in order to establish the keto-enol equilibrium, stored in a dark place, and used in a few days to avoid the decomposition of TTA and chloroform. The chloroform solution, 9.0 mL, and an equal volume of 0.03 M lanthanoid perchlorate aqueous solution at pCH (-log[H+]) ) 3.5 (Tm, Yb, Lu) or 4.3 (Pr, Nd) were placed in a stoppered glass tube and shaken for an hour. This was enough for all relevant species to attain extraction equilibria, since usually the extraction of LnIII with β-diketones is established in less than 10 min. After centrifuging for a few minutes, a portion of the organic phase was transferred into a vial for Karl Fischer titration. The concentration of water extracted into the chloroform as well as into chloroform containing various concentrations of TTA without lanthanoids(III) was measured by Karl Fischer coulometric titration (Hiranuma Sangyo Co. Ltd. Aquacounter model AQ7). The LnIII concentration extracted into chloroform (1.4 × 10-2 M at maximum) was determined by ICP/AES (HITACHI P-4000) after the organic solution was diluted 5 times with fresh chloroform and then the LnIII was back-extracted with 0.1 M perchloric acid aqueous solution. (ii) Measurement of Luminescence Lifetime of Europium(III). Europium(III)-TTA chelate was prepared in a way similar to that described in (i) except that a chloroform solution of 5.0 × 10-3 M TTA was used. The europium(III) concentration was adjusted to 1 × 10-5 M by dilution with chloroform. The excitation spectra of europium(III) in the chloroform were recorded by scanning the dye laser monochromator while monitoring the luminescence intensity at 616 nm. The luminescence decay curves were obtained as described elsewhere.3,10 The laser dye was a 50:50(v/v) mixture of Rhodamine 590 and Rhodamine 610 (Exciton Chemical), and the dye was pumped by the second harmonic output of an Nd:YAG laser at 532 nm. The decay constants kobs(ms-1) were calculated with a computer program using the Simplex procedure (decay curves were fit using a least squares algorithm). Similar experiments were performed using D2O and CDCl3. (10) Choppin, G. R.; Wang, Z. M. Inorg. Chem. 1997, 36, 249.
(
) [HA]o
Ka 1 + +1 Kd K [H+] d
)
(1)
where the subscript “o” means the organic phase and no subscript is the aqueous phase, Ka is the acid dissociation constant (pKa ) 6.33 in 0.1 M NaClO4),11 and Kd is the distribution constant (Kd ) [HA]o/[HA], log Kd ) 1.85 between CHCl3 and 0.1 M NaClO4).11 The TTA may coordinate several water molecules even in chloroform. Figure 1 shows the correlation between the concentration of TTA and water extracted into chloroform. The water concentration increases proportionally to the concentration of TTA in chloroform, and the increment of water content (the balance of water content between the pure chloroform and 0.1 M TTA/CHCl 3) reaches 0.0095 M. The plot of the water concentration in the chloroform versus TTA concentration is linear with a slope of 0.1, indicating hydration of HTTA in the chloroform. For benzene solution of 0.1 M TTA, the increment was reported to be 0.006 ( 0.003.12 The hydration can be represented as follows
[H2O]o,T ) [H2O]0,CHCl3 + [H2O]o,TTA ) [H2O]0,CHCl3 + 0.10[TTA]o (2) where [H2O]0,CHCl3 is the water solubility in chloroform, 7.28 × 10-2 M. The slope can be correlated with the interaction between TTA(HA) and water molecules in the chloroform:
HA(o) + mH2O(o) h HA‚mH2O(o) K)
[HA‚mH2O]o [HA]o[H2O]om
(3)
The total water content in the chloroform can be represented as
[H2O]o,T ) m[HA‚mH2O]o + [H2O]o ) mK [H2O]om[HA]o + [H2O]o
(4)
(11) Sekine, T.; Hasegawa, Y.; Ihara, N. J. Inorg. Nucl. Chem. 1973, 35, 3968. (12) Caceci, M.; Choppin, G. R.; Liu, Q. Solvent Extr. Ion Exch. 1985, 3, 605.
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From the equation, the slope of the plot in Figure 1 should be mK[H2O]om. If we assume m equals unity, the equilibrium constant in eq 3 can be calculated to be 1.4, because [H2O]o can be regarded as the saturated water solubility in chloroform and should be constant in the solutions. (ii) The Hydration Number of LnA3 across the Lanthanoid Series. When lanthanoid ions are extracted with TTA anion, the extracted species can be associated with hydrated water molecules since lanthanoid(III) coordinates with 8-9 water molecules in aqueous solutions, and the three bidentate TTA anions occupy only 6 sites. When the concentration of water in the organic phases is measured by Karl Fischer titration, the water associated with TTA and LnA3 would be in addition to the water dissolved in the pure chloroform. Thus, the total water concentration can be represented as
[H2O]o,T ) [H2O]0,CHCl3 + [H2O]o,TTA + [H2O]o,Ln (5) where [H2O]o,TTA can be determined by using eq 2 or the data in Figure 1, if the free TTA concentration is known. The free TTA concentration is calculated using (AT - 3[LnA3]o) instead of AT in eq 1. Under the present experimental conditions, the most dominant source of water in the organic phase is the water dissolved in pure chloroform because of the low solubility of Ln(TTA)3 in chloroform. The highest concentration of Ln(TTA)3 used to determine the hydration number was 1.4 × 10-2M with 5.8 × 10-2M free TTA, and usually the amount was around 1 × 10-2M with (6.5-7.2) × 10-2M free TTA. When pCH was increased to get better extraction, precipitation was often observed. Accordingly, the water coextracted with Ln(TTA)3 did not exceed 4 × 10-2M, so that the ratio of the concentration of water hydrated to Ln(TTA)3 (4 × 10-2M) to the saturated water solubility in chloroform (7.3 × 10-2 M) is, at most, 0.6. The H2O concentration extracted with LnA3 would be related to the LnA3 concentration by
[H2O]o,Ln ) n[LnA3‚nH2O]o
(6)
where “n” is the hydration number of LnA3 at average. Figure 2 gives an example of the correlation between the H2O concentration and LnIII concentration extracted into the chloroform solution. As the concentration of the LnIII extracted increases, the H2O concentration in the chloroform increases proportionally. The hydration number of LnA3 (n) was calculated from these data. The hydration numbers determined in the present study are listed in Table 1. The hydration numbers are slightly greater than the integral values, which may reflect some small degree of hydration of the chelated TTA in chloroform. (iii) Luminescence Lifetime Measurement of Europium(III)-TTA Chelate in Chloroform. When the europium(III) concentration changed from 8 × 10-6 to 2 × 10-5 M, the decay constants remained almost constant. The measurements were conducted using a europium(III) concentration of 1 × 10-5 M and a TTA concentration of slightly less than 3.0 × 10-3 M. The decay constants of EuA3 were determined for the species extracted from D2O solution into CDCl3 as well as for those extracted from H2O into CHCl3. 5062
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Figure 2. The correlation between the concentrations of water coextracted with LnA3 and LnA3 in the chloroform solution: 0, Nd; b, Er; O, Lu. Table 1. The Hydration Number of Lanthanoid(III) Chelate with TTA in Chloroform lanthanoid(III)
nH2O ( δ
lanthanoid(III)
nH2O ( δ
Pr Nd Sm Eu Gd Tb
3.277 ( 0.085 3.008 ( 0.043 3.147 ( 0.092 3.285 ( 0.125 3.288 ( 0.091 3.285 ( 0.064
Dy Ho Er Tm Yb Lu
3.173 ( 0.066 3.134 ( 0.075 2.858 ( 0.030 2.663 ( 0.049 2.521 ( 0.053 2.401 ( 0.023
In D2O solution the decay constant of europium(III) seems to be about 0.5 ms-1,8 while in CDCl3, a decay constant obtained in the present work was 1.9 ms-1. Since the value of (1.93 ( 0.05) ms-1 has been also reported as the decay constant of Eu(TTA)3 extracted from D2O into benzene,3 the hydration number was also determined from the decay constants by the following equation used for the benzene system.
1.05(k(H2O/CHCl3) - 1.9) ) nH2O
(7)
In eq 7, k(H2O/CHCl3) is the luminescence rate constant (reciprocal lifetime) determined for Eu(TTA)3 prepared by extracting from H2O into CHCl3. The hydration number, nH2O, 2.9 ( 0.1 was obtained. This number agrees within error (2σ) with the value determined by Karl Fischer titration, 3.3 ( 0.1. The hydration number of Nd(TTA)312 in benzene, determined by Karl Fischer titration, has been reported as 3.6. The difference between this number and our value of 3.0 ( 0.0 may be caused by the different organic solvents. (iv) The Trend of the Hydration of Ln(TTA)3 Chelates in Chloroform across the Lanthanoid Series. As seen from Table 1, the hydration number of the Ln(TTA)3 chelates shows no significant change from La to Dy, but decreases with increasing atomic number from Dy to Lu. This trend is very similar to that of the variation of the formation constants of the second adducts of strong monobasic Lewis bases such as TBP (tributyl phosphate)
with Ln(TTA)3, differing from the trend of the adducts of weak Lewis bases such as monobasic carboxylic acids.1 In addition, it has been reported that the second adducts of TBP and also of TOPO (trioctylphosphine oxide) with Nd(TTA)312 as well as Eu(TTA)33 do not coordinate any water molecules in benzene. Accordingly, on the extended line, we can estimate that on the second adduct formation (e.g., Ln(TTA)3‚2TBP), all hydrated molecules are released from Ln(TTA)3 across the lanthanoid series. This estimation would be helpful in explaining the variation of the formation constants of TBP (or TOPO) adducts across the lanthanoid series from the viewpoint of entropy change, i.e., since the β-diketonato chelates of the lighter lanthanoids(III) have a larger number of hydrated water molecules than the heavier LnIII chelates do, besides the hydration number is similar among light lanthanoids(III), and all hydrated molecules are released on adduct formation, the entropy change is more favorable for the lighter lanthanoids and is similar among light lanthanoids(III), however, is less favorable for the adduct formation of heavier lanthanoids(III). This may explain why the formation constants of TBP adducts with Ln(TTA)3 do not significantly change in the lighter lanthanoids and decrease in the heavier lanthanoids.13 Until now, the effect of the coexisting water molecules, including molecules hydrated to the metal species in organic phases, has been ignored in the explanation of liquid-liquid
distribution behavior of metal ions. However, the present work showed that the hydration number would change across the lanthanoid series. The synergistic effects or adduct formation constants would be influenced by the residual hydration. As the organic phases are saturated with water in solvent extraction, the effect of the variable hydration on the extraction behavior should be included as the magnitude of the synergistic effect can be affected by the hydration change of the metal ion and, hence, by the water solubility in the organic solvents.
(13) Farbu, L.; Alstad, J.; Augustson, J. H. J. Inorg. Nucl. Chem. 1974, 36, 2091.
AC990485D
ACKNOWLEDGMENT The authors are very grateful to N. Tsuruta, Science University of Tokyo, for her experimental aid and T. Kimura, Japan Atomic Energy Research Institute, Tokai, and D. Peterman, Florida State University, for their assistance in the laser-induced luminescence lifetime measurements and also for valuable discussion. The luminescence lifetime was measured in the Chemistry Laser Laboratory in Florida State University. This research was supported partly at Florida State University by a Grant from USDOEOBES, Division of Chemical Sciences, and partly by a Grant-inAid for Scientific Research(C), no. 11640614 from the Ministry of Education, Science, Sports, and Culture, Japan. Received for review May 5, 1999. Accepted August 27, 1999.
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