Anal. Chem. 1997, 69, 4814-4818
Ion-Pair Extraction System for the Mutual Separation of Lanthanides Using Divalent Quadridentate Schiff Bases Naoki Hirayama,* Isao Takeuchi, and Takaharu Honjo
Department of Chemistry, Faculty of Science, Kanazawa University, Kanazawa 920-11, Japan Koji Kubono
Division of Natural Science, Osaka Kyoiku University, Kashiwara 582, Japan Hisao Kokusen
Department of Chemistry, Faculty of Education, Tokyo Gakugei University, Koganei 184, Japan
For the selective extraction of trivalent lanthanides, the use of quadridentate divalent phenolic Schiff bases, such as N,N′-bis(5-nitrosalicilidene)ethylenediamine (H2Nsalen) and N,N′-bis(5-nitrosalicilidene)-o-phenylenediamine (H2Nsaloph), was investigated. Lanthanides made anionic 1:2 complexes with these ligands and could be extracted into nitrobenzene as ion-pairs with a suitable monovalent countercation in the aqueous phase, after which the ionpair was dissociated almost perfectly. Although the order of the extractability of lanthanides between these ligands was H2Nsaloph > H2Nsalen, the mutual selectivity of them in the H2Nsalen system was higher than that in H2Nsaloph. Furthermore, with decreasing the size of the countercation, the mutual selectivity was enhanced although the extractability was lowered. A H2Nsalen-KCl extraction system, selected as an example, showed selectivity between lanthanides comparable with those of the better of other systems reported previously. Mutual separation of lanthanides is very difficult due to their remarkable chemical similarities, such as their charges and radii.1-3 The solvent extraction method4-6 is the best choice for separation, and recently, many kinds of novel extraction systems have been investigated to improve their mutual separability.7-17
Quadridentate divalent phenolic Schiff bases, such as N,N′bis(salicilidene)ethylenediamine (H2salen), N,N′-bis(salicilidene)o-phenylenediamine (H2saloph), and N,N′-bis(salicilidene)-1,3propanediamine (H2salpn), are well known as ligands that are easy to synthesize and have structural rigidity, and they are used also as selective extractants for several kinds of metal cations.18-23 However, these Schiff bases have hardly been used for the extraction of trivalent lanthanide cations (Ln3+). Only one success was reported:24 the use of H2salen and N,N′-bis(3-methoxysalicilidene)ethylenediamine (H2MeOsalen) to extract Ln3+ from alkaline solution into chloroform as Ln(OH)salen and Ln(OH)MeOsalen, respectively. We considered that the difficulty of the effective extraction of Ln3+ with H2salen and its analogues is due to their low acidity. Therefore, in this study, we investigated the use of their nitro derivatives, N,N′-bis(5-nitrosalicilidene)ethylenediamine (H2Nsalen), N,N′-bis(5-nitrosalicilidene)-o-phenylenediamine (H2Nsaloph), and N,N′-bis(5-nitrosalicilidene)-1,3-propanediamine (H2Nsalpn), as extractants for Ln3+. The structures of these ligands are shown in Figure 1. As a result, it was found that H2Nsalen and H2Nsaloph are effective extractants for the mutual separation of Ln3+ under the coexistence of a suitable countercation.
* Corresponding author. Fax: (+81) 76-264-5742. E-mail: hirayama@ cacheibm.s.kanazawa-u.ac.jp. (1) Shannon, R. D.; Prewitt, C. T. Acta Crystallogr. 1969, B25, 925-946. (2) Shannon, R. D.; Prewitt, C. T. Acta Crystallogr. 1970, B26, 1046-1048. (3) Shannon, R. D. Acta Crystallogr. 1976, A32, 751-767. (4) Sekine, T.; Hasegawa, Y. Solvent Extraction in Chemistry; Dekker: New York, 1977. (5) Alegret, S. Developments in Solvent Extraction; Wiley: New York, 1988. (6) Rydberg, J., Musikas, C., Choppin, G. R., Eds. Principles and Practices of Solvent Extraction; Dekker: New York, 1992. (7) Ohto, K.; Inoue, K.; Goto, M.; Nakashiro, F.; Nagasaki, T.; Shinkai, S.; Kago, T. Bull. Chem. Soc. Jpn. 1993, 66, 2528-2535. (8) Le, Q. T. H.; Umetani, S.; Takahara, H.; Matsui, M. Anal. Chim. Acta 1993, 272, 293-299. (9) Takayanagi, T.; Yotsuyanagi, T. Bull. Chem. Soc. Jpn. 1994, 67, 18351839. (10) Saitoh, T.; Okuyama, M.; Kamidate, T.; Watanabe, H.; Haraguchi, K. Bull. Chem. Soc. Jpn. 1994, 67, 1002-1006.
(11) Kokusen, H.; Sohrin, Y.; Hasegawa, H.; Kihara, S.; Matsui, M. Bull. Chem. Soc. Jpn. 1995, 68, 172-177. (12) Tsurubou, S.; Mizutani, M.; Kadota, Y.; Yamamoto, T.; Umetani, S.; Sasaki, T.; Le, Q. T. H.; Matsui, M. Anal. Chem. 1995, 67, 1465-1469. (13) Saleh, M. I.; Salhin, A.; Saad, B. Analyst (London) 1995, 120, 2861-2865. (14) Satake, S.; Tsukahara, S.; Suzuki, N. Bull. Chem. Soc. Jpn. 1995, 68, 19291933. (15) Kubono, K.; Hirayama, N.; Matsuoka, Y.; Kokusen, H. Anal. Sci. 1996, 12, 133-135. (16) Sasaki, T.; Umetani, S.; Le, Q. T. H.; Matsui, M.; Tsurubou, S. Analyst (London) 1996, 121, 1051-1054. (17) Ohto, K.; Yano, M.; Inoue, K.; Yamamoto, T.; Goto, M.; Nakashio, F.; Shinkai, S.; Nagasaki, T. Anal. Sci. 1995, 11, 893-902. (18) Aggett, J.; Richardson, R. A. Anal. Chim. Acta 1970, 50, 269-275. (19) Aggett, J.; Richardson, R. A. Anal. Chim. Acta 1970, 51, 528-529. (20) Stronsky, I.; Rekas, M. Radiochem. Radioanal. Lett. 1973, 14, 297-304. (21) Aggett, J.; Richardson, R. A. Analyst (London) 1980, 105, 1118-1120. (22) Aggett, J.; Khoo, A. W.; Richardson, R. A. J. Inorg. Nucl. Chem. 1981, 43, 1867-1872. (23) Panda, C. R.; Chakravortty, V.; Dash, C. K. J. Radioanal. Nucl. Chem. 1987, 108, 65-75. (24) Rekas, M.; Stronsky, I.; Zielinski, A. Z. Phys. Chem. (Leipzig) 1972, 249, 145-153.
4814 Analytical Chemistry, Vol. 69, No. 23, December 1, 1997
S0003-2700(97)00548-9 CCC: $14.00
© 1997 American Chemical Society
a salt (MX) as ion-pair reagent, and 1 × 10-2 mol dm-3 of the buffer (MES or MOPS) having no influence on the extraction were shaken at 25 ( 1 °C for 24 h. (All of the extraction was equilibrated within 10 h.) After the two phases were separated by centrifugation, the pH and the metal concentration in the aqueous phase were determined, and the measured pH was used as equilibrated pH. The metal concentration in the organic phase was determined after back-extraction into 1 mol dm-3 hydrochloric acid. Caution: Nitrobenzene is a highly toxic irritant. Do not get it in eyes or on skin, and do not inhale its vapor.
Figure 1. Structures of H2Nsalen, H2Nsaloph, and H2Nsalpn.
EXPERIMENTAL SECTION Apparatus. A Shimadzu Model ICPS-1000 III inductively coupled argon plasma spectrometer was used for the determination of the concentration of a metal in aqueous solution. A Shimadzu Model UV-160 UV/visible spectrophotometer with a 1.0 cm quartz cell was used for the measurement of absorption spectra. A Horiba Model F-12 pH meter equipped with a Horiba 6366-10D combined glass electrode was used to determine the pH values in aqueous solution. Reagents. The ligands H2Nsalen, H2Nsaloph, and H2Nsalpn were synthesized from 5-nitrosalicylaldehyde and ethylenediamine, o-phenylenediamine, and 1,3-propanediamine, respectively, according to the methods of Choudhary et al.25 Their structures were identified by 1H-NMR spectra and elemental analysis. All other chemicals were reagent-grade materials, and distilled deionized water was used throughout. Acid Dissociation and Distribution of Ligands. The acid dissociation constants and the distribution coefficients of H2Nsalen, H2Nsaloph, and H2Nsalpn were determined by their measuring distribution ratios around pH 2-9. In a 30 cm3 centrifuge tube, an aliquot of nitrobenzene (10 cm3) containing 5 × 10-5 mol dm-3 of a ligand was equilibrated with an equal volume of an aqueous phase containing 1 × 10-1 mol dm-3 of potassium chloride and 1 × 10-3 mol dm-3 of buffer [2-(N-morpholino)ethanesulfonic acid (MES) or 3-(N-morpholino)propanesulfonic acid (MOPS)] at 25 ( 1 °C. After the two phases were separated by centrifugation, the pH of the aqueous phase was measured. The ligand concentration remaining in the aqueous phase was determined by measuring the absorbance at 395 nm after adding an equal volume of 2 mol dm-3 sodium hydroxide solution. Distribution of the Metals. The distribution of metal ions was studied as follows: In a 30 cm3 centrifuge tube, an aliquot of nitrobenzene (10 cm3) containing 1 × 10-4 mol dm-3 of a ligand and an equal volume of an aqueous phase containing 1 × 10-5 mol dm-3 of a trivalent lanthanide (Ln3+), 1 × 10-1 mol dm-3 of (25) Choudhary, C.; Hughes, D. L.; Kleinkes, U.; Larkworthy, L. F.; Leigh, G. L.; Maiward, M.; Marmion, C. J.; Sanders, J. R.; Smith, G. W.; Sudbrake, C. Polyhedron 1997, 16, 1517-1528.
RESULTS AND DISCUSSION Selection of Organic Solvent. As preliminary study, the use of several kinds of popular organic solvents having high permittivities, such as chloroform, 1,2-dichloroethane, and nitrobenzene, was investigated. Because of the low solubility of the ligands, Ln3+ was not extracted into chloroform and 1,2-dichloroethane. To the contrary, the ligands can be dissolved into nitrobenzene enough to extract Ln3+. From the result, nitrobenzene was selected as the solvent in this study. In addition, the upper limit of soluble concentration of H2Nsalen, H2Nsaloph, or H2Nsalpn in nitrobenzene was (1-2) × 10-3 mol dm-3. Acid Dissociation Constants and Distribution Coefficients of Ligands. H2Nsalen, H2Nsaloph, and H2Nsalpn are dibasic acid. The acid dissociation constants of the ligand (H2L) are defined as
Ka1 ) [H+][HL-]/[H2L]
(1)
Ka2 ) [H+][L2-]/[HL-]
(2)
and the distribution coefficient of H2L is
KD ) [H2L]o/[H2L]
(3)
where subscript “o” denotes the species in the organic (nitrobenzene) phase. By using these equations, the distribution ratio of H2L at a fixed pH can be described as follows:
D(H2L) ) [H2L]o/([H2L] + [HL-] + [L2-]) ) KD/{1 + Ka1/[H+] + Ka1Ka2/[H+]2}
(4)
and, therefore, the nonlinear least-squares fitting method is applicable to a log D(H2L) vs pH plot for the determination of Ka1, Ka2, and KD. By the fitting, these values were determined as follows: pKa1 ) 7.20 ( 0.23, pKa2 > 9, log KD) 1.26 ( 0.20 for H2Nsalen, pKa1 ) 7.07 ( 0.17, pKa2 > 9, log KD ) 1.22 ( 0.17 for H2Nsaloph, and pKa1 ) 7.04 ( 0.19, pKa2 > 9, log KD ) 1.27 ( 0.19 for H2Nsalpn. Similar distribution curves were observed when using LiCl or NaCl instead of KCl, and it was found that the distribution of HL- (or L2-) into the organic phase is negligible around pH 2-9. The pKa1 values of these ligands were similar to the pKa of 4-nitrophenol (7.15, ref 26). Higher values of pKa2 might be caused by intramolecular hydrogen bonding between phenol H and phenolate O-. (26) Dean, J. A., Ed., Lange’s Handbook of Chemistry, 13th ed.; McGraw-Hill: New York, 1982.
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Figure 2. Plots of log D of Eu3+ as a function of the aqueous phase pH, equilibrated with organic phase (nitrobenzene) in H2Nsalen-MX systems. (A) MX ) LiCl (9), NaCl (0), NH4Cl (b), KCl (O), RbCl (2), CsCl (4), and (C2H5)4Cl (1). (B) MX ) NaCl (0), NaClO4 (9), KCl (O), and KNO3 (b). Initial concentrations were 1 × 10-4 mol dm-3 for H2Nsalen in nitrobenzene and 1 × 10-1 mol dm-3 for MX in aqueous phase.
Determination of Extracted Species. The following complexes of Ln3+ with H2salen derivatives (H2L′) were reported previously: Ln2L′3,27 NH4LnL′2,28 and Ln(β-diketonate)L′.29 Therefore, also in our Ln3+-H2L-MX-nitrobenzene system, various kinds of possible extracted species, such as Ln2L3, LnL(HL), MLnL2, Ln(HL)2X, and LnLX, should be considered. Figure 2 shows plots of the logarithmic distribution ratio (log D) of Eu3+ between organic and aqueous phases as a function of the aqueous phase pH, equilibrated with nitrobenzene in several kinds of H2Nsalen-MX systems. As shown in Figure 2A, M+ is obviously involved in the extraction of Ln3+. To the contrary, Xdid not influence the extraction (see Figure 2B). From the results and the fact that the typical coordination number of Ln3+ is 8 or 9, it was supposed that Ln3+ is extracted as MLnL2 (ion-pair of M+ and LnL2-). It is well known that an extracted ion-pair in nitrobenzene is often dissociated. If Ln3+ is extracted into nitrobenzene as the following equilibrium shows,
Ln3+ + 2H2Lo + M+ h MLnL2,o + 4H+
(5)
Kex ) [MLnL2]o[H+]4/[Ln3+][H2L]o2[M+]
(6)
(Kex is the extraction constant), then the dissociation of the extracted species can be presented as follows:
MLnL2,o h M
+ o
+
LnL2-o
2
≈ [LnL2 ]o /[MLnL2]o
concentrations of Ln3+ in organic and aqueous phases (CLn,o and CLn, respectively):
CLn,o ) [MLnL2]o + [LnL2-]o ) [MLnL2]o + (Kdis[MLnL2]o)1/2 ) Kex[H2L]o2[M+][Ln3+]/[H+]4 + (KexKdis[H2L]o2[M+][Ln3+]/[H+]4)1/2 ≈ Kex[H2L]o2[M+](CLn1/2/[H+]2)2 + (KexKdis[H2L]o2[M+])1/2(CLn1/2/[H+]2) (9) Therefore, by plotting log CLn,o vs (1/2 logCLn + 2pH) at fixed H2L and MX concentration, the degree of the dissociation of extracted MLnL2 can be estimated. Figure 3 shows several examples of the plots. All these plots gave straight lines with slope close to 1, and it was found that the extracted ion-pair in the nitrobenzene phase is dissociated almost perfectly. From the results mentioned above, the overall extraction equilibrium in this Ln3+-H2L-MX-nitrobenzene system can be represented as follows:
Ln3+ + 2H2Lo + M+ h M+o + LnL2-o + 4H+ and the equilibrium constant ()KexKdis) is
(8)
KexKdis ) [M+]o[LnL2-]o[H+]4/[Ln3+][H2L]o2[M+] ≈ [LnL2-]o2[H+]4/[Ln3+][H2L]o2[M+]
where Kdis is the dissociation constant and [M+]o is nearly equal to [LnL2-]o to keep the electrical neutrality in the organic phase. Then, the following relationship is observed between the total (27) Dutt, N. K.; Nag, K. J. Inorg. Nucl. Chem. 1968, 30, 2493-2499. (28) Isobe, T.; Mizumi, S. Mem. Fac. Sci. Kyushu Univ., Ser. C 1974, 9, 29-38. (29) Dutt, N. K.; Nag, K. J. Inorg. Nucl. Chem. 1968, 30, 2779-2783.
4816
(10)
(7)
Kdis ) [M+]o[LnL2-]o/[MLnL2]o -
Figure 3. Plots of log CLn,o of Eu3+ as a function of (1/2 logCLn + 2pH) in H2Nsalen-MX (A) and H2Nsaloph-MX (B) systems. MX ) LiCl (4), KCl (O), and CsCl (0). Initial concentrations were 1 × 10-4 mol dm-3 for H2Nsalen or H2Nsaloph in nitrobenzene and 1 × 10-1 mol dm-3 for MX in aqueous phase. Solid lines, of which the slope is 1, were obtained by the least-squares fit.
Analytical Chemistry, Vol. 69, No. 23, December 1, 1997
(11)
Therefore, by defining D* as
D* ) CLn,o2/CLn ≈ [LnL2-]o2/[Ln3+]
(12)
Figure 4. Plots of log D* of Lu3+ into nitrobenzene with H2NsalenKCl system as a function of the logarithmic initial concentration of H2Nsalen in nitrobenzene [C(H2Nsalen)o,i] (A) and the aqueous phase pH (B). In (A), the initial concentration of KCl in aqueous phase was 1 × 10-1 mol dm-3, and the aqueous phase pH was 6.0. In (B), initial concentrations were 1 × 10-4 mol dm-3 for H2Nsalen in nitrobenzene and 1 × 10-1 mol dm-3 for KCl in aqueous phase. Solid lines, of which the slope is 2 (A) or 4 (B), were obtained by the least-squares fit.
Figure 5. Plots of the extracted ratios (%E) of three lanthanides as a function of aqueous phase pH in H2L-KCl (A-C) and H2L(C2H5)4NCl (D-F) systems. H2L ) H2Nsalen (A,D), H2Nsaloph (B,E), and H2Nsalpn (C,F). Initial concentrations were 1 × 10-4 mol dm-3 for H2L in nitrobenzene and 1 × 10-1 mol dm-3 for KCl or (C2H5)4NCl in aqueous phase. ], La3+; O, Eu3+; 4, Lu3+. Table 1. Determined log KexKdis Values of Lanthanides [25 °C, I ) 0.1(MCl)]
the following relationship is obtained:
Ln3+
log D* ) log KexKdis + 2log [H2L]o + log [M ] + 4pH (13) To verify eq 10, slope analysis on log D* vs log [H2L]o and log D* vs pH plots was performed. Figure 4 shows typical examples of the plots. The slopes of the former and the latter plots were close to 2 and 4, respectively. From the result, it was proved that Ln3+ is extracted according to eq 10. In addition, the formation of a hydroxo complex such as Ln(OH)2+ in the aqueous phase is negligible at pH < 7.30 Comparison of Extractability and Selectivity of Ln3+ between H2L-MCl Extraction Systems. The value of KexKdis can be represented as
KexKdis ) Ka1Ka2β2KassocKD′Kdis/KD
Li+ Na+ NH4+ K+ Rb+ Cs+ (C2H5)4N+ Li+ Na+ NH4+ K+ Rb+ Cs+ (C2H5)4N+ a
(14)
where β2, Kassoc, and KD′ are stability constant of LnL2- in the aqueous phase, ion association constant between M+ and LnL2in the aqueous phase, and distribution coefficient of MLnL2 between two phases, respectively, defined as follows:
β2 ) [LnL2-]/[Ln3+][L2-]2
(15)
Kassoc ) [MLnL2]/[M+][LnL2-]
(16)
KD′ ) [MLnL2]o/[MLnL2]
(17)
However, all values of β2, Kassoc, KD′, and Kdis cannot be determined separately. To compare the ability of selective extraction of Ln3+ between H2L-MCl-nitrobenzene systems, the extraction behavior of La3+, Eu3+, and Lu3+ in these systems was investigated. Figure 5 shows (30) Rizkalla, E. N.; Choppin, G. R. In Handbook on the physics and chemistry of rare earths; Gshneidner, K. A., Jr., Eyring L., Eds.; Elsevier: Amsterdam, The Netherlands, 1991; Vol. 15, Chapter 103.
La3+
M+
+
Eu3+
Lu3+
H2L ) H2Nsalen -19.53 ( 0.19 -18.79 ( 0.30 -17.63 ( 0.18 -16.49 ( 0.20 -16.18 ( 0.30 -14.89 ( 0.19 -13.35 ( 0.05
-16.02 ( 0.19 -15.57 ( 0.18 -14.16 ( 0.22 -13.58 ( 0.12 -14.45 ( 0.23 -13.21 ( 0.12 -12.90 ( 0.17
H2L ) H2Nsaloph a -16.58 ( 0.19 a -15.57 ( 0.11 a -15.71 ( 0.16 a -15.01 ( 0.12 a -14.49 ( 0.27 -17.78 ( 0.34 -13.19 ( 0.36 -15.76 ( 0.40 -10.69 ( 0.19
-13.52 ( 0.27 -12.67 ( 0.09 -12.95 ( 0.12 -12.63 ( 0.14 -12.54 ( 0.26 -11.36 ( 0.23 -10.04 ( 0.34
a a a a a a a
Not determined because of very low extractability.
the plots of the extracted ratios (%E) of the metals as a function of aqueous phase pH in H2L-KCl and H2L-(C2H5)4NCl systems as examples. Table 1 shows the log KexKdis values in H2NsalenMCl and H2Nsaloph-MCl systems determined by using eq 13. In H2Nsalpn-MCl systems, these lanthanides were not extracted into nitrobenzene, except for Eu3+ and Lu3+ on M+ ) (C2H5)4+ (log KexKdis ) -15.86 ( 0.18 for Eu3+ and -15.27 ( 0.29 for Lu3+). In addition, all non-nitro-substituted H2salen, H2saloph, and H2salpn did not act as extractant for Ln3+ at pH < 8. As shown in Table 1, the order of the extractability of Ln3+ between the investigated extraction systems with a fixed M+ was H2Nsaloph > H2Nsalen . H2Nsalpn for H2L. These three ligands have similar KD and Ka1 values, and the order may be due mainly to the difference of β2 between them, originating from the sizefitting ability of L2- with Ln3+. On the other hand, the order of the extractability with a fixed H2L tended to be (C2H5)4N+ > Cs+ > Rb+ g K+ > NH4+ g Na+ > Li+ for M+. This order is almost the same as the order of effective ionic radii, Cs+ (167 pm) > Rb+ (152 pm) > K+ (138 pm) > Na+ (102 pm) > Li+ (76 pm),3 and seems to be due mainly to the contribution of Kassoc or KD′. Analytical Chemistry, Vol. 69, No. 23, December 1, 1997
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Table 2. Determined pH1/2 and log KexKdis Values of Lanthanides in H2Nsalen-KCl Extraction System [25 °C, I ) 0.1(KCl)] Ln3+
pH1/2a
La3+ Nd3+ Eu3+ Gd3+ Dy3+ Er3+ Yb3+ Lu3+
b 7.48 6.86 6.72 6.40 6.18 6.07 6.09
log KexKdis c -19.01 ( 0.30 -16.49 ( 0.20 -16.00 ( 0.15 -14.81 ( 0.28 -14.02 ( 0.19 -13.56 ( 0.06 -13.58 ( 0.12
a Initial concentrations: 1 × 10-4 mol dm-3 H Nsalen in nitroben2 zene, and 1 × 10-5 mol dm-3 Ln3+ and 1 × 10-1 mol dm-3 KCl in aqueous phase. b Not extracted. c Not determined.
The order of [log Kex(Lu)Kdis(Lu) - log Kex(Eu)Kdis(Eu)] values, as an index to evaluate the mutual selectivity for Ln3+ in several H2L-MCl extraction systems, was H2Nsalen g H2Nsaloph for H2L and Li+ g Na+ ≈ NH4+ g K+ > Rb+ g Cs+ . (C2H5)4N+ for M+, which is almost the opposite of the order of their sizes. Nitrobenzene is classified by Kay et al.31 as a “neutral” solvent, and the dissociation constant of an ion-pair in it increases with the increase of the size of the paired ions,31-33 the opposite of what is observed in “hydrogen bonding” solvent such as water. The selectivity in the systems may originate mainly from the difference of β2 and be reduced by the dissociation of MLnL2 in nitrobenzene. In other words, it is possible that the large size of M+ or L2- makes the Kdis value large and decreases the selectivity between lanthanides. Mutual Separability of Ln3+ in the H2Nsalen-KCl Extraction System. As the extraction system having both favorable extractability and mutual selectivity of Ln3+, the performance of the H2Nsalen-KCl-nitrobenzene extraction system was investigated for comparison with those of other extraction systems reported previously. Table 2 shows the obtained half-extraction pH (pH1/2) and the determined log KexKdis values in the H2NsalenKCl system. Figure 6 shows the [pH1/2 - pH1/2(Eu)] values of several extraction systems,9,15,34,35 including this study. The mutual selectivity of heavy lanthanides (Eu3+-Lu3+) in our system rivaled that in a dialkylphosphoric acid system, such as di(2-ethylhexyl)phosphoric acid,36 used widely. Furthermore, the selectivity of light lanthanides (Nd3+-Eu3+) in it is superior to that in the (31) Kay, R. L.; Evans, D. F.; Matesich, S. M. F. In Solute-Solvent interactions; Coetzee, J. F., Ritchie, C. D., Eds.; Dekker: New York, 1976; Vol. 2, Chapter 2. (32) Okazaki, S.; Sakamoto, I. Solvents and Ions; San-ei: Kyoto, Japan, 1990; Chapter 5. (33) Marcus, Y. Ion solvation; Wiley: Chichester, U.K., 1985; Chapter 8. (34) Paskanzer, A. M.; Foreman, B. M., Jr. J. Inorg. Nucl. Chem. 1976, 38, 309313. (35) Hori, T.; Kawashima, M.; Freiser, H. Sep. Sci. Technol. 1980, 15, 861875. (36) Motomizu, S.; Freiser, H. Solv. Extr. Ion Exch. 1985, 3, 637-665.
4818 Analytical Chemistry, Vol. 69, No. 23, December 1, 1997
Figure 6. Comparison of the [pH1/2 - pH1/2(Eu)] values between several extraction systems. ×, Thenoyltrifluoroacetone (2 × 10-1 mol dm-3)-benzene system (ref 34); 4, N,N′-bis[(2-hydroxyphenyl)methyl]-N,N′-bis(2-pyridylmethyl)ethylenediamine (2 × 10-2 mol dm-3)dibenzoylmethane (2 × 10-2 mol dm-3)-chloroform system (ref 15); ], 2,3-naphthalenediol (1 × 10-2 mol dm-3)-benzyldimethyltetradecylammonium chloride (1 × 10-3 mol dm-3)-NaCl (1 × 10-1 mol dm-3)-chloroform system (ref 9); O, 8-quinolinol (1 × 10-1 mol dm-3)-chloroform system (ref 35); b, H2Nsalen (1 × 10-4 mol dm-3)-KCl (1 × 10-1 mol dm-3)-nitrobenzene system (this work).
thenoyltrifluoroacetone system.34 In addition, log Kex(La)Kdis(La) is obviously lower than -20.5 in the H2Nsalen-KCl system, and [log Kex(Eu)Kdis(Eu) - log Kex(La)Kdis(La)] can be estimated as higher than 4.0, which results in surprisingly high selectivity between light lanthanides. CONCLUSIONS Nitro-substituted quadridentate phenolic Schiff bases such as H2Nsalen and H2Nsaloph acted as effective extractants for mutual separation of Ln3+ when using a suitable countercation M+ as ionpair reagent. With the decrease of the size of M+, the selectivity was increased, although the extractability was decreased. This extraction system has some room for improvement. Particularly, further derivatization of the ligands for the enhancement of their solubilities into organic solvents is very important not only to realize higher extractability by using them at higher concentration but also to suppress the extracted ion-pairs for higher selectivity by using organic solvents having slightly lower permittivities. ACKNOWLEDGMENT We thank Yuka Matsuoka (Tokyo Gakugei University, Japan) for her kind support on the synthesis of the Schiff bases. This study was financially supported in part by Grants-in-Aid for Scientific Research No. 07854045 and No. 08740572 from the Ministry of Education, Science and Culture, Japan. Received for review May 28, 1997. Accepted September 10, 1997.X AC970548X X
Abstract published in Advance ACS Abstracts, October 15, 1997.
