Anal. Chem. 1997, 69, 2835-2841
Centrifugal Partition Chromatographic Separation of Tervalent Lanthanides Using Acylpyrazolone Extractants Genxiang Ma,† Henry Freiser, and Subramaniam Muralidharan*
Strategic Metals Recovery Research Facility, Department of Chemistry, University of Arizona, Tucson, Arizona 85721
The separation of tervalent lanthanides (M3+) by centrifugal partition chromatography (CPC) with the extractants 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone (HPMBP, HL) and 1-phenyl-3-methyl-4-capryloyl-5-pyrazolone (HPMCP, HL) in the toluene-water phase pair and the factors influencing the separation efficiencies have been investigated. The CPC efficiencies are mainly limited by the slow dissociation of the M3+-acylpyrazolone complexes (ML3) occurring exclusively at the toluene-water interface, as indicated by a direct linear correlation between the reduced plate heights (CETPck) and the half-lives (t1/2) of the dissociation of the ML3 complexes determined by independent kinetic studies. The lanthanide-acylpyrazolone system represents the first example of a separation by multistage countercurrent distribution wherein the efficiencies are mainly limited by interfacial processes analogous to conventional liquid chromatographic systems. Dramatic improvements in the efficiencies of these separations could be obtained by the addition of the neutral surfactant Triton X-100 to the toluene phase and the metallochromic indicator Arsenazo III (AZ) to the aqueous phase. The improvement in the efficiencies with Triton X-100 was due to the increased interfacial areas resulting from the adsorption of the surfactant, and that in the case of AZ was due to the increased interfacial area and interfacial catalysis of the formation and dissociation of the ML3 complexes. As a result, the addition of AZ provided much higher efficiencies than did the addition of Triton X-100. Further, the addition Triton X-100 did not significantly alter the selectivities of the ligands for M3+, but the addition of AZ resulted in much poorer selectivities. Significant differences were observed in the efficiencies of separations with HPMBP and HPMCP under various experimental conditions, which stem from differences in the interfacial dissociation rate constants of ML3 complexes and the interfacial areas generated, indicating that CPC separation of tervalent lanthanides with acylpyrazolones in the toluene-water phase pair is driven mainly by interfacial processes. The complete separation of tervalent lanthanide ions (M3+) is an area of continuing interest for several applications, including the treatment of nuclear wastes to remove radioactive nuclei and the recovery of M3+ from natural sources for a variety of † On study leave from The Laboratory of Rare Earths Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China.
S0003-2700(96)01192-4 CCC: $14.00
© 1997 American Chemical Society
applications. Their separation is complicated by the similarities of their properties and requires the employment of multistage methods such as continuous ion exchange,1 cation exchange chromatography,2,3 micellar high-performance chromatography,4 capillary zone electrophoresis,5 and supported liquid membranes.6 Yet another multistage method that has proven valuable for the separation of tervalent lanthanides is centrifugal partition chromatography (CPC).7-10 CPC is distinctly different from the various conventional liquid chromatographic (LC) methods mentioned in that it is a multistage countercurrent distribution technique involving two immiscible liquid phases, usually an organic phase and an aqueous phase, with the organic phase usually serving as the stationary phase by the action of a centrifugal force and the aqueous phase serving as the mobile phase. As a result, CPC is a multistage solvent extraction method employing two bulk phases, unlike conventional LC techniques, which employ a solid stationary phase and a liquid mobile phase, where separations are predominantly effected by physical and chemical interactions at the solid-liquid interface. We have demonstrated in previous studies that CPC separations are influenced by both bulk and interfacial equilibrium and kinetic processes, and, in particular, the efficiencies of CPC separations of metal ions by complexation with appropriate ligands are limited mainly by the kinetics of metal complex formation and dissociation reactions that have both a bulk aqueous component and an interfacial component.8,11-14 In the current study on the separations of tervalent lanthanides using the acylpyrazolone ligands, we demonstrate for the first time CPC separations whose efficiencies are entirely influenced by interfacial kinetics and are thus analogous in their behavior to conventional LC separations. (1) Byers, C. H.; Williams, D. F. Ind. Eng. Chem. Res. 1996, 35, 993-998. (2) Strelow, F. W.; Victor, A. H. Talanta 1990, 37, 1155-1161. (3) Bruzzoniti, M. C.; Mentasti, E.; Sarzanini, C.; Braglia, M.; Cocito, G.; Kraus, J. Anal. Chim. Acta 1996, 322, 49-54. (4) Elchuk, S.; Burns, K. I.; Cassidy, R. M.; Lucy, C. A. J. Chromatogr. 1991, 558, 197-207. (5) Vogt, C.; Conradi, S. Anal. Chim. Acta 1994, 294, 145-153. (6) Hrdlicka, A.; Fialova, I.; Dolezalova, J. Talanta 1996, 43, 649-657. (7) Cai, R.; Muralidharan, S.; Freiser, H. J. Liq. Chromatogr. 1990, 13, 36513672. (8) Inaba, K.; Freiser, H.; Muralidharan, S. Solv. Extr. Res. Dev., Jpn. 1994, 1, 13-29. (9) Abe, H.; Usuda, S.; Tachimori, S. J. Liq. Chromatogr. 1994, 17, 1821-1835. (10) Akiba, K.; Hashimoto, H.; Nakamura, S.; Saito, Y. J. Liq. Chromatogr. 1995, 18, 2723-2741. (11) Surakitbanharn, Y.; Muralidharan, S.; Freiser, S. Anal. Chem. 1991, 63, 2642-2645. (12) Surakitbanharn, Y.; Freiser, H.; Muralidharan, S. Anal. Chem. 1996, 68, 3934-3938. (13) Chen, F. Y.; Freiser, H.; Muralidharan, S. Langmuir 1994, 10, 2139-2144. (14) Chen, F. Y.; Freiser, H.; Muralidharan, S. Langmuir 1995, 11, 3235-3242.
Analytical Chemistry, Vol. 69, No. 14, July 15, 1997 2835
The correlation between chromatographic efficiencies and chemical kinetics in LC is entirely lacking, and we are currently addressing this through the separation of metals by micellar highperformance liquid chromatography (MLC).15 Our ultimate goal is to determine if the efficiencies of interfacially driven separations, irrespective of the separation technique adopted (MLC or CPC), yield a general correlation with interfacial chemical kinetics, that is, if a general reduced plate height vs half-life (t1/2) correlation can be obtained.11,14 The acylpyrazolone family of ligands, with their low pKa values (∼4), are ideally suited for the extraction and separation of tervalent lanthanides.16-18 Even though batch extraction experiments indicate that their selectivities for a given pair of lanthanides are not as good as the phosphinic acids, which possess some of the best selectivities,7 complete separation of the tervalent lanthanides using the acylpyrazolones is still possible using multistage methods such as CPC. We have conducted a systematic study of the kinetics of formation and dissociation of the tervalent lanthanide complexes (ML3) of 1-pheny-3-methyl-4-benzoyl-5pyrazolone (HPMBP, HL) and 1-phenyl-3-methyl-4-capryloyl-5pyrazolone (HPMCP, HL) in the toluene-water phase pair and found these reactions to occur exclusively at the toluene-water interface.19 We have also shown for the first time that the formation and dissociation of the ML3 complexes can be interfacially catalyzed by Arsenazo III (AZ).19 The separation of M3+ using HPMBP and HPMCP, the correlation of the kinetic component of the reduced plate height, CETPCK, with the interfacial t1/2 values, and the effect of a neutral surfactant Triton X-100 and the metallochromic indicator AZ on the CPC separation selectivities and efficiencies have been examined to elucidate the influence of the liquid-liquid interface in multistage separations. EXPERIMENTAL SECTION Apparatus. A Sanki Co. (Kyoto, Japan) assembly consisting of a Model SPL centrifuge containing six analytical/semipreparative cartridges each having 400 channels (2400 total channels), a Model CPC FCU-V loop injector, and a Model LBP-V pump were used for CPC experiments. The lanthanides were analyzed by postcolumn derivatization using an Altex 110A HPLC pump and a 100 cm loop for efficient mixing of the eluent and Arsenazo III. A Schoeffel Instrument Co. Model 770 UV-visible spectrophotometric detector with a 0.1 mL volume and 8 mm path length cell was used. It was set at 654 nm for analysis of the lanthanide Arsenazo III complexes. The chromatograms were recorded with an Alltech 1200 linear chart recorder. A Hewlett-Packard 8452A diode array spectrophotometer was used for the measurement of UV-visible spectra. All pH measurements were made with a Fisher Scientific 925 pH meter. Reagents. The extractants, HPMBP and HPMCP, were synthesized and purified as described in the literature.20 Stock solutions of Pr, Eu, Tb, Ho, and Yb were prepared from their chloride salts (99.9% purity, Alfa Products) by dissolving in 0.1 M hydrochloric acid. Arsenazo III (Aldrich Chemical) was recrystallized from a mixture of 90% H2O and 10% ethanol (v/v) twice and (15) Ma, H.; Freiser, H.; Muralidharan, S. Unpublished results. (16) Tochiyama, O.; Frieser, H. Anal. Chim. Acta 1981, 131, 233-238. (17) Umetani, S.; Freiser, H. Inorg. Chem. 1987, 26, 3179-3181. (18) Santhi, P. B.; Reddy, M. L. P.; Ramamohan, T. R.; Damodaran, A. D.; Mathur, J. N.; Murali, M. S.; Iyer, R. H. Solv. Extr. Ion Exch. 1994, 12, 633-650. (19) Ma, G.; Freiser, H.; Muralidharan, S. Anal. Chem. 1997, 69, 2827-2834 (preceding paper in this issue). (20) Jensen, B. S. Acta Chem. Scand. 1959, 13, 1668-1670.
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Analytical Chemistry, Vol. 69, No. 14, July 15, 1997
dried under vacuum. Triton X-100 (Fluka, >99%) was used as received. The ionic strength of the aqueous solution was maintained at 0.1 by the addition of sodium perchlorate (Alfa Products). All the other reagents were analytical grade. Procedures. All the experiments were carried out at 298 K. The pH values of the metal complex solutions and the solutions after mixing with HCl were determined using a Fisher Scientific Accumet 925 pH meter, calibrated with standard buffer solutions. The CPC experiments were conducted with toluene containing 0.04-0.14 M HPMBP or HPMCP as the stationary phase and an aqueous solution in the pH range 2.0-3.8 as the mobile phase, pumped in the descending mode. The aqueous and organic phases were equilibrated for 1-3 h, separated, and filtered before the CPC experiments were conducted. The buffers were prepared with hydrochloric acid and succinic acid. (Since the maximum concentration of succinic acid used was 0.01 M and the log of the stability constants for the 1:1 and 1:2 lanthanide-succinate complexes are about 1.5 and 3.0, it is evident that the succinate complexes are insignificant.) The CPC cartridges were loaded with the toluene stationary phase and equilibrated with the aqueous mobile phase at a rotational speed of 200 rpm, with the total internal volume in the present studies being 125 mL. The separation experiments were conducted under conditions optimized in our previous studies,7,11 with organic and aqueous phase volumes of 25 and 100 mL, respectively, at a rotational speed of 800 rpm and a mobile phase flow rate of 1 mL/min. Sample solutions (4 × 10-4-2 × 10-3 M) of Pr, Eu, Tb, and Yb were prepared by dilution of their stock solutions with the equilibrated mobile phase. A 0.4-1.5 mL aliquot of the sample solution was injected into the CPC system for the separation run. The mass transfer and diffusion component, CETPdif, of the observed experimental reduced plate height, CETPobs, of the lanthanide metal ions was determined from the chromatogram of 3-picoline obtained at the same D value of the metal ion by adjusting the pH of the aqueous mobile phase.11 RESULTS AND DISCUSSION CPC Separation of Lanthanides. Extraction equilibrium was achieved in the separations of the tervalent lanthanides as indicated by the log D vs pH and log D vs log [HL] plots having slopes of +3 (D is the distribution ratio of M3+ at extraction equilibrium ) [ML3] in the toluene phase/[M3+] in the aqueous phase). The extraction equilibrium based on these dependencies is shown in eq 1, and the relationship of the extraction equilibrium constant Kex to the distribution constant of the metal complex KDC, the distribution constant of the ligand KDR, the acid dissociation constant of the ligand Ka, and the stability constant of the ML3 complex βLL is given in eq 2. The averages of log Kex from batch Kex
M3+ + 3HLo y\z ML3,o + 3H+ Kex )
KDC 3 KaβLL K3DR
(1) (2)
and CPC experiments, and log βLL calculated from the Kex values, using the KDR, KDC, and Ka values determined independently,19 are listed in Tables 1 and 2, respectively, where it can be seen that these values increase with the atomic numbers of the lanthanides, similar to previous observations with other
Table 1. Log Kex Values in Toluene-Water Phase Pair log Kex HPMBP
HPMCP
metal nonea Tritonb Arsenazo IIIc Pr Eu Tb Yb
-4.08 -3.42 -2.86 -1.83
-1.02 -0.45 0.26
-2.84 -2.25 -2.10 -1.99
nonea Tritonb Arsenazo IIIc -6.98 -5.77 -5.44 -4.40
-3.18 -2.69 -2.51
-5.08 -4.32 -4.30 -4.18
T T AZ AZ a Log K , K b -1 c ex ex unitless. Log Kex, Kex in M . Log Kex , Kex in M-1.
Table 2. Log β Values in the Toluene-Water Phase Pair log β HPMBP
HPMCP
metal nonea Tritonb Arsenazo IIIc nonea Tritonb Arsenazo IIIc Pr Eu Tb Yb
13.52 14.18 14.74 15.77
3.06 2.97 2.60
a Log β , β -3 LL LL in M M-1.
8.43 8.50 8.91 9.83 b
16.02 17.23 17.56 18.60
Log βAT, βAT in M-1
3.80 3.08 2.93 c
Figure 1. CPC separation of Pr3+, Eu3+, and Yb3+ with [HPMBP] ) 0.08 M at I ) 0.1 and pH ) (a) 2.36, (b) 2.41, and (c) 2.46 (displaced by arbitrary A values).
7.77 8.22 8.53 9.45
Log βMAZ, βMAZ in
ligands.7,8,16-19,21,22 It is also evident that the tervalent lanthanides have better extractabilities with HPMBP than with HPMCP, but the separation factors with the two ligands for a given pair of lanthanides, as indicated by the differences in their log Kex values, are about the same. The log βLL values, shown in Table 2, for the HPMCP complexes are 3 orders of magnitude larger than those for the HPMBP complexes, indicating that HPMCP forms much stronger complexes than does HPMBP. This is also reflected in the rate constants of formation and dissociation of the HPMBP and HPMCP complexes.19 The Kex values for HPMCP, however, are lower than the values for HPMBP due to the larger KDR values for HPMCP than for HPMBP (eq 2).19 The CPC separations of Pr3+, Eu3+, and Yb3+ with HPMBP as the extractant at different pH values are shown in Figure 1, indicating base line separation of the three lanthanide metal ions. It is also evident that the bandwidths and, hence, the efficiencies of the three metal ions are different at a given pH and the efficiencies decrease with increasing pH and with increasing concentrations of ligand. The ligand HPMBP cannot separate the heavy lanthanides Tb3+ and Yb3+ like HPMCP (Figure 2). This is due to the much poorer efficiencies obtained with HPMBP compared to HPMCP, as can be seen from Table 3, where the chemical kinetic contribution to the reduced plate height values (CETPck ) CETPobs - CETPdif; CETPdif determined for each D value using 3-picoline) for the various lanthanides are compared at identical D values.11 This is discussed in detail below. Correlation between CPC Efficiency and Kinetics. The plots of log CETPck as a function of pH and log [HL] have slopes of +1 for all the metals and the ligands HPMBP and HPMCP. We have shown in our kinetic studies that the formation and dissociation of the ML3 complexes in the toluene-water system occur exclusively in the interfacial region, and the observed (21) Kumar, K.; Chang, C. A.; Tweedle, M. F. Inorg. Chem. 1995, 32, 587-593. (22) Kodama, M.; Koike, T.; Mahatma, A. B.; Kimura, E. Inorg. Chem. 1991, 30, 1270-1273.
Figure 2. CPC separation of Tb3+ and Yb3+ with HPMCP ) 0.06 M at I ) 0.1 and pH ) (a) 2.85, (b) 2.90, and (c) 2.95 (displaced by arbitrary A values). Table 3. Comparison of CETPck Values for HPMBP and HPMCP in the Toluene-Water Phase Paira D ) 2.0 HPMBP
D ) 3.0
HPMCP
HPMBP
HPMCP
metal
pH
CETPck
pH
CETPck
pH
CETPck
pH
CETPck
Pr Eu Tb Yb
2.56 2.34 2.15 1.81
93.5 166.7 263.6 297.2
3.52 3.12 3.01 2.66
13.9 31.6 34.9 63.8
2.62 2.40 2.21 1.87
111.7 187.5 304.1 310.5
3.58 3.18 3.07 2.72
16.9 36.4 40.1 71.9
a
Concentrations of HPMBP and HPMCP in toluene, 0.08 M.
d interfacial pseudo-first-order dissociation rate constant, kobs , at i constant pH and excess HL is given by eq 3.19 Here, k-1 is the
kdobs ) kdi obsas )
( )
ki-1K-2 KDR [H+] i (K d)a Ka KDC [HL]o DC s
(3)
rate constant for the dissociation of ML2+ at the interface, K-2 is the dissociation equilibrium constant for ML2+ h ML2+ + L-, KiDC is the distribution constant for the ML3 complex between the toluene phase and the interface, d is the thickness of the Analytical Chemistry, Vol. 69, No. 14, July 15, 1997
2837
interface, and as is the specific interfacial area generated in the d CPC experiment. The t1/2 (0.693/kobs ) for the dissociation reaction at a fixed specific interfacial area, as, is clearly a function of [HL]o/[H+]. We have shown for several families of metal ions, namely the transition metals, platinum group metals, and lanthanides, that CETPck ∝ t1/2.8,11-14 The CPC separations were conducted at a fixed flow rate of the mobile phase (1 mL/min), and as a result the specific interfacial area generated for a given ligand is constant. If the dissociation of the ML3 complexes is the major factor limiting the efficiencies of separations of the lanthanides, then log CETPCK vs pH at constant [HL] and log CETPck vs log [HL] at constant pH should have slopes of +1. As mentioned above, this, indeed, was the case for the all the lanthanides separated with HPMBP and HPMCP. It is evident from eq 3 that the specific interfacial area, as, generated under the conditions of the CPC experiments (internal volumes: organic phase 25 mL, aqueous phase 100 mL; rotational speed, 800 rpm; aqueous mobile phase flow rate, 1 mL/min) can be calculated from the intercepts of the plots of log CETPCK vs pH and log CETPck vs log [HL], as all the other quantities are known.19 The as values for HPMBP and HPMCP determined from such plots are 183.43 and 130.17 cm-1, respectively, which are similar to the a values generated at a stirring speed of 5000 rpm in the AMES experiments, namely 160.74 cm-1 in the case of HPMBP and 114.96 cm-1 in the case of HPMCP.19 The areas generated in CPC correspond to aqueous mobile phase droplet sizes of 163.6 µm for HPMBP and 230.5 µm for HPMCP, which are, again, similar to the droplet sizes generated at 5000 rpm in the AMES experiments, namely 186.6 µm for HPMBP and 261.0 µm for HPMCP. We have studied the kinetics of dissociation of the ML3 complexes in the toluene-water phase pair and in the micelles formed by the neutral surfactant Triton X-100.19 The rate constants determined can be used to calculate the t1/2 values for the dissociation of the ML3 complexes under the conditions of the CPC experiments using eq 3 and the correlation between the CETPck values and these t1/2 values examined. The plot of CETPck vs t1/2, for t1/2 values calculated using the dissociation rate constants in Triton X-100 micelles, yields a separate straight line for each tervalent lanthanide ion and ligand (Figure 3). When the rate constants and equilibrium constants determined in the toluene-water phase pair are used to calculate the t1/2 values, as evident from Figure 4, a single CETP1/2-t1/2 correlation is, indeed, obtained for all the lanthanide metal ions and the ligands HPMBP and HPMCP. A comparison of the equilibrium and kinetic results in the toluene-water and Triton X-100 micellar systems indicate that three factors contribute to the difference in the CETPck-t1/2 correlations when the t1/2 values in micelles and two-phase systems are used:19 (i) differences in the distribution constants of the metal complexes (KDC) and the ligands (KDR) between the two-phase system and the micellar system, (ii) differences in the rate constants between the two-phase system and the micellar system, and (iii) dissociation reactions in the two-phase system which can occur both in the bulk aqueous phase and at the aqueous-organic interface,13,14 while in the micellar systems they occur exclusively at the aqueous-micelle interface.23 In the present studies, only factors i and ii are important, as the dissociation reactions in both the toluene-water phase pair and the micelles occur exclusively in the interfacial region. The CPC separation of lanthanides with (23) Cai, R.; Freiser, H.; Muralidharan, S. Langmuir 1995, 11, 2926-2930.
2838 Analytical Chemistry, Vol. 69, No. 14, July 15, 1997
Figure 3. CETPck vs t1/2 for CPC separations of lanthanides with HPMBP and HPMCP for t1/2 values calculated with dissociation rate constants in Triton X-100 micelles. (a) HPMBP and (b) HPMCP.
Figure 4. CETPck vs t1/2 for CPC separations of lanthanides with HPMBP and HPMCP for t1/2 values calculated with dissociation rate constants in the toluene-water phase pair.
the toluene-water phase pair and the ligands HPMBP and HPMCP is the first demonstration of a countercurrent separation with two bulk phases, where the efficiency is determined exclusively by interfacial kinetics, analogous to conventional LC separations. Many dissociation reactions in two-phase systems are too fast for spectrophotometric measurements using the automated membrane extraction system (AMES), necessitating the use of homogeneous media like micelles, which are good models for the twophase system, for the measurement of dissociation rate constants by rapid kinetic techniques like stopped-flow.8,11,12 Even though the dissociation rate constants (k-1K-2) for the HPMCP complexes in the toluene-water phase pair are smaller than those for the HPMBP complexes,19 the efficiencies for the former ligand are significantly larger than those for the latter ligand (Table 3). This is because CETPck is a function of not only the metal complex
Figure 5. CPC separation of Pr3+ and Eu3+ with HPMBP ) 0.08 M and Triton X-100 ) 0.001 M at I ) 0.1 and pH ) (a) 2.37, (b) 2.41, and (c) 2.46 (displaced by arbitrary A values).
dissociation rate constant but also the various equilibrium constants and the interfacial area generated. Effect of Triton X-100 on the CPC Efficiencies and Separations. To further understand the influence of the liquidliquid interface in CPC separations, the neutral surfactant Triton X-100 (TX) was added to the toluene phase and its effect on the separation efficiency and resolution examined. The separation of Pr3+ and Eu3+ using 0.08 M HPMBP with 0.001 M Triton X-100 added is shown in Figure 5 as an example. It is evident from Table 4 that the addition of Triton X-100 not only improves the chromatographic efficiencies but also changes the D values of the metal ions, indicating that it affects both interfacial kinetics and extraction equilibrium. To understand better the role of the added Triton X-100 in the CPC separations, these separations were conducted as a function of the concentrations of HL, H+, and TX, and the extraction equilibrium in eq 4 was deduced from the dependencies of log D of the metal ion as a function of the log of the concentrations. The equilibrium studies indicate that the T Kex
M3+ + 3HLo + TXo y\z ML3‚TXo + 3H+
(4)
complex ML3 forms a 1:1 adduct with Triton X-100 in the toluene phase. The corresponding extraction equilibrium constant KTex is given in eq 5, and the log KTex values are listed in Table 1 for HPMBP and HPMCP.
KTex )
[ML3‚TX]o [H+]3 [M3+]
[H+]3 1 1 ) D (5) [HL]o3 [TX]o [HL]o3 [TX]o
It is clear from eqs 4 and 5 that, due to adduct formation, the D values for the metal ions should be different in the presence of Triton X-100 from those in its absence, and this is evident from Table 4 and Figure 5. Interestingly, the presence of Triton X-100 does not significantly change the separation factor for a given pair of lanthanides. The log of the equilibrium constant for the formation of the Triton X-100 adduct of the ML3 complexes in the toluene phase, βAT, is equal to log KTex - log Kex, and these values are listed in Table 2. The βAT decreases slightly from the
light to the heavy lanthanides for both ligands HPMBP and HPMCP, in contrast to the stability constant βLL for the ML3 complexes, which exhibits a substantial increase. Adduct formation constants for lanthanide-acylpyrazolone complexes with ligands such as trioctylphosphine oxide and 1,10-phenanthroline also display similar variations between the light and heavy lanthanides.16,17 The variation in βAT values in toluene is different from the variation of the adduct formation constant βAM in Triton X-100 micelles, where the βAM decreases substantially from the light to the heavy lanthanides.19 Unlike βAM, βAT is insensitive to the ligand, being about the same for the HPMBP and HPMCP complexes. Triton X-100, however, has different effects in the separations with HPMBP and HPMCP, as is evident from Table 4, where the CETPck values for different lanthanides with HPMBP and HPMCP at a given [HL] and [H+] in the absence and presence of Triton X-100 are compared. Its presence improves efficiency and resolution in the case of separations with HPMBP but has the opposite effect in separations with HPMCP. Since adduct formation does not alter the dissociation mechanism of ML3 complexes in Triton X-100, we can expect this to be the case in the toluenewater phase when Triton X-100 is added to toluene to form the adduct.19 Equation 3 indicates that the changes in the CETPck values in the presence of Triton X-100 must stem from changes in as, KiDCd, and KDC. The specific interfacial area, as and KiDCd at the toluene-water interface for both HPMBP and HPMCP using the AMES apparatus were determined to increase by a factor of 2 in the presence of 0.001 M Triton X-100 (as at 5000 rpm, for PMBP- 335.23 cm-1, PMCP- 241.45 cm-1; KiDCd, for Eu(PMBP)3 0.00969 L/cm2, for Eu(PMCP)3 0.01176 L/cm2), and these changes would also occur in the CPC experiments. The CETPck values in Table 4 indicate that, accounting for increases in as and KiDCd in the presence of Triton X-100, the KDC values of the M(PMBP)3 and M(PMBP)3‚TX complexes are the same, but the KDC values of M(PMCP)3‚TX complexes are larger than the values for M(PMCP)3 by a factor of 5-10, depending on the lanthanide metal ion. For the complexes of HPMCP, the much larger increase in KDC in the presence of Triton X-100 than the increases in as and KiDCd is responsible for the efficiencies being poorer in the presence of Triton X-100 compared to those in its absence. The experiments in the presence of Triton X-100 again emphasize the critical role played by the liquid-liquid interface in CPC separations and the interplay of the kinetic and equilibrium parameters in determining the efficiencies of separations. CPC Separations in the Presence of Arsenazo III. In our studies of the kinetics of formation and dissociation of the ML3 complexes in the toluene-water phase pair using the metallochromic indicator Arsenazo III (AZ), we found that these interfacial reactions are catalyzed by the reaction of the MAZ complex adsorbed at the toluene-water interface with L-.19 We conducted CPC separations of the tervalent lanthanides with HPMBP and HPMCP in the presence of AZ in the aqueous mobile phase to determine if interfacial catalysis resulted in significant improvement in the efficiencies. In the presence of AZ, the M3+ is present as MAZ,19 and its extraction equilibrium was established by examining the dependence of its log D on pH, log [HL], and log [AZ], which yielded slopes of +1, +3, and -1, respectively. These observations can be rationalized on the basis of the extraction Analytical Chemistry, Vol. 69, No. 14, July 15, 1997
2839
Table 4. Comparison of D, N, and CETPck Values for HPMBP and HPMCP in the Absence and Presence of 0.001 M Triton X-100 and 2 × 10-5 M Arsenazo III [HPMBP] ) 0.08 M, pH ) 2.42 Triton X-100
none
Arsenazo III
metal
D
N
CETPck
D
N
CETPck
D
N
CETPck
Pr Eu Tb Yb
0.78a 3.54 12.88 137.81a
27.9 12.4 8.3 4.0
76.7 184.6 279.9 586.1
3.38 5.16
43.7 26.4
45.4 81.5
0.72a 3.24 10.38 45.94a
101.8 91.3 66.9 56.9
14.1 16.8 26.4 32.7
[HPMCP] ) 0.08 M, pH ) 3.25 Triton X-100
none metal Pr Eu Tb Yb a
D 0.30a 4.89 10.45 114.62a
N 122.1 37.0 35.7 9.5
CETPck 10.2 55.3 57.8 244.6
D 5.89 8.86
N 30.8 12.3
Arsenazo III CETPck 68.5 185.7
D
N
CETPck
0.17a
107.7 69.4 57.1 24.2
12.8 25.1 32.6 89.8
1.51 1.87 3.51a
Calculated for the reported conditions from the experimental log Kex values and N vs D, CETPck vs D correlations.
The higher efficiencies in the presence of AZ unfortunately do not lead to better separations, as the resolution of a pair of analytes is a direct function of efficiency, selectivity, and retention. While the efficiency is higher in the presence of AZ, both selectivity and retention are lower, and a combination of these factors leads to poor resolution. We can understand further the reason for the poor selectivity in the presence of AZ using eq 8, which relates the log of the stability constant, βMAZ, of the MAZ complex to the log of the extraction equilibrium constants Kex and KAZ ex values and the pK4 (3.40) and pK5 (6.27) of AZ.24 The log βMAZ values calculated using
log βMAZ ) log Kex - log KAZ ex + pK4 + pK5
Figure 6. CPC separation of Pr3+ and Yb3+ with HPMBP ) 0.06 M at pH ) 2.82, I ) 0.1, and [Arsenazo III] ) (a) 2.0 × 10-5, (b) 4.0 × 10-5, and (c) 6.0 × 10-5 M (displaced by arbitrary A values).
equilibrium in eq 6 with the corresponding extraction equilibrium constant KAZ ex given by eq 7. KAZ ex
M(H3AZ)2- + 3HLo y\z ML3,o + H+ + H5AZ3KAZ ex )
[ML3]o[H+][H5AZ3-] [M(H3AZ)2-][HL]o3
(6) (7)
An examination of the extraction equilibrium constants with HPMBP and HPMCP in Table 1 indicates that, for a lanthanide metal ion, KTex > KAZ ex > Kex. Table 1 also indicates that the separation factor for a given pair of lanthanides in the presence of AZ is much poorer than that in its absence or in the presence of Triton X-100. This poor selectivity in the presence of AZ only allows the partial separation of Pr3+ and Yb3+, as can be seen from Figure 6, where their separations with HPMBP at various concentrations of AZ are displayed. The efficiencies, as shown in Table 4, are much higher in the presence of AZ, as expected from the kinetic studies,19 compared to the efficiencies in its absence and when Triton X-100 is added to the toluene phase. 2840
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(8)
eq 8 from the extraction equilibrium constants for HPMBP and HPMCP are listed in Table 2. These log βMAZ values were also independently determined in our studies by Job’s method19 and found to be 8.1, 8.4, 8.8, and 9.3 for the lanthanides Pr3+, Eu3+, Tb3+, and Yb3+, respectively. It can be seen that the averages of the log βMAZ values determined from the extraction equilibrium constants with HPMBP and HPMCP are in excellent agreement with the values determined in our work19 and the recently reported literature values.25 This analysis also indicates that the smaller separation factors in the presence of Arsenazo III are due to the closeness in the βMAZ values (eq 8). A ligand such as AZ can simultaneously improve efficiency and selectivity if its stability constants for the different lanthanides are sufficiently different. Comparing the CETPck values (Table 4) in the presence and absence of AZ for HPMBP and HPMCP at a given [HL] and pH, it is seen that these values are substantially lowered in the presence of AZ. The factors responsible for the lower CETPck in the presence of AZ can be better understood upon examination of the pseudo-first-order dissociation rate constant in the presence of AZ (eq 9), where all the kinetic and equilibrium constants have been determined.19 A comparison of eqs 3 and 9 indicates that lowering of the CETPck in the presence of AZ can be expected if the AZ-catalyzed dissociation pathway is significant. Based on the various rate and equilibrium constants in eq 9,19 the catalytic (24) Budesinsky, B. Talanta 1969, 16, 1277-1288. (25) Rohwer, H.; Collier, N.; Hosten, E. Anal. Chim. Acta 1995, 314, 219-223.
kdobs ) kdi obsas ) K-2 KDR [H+] i (K d)a (9) Ka KDC [HL]o DC s
i KiAZ[H3AZ5-]) (ki-1 + k-AZ
pathway is significant for HPMCP and not significant for HPMBP due to the pH employed in these experiments. Since the CETPck is lowered for both HPMBP and HPMCP (Table 4), it is clear that, for both ligands, as must be higher in the presence of AZ. The as values calculated from the CETPck values and eq 9 are 2625.8 ( 963.0 cm-1 (mobile phase drop size, 12.9 ( 0.4 µm) for HPMBP and 225.5 ( 81.9 cm-1 (mobile phase drop size, 148.7 ( 4.4 µm) for HPMCP. Comparing these values with the values of as generated in the absence of AZ, we find that as increases by a factor of 14 for HPMBP and by a factor of 2 for HPMCP. The improvement in efficiency in the presence of AZ for HPMBP is entirely due to an increase in the specific interfacial area, while in the case of HPMCP it is due to the catalysis of the dissociation reaction by AZ as well as an increase in the specific interfacial area. The reasons for the large increase in as in the case of HPMBP are not clear, but this is similar to the observation we had previously made in the case of the nickel-dodecylsalicylaldoxime system.14 In CPC, where the mobile phase is moved through capillary ducts, the coadsorptions of the ligands and their lanthanide complexes along with AZ and MAZ could lead to much different as values for HPMBP and HPMCP, and this is under further invesitgation. CONCLUSIONS The studies described here are the first example of separation by a multistage countercurrent distribution where the efficiencies are mainly determined by interfacial processes like in conventional LC separations. CPC, where the separations are performed with two immiscible liquid phases, allows a clear identification of the various factors that influence the selectivity and efficiency, and hence the resolution, of chemically similar analytes. A significant demonstration of the current studies is the role of interfacial kinetics in determining chromatographic efficiencies and the identification of the kinetic and equilibrium parameters that affect them. The interfacial dissociation rate constant is a composite of the dissociation rate constant, the distribution constants of the ligand and metal complex between the bulk organic and bulk aqueous phases, the specific interfacial area, and the distribution constant of the metal complex between the bulk organic phase and the organic-aqueous interface. The interplay of all these factors needs to be considered in order to understand the correlation between chromatographic efficiency and chemical kinetics, especially for metal and ligand systems that are closely related. The role of the interface in the chromatographic separations has been further demonstrated by the addition of surfactants and by employing reagents that catalyze complex formation and dissociation reactions at the interface. The addition of surfactants does not always lead to improved efficiency, again due to the
interplay of the different factors that affect interfacial kinetics. Interfacial catalysis leads to much higher efficiencies compared to those of the uncatalyzed systems but could result in poorer selectivities if the reagent forms complexes with the analytes with similar stability constants. Adduct formation in the organic phase and the formation of intermediate complexes in the aqueous phase are mechanisms capable of influencing chromatographic selectivities and efficiencies and, as a result, the resolution of the analyte components. ACKNOWLEDGMENT This research was supported by a grant from the Chemistry Division of the National Science Foundation. GLOSSARY CPC
centrifugal partition chromatography
HPMBP
1-phenyl-3-methyl-4-benzoyl-5-pyrazolone
HPMCP
1-phenyl-3-methyl-4-capryloyl-5-pyrazolone
AZ
Arsenazo III
AMES
automated membrane extraction system
M3+
tervalent lanthanide metal ion
HL
HPMBP, HPMCP
L-
PMBP-, PMCP-
ML3
1:3 M3+:HL complex
Kex
extraction equilibrium constant, unitless
KTex
extraction equilibrium constant in the presence of Triton X-100, M-1
KAZ ex
extraction equilibrium constant in the presence of AZ, M-1
KDR
distribution constant of HL ) [HL] in the organic phase/[HL] in the aqueous phase
KDC
distribution constant of the metal complex ) [ML3] in the organic phase/[ML3] in the aqueous phase
KiDC
adsorption equilibrium constant of the metal complex ) [ML3] in the organic phase/[ML3] at the organicaqueous interface
Ka
acid disassociation constant, M
βLL
stability constant of ML3, M-3
βAT
adduct formation constant in the organic phase between ML3 and Triton X-100, M-1
βAZ
stability constant of MAZ complex, M-1
t1/2
half-life, s
at
total interfacial area, cm2
as
specific interfacial area ) at/total volume, cm-1
Received for review November 22, 1996. Accepted April 28, 1997.X AC961192C X
Abstract published in Advance ACS Abstracts, June 15, 1997.
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