Anal. Chem. 1988,60,2527-2531 (35) Nakagawa, T. Nonionic Surfactants; Shick, M. J., Ed.; Dekker: New York. 1966. (36) Pelizzetti, E.; Pramauro. E. J . Phys. Chem. 1984, 88, 990. (37) Pramauro, E.; Saini, G.; Pelizzetti, E. Anal. Chlm. Acta 1984, 166, 233. (38) Bunton, C. A.; Sepulveda, L. J . Phys. Chem. 1979, 8 3 , 680. (39) Sepulveda, L.; Lissi, E.; Quina, F. Adv. ColloM Interface Sci. 1986, 25, 1-57 and references therein. (40) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biologlcal Membranes; Wiley: New York, 1980. (41) CRC Handbook of Chemistryand Physics, 59th ed. Weast, R. C., Ed.; CRC: Boca Raton, FL 1978.
2527
(42) The Merck Index, 9th ed. Windholz, M., Ed.; Merck: Rahway, NJ, 1976. (43) Sutton, C.; Calder, J. A. J . Chem. Eng. Data 1975, 2 0 , 320. (44) Chan, A. F.; Evans, D. F.; Cussler, E. L. AIChEJ. 1976, 22, 1006. (45) Shaeiwitz, J. A. Chem. Eng. Commun. 1987, 55, 225.
RECEIVED for review April 11, 1988. Accepted August 1, 1988. The authors thank the R. J. Tobacco cO.$ Winston-Salem, NC, for their generous support of this research.
Solvent Extraction of Lanthanoid Picrates with Crown Ethers: Preferential Sandwich Complexation and Unique Cation Selectivities Kazuharu Nakagawa and Shouhichi Okada Leather Research Institute of Hyogo Prefecture, 3 Higashikawara, Nozato, Himeji, Hyogo 670, Japan Yoshihisa Inoue* and Akira Tai Basic Research Laboratory, Himeji Institute of Technology, 2167 Shosha, Himeji, Hyogo 671-22, Japan Tadao Hakushi Department of Applied Chemistry, Himeji Institute of Technology, 2167 Shosha, Himeji, Hyogo 671 -22, Japan
Quantltatlve solvent extractions of aqueous lanthanoid plcrates with 15-crown-5 and 18crown-6 were conducted at low ionic strength In the absence of background salts. An overwheiming preference for the sandwlch complexation and unique cation selectivities were observed. The peak extraction constants were found for samarium with 15-crown-5 (1:2 stoichiometry) and for cerlum and praseodymium with 18crown-6 (1:l and 1:2 stolchiometrles, respectively). The facile sandwich complexation and unique cation seiectivlties are interpreted in terms of the heavy hydration of ianthanoid ions of high charge density.
Possessing close similarity in the chemical properties, the elements of the lanthanoid family are difficult in general to separate from each other (1). The crown ethers and the related macro(bi)cyclic ligands are known to recognize fairly strictly the size of the guest cation accommodated in the cavity (2), and are expected to discriminate the lanthanoid cations through the minimal difference in cation size owing to the lanthanoid contraction. Besides the earlier studies on the isolated lanthanoid complexes of the macrocyclic ligands (3), works on solvent extraction (4-14), as well as homogeneousphase complexation (15-20), have recently been conducted under a wide variety of conditions including varied solvents, ligands, counteranions, and ionic strengths. Some solvent extraction studies on the lanthanoids show apparent contradictions about the complex stoichiometry and the cation-selectivity sequence. In all preceding experiments, the lanthanoid picrates were prepared in situ and the aqueous phase contained a large excess of lithium salt as a background salt maintaining constant ionic strength. It has been demonstrated however that not only the concentration but also the counteranion of the background salt affect significantly 0003-2700/88/0360-2527$0 1.50/0
the extractability of aqueous metal picrates (21, 22). Furthermore our recent investigation (23) reveals that simple crown ethers extract aqueous lithium picrate in a comparable order of magnitude as lanthanoid picrates. In order to investigate the solvent extraction of lanthanoid picrates unaffected by the dense background salt, we first synthesized and isolated a series of lanthanoid picrates as crystal (24). In the present paper, we report our results of a quantitative solvent extraction study that uses simple crown ethers and aqueous lanthanoid picrates of low ionic strength and shows unique extraction behavior different from the previous reports.
EXPERIMENTAL SECTION Reagents and Instruments. Commercially available picric acid (Nakarai), lanthanoid carbonates and oxides (Mitsuwa, Nakarai, Wako, or Rare Metallic Co.), 15-crown-5(Nisso),and 18-crown-6 (Nisso) were used without further purification. The lanthanoid picrates of La-Gd (undecahydrate) and of Dy and Yb (octahydrate)were prepared as reported previously (24). Similar procedures gave terbium and holmium picrate octahydrates in 84% and 73% yield, respectively. Tb(Pic),-8HzO:decomposition point, 295 "C (explosion point (ep) 351 "C). Anal. Calcd for TbCl8HZ2N9Oz9: C, 21.90; H, 2.25; N, 12.77. Found: C, 22.24; H, 2.16; N, 12.76. Ho(Pic),: decomposition point, 291 "C (ep 357 "C). Anal. Calcd for C, 21.76; H, 2.23; N, 12.69. Found: C, 22.34; H, 1.99; N, 13.67. Deionized water and distilled dichloromethane were used throughout the study. Electronic spectra were recorded on a Hitachi 228 spectrophotometer. Inductively coupled plasma (ICP) atomic emission analyses were performed on a Shimadzu GVM lOOP instrument, which was calibrated for each lanthanoid as reported (24). Proton NMR spectra were recorded on a Jeol JNM-GX400 instrument in dichloromethane-d, (Merck, 99.5% deuteriated) solution containing 2 % chloroform added as an internal standard for field calibration and intensity normalization. Extraction. The general procedures were analogous to those employed in the previous papers (25, 26). The solvents, dichloromethane and water, were saturated with each other prior 0 1988 American Chemical Society
2528
ANALYTICAL CHEMISTRY, VOL. 60, NO. 22, NOVEMBER 15, 1988
Table I. Solvent Extraction of Aqueous Lanthanoid Picrates" ligand
concn, M
15-crown-5
0.003 0.13 0.26
18-crown-6
0.003 0.075
0.15
anal. methodb La3+
Ce3+
Pr3+
Nd3+
% extractability* Sm3+ E u ~ + Gd3+
Tb3+
Dy3+
Ho3+
ybS+
0.3 24.3 lgd 44.2 3ad 1.85 27.9 26.5 44.4 41.0
0.3 27.4 26.5 46.4 47.1 1.90 29.3 30.2 45.8 47.7
0.3 30.1 28.9 49.2 49.3 1.51 29.7 28.5 46.8 46.8
0.5 30.9 29.4 50.2 49.5 1.10 29.2 28.3 47.3 46.5
0.3 33.3 32.9 52.1 52.3 0.63 28.7 27.8 46.9 47.4
1.5 34.8 21.4 50.9 39.0 12.2 29.9 15.5 45.6 32.8
1.1 27.6 18.6 43.9 37.2 8.4 23.5 13.9 38.1 30.8
c 42.3 10.6 53.3 23.8 c 41.3 6.3 50.3 18.6
2.2 57.6 5.6 67.2 15.4 10.0 60.6 3.5 65.5 13.0
UV UV ICP UV ICP
uv
UV ICP Uv ICP
0.3 32.5 31.3 51.5 51.0 0.57 27.2 26.0 45.4 44.1
0.6 26.8 26.6 45.6 46.1 0.66 22.5 22.0 40.0 39.7
*
"Temperature 25.0 0.2 "C; aqueous phase (10 mL): [picrate] = 0.003 M; organic phase (CH2C12,10 mL). *Extractabilities were dethis partermined by spectrophotometric means (UV) or by inductively coupled plasma emission (ICP) analysis. 'Not determined. ticular run. a Door calibration curve for La3+gave a less-reliable value in ICP analvsis. to use in order to prevent the volume change of both phases during extraction. Equal volumes (10 mL) of a dichloromethanesolution of the respective crown ether (0.008-0.260 M for 15-crown-5 and 0.001-0.150 M for 18-crown-6) and of an aqueous lanthanoid picrate (0.003 M) were introduced into an Erlenmeyer flask, which was stoppered and then shaken vigorously for 40 min in a Toyo incubator thermostated at 25.0 f 0.2 "C. This period of shaking was long enough to establish equilibrium between the two phases, since the shaking periods of 10, 20, and 40 min gave practically identical results. The mixture was then allowed to stand for at least 2 h at that temperature in order to complete phase separation. Three milliliters of the dichloromethane phase was withdrawn and evaporated to dryness in vacuo. After appropriate dilution of the resultant with water, the picrate concentration was determined from its absorption maximum at 354 nm. The molar extinction coefficients at 354 nm of light lanthanoid picrate undecahydrates (La-Gd) and heavy lanthanoid octahydrates (TbYb) are as follows: La, 42 100 M-' cm-'; Ce, 42 200; Pr, 41 600; Nd, 42 100; Sm 42 100,Eu 40900; Gd, 42 100, Tb, 42 100,Dy, 42 700; Ho, 43 200; Yb, 43 000. In control experiments, no detectable amount of any picrate was extracted into the organic phase in the absence of crown ether; extractability was well less than 0.1%.
RESULTS AND DISCUSSION Extracted Species. For comparison purpose, the solvent extractions of aqueous lanthanoid picrates with 15-crown-5 and 18-crown-6 were carried out under our standardized condition for evaluation ([ligandIi = [picrateIi = 0.003 M), where some alkali, alkaline-earth, and heavy-metal picrates show moderate extractabilities for common ligands (27,281. Conventionally, the concentration of picrate, not metal, ion extracted into the organic phase was measured spectrophotometrically to give the percent extractability for each lanthanoid picrate. The results are listed in Table I; see the first line of each ligand at 0.003 M concentration. From these UV data, one might presume that the heavy lanthanoid picrates (Tb-Yb) are extracted fairly feasibly by the simple crown ethers, whereas the light lanthanoid picrates (La-Gd) are extracted only negligibly under the conditions employed. However this dramatic difference in the extractabilities for light and heavy lanthanoids turned out not to be true, as the comparative UV and ICP analyses of the organic phase from the solvent extraction a t higher ligand concentrations revealed that the real concentrations of heavy lanthanoids extracted are much lower than those calculated from the absorbance of picrates extracted; see Table I. Since there is no background salt in the aqueous phase, the extra picrate detected photometrically in the organic phase must be picric acid. However, in the absence of the crown ether, no extraction of the heavy lanthanoid picrate was observed, and therefore the otherwise-negligible hydrolysis reaction of the heavy lanthanoid picrates is facilitated by the preferential extraction of the resulting picric acid (PicH) by the crown ether (CE). The
picric acid extracted may form a monohydrate of oxonium crown ether and/or a crown ether complex of hydronium picrate (29). Yb(Pic),
+ xH,O
F!
Yb(Pic),-,(OH),
CEOW
+ xPicH r [H(CE)PicI.,,
I t is concluded therefore that the methodology of solvent extraction followed by the photometric analysis of the counteranion picrate is still applicable to the light lanthanoids (La-Gd) but is invalid for the heavy lanthanoids beyond gadolinium, for which the ICP analysis should be employed. Stoichiometry. The overall extraction equilibrium between aqueous lanthanoid picrate (LnA,) and n molecules of crown ether (CE) in the organic phase is expressed as Ln3+aq+ 3A-,,
+ nCEorg e [Ln(CE),A310rg
The extraction equilibrium constant (p,) is given by Pn
=
DLn
[A-Iaq3[CEIorc
(1)
where the distribution ratio of lanthanoid is [Ln(CE)nA31org DLn =
[Ln3+Iaq
(2)
Modification of eq 1 leads to the following equation:
In order to examine complex stoichiometry in detail and determine equilibrium constants (p,) accurately, extraction experiments were carried out over a wide range of ligand concentrations from 0.008 to 0.26 M for 15-crown-5 and from 0.001 to 0.15 M for l&crown-6 with fixed picrate concentration a t 0.003 M. Even a t the lowest ligand concentrations, the extractabilities, though less than 1% in some cases, were reproducible. The results were analyzed according to eq 3. With each picrate, log (DLn/[A-Iaq:)values were plotted as a function of log [CE],, to give a single straight line of slope 2 for 15-crown-5 but a bent line with slopes 1 and 2 for 18crown-6. As exemplified with lanthanum picrate in Figure 1, 15-crown-5 forms a 1:2 sandwich complex almost over the entire ligand concentration range employed, whereas 18crown-6 affords a 1:l complex a t low concentrations but 1:2 a t high concentrations. The above complex stoichiometries coincide mostly but conflict in part with those reported earlier (4-6,9, 14). This discrepancy would be ascribed to the different ligand concentration, solvent, counteranion, and/or ionic strength em-
ANALYTICAL CHEMISTRY, VOL.
60, NO. 22, NOVEMBER 15, 1988 2529
Table 11. Extraction Equilibrium Constants (8, and 8,) for 1:1 and 1:2 Complexation of Lanthanoid Picrates with Crown Ethersa
15-crown-5 18-crown-6
2
8.03 7.05 8.66 1.61
1 2
1%
(P2IP1)
8.11 7.07 8.71 1.64
8.18 6.95 8.73 1.78
8.19 6.79 8.70 1.91
8.27 6.57 8.68 2.11
8.22 6.52 8.63
8.09 6.55 8.43 1.88
2.11
7.91'
7.8OC
7.35c
6.97'
d
d
d
d
8.16'
8.06'
7.58'
7.28'
Dichloromethane-water system at 25.0 O C . *Determined by spectrophotometry, unless stated otherwise. Determined by ICP analysis. Not determined due to poor extractability. Table 111. Relative Cation Selectivity as Measured by K,, Ratios for 1:2 Sandwich Complexation by 15-Crown-5
Ln La Ce Pr Nd Sm Eu Gd Tb Dy Ho
Yb
Ln/La 1.20 1.41 1.45 1.74 1.55 1.15 0.76 0.59 0.21 0.09
Ln/Ce
Ln/Pr
Ln/Nd
Ln/Sm
Ln/Eu
Ln/Gd
Ln/Tb
Ln/Dy
Ln/Ho
Ln/Yb
0.83
0.71 0.85
0.69 0.83 0.98
0.58 0.69 0.81 0.83
0.65 0.78 0.91 0.93 1.12
0.87 1.05 1.23 1.26 1.51 1.35
1.32 1.58 1.86 1.91 2.29 2.04 1.51
1.70 2.04 2.40 2.45 2.95 2.63 1.95 1.29
4.79 5.75 6.76 6.92 8.32 7.41 5.50 3.63 2.82
11.5 13.8 16.2 16.6 20.0 17.8 13.2 8.71 6.76 2.40
1.17 1.20 1.45 1.29 0.95 0.63 0.49 0.17 0.07
1.02 1.23 1.10 0.81 0.54 0.42 0.15 0.06
1.20 1.07 0.79 0.52 0.41 0.14 0.06
0.89 0.66 0.44 0.34 0.12 0.05
0.74 0.49 0.38 0.13 0.06
0.66 0.51 0.18 0.08
0.78 0.28 0.11
La CePr
0.35 0.15
I
0.42
Sm JGdl Dy 1
9
C
e ol
0
8-
7-
la
0,'
-3
-2 logCCE1,,g
-1
Extraction of aqueous lanthanum picrate (3 mM) with 15crown-5 (8-260 mM) (0)and 18-crown4 (1-150 mM) (0) in dichloromethane.
Figure 1.
ployed. However the present results do not immediately exclude the potential formation. of the 1:l complex with 15crown-5, which is rather implied by the curvature at the lower end of the straight line shown in Figure 1. We infer that 15-crown-5 could also form 1:l complex as demonstrated in the homogeneous phase (19),but its lipophilicity is insufficient to be extracted into the organic solvent in an appreciable amount. For this reason, the contribution of lipophilic 1:2 sandwich complexes is likely to be exaggerated in general in the solvent extraction as compared with the homogeneous phase complexation. Cation Selectivity. The log /I, values are given as the intercepts of the log [CE], - log (Dh/[A-lq3) plots, like those in Figure 1,and are listed rn Table I1 along with the logarithm of the equilibrium constant (/12//11), or K,, for the ligation of the second crown ether. Although the variation of extraction equilibrium constant is relatively small throughout the lan-
1 .o
1 .l
1/r(A-') Flgure 2. Extraction equilibrium constants (0, and /I2)for 1:l and/or 1:2 complexation of lanthanoid picrates with 15-crown-5(0)and 18crown-6 (A and 0). Table IV. Relative Cation Selectivity as Measured by K,, Ratios for 1:l Complexation by 18-Crown-6
Ln Ln/La Ln/Ce Ln/Pr Ln/Nd Ln/Sm Ln/Eu Ln/Gd La Ce
Pr Nd Sm Eu Gd
0.95 1.05 0.79 0.55 0.33 0.30 0.32
0.76 0.52 0.32 0.28 0.30
1.26 1.32 0.69 0.42 0.37 0.40
1.82 1.91 1.45 0.60 0.54 0.58
3.02 3.16 2.40 1.66 0.89 0.95
3.39 3.55 2.69 1.86 1.12
3.16 3.31 2.51 1.74 1.05 0.93
1.07
thanoid series, the cation selectivity differs distinctly for the ligand employed and the stoichiometry applied, as shown in Figure 2. Thus, 15-crown-5with 1:2 stoichiometry exhibits the highest extractability for samarium, while 18-crown-6 exhibits the highest extractability for praseodymium with 1:2 stoichiometry and for cerium with 1:l stoichiometry. The mutual cation selectivities for each lanthanoids are listed in Tables 111-V.
2530
ANALYTICAL CHEMISTRY, VOL. 60, NO. 22, NOVEMBER 15, 1988
Table V. Relative Cation Selectivity as Measured by K , , Ratios for 1:2 Sandwich Complexation by 18-Crown-6 Ln
La Ce
Pr Nd Sm Eu Gd
Tb DY
Ho Yb
Ln/La 1.12 1.17 1.10 1.05 0.93 0.59 0.32 0.25 0.08 0.04
Ln/Ce
Ln/Pr
Ln/Nd
Ln/Sm
Ln/Eu
Ln/Gd
Ln/Tb
Ln/Dy
Ln/Ho
Ln/Yb
0.89
0.85 0.95
0.91 1.02 1.07
0.95 1.07 1.12 1.05
1.07 1.20 1.26 1.17 1.12
1.70 1.91 2.00 1.86 1.78 1.58
3.16 3.55 3.72 3.47 3.31 2.95 1.86
3.98 4.47 4.68 4.37 4.17 3.72 2.34 1.26
12.0 13.5 14.1 13.2 12.6 11.2 7.08 3.80 3.02
24.0 26.9 28.2 26.3 25.1 22.4 14.1 7.59 6.03 2.00
1.05 0.98 0.93 0.83 0.52 0.28 0.22 0.07 0.04
0.93 0.89 0.79 0.50 0.27 0.21 0.07 0.04
0.95 0.85 0.54 0.29 0.23 0.08 0.04
0.89 0.56 0.30 0.24 0.08 0.04
Table VI. Cavity Size of Crown Ethers and Diameter and Hydration Energy (-AGho) of Cations
ligand 15-crown-5 18-crown-6
cavity ionic diameter, A - A ~ h O , d diameter," A cation crystalb hydratedC kcal/mol 1.7 2.6
Na+ K+ Rb+ CS'
Ag'
T1+ Mg2+
Ca2+ Sr2+
Ba2+ La3' Ce3+
Pr3' Nd3+ Sm3+ Eu3+ Gd3+ Tb3+ Dy3' Ho3+ Yb3+
2.04 2.76 3.04 3.34 2.30 3.00 1.44 2.00 2.36 2.70 2.06 2.02 1.98 1.97 1.92 1.89 1.88 1.85 1.82 1.80 1.73
7.16 6.62 6.58 6.58 6.82 6.60 8.56 8.24 8.24 8.08 9.04 9.04
90 73 67 62 105 74 439 362 331 301 754 767 777 786 796 805 808 814 820 831 855
"Estimated by CPK molecular models. bReference 31; coordination number = 6. cReference 32. dReference 30. The conventional cation-cavity size relationship, though claimed valid for the homogeneous-phase complexation of lanthanoid ions (19) and for the solvent extraction of monovalent cations (2,25,27),obviously fails to explain the peculiar cation selectivities observed in the present study. In the solvent extraction of alkaline-earth metal picrates reported previously (28),hydration of cation plays the major role; the relative cation selectivities and extractabilities of divalent cations are determined not by the size relationship but by the hydration energy of the cation extracted. Thus barium ion of the lowest hydration energy in the series is favored over any other alkaline earths, irrespective of the cavity size of the ligand (28). In this context, the extractability sequence for the lanthanoid series would decline monotonically with increasing atomic number, but this is the case only with the heavier lanthanoids. As listed in Table VI, the lanthanoid ions possess much higher free energy of hydration than the mono- and divalent ions; -AGho values are 60-90 and 300-450 kcal/mol for common alkalis and alkaline earths, respectively, but leap up to 750-850 kcal/mol for the trivalent lanthanoid ions (30). This high energy of hydration may assist stronger ion-dipole interaction with the donor oxygens of ligand especially in the homogeneous-phase complexation. However, the extraction of aqueous lanthanoid picrates is highly discouraged by the heavy hydration. The overwhelming preference for the sandwich complexation and the unique cation selectivities observed may be reasonably understood, provided that the crown ether extracts and lanthanoid ion with the
0.63 0.34 0.27 0.09 0.04
0.54 0.43 0.14 0.07
0.79 0.26 0.13
0.33 0.17
0.50
Table VII. Chemical Shifts and Intensities of Crown Ether and Water Protons in Organic Phase Separated from the Equilibrium Mixture between Aqueous and Dichloromethane-d2Phases in the Absence/Presence of Lanthanum Picrate and 18-Crown-6, Respectively
additive chemical shift 6 (re1 int)a _______ entry
phase
1 2 3
none
4
LaPicb
LaPicb none
phase none none 18-crown-6c 18-crown-6c
-OCH2CH2-
H,O
3.52 (22.83) 3.52 (24.29)
1.49 (0.74) 1.49 (0.79) 1.97 (3.06) 1.98 (3.33)
a Intensity normalized by the integrated area of 2% chloroform added as an internal standard. [LaPic] = 0.003 M. [18-Crown-
61 = 0.060 M.
remaining water of hydration bound tightly in the first solvation shell. The effective diameter of each lanthanoid ion extracted will be greater than the crystal ionic diameter (31) but smaller than the diameter of the fully hydrated ions (32). The participation of water of hydration in the lanthanoid picrate complex extracted was studied by NMR spectroscopy. The solvent extractions were carried out with and without aqueous lanthanum picrate (3 mM) and 18-crown-6 (60 mM) in dichloromethane-d2 under comparable conditions, and the organic extract separated was analyzed by IH NMR. The chemical shifts and the integrated area of oxyethylene and water protons are listed in Table VII. In the absence of the salt and ligand (entry l),the signal of water dissolved in the organic phase appeared at 6 1.49 with the normalized intensity of 0.74. The addition of lanthanum picrate to the aqueous phase raised slightly the water content without changing the chemical shift (entry 2). By contrast, the addition of crown ether (entry 3) led to a marked downfield shift of water peak by 0.48 ppm, as well as the increased water content up to 2.95 in the organic phase. The downfield shift indicates that all the water molecules in the organic phase are interacting with the donor oxygens of crown ether. The relative proton intensity between oxyethylene and water is calculated as 24.0:3.22, which may suggest 2:3 complex formation between 18-crown-6 and water. Under the actual solvent extraction conditions (entry 4), where the extractability was measured as 24.0%, the water content is further raised to 3.33 (8.5% increment), while the intensity of crown ether is increased to a smaller extent (6.4% increment) probably due to the salting-out effect by lanthanum picrate. Then the intensity ratio increases from 24.0:3.22 to 24.0:3.29. The difference may be attributed to the water brought into the organic phase through the extraction of hydrated lanthanum picrate. With the extractability taken into account (24.0%), the number of hydration waters brought into the organic phase through extraction of lanthanum picrate is calculated as ca. 3. It is interesting to note that this figure coincides incidentally with the number of waters of hydration that is not removed by mild
Anal. Chem. 1988, 60, 2531-2534
drying (Si0-J or heating (
