Anal. Chem. 1986, 58,3233-3235
3233
Solvent Extraction of Lanthanides with a Crown Ether Carboxylic Acid Sir: Since Pedersen published his pioneering work on the preparation of crown ethers and their complexing properties, numerous papers have been written that describe the metal cation binding properties of various synthesized crown ethers as determined by solvent extraction methods ( I , 2). In the previously reported metal ion-crown ether systems, counteranions generally played an important role in determining the efficiency of the extraction. The extractibility of a metal ion-crown ether pair into an organic solvent is strongly influenced by the lipophilic nature of the counteranion. A counteranion with a large molar volume, such as picrate, dinitrophenolate, dipicrylamine, and tetraphenylborate, tends to make the extraction of a metal ion-crown ether pair more effective. An alternative approach is to eliminate the need to extract an aqueous-phase anion by use of a macrocyclic host to which is attached an appropriate negatively charged functional group. This approach is illustrated by recent reports on the extraction of alkali metal and alkaline earth metal ions from aqueous solution into chloroform by crown ethers bearing carboxylic acids as functional groups (3-5). It has been shown that the extraction of these metal ions by crown ether carboxylic acids is independent of counteranions, although the extraction efficiencies in these systems are insufficient for use in chemical analysis. In this paper, we wish to report the results of an investigation in which a crown ether carboxylic acid) was utilized to acid (sym-dibenzo-16-crown-5-oxyacetic effectively extract lanthanides from aqueous solution into an organic phase. In addition to high efficiencies, significant selectivities were noted in lanthanide extraction. The details of the extraction procedures and the results of our experiments are described in the following sections. EXPERIMENTAL SECTION Reagents and Instruments. The lanthanides either in the chloride or nitrate form were obtained from Alfa Products. Other chemicals including LiOH, NaOH, NaC1, HN03, H2S04,acetic acid, chloroform,heptanol, etc., were purchased from J. T. Baker. sym-Dibenzo-16-crown-5-oxyacetic acid was synthesized in our laboratory according to the literature procedure (6). Deionized water was prepared by passing distilled water through an ionexchange column (Barnstead ultrapure water purification cartridge) and a 0 . 2 - ~ mfilter assembly (Pall Corp., Ultipor DFA). All containers used in the experiments were acid washed, rinsed with deionized water, and dried in a class 100 clean hood. Rare-earth radioisotopes were used as tracers in the extraction experiments. The radioisotopes were produced in a 1-MW TRIGA nuclear reactor located 8 miles away from our campus. The half-lives of the isotopes produced and the y radiations used for their identification are given as follows: 140La(40.2 h, 487 keV), 142Pr(19.2 h, 1576 keV), 153Sm(46.8 h, 103 keV), 1 5 2 m E(9.3 ~ h, 122 keV), '60Tb (72 days, 299 keV), 171Er (7.5 h, 308 keV), 175Yb (4.2 d, 283 keV), and 177Lu(6.7 d, 208 keV). A large-volume ORTEC Ge(Li) detector with a resolution of 2.3 keV at the 1332-keV y from 6oCowas used for y counting. The details of neutron irradiation and y spectrometry are described elsewhere (7).
Measurements of pH were made with an Orion Research Model 701A pH meter and an Orion 91-03 semimicro glass electrode. Absorption spectra of the crown ether carboxylic acid were measured with a Varian Cary 2200 ultraviolet-visible spectrometer. NMR and IR spectra were obtained with a 200-MHz Nicolet NT-200WB spectrometer and a Bio-Rad FTS-80 spectrometer, respectively. Extraction Procedures. Extraction experiments were normally carried out with 10-mL aliquots of a lanthanide solution spiked with an appropriate amount of radioisotope in 50-mL ground-glass stoppered Erlenmeyer flasks. The pH of each solution was adjusted to a desired value using LiOH and HNO,.
The extraction solution was prepared by dissolving a weighed amount of the crown ether carboxylic acid in several hundred milliliters of a chloroform-heptanol mixture in a beaker with magnetic stirring. After the crown ether carboxylic acid was completely dissolved, the organic phase was shaken with a pH 2 HC1 solution to remove potential impurities. Since the pK, value of sym-dibenzo-16-crown-5-oxyacetic acid is 4.6 (3),the acid has low solubility in water at pH 2. After washing, the organic phase was kept in contact with a small amount of water to which LiOH was added to maintain a neutral solution (pH 7 ) . To each aqueous lanthanide ion sample, 10 mL of this extraction solution was added. The mixtures were shaken vigorously on a mechanical wrist-actionshaker (Burrell Model 75) for several minutes at room temperature. After shaking, the mixtures were allowed to stand for a period of time for phase separation. Five milliliters each of the organic and the aqueous phase were then removed from the flask and placed in 10-mL glass vials with fast-turn caps for y counting. Normally, a total of 103-104 counts were collected from each measurement. The statistical errors associated with the radioactivity measurements were 1-3%. The final equilibrium pH of the aqueous phase was also measured with the semimicro glass electrode.
RESULTS AND DISCUSSION In our extraction system, the chelating agent was dissolved in an organic phase (chloroform-heptanol mixture), which was in contact with an aqueous solution of the lanthanides. The acid in water solubility of syrn-dibenzo-16-crown-5-oxyacetic in this liquid-liquid extraction system a t different pH values was measured by its ultraviolet absorption. The crown ether carboxylic acid exhibits a strong absorption with a maximum centered at 273-274 nm. The absorbance a t 273-274 nm in the water layer increases rapidly above pH 4.5. The concentration of the crown ether carboxylate in water a t pH 6 is 5.8 X M according to our measurements. In these experiments, the concentration of the spiked Lu3+ in the M, and the concentration of the aqueous phase was 1 X chelating agent in the organic phase was 3 X M. The ultraviolet absorption curve of syrn-dibenzo-15-crown-6-oxyacetic acid corresponds well with the efficiency of extraction of 177Luas a function of pH from aqueous phase into a mixture of 8020 chloroform-heptanol (v/v) containing the extracting agent (Figure 1). Thus, the extraction of Lu3+into the organic phase becomes significant at about pH 4.5 and increases very rapidly above this pH value. The extraction of 177Luis virtually quantitative in the pH range 6-7. The maximum efficiency (over 98% extraction) appears to occur around pH 6.5. The kinetics of the extraction is fast, requiring only several minutes of shaking to reach the observed efficiency. The reaction appears reversible; i.e., at lower pH values (e.g.,
