Anal. Chern. 1981, 53, 2247-2250
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Extraction of Alkaline Earth Cations from Aqueous Solutions by Crown Ether Carboxylic Acids Jerzy Strzelbicki' and Richard A. Bartsch" Depafimont of Chemisttyn Texas Tech University, Lubbock, Texas 79409
Solvent extraction of Mg, Ca, Sr, and Ba cations from aqueous solutions by sym-dibenro-16-crown-5-oxyacetlc acid (1 ), sym-dlbenro-13-crown-4-oxyacetlc acld (2), and sym-dibenro-19-crown-6-oxyacetlc acld (3) In chloroform has been Investigated. Influences of pH and metal Ion concentration In the aqueous phase upon the concentrations of metal(s) and complexing agent In the organlc phase are assessed for slngle Ion (wlth 1) arid competltlve (wlth 1, 2, or 3) extractlons. Marked differences In the efficlency and selectivity order for single Ion and competitive extractions uslng 1 are noted. Although chloroform phase extraction complexes of MA, are lndlcated for each crown ether carboxylic acld, 1 surpasses 2 and 3 In extractlon efficiency and selectlvlty. The crown ether carboxylk aclds 1, 2, and 3 are superior extractlon agents to closely relaled neutral crown1 ethers and to phenoxyacetlc acld,,
Recently, we reported the use of the crown ether carboxylic acid 1 for extraction of alkali metals frorn aqueous media into chloroform ( I ) The crown ether carboxylic acid exhibits extraction efficiency and selectivity which surpass those of the closely related neutral crown ether 4 or of phenoxyacetic acid.
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I t was also demonstrated that metal extraction did not involve the transfer of the aqueous phase anion into the organic medium. This latter factor is of immense importance to potential practical applications of crown ethers for metal extraction from aqueous media. For process solvent extraction, the anions normally encountered are chloride, nitrate, and sulfate. Distribution coefficients for metal chlorides, nitrates, and sulfates between an aqueous phase and a hydrocarbon or chlorocarbon phase which contains nonionizable crown ethers are too low to be useful (2-5). This problem is circumvented for complexing agents such as 1 in which the crown ether bears pendant ionizable functionality. The previous investigation involved only alkali metals and one crown ether carboxylic acid. We now describe a study of alkaline earth metal extraction from water into chloroform which utilizes the three crown ether carboxylic acids 1-3 and includes for Mg, Ca, Sr, and Ba: (a) single ion extractions by 1 as a function of the p I l and metal ion concentration of the aqueous phase; (b) competitive extractions by 1-3 as a function Present address: Institute of Inorganic Chemistry and Metallurgy of Rare Elements, Technical University of Wroclaw, 50-370 Wroclaw, Poland.
of the pH and metal ion concentration of the aqueous phase; and (c) competitive extractions by the neutral crown ether 4 and by phenoxyacetic acid. EXPERIMENTAL SECTION Apparatus. The apparatus was the same as in the previous study ( I ) . Reagents. Synthetic procedures for the preparation of symdibenzo-16-crown-5-oxyacetic acid (l),sym-dibenzo-13-crown-4acid (3), and oxyacetic acid (2), sym-dibenzo-19-crown-6-oxyacetic sym-dibenzo-16-crown-5methyl ether (4) are reported elsewhere (6). Sources of other chemicals include: Aldrich (Milwaukee, WI), phenoxyacetic acid; Baker (Phillipsburg, NJ), MgC12,SrC1,; Fisher (Fair Lawn, NJ), LiOH, BaCl,; MCB (Cincinnati, OH), CaC12. Water and chloroform w6re prepared as previously described ( I ) . Procedure. A chloroform solution (5.0 mL) of the complexing agent and 5.0 mL of an aqueous solution of alkaline earth chloride and LiOH were shaken for 15 min in a 30-mL separatory funnel at room temperature (2&23 "C). Concentrated LiOH solution (5 M) was used for pH regulation because of the low solubilities of alkaline earth hydroxides in water and the low extractability of Li from aqueous solutions into chloroform by crown ether carboxylic acids 1-3 (I, 7). The 5.0 mL phases were separated and the equilibrium pH of the aqueous phase was measured. A small sample (0.025 mL) of the organic phase was removed and diluted with chloroform in a 10-mL volumetric flask. Measurement of the absorption at 273-274 nm determined the concentration of all forms of the complexing agent in the chloroform layer. Compounds 1-3 in chloroform exhibit maxima at 273-274 nm with 6 = 5100,3600,4170, respectively. Neither the position nor intensity of the maxima varied significantlywhen the crown ether carboxylic acids were converted into their carboxylate forms. The remainder of the organic phase was shaken with 5.0 mL of 0.1 N HC1 for 20 min to strip the metals from the organic phase into aqueous solution for analysis by ion chromatography ( I ) . RESULTS AND DISCUSSION Single Ion Extractions of Mg, CaySr, and Ba by symDibenzo-16-crown-5-oxyacetic acid (1). The influence of the aqueous phase equilibrium pH upon the extractability of individual alkaline earth metal species was probed by using initial aqueous phase metal chloride concentrations of 0.060 N (2 equiv/mol) and an initial concentration of 1 in the chloroform phase equal to 0.050 N. Data for the resultant concentrations of metal and all forms (both carboxylate and carboxylic acid) of the complexing agent in the chloroform phase as a function of the equilibrium pH are correlated in Figure 1. The curve shapes for metal concentration in the chloroform phase vs. the equilibrium pH of the aqueous phase (open symbols, Figure 1) are commonly encountered in extraction processes involving carboxylic acids (8). Thus at low pH ( Ba > Sr > Mg in basic pH regions. Low selectivity is indicated since the organic phase Ca concentration is only 1.6 times that of Mg above pH 8.
0003-2700/81/0353-2247$01.25/0 0 1981 American Chemical Society
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* ANALYTICAL CHEMISTRY, VOL. 53, NO.
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PH Flgure 1. Normal concentrations of metal (X10') and complexing agent (X102)in the chloroform phase vs. the pH of the aqueous phase for single ion extractions of 0.060 N alkaline earth cations by 0.050 N 1: 0 = Ba, 0 = Sr, A = Ca, 0 = Mg; filled symbols = complexlng agent for cation system by symbol shape. The curves for the concentration of complexing agent in the organic phase vs. the equilibrium pH of the aqueous phase (filled symbols, Figure 1)are markedly different for Ca, Sr, and Ba on the one hand and Mg on the other. For the former groups, 85% of the initial complexing agent remains in the organic phase even at high pX. However with Mg, only 45% of the complexing agent is found in the chloroform layer after extractions at pH 8-10. This more nearly resembles the behavior of l in analogous extractions of alkali cations where only 20-30% of the initial complexing agent remains in the chloroform phase when the pH of the aqueous phase is 8-12 (1).
For Ca and Mg, the normal concentrations of metal and 1 in the organic phases are nearly equal above pH 8. This
is consistent with extraction complexes which contain one alkaline earth metal and two crown ether carboxylate species, MA2, but contrasts sharply with the apparent extraction complex stoichiometry of MA.2HA found for extractions of alkali metals by 1 (1) and the stoichiometry of MA-BHA or MA.3HA reported for the solvent extraction of Sr from aqueous media into n-octane using branched carboxylic acids with 15-3 9 carbon atoms (9). Competitive Extractions of Mg, Ca, Sr, and Ba by syrn-Dibenzo-16-crown-5-oxyacetic Acid (1). Results from competitive extractions of aqueous solutions of alkaline earth chlorides in which each cationic species concentration was 0.,020, 0.060, 0.125, and 0.25 N by chloroform solutions of 1 (initial concentration = 0.050 N) are recorded in Figure 2. Although the general curve shapes for the metal concentration in the organic phase vs. the equilibrium pH are similar for the competitive and single ion extraction, there are several important differences. Most striking is the change in selectivity order to Ba > Ca > Sr > Mg for the competitive extractions. Thus, the selectivity order for Ba and Ca is the opposite of that which would be anticipated on the basis of the single ion extraction studies. A simiIar lack of correlation between selectivity orders in single and multiion systems was previously noted in alkali metal extractions by 1 (1). The present results further underscore the dangers of extrapolating single ion extraction data to multiion systems.
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Figure 2. Normal concentrations of metals (X lo3) and complexing agent (X10') in chloroform phase vs. the pH of the aqueous phase for competitive extraction of (a) 0.020 N, (b) 0.060 N, (c) 0.125 N, and (d) 0.25 N alkaline earth cations by 0.050 N 1: 0 = Ba, 0 = Sr, A = Ca, = Mg, 0 = complexing agent. Comparison of the metal concentration data in Figure 2 with that in Figure 1 reveals a better selectivity in the competitive extractions than would have been anticipated from the results of the single ion extraction experiments. For the competitive experiments, the selectivity for Ba improves substantially as the concentration of each metal ion in the aqueous phase is raised from 0.020 N (Figure 2a) to 0.060 N (Figure 2b). Further concentration increases (Figure 2c,d) produce less dramatic enhancements in the selectivity for Ba. The percentage of the complexing agent which remains in the chloroform layer after extraction at alkaline pH is slightly dependent upon the aqueous phase salt concentration. The proportion decreases from 88% when each cation in the aqueous phase is initially 0.020 N to 71% when each cation concentration is 0.25 N. With the exception of competitive extractions in which the initial concentration of each metal ion in the aqueous phase was 0.25 N (vida infra), the total metal and complexing agent normal concentrations at alkaline pH are roughly equivalent. This indicates a dominant extraction complex of one alkaline earth metal and two crown ether carboxylate species, MA2. In the competitive extractions at the highest aqueous phase metal ion concentrations (Figure 2d), the behavior of Mg is unusual. At lower aqueous phase salt concentrations (Figure 2a-c), the resulting Mg concentration in the chloroform phase is uniformly low and does not exceed a value of 1.5 X N. However, when the initial concentration of each aqueous phase metal ion is 0.25 N, the organic phase Mg concentration increases very sharply above pH 7.5 and reaches 0.0143 N a t pH 9.05. In this case, the sum of the metal concentrations in the chloroform phase significantly exceeds the concentration of complexing agent. However, the sum of the Ba, Ca, and Sr concentrations remains consistent with a complex stoichiometry of MA2. The possibility that the chloroform phase complex stoichiometry has changed to (MA)+Cl- was eliminated by establishing the absence of chloride ion in the
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chloroform layer using anionic ion chromatography. Therefore, the high concentration of Mg in the chloroform layer appears to result from some type of physical phenomenon, such as a colloidal system formed by interactions of Mg(OH)Z particles with the complexed Ba, Ca, and Sr species. Neither centrifuging nor filtering of the chloroform solution reduced the anomalously high Mg concentrations. Competitive Extractions of Mg, Ca, Sr, and Ba by sym-Dibenzo-16-crown-5Methyl Ether (4) and by Phenoxyacetic Acid. In (order to compare the results obtained for the crown ether carboxylic acid 1 with those for closely related complexing agents which contain only crown ether or carboxylic acid functional groups, we conducted competitive extractions using the neutral crown ether 4 and phenoxyacetic acid. Extraction of aqueous solutions which were 0.25 N in each alkaline earth chloride with chloroform solutions of phenoxyacetic acid (0.050 N ) gave only very low metal concentrations ( Ca > Sr > Mg, but the selectivity is poorer than that found with 1. Reversal of the Ca:Sr selectivity at more acidic pHs is unique for the crown ether carboxylic acid 3. Similar reversals of the Na:K selectivity in alkali metal extractions by 1 have been ascribed to an effect of pH upon the balance between the complexing effectiveness of the crown ether cavity and carboxyl group portions of the complexing agent (1). In one case (Figure 4a), the Mg concentration in the chloroform phase became quite high at pH >9. It is postulated that this results from the same type of colloidal system formed by interactions of Mg(OH)2 particles with organic phase complexes as was proposed for extractions using 1 (vide supra). Comparison of Alkaline Earth Metal Extractions by Crown Ether Carboxylic Acids 1-3. In terms of extraction efficiency and selectivity, the order for the crown ether carboxylic acids is 1 > 3 > 2. This order may be rationalized by considering the relative complexing abilities of the crown ether cavity and the carboxyl group portion of the complexing agent. Diameters for the cavities of the crown ether carboxylic acids 1, 2, and 3 as estimated from Corey-Pauling-Kortum (CPK) space-filling models are 2.0-2.4 A, C1.2 A, and 3.C-3.5 A, respectively. Alkaline earth cation diameters are as follows:
Mg, 1.56 A; Ca, 2.12 A; Sr, 2.54 A; and Ba, 2.83 8, (10). Due to the very small cavity in the carboxylate form of 2, interactions with alkaline earth metals should be confined largely or solely to the carboxylate group. Support for this contention is the selectivity for Ba noted with 2 which is consistent with the results of alkaline earth metal extractions using ordinary carboxylic acids (8). The low level of metal extractability and high aqueous phase solubility of the complexing agent are also consistent with metal-complexing agent interactions which involve the carboxylate group. Remarkable enhancements in metal extractability and chloroform phase solubility of the complexing agent are observed in going from 2 to 1which strongly suggests interactions of alkaline earth metals with both the polyether and carboxylate portions of the ionized form of 1. The extraction selectivity order found for 2 (Ba > Ca > Sr > Mg) is somewhat different from that observed (Ba > Sr > Ca > Mg) for alkaline earth extractions using ordinary carboxylic acids (8). This again indicates the involvement of metal-polyether interactions. Although the most extractable metal Ba is too large to fit within the cavity of 1, it is appropriate for the formation of a HA2 complex in which the metal is “sandwiched” by two crown ether carboxylate species. Although three configurations of the sandwich complex are possible (both carboxylate groups inside, one carboxylate group inside and one outside, and both carboxylate groups outside), we prefer the one with both carboxylate groups inside because it provides the best possibility for metal interactions with both the polyether and carboxylate group portions of the complexing agent. The diminutions of metal extractability and chloroform phase solubilities of the complexing agest produced by the ring enlargement of 1 to 3 may also be understood on the basis of competitive polyether and carboxylate group interactions. Although the carboxylate portion remains constant in going from 1 to 3, the crown ether cavity of 3 is now large enough to accommodate any of the alkaline earth metals. This diminishes the tendency to form the chloroform phase soluble sandwich complex MA2 by providing greater stability to a 1:l complex (MA)+which remains in the aqueous phase due to its hard chloride counterion.
LITERATURE CITED (1) Strzelbickl, J.; Bartsch, R. A. Anal. Chem. 1981, 53, 1894-1899. (2) McDowell, W. J.; Shoun, R. R. Energy Reg. Abstr. 1978, 3, 4537. (3) McDowell, W. J.; Shoun, R. R. CIM Spec. Vol. 1879, 27, 95-100; Chem. Abstr. 1979. 92, 7 9 9 4 8 ~ . (4) Gerow, I.H.; Davis, M. W. S e p . Scl. Technol. 1979, 14, 395-414. (5) Kolthoff, I. M. Anal. Chem. Ig79, 51, 1R-22R. (6) Bartsch, R. A,; Heo, G. S.;Kang, S. I.; Liu, Y.; Strzelblckl, J., submitted for publlcation in J. Org. Chem. (7) Strzelbicki, J.; Heo, G. S.;Bartsch, R. A. Sep. Sci. Technol., In press. ( 8 ) Flett, D. S.;Jeycock, M. J. “Ion Exchange and Solvent Extraction”; Marlnsky, J. A,, Marcus, Y., Eds.; Marcel Decker: New York, 1978; Vol. 3, Chapter 1. (9) McDowell, W. J.; Harmann, H. D. J . Inorg. Nucl. Chem. 1969, 31, 1473- 1405. (10) Moore, W. E.; Amman, D.; Bisslg, R.; Pretsch, E.; Simon, W. I n Progress in Macrocyclic Chemistry”; Izatt, R. M.. Christensen, J. J., Eds.; Wiley-Interscience, New York, 1979; Vol. 1, p 9.
RECEIVED for review June 8,1981. Accepted August 28,1981. This research was supported by the Department of Energy (Contract DE-80ER-10604) and the Texas Tech University Center for Energy Research (postdoctoral fellowship to J.S.).
