Anal. Chern. 1981, 53, 2251-2253
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Solvent Extraction of Alkali Metal Cations from Aqueous Solutions by Highly Lipophilic Crown Ether Carboxylic Acids Jerzy Strzelblcki' and Richard A. Bartsch" Department of Chemistry, Texas Tech University, Lubbock, Texas 79409
Competitive solvent extractions of alkali metal catlons from water into chloroform by 2 4 sym-dibenzo-16-crown-5-oxy)butanoic acid (5), 2-( s,~m-dlbenro-l6-crown-5-oxy)hexanoIc acid (6),2 4 sym-dibenzo-16-crown-5oxy)octanoic acid (7), 2 4 sym-dibenzo-16-crown-5-oxy)decanolc acid (8), and 5(sym-dibenzo-l6-crown-5-oxy)pentano#c acid (9) are reported. For 6, 7, and 8 the complexlng agents are sufficlently lipophlllc that they remaln totally In the organic phase when chloroform solutions of the crown ether carboxyllc acids are contacted with highly alkaline aqueous alkall metal salt solutlons.
This paper is a continuation of our study of the synthesis of novel crown ether carboxylic acids and their use as complexing agents in the solvent extraction of alkali metal arid alkaline earth cations from aqueous media into organic solvents (1-3). Compared with neutral crown ethers, these ionizable species have the advantage that extraction of the metal into the organic phase does not require the simultaneous transfer of an aqueous phase anion. In previous investigations, competitive solvent extractions of alkali and of alkaline earth metals from water into chloroform by crown ether carboxylic acids 1-4 have been explored (1-3). In these studies it was
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discovered that the ionized forms of 1-41 are of insufficient lipophilicity to remain completely in the organic phase. In extreme cases, only a few percent of the complexing agent remains in the chloroform layer after extracting a basic aqueous salt solution with a chloroform solution of the crown ether carboxylic acid. The phenomenon significantly decreases the extraction efficiency and results in important loss of the complexing agent to the aqueous phase. Bradshaw, Parish, and co-workers (4,511have increased the lipophilicity of neutral crown ethers by attaching hydrophobic alkyl groups through a process of Friedel Crafts acylation of aromatic crown ethers followed by reduction of the resultant ketone function. However in our system, it is easier to increase the lipophilicity of the carboxylic acid portion of the crowii ether carboxylic acids. We now report results for competitive solvent extractions of alkali metals using the series of crown ether carboxylic acids 5-9 which possess a common dibenzo-16-crown-5 polyether ring system and carboxylic acid portions of varying lipophilicity (5-8) or of a different spacial Present address: Institute of Inorganic Chemistry and Metallurgy of Rare Elements, Technical University of Wroclaw, 50-370 Wroclaw, Poland.
relationship with the polyether cavity (9).
7
EXPERIMENTAL SECTION Apparatus. The apparatus was the same as that in previous studies in this series (1-3). Reagents. Reaction of sym-hydroxydibenzo-16-crown-5 (6) with NaH in THF followed by the addit'on of the appropriate a-bromocarboxylic acid produced 2-(syrn-dibenzo-16-crown-50xy)butanoic acid (5), 2-(sym-dibenzo-16-crown-5-oxy)hexanoic acid (6), 2-(syrn-dibenzo-16-crown-5-oxy)octanoic acid (7), and 2-(syrn-dibenzo-16-crown-5-oxy)decanoicacid (8) (7). The 5(syrn-dibenzo-16-crownn-5-oxy)pentanoic acid (9) was synthesized by the reaction of sym-hydroxydibenzo-16-crown-5(6) with NaH in THF followed by the addition of ethyl 5-bromovalerate (Aldrich, Milwaukee, WI). Subsequent basic hydrolysis of the resulting crown ether carboxylic acid ester produced 9 (7). A complete description of the synthesis, characterization,and analysis of the new crown ether carboxylic acids 5-9 will be published separately (7). Sources of reagent grade inorganic chemicals and solvent purification methods were as previously described ( 1 , 3). Procedure. The procedure was the same as that utilized in previous studies (1-3). Compounds 5-9 exhibit ultraviolet absorption maxima in chloroform at 273-274 nm with t = 4140,4300, 4380,4480,and 4450, respectively. For 6,7, and 8 the absorptions show either two maxima at 273 and 274 nm or a maximum and a distinct shoulder at 273 and 274 nm, respectively. The absorption at 273 nm for 6-8 increases significantly when the carboxylic acids are converted into metal carboxylates. Much smaller increases occur for the absorption at 274 nm, so the absorption at this wavelength was used to determine the complexing agent concentration in the chloroform layer. RESULTS AND DISCUSSION In our previous studies of alkali metal extraction by crown ether carboxylic acids ( I ) , it was determined that the efficiencies and selectivity orders for competitive extractions are often quite different from expectations based upon the results of single ion extractions. Therefore, competitive extractions were used throughout this investigation. Competitive Extractions of Alkali Metals by 2-(symDibenzo-16-crown-5-oxy)butanoic Acid (5). Results for extractions of aqueous solutions in which the concentration of each alkali metal chloride was 0.06 M and 0.25 M with 0.050 M chloroform solutions of 5 are presented in Figure 1. The curve shapes for the concentrations of the dominant alkali metal (Na) in the chloroform phase as a function of the aqueous phase p H are typical for metal extractions which utilize highly lipophilic carboxylic acids. Thus, the low Na concentration observed at low pH increases sharply as the pH is raised from 6 to 9. Above pH 9 an alkaline plateau is
0003-270018110353-2251$01.2510 0 1981 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 14, DECEMBER 1981
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Figure 1. Concentrations of metals (X103) and complexing agent (XIO') in the chloroform phase vs. the pH of the aqueous phase for competitive extractions of (a) 0.060 M and (b) 0.25 M alkali metal cations by 0.050 M 5: R = complexing agent, A = Na,o = K, 0 = Rv, V = Cs. reached in which the Na concentration of the organic phase is insensitive to changes in the aqueous phase pH. The behavior for K is a bit different with a concentration maximum being reached at a p H of about 8 followed by concentration decreases at more alkaline pH. Concentrations of Rb and Cs in the chloroform phase are very low and appear to reach maximum concentration plateaus above pH 7 . The Li concentration in the organic phase was too low to be measured by the ion chromatographic analytical method. Selectivity in extractions employing 5 is quite good. At pH 10 and a 0.25 M concentration of each alkali metal cation in the aqueous phase, the chloroform phase concentration of Na exceeds that of K, Rb, and Cs by factors of 4, 23, and 35, respectively. The total concentrations of the metals in the organic phases are also quite good for these extractions. The maximum concentrations of metals are equal to 0.006 33 M and 0.0129 M for extractions of aqueous solutions in which each alkali metal cation was 0.060 M and 0.25 M, respectively. A pronounced influence of the aqueous phase salt concentration upon the metals concentration of the organic phase is evident. The organic phase partitioning of the complexing agent derived from 5 (broken lines in Figure 1)is considerably better than that reported for 3 (1). Nevertheless after contacting chloroform phases containing 5 with the lower salt concentration and higher salt concentration aqueous phases at pH 10, only 40% and 7070, respectively, of the complexing agent remains in the organic phase. Competitive Extractions of Alkali Metals by 6,7, a n d 8. Since the lipophilicity of 5 is insufficient to confine the complexing agent to the organic phase when the crown ether carboxylic acid is contacted with alkaline aqueous solutions, extractions were performed with 6, 7, and 8 for which the lipophilicity is systematically increased by attaching longer hydrocarbon chains to the a position of 2-(sym-dibenzo-16crown-5-oxy)acetic acid. Aqueous solutions which were 0.060 M and 0.25 M in each of the alkali metal chlorides were extracted with 0.050 M chloroform solutions of the crown ether carboxylic acids. Results for alkali metal extractions using 6, 7, and 8 are recorded in Figures 2, 3, and 4, respectively. Examination of the data for the concentrations of complexing agent in the chloroform phase as a function of the aqueous phase pH (broken lines in Figures 2-4) reveals that the complexing agents derived from 6-8 remain completely in the organic phase even when the aqueous phase is highly alkaline. As mentioned in the Experimental Section, the modest absorption enhancements which occur when 6-8 are converted into their carboxylate forms may cause a slight overestimation (up to 10%)in the chloroform phase concen-
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Figure 2. Concentrations of metals (X103) and complexing agent (X102) in the chloroform phase vs. the pH of the aqueous phase for competitive extractions of (a) 0.060 M and (b) 0.25 M alkali metal cations by 0.050 M 6: R = complexing agent, 0 = Li, A = Na, 0 = K, 0 = Rb, V = Cs.
Figure 3. Concentrations of metals (X 103)and complexing agent (X102) in the chloroform phase vs. the pH of the aqueous phase for competitive extractions of (a) 0.060 M and (b) 0.25 M alkall metal cations by 0.050 M 7: W = complexing agent, 0 = Li, A = Na, 0 = K, 0 = Rb, V = Cs. 25
20
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Figure 4. Concentrations of metals (X103) and complexing agent (X102) in the chloroform phase vs. the pH of the aqueous phase for competitive extractions of (a) 0.060 M and (b) 0.25 M alkali metal cations by 0.050 M 8: W = complexing agent, 0 = Li, A = Na, 0 = K, 0 = Rb, V = Cs. tration of the complexing agents. Therefore, the complexing agent concentrations in Figures 2-4 may appear to be higher than the 0.050 M initial concentration of the crown ether carboxylic acid in the chloroform phase. The dependence of the organic phase metal concentrations upon the aqueous phase pH for 6-8 is very similar to that observed for 5 (vide infra). In contrast to the resulh obtained in this study with 5 and in a previous investigation with 3 (I), the total concentrations of metals in the chloroform phases are quite insensitive to changes in the aqueous phase salt
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Competitive Extraction of Alkali Metals by 54symDibenzo-16-crown-5-oxy)pentanoic Acid (9). An alter-
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Flgure 5. Concontrationis of metals (X103) and complexing agent
(X102) in the chloroform phase vs. the pH oi the aqueous phase for competitive extractions of (a) 0.060 M and (b) 0.25 M alkali metal = complexing agent, A = Na, 0 = K, 0 cations by 0.050 M 9:
= Rb, V = Cs.
concentrations for extractions with 6-13. The average concentration of metals in the chloroform ph,ases with 6-8 is (0.034 f 0.003) M at pH 11. When compared with the chloroform phase complexing agent concentration of 0.050 M, predominant extraction complexes of MA (where M is the metal and A is the crown ether carboxylate) are indicated with some contribution from MA.HA and/or MA-BI-IA complexes. Good evidence for MA.2HA extraction complexes was obtained in a previous investigation of alkali metal extraction by 3 (1). Although changes of the aqueous phase salt concentration have little effect upon the total concentrations of metals in the organic phases for extractions with 6-8, the extraction selectivity is affected. Increasing the aqueous phase salt concentration produces enhancements in the extraction Belectivity €or Na. When the initial concentration of each alkali metal cation in the aqueous phase is 0.25 M and the pH is 11, the chloroform phase concentration of Na exceeds that of K, Rb, and Cs by factors of approximately 6, 40, and 110, respectively, for 7 and 8.
native method for enhancing the lipophilicity of a dibenzo16-crown-5 carboxylic acid is lengthening of the hydrocarbon linkage which joins the polyether and carboxylic acid groups. The crown ether carboxylic acid 9 represents such a structural modification. Results for extractions of aqueous solutions which were 0.060 M and 0.025 M in each of the alkali metal chlorides with 0.050 M solutions of 9 in chloroform are presented in Figure 5. In all cases, the Li concentrations in the organic phases were below the detection limit of the ion chromatographic analytical technique. With respect to the influence of the aqueous phase pH and salt concentration upon the concentrations of metals and complexing agents in the chloroform phase, the results for 9 are very similar to those noted with 5. Although the two crown ether carboxylic acids are not exactly equivalent since 9 contains an extra methylene group, the similarity of results suggests that total lipophilicity is a more important factor than the precise spatial relationship between the polyether and carboxylate groups of the complexing agent for the extraction process. LITERATURE CITED Strzelbicki, J.; Bartsch, R. A. Anal. Chem. 1080, 53, 1894. Strzelbicki, J.; Bartsch, R. A. Anal. Chem., in press. Strzelbickl, J.; Heo, G. S.; Bartsch, R. A. Sep. Sci. Techno/.,In press. Parish, W. W.; Stott, P. E.; McCausiand, C. W.; Bradshaw, J. S. J . Org. Chem. 1078, 43, 4577-4581. (5) Stott, P. E.; Bradshaw, J. S.; Parish, W. W.; Copper, J. W. J . Org. Chem. 1080, 45. 4716-4720. (6) Heo, G. S.; brtsch, R. A.; Schlobohm, L. L.; Lee, J. G. J . Org. Chem. 1081, 46, 3574-3575. (7) Bartsch, R. A.; Heo, G. S.; Kang, S. I.; Llu, Y.; Strzelbicki, J. J . Org. Chem., In press. (1) (2) (3) (4)
RECEIVED for review July 15, 1981. Accepted September 15, 1981. This research was supported by the Department of Energy (Contact DE-ASOS-80ER-10604) and the Texas Tech University Center for Energy Research (postdoctoral fellowship to J.S.).
