Anal. Chem. 1985, 57, 1713-1717
Other ionic lines than resonance lines were observed. For group 13 elements, transitions between some triplet states were observed, reflecting the high population of the ions. For boron, there also existed a transition between excited singlet states. For antimony and bismuth, transitions between singlets were observed as well. Transitions between different spin multiplicities were observed for lead (quartets to doublets) and bismuth (a singlet to a triplet) in the studied region. For gallium, indium, or thallium, the transition between lowest triplet and ground state singlet has been observed in conventional UV and used as an ionic analytical line (Ga 209.1 nm, In 230.6 nm, T1190.9 nm) (1). Recently, Anderson and Parsons reported the same transition for aluminum (266.9 nm) but the sensitivity of the line was poor (11). Neutral Atomic Lines Observed. For boron and silicon, there existed strong neutral atomic lines (B 182.6,181.8,166.7, and 160.0 nm; Si 185.1, 169.8, 169.6, 167.5, and 163.0 nm). Some of them could be used for analytical lines. For carbon or nitrogen, strong lines of neutral atoms were observed, but BEC's could not be obtained due to contamination from argon gas and/or the air. For phosphorus or arsenic, vacuum UV atomic emission lines have been used as analytical lines. Other elements have neutral atomic emission lines, but their sensitivities were not so high. Transitions from Highly Excited States. In this study, we observed transitions from ionic excited states which require more energy than IP of argon if they are excited directly from atomic ground states. They are lead lines at 179.7 nm and 182.2 nm and an indium line at 170.0 nm; energy levels from atomic ground states are 14.3, 14.9, and 14.9 X lo6 m-l, respectively. This fact would support our assumption together with the previous aluminum data (3). Potential for Analytical Use. Among the lines shown in this study, In 158.7 nm, Ge 164.9 nm, Sn 175.8 nm, and Pb 168.2 nm are considered promising as well as A1 167.1 nm. These lines were checked for practical detection limits and spectral interferences of other elements-B, Na, Mg, Al, Si, P, S, K, Ca, Ti, Mn, Fe, Cu, and Zn. The detection limit was estimated by two methods: the signal to noise ratios (SIN)of the recorder trace (time constant = 1 s) and the standard deviation ( u ) of five measurements
1713
of 10 s integration of blank solution. The results from recorder traces (SIN = 2) were 7 nglg, 30 nglg, 80 nglg, and 50 ng f g for In, Ge, Sn, and Pb, respectively. Integration measurements (3a) gave the detection limits of 3 nglg, 10 nglg, 25 nglg, and 15 nglg for In, Ge, Sn, and Pb, respectively. The results of interference surveys are summarized in Table IV. Magnesium, aluminum, and calcium interfered with all the elements more or less, because of recombination continuum; however, these interferences were flat background increases and hence easy to correct by various background correction methods. Severe interferences due to line coincidence were not observed for these elements, e.g., less than 0.5 X loT3. For bismuth 179 nm, a plasma background structure (probably due to NO molecular bands) was overlapping; therefore, the line is not good for the determination of low concentrations. Registry No. In, 7440-74-6; Ge, 7440-56-4;Sn, 7440-31-5;Pb, 7439-92-1;Mg, 7439-95-4;Al,7429-90-5;Si, 7440-21-3;P, 7723-14-0; Ca, 7440-70-2; Ti, 7440-32-6; Mn, 7439-96-5; Fe, 7439-89-6; Cu, 7440-50-8;B, 7440-42-8;Ga, 7440-55-3;T1,7440-28-0;Sb, 7440-36-0; Bi, 7440-69-9.
LITERATURE CITED Wlnge, R. K.; Peterson, V. J.; Fassel, V. A. Appl. Spectrosc. 1979, 33, 206-219. Boumans. P. W. J. M. Spectrochim. Acta, Part 8 1981, 368, 169-203. Uehiro, T.; Morlta, M.; Fuwa, K. Anal. Chem. 1984, 56, 2020-2024. Heine, D. R.; Babis, J. S.; Denton, M. B. Appl. Spectrosc. 1980, 3 4 , 595-596. Mlles, D. L.; Cook, J. M. Anal. Chim. Acta 1982, 141, 207-212. Hayakawa, T.; Kikui, F.; Ikeda,S. Spectrochim. Acta, Part B 1982, 378, 1069-1073. Carr, J. W.; Blades, M. W. Spectrochim. Acta, Part 8 1984, 398, 567-574. Moore, C. E. Natl. Stand. Ref. Data Ser. ( U S . , Natl. Bur. Stand.) 1971, NSRDS-N8S 35, VOIS. 1-111. Bashkin, S.; Stoner, J. O., Jr. "Atomic Energy Levels and Grotrlan Diagrams, Volume I. Hydrogen I-Phosphorus XV"; North Holland Publishing Co.: Amsterdam, Nethdands, 1975. Reader, J., Corliss, C., Eds. CRC Handbook of Chemistry and Physics", 5gth ed.; CRC Press: Boca Ratan, FL, 1976; Line Spectra of the elements, pp E-216-E-364. Anderson, T. A.; Parson, M. L. Appl. Spectrosc. 1984, 38, 625-634.
RECEIVED for review November 27, 1984. Accepted March 13, 1985.
Competitive Solvent Extraction of Alkaline-Earth Cations into Chloroform by Lipophilic Acyclic Polyether Dicarboxylic Acids Sang Ihn Kang, Anna Czech, Bronislaw P. Czech, Louis E. Stewart, and Richard A. Bartsch* Department of Chemistry, Texas Tech University, Lubbock, Texas 79409
Competitive solvent extractlon of alkaline-earth cations from aqueous s6lutlons Into chloroform by a series of iipophllic acyclic polyether dicarboxylic acids Is reported. The influence of polyether chain length and of terminal carboxylic acid group variation upon extraction selectivity and efficiency is assessed. I n the competitive extractlon of concentrated magnesium, calcium, strontium, and barium chloride solutions, one complexing agent exhibits pronounced selectivity for barium wlth Ba2+/Sr2+ = 50, Ba2+/Ca2' = 250, and no detectable Mg2+ extractlon. 0003-2700/65/0357-1713$01.50/0
Recently Marchelli and co-workers (1)reported the highly selective extraction of uranyl ions from aqueous solutions containing other divalent metal ion species into dichloromethane by acyclic polyether ligands which had two phenylalanine end groups. Such selectivity encourages the preparation and testing of diionizable polyether ligands for the solvent extraction of other divalent metal ions, such as the alkaline earths. In 1974, T6ei and co-workers (2) noted that the chelate stability constant ordering for alkaline-earth cation com0 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 8, JULY 1985
1714
plexation by HOzCCHzO(CHzCHzO).CHzCO2H in water varied with changes in the polyether chain length. This finding suggests that acyclic polyether dicarboxylic acids with appropriate lipophilicity might be useful agents for the extraction of alkaline-earth cations into organic media. Acyclic polyether dicarboxylic acids 1 and 2 have been prepared and utilized in their un-ionized forms for compexation of alkali-metal cations (3-7). A complex of 1 with calcium thiocyanate also involved the un-ionized form of the multidentate ligand (3). Ligands 1 and 2 would have insufficient lipophilicity for use in solvent extraction. Very recently Hiratani and co-workers (8,9) reported in preliminary form studies of competitive alkaline-earth cation (Mg2+,Ca2+,Ba2+) transport through CHC13 liquid membranes by the lipophilic acyclic polyether dicarboxylic acids 3-10 and of solvent extraction into CHC13 by 4. For the competitive solvent extraction of magnesium, calcium, and barium chlorides from aqueous solutions into CHC13by 4, modest calcium selectivity was noted (8). P0-l
O n -
aoo&? aou I@? 1
I
-
J
co2H
I 5
- C H2CH2 -CH,CH$H*-CH.fH20CH2CH2-
H02C
R -
R -
2
1I
6
1 4
9
- CH2CH20CH2CH2-C H2C H2CH2
-CH2CH2(0CYH2I2-CHfH20
OCH2CH~
@
C(CH3)3
E
To provide greater insight into the factors which control the selectivity and efficiency of alkaline-earth cation extraction by lipophilic acyclic polyether dicarboxylic acids, a series of structurally related ligands 11-19 has now been synthesized and tested. Results of this study are reported. EXPERIMENTAL SECTION Apparatus. Concentrations of alkaline-earth cations in aqueous phases were determined with a Dionex Model 10 ion chromatograph. Organic complexing agent concentrations in the CHC1, phases were measured with a Cary Model 17 ultravioletvisible spectrophotometer. Measurements of pH were made with a Fisher Accumet Model 620 pH meter using a Corning No. 476050 combination electrode. IR and 'H NMR spectra were determined with a Varian EM-360A nuclear magnetic resonance spectrometer and a Nicolet MS-X infrared spectrophotometer. Reagents. The sources of reagent grade inorganic chemicals and the solvent purification method were the same as before (IO, 11). Methyl 3-hydroxy-2-naphthoate (12),methyl l-hydroxy-2naphthoate (13), 2-bromodecanoic acid ( I 4 ) , methyl 5-n-decyl-
rc\
o n o n o n 0
n
E
1
B
19
salicylate (15),1,9-dihydroxy-3,7-dioxanonane (16), and pentaethylene glycol (17) were prepared by known methods. Ethylene, diethylene, triethylene, and tetraethylene glycols were obtained from Aldrich (Milwaukee,WI). Ditosylates were prepared from the glycols according to a reported general procedure (18). General Procedure for the Synthesis of 11-17 and 19. Under nitrogen, NaH (50% dispersion in mineral oil, 0.86 g, 18 mmol) was washed with n-pentane to remove the mineral oil and was suspended in 50 mL of dry THF. To this stirred suspension, 15 mmol of the appropriate o-hydroxynaphthoic or o-hydroxybenzoic acid methyl ester in 20 mL of THF was added dropwise. After the mixture was stirred for 2 h at room temperature, 7.5 mmol of the requisite glycol ditosylate in 15 mL of THF was added and the reaction mixture was refluxed with stirring for 2 days. The solvent was removed in vacuo and the polyether diester was isolated by column chromatography on silica gel with EtOAc as eluent. The pure polyether diester (2.4 mmol) was dissolved in 15 mL of EtOH and a solution of 1.0 g (25 mmol) of NaOH in HzO (10 mL) was added. The mixture was refluxed for 4 h after which most of the solvent was evaporated in vacuo. Water (50 mL) was added to the residue and the solution was acidified with 6 N HCl and extracted three times with CHZClz. The CHzClz solution was washed with HzO,dried over MgS04,and evaporated to afford the analytically pure polyether diacid. Yields, physical properties, spectral data, and elemental analysis data for 11-17 and 19 are collected in Table I. Synthesis of 18. Under nitrogen, NaH (50% dispersion in mineral oil, 11.5 g, 0.24 mol) was washed with n-pentane to remove the mineral oil and was suspended in 40 mL of dry THF. The mixture was stirred for 0.5 h at room temperature and 3.00 g (0.020 mol) of triethylene glycol in 100 mL of THF was added dropwise during 2 h. After the mixture was stirred at room temperature for an additional 1h, 10.0 g (0.040 mol) of 2-bromodecanoic acid dissolved in 100 mL of THF was added dropwise during 12 h. The reaction mixture was stirred for 24 h at room temperature and the solvent was evaporated in vacuo. The residue was carefully added to 250 mL of HzO to destroy the excess NaH and the aqueous solution was extracted three times with CHZClz. The combined extracts were washed with HzO,dried over MgS04,and evaporated in vacuo to give the crude polyether diacid which was purified by column chromatography on silica gel with CHZClz:EtOAc(3:l) as eluent to give 3.90 g (40%) of pure 8 as a yellow viscous oil. Spectral and elemental analysis data for 18 are presented in Table I. Procedure. A CHC1, solution (5.0 mL) of the complexing agent (0.050 M) and 5.0 mL of an aqueous solution of alkaline-earth chlorides (0.25 M in each) and LiOH (for pH regulation) were shaken for 15 min a 30-mL separatory funnel at room temperature (2G-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 CHCl, by cyclic polyether carboxylic acids (IO,19). 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 CHC1, in a 10-mL volumetric flask. Measurement of the absorption at 273-274 nm determined the concentrations of all forms of the complexing agent in the CHC1, layer. Of the remaining organic phase, 4.0 mL was
ANALYTICAL CHEMISTRY, VOL. 57, NO. 8, JULY 1985
1715
Table I. Yields, Physical Properties, Spectral Data, and Elemental Analysis Data for Lipophilic Polyether Dicarboxylic Acids 11-19 compd no.
yield, %
mp, "C
11
68 (20)*
256-2598
12
87 (42)
201-203
13
97 (54)
149-150
14
74 (48)
oil
15
95 (25)
oil
16
88 (19)
129-130
17
68 (40)
paste
18
40
oil
19
74 (48)
116-1 17
'H NMR spectra (60 MHzL ppm' 4.63 (s, 4 H), 7.3-8.15 (m, 10 H), 8.30 (s, 2 H) 3.7-4.5 (m, 8 H), 7.3-8.1 (m, 10 H), 8.25 (s, 2 H) 3.7-4.6 (m, 12 H), 7.1-7.9 (m, 10 H), 8.50 (s, 2 H) 3.4-4.5 (m, 16 H), 7.0-7.8 (m, 10 H), 8.45 ( 8 , 2 H) 3.5-4.55 (m, 20 H), 7.1-7.9 (m, 10 H), 8.56 ( 8 , 2 H) 3.83-4.55 (m, 12 H), 7.2-8.35 (m, 12 H) 0.5-2.0 (m, 38 H), 2.25-2.85 (m, 4 H), 3.43-4.50 (m, 12 H) 6.8-8.15 (m, 6 H) 0.6-2.0 (m, 34 H), 3.55-4.13 (m, 14 H) 1.90 (pentet, 2 H), 3.5-4.0 (m, 8 H), 4.33 (t, 4 H), 7.1-7.8 (m, 10 H), 8.55 ( 8 , 2 H), 10.35 (br s, 2 H)
elem analysis
IR spectra, cm-'
theory
found
3300-2300 (COOH),d 1734, 1691 (C=O)
CC H
70.07 4.65
70.13 4.57
3300-2300 (COOH): 1730-1701 (C=O)
C H
69.95 4.96
69.72 5.16
3300-2300 (COOH),d 1736, 1716 (C=O)
C H
68.58 5.31
68.39 5.38
3600-2350 (COOH),' 1734 (C=O)
C H
66.29 5.56
66.55 5.76
3700-2400 (COOH)! 1732 (C=O)
C H
66.42 5.92
66.46 6.14
3600-2300 (COOH): 1722, 1691 (C=O) 3700-2600 (COOH): 1734 (C=O)
C H C H
68.58 5.31 71.61 9.31
68.50 5.40 71.39 9.55
3700-2300 (COOH),' 1747, 1720 (C=O) 3700-2600 (COOH),e 1734 (C=O)
C H
63.64 10.27 71.61 9.31
63.39 10.21 71.39 9.55
C'
H
Measured in CDC1, except for compounds 11 and 12 which were dissolved in Me,SO-ds. *Yieldsgiven in parentheses are for the diesters. 'Analyzed as a 0.5 hydrate. dMull. ODeposit on sodium chloride plate. f Neat. #Decomposition. removed and shaken with 5.0 mL of 0.1 N HCl for 20 min to strip the metal cations from the organic phase into aqueous solution for analysis by ion chromatography (10). For 18, which does not contain aromatic ring components, concentrations of the complexing agent in the chloroform phase after stripping were determined by titration to a phenolphthalein end point with standard methanolic NaOH.
RESULTS AND DISCUSSION Synthesis of New Lipophilic Acyclic Dicarboxylic Acids 11-19. For the preparation of 11-17 and 19, the sodium salt of an o-hydroxybenzoic or o-hydroxynaphthoic acid methyl ester was reacted with a glycol ditosylate to form the polyether diester. Hydrolysis of the diester under basic conditions produced the lipophilic acyclic polyether dicarboxylic acid. Synthesis of 18 required an alternative approach in which triethylene glycol in the presence of a large excess of NaH was treated with 2 equiv of 2-bromodecanoic acid to afford the lipophilic acyclic polyether dicarboxylic acid directly. The structures of 11-19 are supported by IR and 'H NMR spectra and elemental analyses (Table I). Compounds 11-15 provide a series of acyclic polyether dicarboxylic acids in which the lipophilic end groups are held constant, but the polyether chain length is systematically varied. Compound 19 also has the same carboxylic acid end groups but the central two-carbon bridge in the polyether fragment of 13 is replaced with a three-carbon bridge. For the series of 13 and 16-18, the polyether chain length is invariant, but four different carboxylic acid containing end groups are utilized. Solvent Extraction of Alkaline-Earth Cations from Aqueous Solutions into Chloroform by Lipophilic Acyclic Polyether Dicarboxylic Acids. In an earlier study of alkaline-earth cation extraction by cyclic polyether monocarboxylic acids ( l l ) ,it was shown that efficiencies and selectivity orders for competitive extractions were quite dif-
ferent from expectations based upon the results of single ion extractions. Therefore, competitive extractions were used in this investigation. In an extraction, the aqueous solution which was 0.25 M in magnesium, calcium, strontium, and barium chlorides and contained varying concentrations of LiOH for pH adjustment was shaken with a 0.050 M solution of the lipophilic acyclic polyether dicarboxylic acids in CHC13. The two phases were separated and the pH of the aqueous phase was measured. A small portion of the CHC13phase was removed and diluted and the concentration of complexing agent was determined by ultraviolet spectrometry. A large portion of the organic phase was shaken with dilute HCl to strip the metal cations from the CHCl, phase into an aqueous phase for analysis by ion chromatography. For 18 which lacks aromatic ring components, the complexing agent concentration was determined by titration of the stripped CHC13 layer with methanolic NaOH. Effect of Polyether Chain Length. For the series of lipophilic polyether dicarboxylic acids 11-15, neither 11 nor 12 possessed sufficient solubility in CHC13 to be utilized in the solvent extraction studies. Shaking CHCl, solutions of 13-15 with aqueous alkaline-earth chloride solutions in which the pH was adjusted by addition of LiOH (see Experimental Section) produced the extraction data which are presented in Figure la-c for 13, 14, and 15, respectively. Resultant concentrations of metal cations and all forms of the complexing agent (carboxylate and carboxylic acid) in the CHC13 phase as a function of the equilibrium pH (after extraction) are shown. Although there is no apparent loss of polyether dicarboxylic acid 13 from the CHCl3 layer during the extractions (Figure la, solid squares), slight loss of 14 to contacting weakly acidic, neutral, and alkaline aqueous phases is evident (Figure lb). For 15 (Figure IC),the complexing agent concentration in the
1716
ANALYTICAL CHEMISTRY, VOL. 57, NO. 8, JULY 1985
25
I
c,
.-520
I
c
c
e
C 0
IO 5
in0 Mg2*i
I.
,
,2'
5 6 7 8 9 PH
P
Molar concentrations of metals (X103) and complexing agent the chloroform phase vs. the equilibrium pH of the aqueous phase for competitive extractions of 0.25 M alkaline-earth cations by Flgure 2.
(W, X102) in
0.050 M 19.
PH Flgure 1.
Molar concentrations of metals (X103) and complexing agent
(m,X102) in the chloroform phase vs. the equilibrium pH of the aqueous phase for competitive extractions of 0.25 M alkaline-earth cations by 0.050 M (a) 13, (b) 14, and (c) 15. CHC13 phase drops rapidly above pH 5.5 to only half of the initial value a t pH 6 and about one-fifth of the initial concentration at pH 7 with even heavier losses at alkaline pH. The increased aqueous phase solubilities of the dicarboxylate forms of 14 and particularly 15 (compared with 13) are unanticipated since the lipophilicity increment T~ (20) for the fragment -CH20CH2- is +0.03. Thus, 14 and 15 would be predicted to be more lipophilic than 13. Data for the CHCl,-phase metal cation concentrations for 13-15 reveal that both the efficiency and selectivity of alkaline-earth cation extraction are markedly influenced by the polyether chain length. For lipophilic polyether dicarboxylic acid 13, metal cation loading of the organic phase is complete (formation of M2+A2-complexes) and high selectivity for barium extraction is evident (Figure la). Only very low levels of strontium and calcium are observed in the CHC13 phase with Ba2+/Sr2+= 50 and Ba2+/Ca2+= 250. The Ba2+/Mg2+ selectivity is even higher since no Mg2+could be detected. Insertion of an additional ethylene oxide unit into the polyether backbone of 13 to form 14 produces pronounced diminution in extraction selectivity (Figure Ib). The Ba2+/Sr2+ and Ba2+/Ca2+selectivities plummet to values of 2 and 3, respectively. Again no detectable Mg2+was extracted. Alkaline-earth cation loading of the CHClB phase was approximately 85% with 14. Due to heavy loss of the dicarboxylate form of 15 into the contacting aqueous phases, very low efficiency and poor selectivity are observed (Figure IC). Differences between the extraction results obtained with 13 and those reported by Hiratani and co-workers (8) for competitive extractions of aqueous magnesium, calcium, and barium chloride solutions with the structurally related polyether dicarboxylic acid 4 dissolved in CHC1, are striking. Although each complexing agent contains four polyether oxygens and two aromatic carboxylic acid end groups, 13 exhibits high selectivity and efficiency for barium extraction with Ba2+/Ca2+= 250, whereas only low loading (7%) and modest calcium selectivity (Ca2+/Ba2+= 6)are reported for extractions with 4. This comparison suggests that not only the number
but also the precise positioning of oxygens within the polyether chain is important. To further probe the sensitivity of cation complexation selectivity and efficiency to changes in the polyether structure, the central two-carbon bridge of 13 was replaced with a three-carbon bridge in 19. Extraction data for 19 are presented in Figure 2. Comparison of data for 13 (Figure la) with that for 19 (Figure 2) reveals dramatic differences. The outstanding barium selectivity and extraction efficiency which are observed with 13 disappear with 19. Surprisingly when CHC13solutions of 19 were contacted with neutral or alkaline solutions of alkaline-earth chlorides, marked complexing agent loss from the organic phase occurred. Results for the series of lipophilic acyclic polyether dicarboxylic acids 13, 14, 15, and 19 reveal a pronounced influence of the polyether fragment structure upon both the selectivity and efficiency of alkaline-earth cation extraction. An examination of Corey-Pauling-Koltun (CPK) space-fiiing models indicates a favorable conformation in which the four ethereal oxygens of diionized 13 surround a barium cation in a planar pseudocyclic arrangement with one oxygen from each carboxylate end group located slightly above and below the ethereal oxygen plane. Increasing the number of oxyethylene units (in 14 and 15) or changing a two-carbon bridge to a three-carbon bridge (in 19) destroys this apparently optimal spatial arrangement. Dependence of selectivity upon the number and positioning of polyether oxygens has also been noted in competitive transport of alkaline-earth cations (Mg2+,Ca2+,Ba2+)across CHC13 membranes by the lipophilic acyclic polyether dicarboxylic acids 3-10 (8,9).Although solvent extraction and liquid membrane transport are not directly comparable since the metal ion complexation step of the former is combined with a decomplexation step for the latter, it is interesting to note that for transport across CHCl:, membranes, 4, 6, and 10 exhibit the selectivity order of Ca2+> Ba2+> Mg2+,whereas 3,5, and 7-9 provide varying levels of selectivity in the order Ba2+> Ca2+> Mg2+. Effect of End Groups. To probe the influence of varying the lipophilic carboxylic acid end groups, alkaline-earth cation extractions into chloroform by 16,17, and 18 were conducted. These compounds have the same polyether fragment as does the highly selective and efficient 13, but different carboxylic acid end groups. Results from extractions of aqueous solutions of magnesium, calcium, strontium, and barium chlorides with CHC1, solutions of polyether dicarboxylic acids 16-18 are shown in Figure 3, parts a-c, respectively. For all three compounds, there was no apparent loss of the complexing
ANALYTICAL CHEMISTRY, VOL. 57, NO. 8 , JULY 1985
1717
extracted any Mg2+ into the chloroform phase. The poor extraction selectivity of 18 demonstrates the importance of structural rigidity within the end groups. Such rigidity controls and positioning of the carboxylate ion binding sites.
CONCLUSIONS
.*.... ..e1
..
3 4 5 6 ’ 7 5 6 ’ 7 8 9
.
3 4 5 6 ’ 7 8 9
PH
Figure 3. Molar concentrations of metals (X103) and complexing agent @, X102) in the chloroform phase vs. the equilibrium pH of the aqueous
phase for competitive extractlons of 0.25 M alkaline-earth cations by 0.050 M (a) 16, (b) 17, and (c) 18. agents from the CHC1, phases (filled squares in Figure 3), during the extractions. For 16, which is a structural isomer of 13, data could be obtained only when the equilibrium pH of the aqueous phase was 6.0 or less. At pH 6.3 and above, massive precipitates formed in the CHC13 layer. IR spectral examination and melting point behavior suggest that the precipitate was a complex of diionized 16 and alkaline-earth cations. The data which were obtained for extractions by 16 (Figure 3a) reveal that the selectivity is less than that observed for extractions of alkaline-earth cations from acidic aqueous solutions by 3 (Figure la). Examination of CPK models indicates that steric interactions of the naphthalene rings with the polyether unit in 16 makes it difficult to obtain the optimal carboxylate group positioning with respect to a complexed alkaline-earth cation that is suggested for complexes with 13 (vide supra). Lipophilic acyclic polyether dicarboxylic acid 17 exhibits essentially complete metal cation loading of the organic phase (Figure 3b) and an extraction selectivity order of Ba2+> Sr2+ = Ca2+,with no detectable Mg2+extraction. With Ba2+/Sr2+ and Ba2+/Ca2+ratios of approximately 7, the extraction selectivity for barium is much less than that observed with 13 (Figure la). The lipophilic acyclic polyether dicarboxylic acid 18 differs from 13, 16, and 17 in that is has alkanoic acid end groups in contrast to the substituted benzoic and naphthoic acid termini of the latter. Although the organic phase metal cation loading with 18 is high (Figure 3 4 , the extraction selectivity is very poor with only slight differentiation among Ba2+,Ca2+, and Sr2+. Noteworthy is the observation that of the lipophilic polyether dicarboxylic acids examined in this study, only 18
The efficiency and selectivity with which lipophilic acyclic polyether dicarboxylic acids extract alkaline-earth cations from aqueous solutions into chloroform are strongly influenced by the structure of the complexing agent. The high barium selectivity observed in competitive alkaline-earth cation extraction by diionized 13 appears to arise from a unique combination of the number and positioning of polyether oxygens and the lipophilic, but rigid, carboxylic acid containing end groups. Registry No. 11,96129-22-5;12,96129-23-6;13,96129-24-7; 14,96129-25-8; 15,96129-26-9; 16,96129-27-0; 17,96129-28-1; 18, 96129-29-2; 19, 96150-64-0; Mg, 7439-95-4; Ca, 7440-70-2; Sr, 7440-24-6; Ba, 7440-39-3.
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RECEIVED for review August 31, 1984. Resubmitted March 14,1985. Accepted March 14, 1985. This research was supported by the Division of Basic Chemical Sciences of the United States Department of Energy (Contract DE-AS0580ER10604).
