2022
Anal. Chem. 1990, 62,2022-2026
Effect of Ring Size Variation within Lipophilic Crown Ether Carboxylic Acids on the Selectivity and Efficiency of Competitive Alkali-Metal Cation Solvent Extraction into Chloroform Wladyslaw Walkowiak,' Sang Ihn Kang, Lewis E. Stewart, Grace Ndip, and Richard A. Bartsch*
Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061
Competltlve solvent extractlon of alkall-metal cations from aqueous solutlons Into chloroform by a series of llpophlllc crown ether carboxyllc acids wlth varylng rlng skes is reported. Extractkn selectivity for Ll' Is observed for #pophlHc crown ether carboxyllc a c h whh 12-ltimembered wether rlngs contalnlng four oxygen atoms. For lipophilic 14crown4-carboxylic acids, very hlgh LI+/Na+ selectlvlty coefflclents of 17-20 are observed wlth no detectable extraction of K+, Rb+, or C s ' . Lipophlllc crown ether carboxylic aclds which contain 15crown-5, 18crown-6, and 21-crown-7 rings exhiblt good selectivltles for Na', K', and Cs', respectlvely. In contrast, poor extractlon selectlvlty is observed for llpophlllc crown ether carboxyllc acids wAh 24-crown-8, 27-crown-9, and 30-crown-10 rings.
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CE INTRODUCTION The potential of crown ethers as a new generation of specific extracting agents for metal ions was markedly enhanced by the introduction of crown ethers which bear pendant proton-ionizable groups (1-5).Attachment to the cyclic polyether framework of a sidearm which contains an acid group allows the extractant to function both as a cation exchanger and a coordinator (6). In previous research, competitive extraction of alkali-metal cations from aqueous solutions into chloroform by lipophilic dibenzocrown ether carboxylic acids 1 was studied (7).These ionophores provided a versatile series of proton-ionizable crown ethers with which the influence of structural variation within the complexing agent upon extraction selectivity and efficiency could be assessed. These variations included the crown ether ring size (n variation) and the nature and attachment site of the lipophilic group (R, R1,R2variation). Appropriate combinations of the crown ether ring size and lipophilic group attachment site produced extractants with good selectivity for Li+, Na+, and K+. An outstanding example is IC with n = 1, which exhibits Na+/K+ and Na+/Li+ selectivity ratios of 32 and 66, respectively, in competitive alkali-metal cation extractions from aqueous solutions into chloroform. We now report results for solvent extractions of alkali-metal cations by a new series of lipophilic crown ether carboxylic acids 2-15 in which the crown ether rings do not possess rigidifying benzo groups. In addition ring size variation in 2-15 does not involve changes in the relative numbers of alkyl aryl ether oxygens (less basic) and dialkyl ether oxygens (more basic) as occurs when the crown ether ring in 1 is expanded. Present address: Institute of Inorganic Chemistry and Metallurgy of Rare Elements, Technical University of Wroclaw, 50-370 Wroclaw. Poland. 0003-2700/90/0362-2022$02.50/0
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EXPERIMENTAL SECTION Reagents. Sources of inorganic reagents and chloroform were the same as those reported previously (8-11).The 4-octylbenzoic acid was obtained from Fluka Chemical Corp. and was used as received. Syntheses of lipophilic crown ether carboxylic acids 2-15 have been reported (12, 13). Apparatus and Procedure. The apparatus and procedure were as described in the preceding paper (7). RESULTS AND DISCUSSION In an earlier investigation of alkali-metal cation extractions from aqueous solution into chloroform, it was found that selectivity orders and efficiencies for competitive extractions in multi-ion systems were quite different from expectations based upon the results of single ion extractions (8). Therefore competitive extractions were utilized in the present study. Extractants 2, 8, 10, and 12-15 are a series of lipophilic crown ether carboxylic acids in which the number of ethy0 1990 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 62, NO. 18, SEPTEMBER 15, 1990
D/
2023
Table I. Effect of Crown Ether Ring Size upon Selectivity and Efficiency of Competitive Alkali-Metal Cation Extraction from Aqueous Solutions into Chloroform by Crown Carboxylic Acids 2 , 8 , 10, 12, 13, 14, and 15
a'
compound ring size 2
12C4
selectivity order and selectivity coefficients'',*
15C5
Na+ > Kt
2.1
10
18C6
5
7
9
1
1
1
3
5
7
9
1
1
1
3
Figure 1. Molar concentrations of metals (X103) in the chloroform phase vs the equilibrium pH of the aqueous phase for competitive extraction of 0.25 M alkali-metal cations by 0.050 M (a) 2, (b) 8, and (c) 10. lenoxy units in the polyether ring is systematically increased from a 12-crown-4unit through a 30-crown-10 ring. Lipophilic crown ether carboxylic acids 9 and 11 are closely related to 8 and 10 but have one three-carbon bridge in the polyether ring. Finally compounds 2-7 are a series of lipophilic crown ether carboxylic acids which have four oxygens and ring sizes varying from 12 to 15 members. Pairs of extractants 3 , 4 and 5, 6 are structural isomers in which the crown ether ring attachment site changes from a two-carbon bridge to the central carbon of a three-carbon bridge. Extractants 2-15 were found to be sufficiently lipophilic that there was no detectable loss of the complexing agent from the chloroform phases during extraction (as assessed by ultraviolet spectroscopy). Solvent Extraction of Alkali-Metal Cations from Aqueous Solutions into Chloroform by Lipophilic Crown Ether Carboxylic Acids 2,8,10, and 12-15. For competitive solvent extractions of aqueous alkali-metal cation (0.25 M in
21C7
13
24C8
14
27C9
15
30C10
>
3.0 Rb+ Lit > Cst 4.3 5.5 6.7
90
2.2
94
K+ > Rbt > Ca+ > Na+ > Li+
64
Cs+ > Rb+ > K + > Li+ > Nat 3.0 3.6 13 15 Cst > Rbt > Na+, > Li+ 1.4 1.5 2.0 K+ > Rbt > Lit > Cs+ > Na+ 1.1 1.2 1.3 1.5 Rbt > K+ > Cs+ > Lit > Na+ 1.2 1.4 3.0 3.6
83
3.0
12
9i
Li+ > Na+ > Kt > Rbt, Cs+ 1.5
8
maximal metals loading,
4.6
13
64
100 52 41
Ratio of chloroform phase concentration of best extracted metal ion and indicated metal cation. * A t pH = 10.0.
each) solutions by 0.050 M lipophilic crown ether carboxylic acids 2, 8, 10, and 12-15 in chloroform, data for the metals concentrations in the chloroform phase vs the equilibrium pH of the aqueous phase are shown in Figures 1and 2. Selectivity orders, selectivity coefficients, and maximal metals loadings are summarized in Table I. (The selectivity coefficient is the ratio between the chloroform phase concentration of the best extracted cation and the indicated cation a t pH 10.0. The percent metals loading was calculated assuming that a 1:l metal ion to crown carboxylate extraction complex is formed.) Lipophilic crown ether carboxylic acids 2,8,10, and 12 with ring sizes of 12-crown-4, 15-crown-5, 18-crown-6, and 21crown-7 exhibit extraction selectivities for Li+, Na+, K+, and Cs+, respectively, as would be predicted from the relationship between the crown ether cavity and metal ion diameters (Table 11). Of the series, the lipophilic 18-crown-6carboxylic acid 10 has the best selectivity (Figure IC)with K+/Na+ = 13 and K+/Rb+ = 3.0. Of particular interest is the selectivity observed for the lipophilic 21-crown-7-carboxylic acid 12 (Figure 2a) in which Cs+ is the best extracted species and Na+ is the poorest. Extractants with high Cs+/Na+ selectivity have potential application in the separation of Cs+ from radioactive waste solutions (14).
20
15 C 0
.-e
e
c C
IO 0
V
5
Figure 2. Molar concentrations of metals (X103) in the chloroform phase vs the equilibrium pH of the aqueous phase for competitive extraction of 0.25 M alkali-metal cations by 0.050 M (a) 12, (b) 13, (c) 14, and (d) 15.
2024
ANALYTICAL CHEMISTRY, VOL. 62,NO. 18, SEPTEMBER 15, 1990
PH
Figure 3. Molar concentrations of metals (X103)in the chloroform phase vs the equilibrium pH of the aqueous phase for competitive extraction of 0.25 M alkali-metal cations by 0.050 M (a) 3, (b) 4, (c) 5, and (d) 6.
Table 11. Comparison of Alkali Metal Cation and Crown Ether Cavity Diameters cation
Li+ Na+ K+ Rb+ cs+
ionic diameter (15),A
crown ether cavity
cavity diameter,' A
1.20 1.90 2.66 2.96 3.38
12-crown-4 14-crown-4 15-crown-5 16-crown-5 18-crown-6 19-crown-6 21-crown-7
1.2-1.5 1.2-1.5 1.7-2.2 2.0-2.4 2.6-3.2 3.0-3.5 3.4-4.3
a Estimated from Corey-Pauling-Koltun models.
(CPK) space-filling
Maximal metals loadings for lipophilic crown ether carboxylic acids 2, 8, 10, and 12 range from 64 to 94% (Table I) with a possible inverse relationship between extraction efficiency and selectivity. Thus while selectivity decreases in the order 10 > 12 > 8 > 2, the metals loadings are highest for 2 (90%) and 8 (94%) but diminish with 12 (83%) and then for 10 (64%). Compared with 2, 8, 10, and 12, lipophilic crown ether carboxylic acids 13, 14, and 15 which have 24-crown-8, 27crown-9, and 30-crown-10 rings exhibit poor extraction selectivity (Figure 2b-d). Although 13 extracts Cs+ best, the selectivity is much poorer than that for the analogous 21crown-7 compound 12. The quantitative metals loading for 13 appears to continue the inverse relationship between extraction efficiency and selectivity which was noted above for chelating agents 2, 8, 10, and 12. Lipophilic 27-crown-9-carboxylic acid 14 exhibits the poorest extraction selectivity of the series (Figure 2c). The selectivity coefficient between K+, the best extracted metal ion, and Na+, the worst extracted, is only 1.5. For the lipophilic 30-crown-10-carboxylicacid 15, the extraction selectivity remains low, but there is selectivity for K+, Rb+, and Cs+ as a group over Li+ and Na+ (Figure 2d). Presumably the low selectivities for the large ring extractants 14 and 15 result from nonplanar geometrices of the crown ether rings which provide three-dimensional, "wrap-around" coordination of the alkali-metal cations (15). For the 27-crown-9 and 30-crown-10 compounds 14 and 15, respectively, the extraction efficiencies are the lowest for this
Figure 4. Molar concentrations of metals (X103) in the chloroform phase vs the equilibrium pH of the aqueous phase for competitive extraction of 0.25 M alkali metal cations by 0.050 M (a) 7, (b) 9, and (c) 11.
series of lipophilic crown ether carboxylic acids (Table I). Although maximal metals loadings of 50% and below might indicate the formation of 1:2 metal-to-ligand complexes for these large ring crown ethers (17-19), such extraction complex stoichiometries would be expected to favor the smaller alkali-metal cations. Since alkali-metal cation extraction by 14 exhibits so little selectivity, extraction complexes with 1:l stoichiometries appear to be more reasonable. Solvent Extraction of Alkali-Metal Cations from Aqueous Solutions into Chloroform by Lipophilic Crown-4-carboxylic Acids 2-7. In an attempt to enhance the limited Li+ selectivity found for the lipophilic 12-crown4-carboxylic acid 2 (Figure la), a series of analogous compounds 3-7 with 13-crown-4,14-crown-4,and 15-crown-4rings was examined. Results for competitive solvent extractions of alkali-metal cations from aqueous solutions into chloroform
ANALYTICAL CHEMISTRY, VOL. 62, NO. 18, SEPTEMBER 15, 1990
Table 111. Effect of Crown Ether Ring Size upon Selectivity and Efficiency of Competitive Alkali-Metal Cation Extraction from Aqueous Solutions into Chloroform by Crown Carboxylic Acids 3,4,5,6,7,9, and 11
compound ring size 3 4 5
selectivity order and selectivity coefficientso
13C4(2)b Li+ > Na+ > K+ > Rb+ > Cs+ . . 2.4 4.6 6.9 8.6 13C4(3)' Li+ > Na+ > K+ > Rb+. Cs+ 2.6 3.6 5.2 14C4(2)b Li+ > Na+ > K+, Rb+, Cs+ 17
IO
I
2025
A
maximal metal loading, %
100 100 90
NDd
14C4(3)b Li+ > Na+ > K+,-Rb+,Cs+ 100 20 NDd 7 15C4 Li+ > Na+ > K+ > Cs+ > Rb+ 92 3.7 4.9 5.4 7.1 88 Na+ > Li+ > K+ > Rb+ > Cs+ 16C5 2.5 5.4 9.3 18 11 19C6 K+ > Rb+ > Li+ > Cs+ > Na+ 100 1.5 5.5 5.9 7.8 OAt DH = 10.0. *Attachment to one carbon of a two-carbon
6
bridge In the crown ether. cAttachment to the central carbon of a three-carbon bridge in the crown ether. d N o t detected. eSelectivity coefficients a t pH = 11.0.
by 3-7 are shown in Figures 3 and 4. Selectivity orders, selectivity coefficients, and maximal metal loadings are presented in Table 111. When the ring size for the lipophilic crown ether carboxylic acid is expanded from 12-crown-4 in 2 to 13-crown-4 in 3 and 4, the Li+/Na+ selectivity coefficient increases from 1.5 (Table I) to 2.4-2.6 (Table 111). Concomitantly, the Li+/K+ selectivity coefficient increases from 2.2 for 2 to 3.6-4.6 for 3 and 4. Lipophilic crown ether carboxylic acids 3 and 4 differ in the attachment of the sidearm to a two-carbon bridge of the 13-crown-4 ring in the former and to the central carbon of a three-carbon bridge in the latter. Both 3 and 4 gave quantitative metals loadings. Further expansion of the ring size to 14-crown-4 produces very high Li+ extraction selectivity. For lipophilic 14crown-4-carboxylic acids 5 and 6 (Figures 3, parts c and d, respectively), only Li+ and Na+ are extracted into the chloroform phase and the Li+/Na+ selectivity coefficients are 17-20. These results are in agreement with earlier reports of strong Li+ complexation by 14-crown-4 compounds (19-23). In terms of both extraction selectivity and efficiency,lipophilic 14-crown-4-carboxylicacid 6 in which the sidearm is attached to the central carbon of a three-carbon bridge in the polyether ring appears to be slightly better than 5 for which the attachment site is a carbon of a two-carbon bridge (Table 111). With the further ring size increase in the lipophilic 15crown-4-carboxylic acid 7, there is a marked diminution in Li+ extraction selectivity (Figure 4a). The Li+/Na+ selectivity coefficient drops to 3.7 and K+, Rb+, and Cs+ me also extracted into the chloroform phase. These results demonstrate a very strong influence of ring size upon the selectivity of Li+ extraction by chelating agents 2-7. For a group of 11 lipophilic crown-4-carboxylic acids, compounds 5 and 6 provide the highest Li+ selectivity in competitive solvent extractions of alkali-metal cations from aqueous solutions into chloroform (7, 24). It should be noted that lipophilic crown carboxylic acids 7 and 8 both contain 15-membered polyether rings. However, the former has four ring oxygens while the latter has five. The change in selectivity order of Li+ > Na+ > K+ > Cs+ > Rb+ for the 15-crown-4 compound 7 (Table 111) to Na+ > K+ > Rb+ > Li+ > Cs+ for the 15-crown-5 compound 8 underscores the importance influence of the number of donor atoms in the polyether ring on metal ion complexation.
5
6
7
8
9
1
0
1
1
PH
Figure 5. Molar concentrations of metals (X103) in the chloroform phase vs the equilibrium pH of the aqueous phase for competitive extraction of 0.25 M alkali-metal cations by 0.050 M 4-octylbenzoic acid. Molar chloroform-phase concentrations of 4-octylbenzoic acid (X102)are shown as solid squares.
Solvent Extraction of Alkali-Metal Cations, from Aqueous Solutions into Chloroform by Lipophilic 16Crown-5- and 19-Crown-6-carboxylicAcids. In research conducted with neutral crown ether ligands, it has been found that Na+/K+ selectivity (as extrapolated from single ion species picrate extraction experiments) is higher for 16-crown-5 than 15-crown-5 compounds (25,26).Results for competitive solvent extractions of alkali-metal cations from aqueous solutions into chloroform by lipophilic 16-crown-5- and 15crown-5-carboxylic acids 9 and 8 are presented in Figures 4c and l b , respectively. Although the metal loadings are quite similar for the two extractants, the selectivity order of Na+ >> K+ >> Rb+ > Li+ > Cs+ for 8 changes to Na+ >> Li+ >> K+ > Rb+ > Cs+ for 9 (Tables I and 111). The Na+ selectivity relative to K+, Rb+, and Cs+ is enhanced in going from 8 to 9, but the Na+/Li+ selectivity coefficient decreases when the 15-crown-5 ring in the extractant is expanded to 16-crown-5. The effect of replacing a two-carbon bridge in the polyether ring with a three-carbon bridge was also examined for crown-6 compounds. Results for competitive solvent extractions of alkali-metal cations from aqueous solutions into chloroform by lipophilic 18-crown-6- and 19-crown-6-carboxylic acids 10 and 11, respectively, are presented in Figures ICand 4c. The selectivity order of K+ >> Rb+ > Cs+ >> Na+ > Li+ found for 10 changes to K+ > Rb+ >> Li+ > Cs+ > Na+ with overall lower selectivity when the polyether ring is expanded by one member in 11 (Tables I and 111). Once again Li+ extraction is enhanced when a two-carbon bridge in the polyether ring is replaced with a three-carbon bridge. Interestingly, quantitative metals loading of the chloroform phase was obtained with the lipophilic 19-crown-6-carboxylic acid 11, whereas the maximal metals loading was only 64% for the more selective 18-crown-6 analogue 10. Competitive Solvent Extraction of Alkali-Metal Cations from Aqueous Solutions into Chloroform with a Lipophilic Benzoic Acid. For comparison, solvent extractions were conducted with 4-octylbenzoic acid (161, a lipophilic carboxylic acid which does not possess a crown ether ring. Results for the competitive solvent extraction of alkali-metal cations from aqueous solutions into chloroform by model compound 16 are shown in Figure 5. As can be seen, the lipophilic carboxylic acid is found to be very unselective. Furthermore for aqueous phase pH > 8, there is significant
Anal. Chem. 1990, 62, 2026-2032
2026
loss of 16 from the chloroform phase due to the formation of a precipitate at the water-chloroform interface. Hence the polyether rings in lipophilic crown ether benzoic acids 2-15 not only provide extraction selectivity but also give extractants with improved solubility characteristics compared with 16.
McDowell, W. J. Sap. Sci. Technol. 1988, 23, 1251-1268. Walkowiak, W.; Charewicz, W. A.; Kang, S. I . ; Yang. I.-W.; Pugia, M. J.; Bartsch, R. A. Anal. Chem., preceding paper in this issue. Strzelbicki, J.; Bartsch, R. A. Anal. Chem. 1981, 5 3 , 1894-1899. Strzelbicki, J.; Bartsch. R. A. Anal. Chem. 1981, 5 3 , 2251-2253. Charewicz, W. A.; Heo, G. S.; Bartsch, R . A. Anal. Chem. 1982, 5 4 , 2094-2097. Charewicz, W. A.; Bartsch, R. A. Anal. Chem. 1982, 5 4 , 2300-2303. Czech, B.; Son, B.; Bartsch, R. A. Tetrahedron Lett. 1983, 2 4 , 2923-2936. Czech, B. P.; Czech, A.; Son, B.; Lee, H. K.; Bartsch, R. A. J . Heterocycl. Chem. 1986. 2 3 , 465-471. Shuler, R. G.;Bowers, C. B., Jr.; Smith, J. E., Jr.; Van Brunt, V.; Davis, M. W., Jr. Solvent Exk. Ion Exch. 1985, 3 , 567-604. Bush, M. A.; Truter, M. R. J . Chem. Soc., Perkin Trans 2 1972, 345-350. Mercer, M.; Truter, M. R . J . Chem. SOC., Dalton Trans. 1973, 2469-2473. Hughes, D. L. J . Chem. SOC.,Dalton Trans. 1975, 2374-2378. Owen, J. D.; Truter, M. R. J . Chem. SOC., Dalton Trans. 1979, 183 1- 1835. Kimura, K.; Kitazawa, S . ; Shono, T. Chem. Lett. 1984, 639-640. Kitazawa, S.; Kimura, K.; Yano, H.; Shono, T. J . Am. Chem. SOC. 1984, 106, 6978-6983. Kimura. K.; Yano, H.; Ktazawa, S . ; Shono, T. J . Chem. SOC.,Perkin Trans. 2 1986, 1945-1951. Kimura, K.; Tanaka, M.; Iketani, S . ; Shono, T. J . Org. Chem. 1987, 52,836-844. Attiyat, A. S.; Christian, G. D.; Xie, R . Y.; Wen, X.; Bartsch, R. A. Anal. Chem. 1988, 6 0 , 2561-2564. Bartsch, R. A.; Czech, B. P.; Kang, S. 1.; Stewart, L. E.; Waikowiak, W.; Charewicz, W. A.; Heo. G. S.; Son, B. J . Am. Chem. SOC.1985, 107, 4997. Ouchi, M.; Inoue, Y.; Sakamoto, H.; Yamahira, A.; Yoshinaga, M.; Hakushi, T. J . Org. Chem. 1983, 46, 3168-3173. Inoue, Y.; Wada, K.; Liu, Y.; Ouchi. M.; Tai, A,; Hakushi, T. J . Org. Chem. 1989, 5 4 , 5268-5272.
CONCLUSIONS For competitive solvent extraction of alkali-metal cations from aqueous solutions into chloroform by lipophilic crown ether carboxylic acids 2-15, the selectivity is strongly influenced by the crown ether ring size and number of oxygen atoms. Lipophilic crown ether carboxylic acids 2-7 with 12crown-4, 13-crown-4, 14-crown-4,and 15-crown-4rings exhibit extraction selectivity for Li+, with outstanding Li+ selectivity for the 14-crown-4 compounds 5 and 6. The extractants 8 and 9 with 15-crown-5 and 16-crown-5 rings are Na+ selective. Chelating agents 10 and 11 with 18-crown-6 and 19-crown-6 rings, respectively, are K+ selective, with a K+/Na+ selectivity coefficient of 13 for 10. The lipophilic 21-crown-7-carboxylic acid 12 is Cs+ selective and has a Cs+/Na+ selectivity coefficient of 15. Lipophilic crown ether carboxylic acids with 24-crown-8, 27-crown-9, and 30-crown-10 rings exhibit poor extraction selectivities, presumably due to deformation of the polyether rings from planar to three-dimensional “wraparound” geometries. LITERATURE CITED Helgeson, R. C.; Timko, J. M.; Cram, D. J. J . Am. Chem. SOC.1973, 9 5 , 3023-3025. Newcomb. M.; Cram, D. J. J . A m . Chem. SOC. 1977, 9 7 , 1257-1259. Nakamura, H.; Takagi. M.; Ueno, K. Talanta 1979, 26, 921-927. Frederick, L. A.; Fyles, T. M.; Gorprasad, N. P.; Whtfield, D. M. Can. J . Chem. 1981, 5 9 , 1724-1733. Bartsch, R. A.; Heo, G. S.;Kang, S. I.; Liu, Y.; Strzelbicki, J. J . Org. Chem. 1982, 47, 457-460.
RECEIVEDfor review March 20,1990. Accepted May 31,1990. The research was supported by the Division of Chemical Sciences of the Office of Basic Energy Sciences of the U S . Department of Energy (Contract DE-AS05-80ER10604 and Grant DE-FG05-88ER13832).
Design of Coaxial Segmentors for Liquid-Liquid Extraction/Flow Injection Analysis Vlastimil Kuban,’ Lars-Goran Danielsson,* and Folke Ingman Department of Analytical Chemistry, The Royal Institute of Technology, 27-100 44 Stockholm, Sweden
The process of the segmentation of two immiscible solvents by newly Introduced coaxial segmentors of dMerent geometry was studied In a contlnuous Ilquid-liquid extraction flow system. A fast readlng “on-tube” photometrlc detection system (-3 ms time resolutlon) controlled by a computer was used to measure optical transparency directly across the flowing stream. The influence of flow rates, flow rate ratio, the density and Interfacial tension of the phases as well as the geometry of the segmentors was studled. A segmentor wHh a confluence chamber made of a glass tube wlth a conical PVDF insert and an allglass segmentor wHh a conlcai m o w channel gave the most repeatable segmentation ( 8 , < 2 % ) . They work well at a total flow rate Q, up to 10 mL mln-‘ and a flow rate ratlo Q,/O, from 2 to 40. The length of the organic segments can be varied over a wide range from 2 to 50 mm and the length is only weakly influenced by the total flow rate. ‘On leave from the Department of Analytical Chemistry, J. E. Purkyne University, Kotlarska 2, CS-61137 Brno, Czechoslovakia.
Liquid-liquid extraction is a frequently used separation/ preconcentration method in flow injection analysis (FIA)( I d ) . In any method utilizing liquid-liquid extraction, be it a manual batch procedure or a method for a mechanized or automated system, three basic operations must be performed. The immiscible organic and aqueous phases must be dispensed in defined volumes, the phases must be brought into intensive contact with each other for the extraction to take place, and finally they must be physically separated from each other after the extraction in order to make the chemical separation meaningful. These three principal operations are connected to the three basic liquid-liquid extraction FIA units-a segmentor or confluence point for the organic and aqueous phase streams providing alternate and regular segments of both solvents to one uniform segmented flow in a single channel, an extraction coil, in which the solute is transferred from one phase to the other, and finally, a phase separator, the mission of which is to continuously and quantitatively separate the segmented outlet stream into two or three parts, a t least one of them consisting of only one phase.
0003-2700/90/0362-2026$02.50/0 1990 American Chemical Society
