Anal. Chem. 1987, 59, 494-496
494
Effect of Solvent upon Competitive Liquid-Liquid Extraction of Alkali Metal Cations by 2-[ (sym-Dibenzo- 16-crown-5)oxy]decanoic Acid Witold A. Charewicz,’ Wladyslaw Walkowiak,’ and Richard A. Bartsch* Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-4260
Compdtlve ik@Wk@d exlractbn d alkali metal catkns from water Into organlc solvents containing the bnlzable crown ether 2-[(sym-dibenzo-l6-crown-5)oxy~canok acld is reported. Extrac#oneffidency decreases BS the organlc solvent is varied in the order chloroform > l,l,l-trlchioroethane > tetrahydronaphthalene > benzene > toluene > p-xylene. Extractlon sekctlvlty for Na’ Is much higher In chloroform than In the other organk solvents. The relatknrhlp between emplrlcal solvent pokdly parameters and the seiectlvlty and efficiency of extraction is explored.
In previous studies of competitive alkali metal solvent extraction from aqueous solution into chloroform (1) and toluene (2) by lipophilic crown ether carboxylic acids, a substantial influence of solvent variation upon extraction selectivity and efficiency was noted. In addition, formation of a second oily organic phase was observed when highly alkaline aqueous solutions of alkali metal cations were contacted with toluene acid (1). solutions of 2-[ (sym-dibe~16-crown-5)oxy]decanoic
-1 T o better understand the effect of organic solvent variation upon the selectivity and efficiency of alkali metal solvent extraction by lipophilic crown ether carboxylic acids, we have now examined competitive solvent extractions with 1 as the complexing agent and chloroform, l,l,l-trichloroethane, tetrahydronaphthalene, benzene, toluene, and p-xylene as the organic solvents under standardized conditions.
EXPERIMENTAL SECTION Apparatus and Reagents. The apparatus and sources of inorganic reagents, chloroform, and toluene were the same as reported previously (1-4). Additional solvents were l,l,l-trichloroethane (MC&B),tetrahydronaphthalene (Pfaltz & Bauer), benzene (MC&B),and p-xylene (Eastman). The synthetic route to 24(sym-dibenzo-16-crown-5)oxy]decanoic acid has been published (5). Procedure. An aqueous solution of the alkali metal chlorides with CsOH added for pH adjustment (5.0 mL, 0.05 M in each alkali metal cation) and 5.0 mL of a 0.125 M solution of 1 in the desired organic solvent were shaken for 30 min in a 30-mL separatory funnel at room temperature. The 5.0-mL phases were separated and the equilibrium pH of the aqueous phase was measured. Of the organic phase, 4.0 mL was removed and shaken with 4.0 mL of 0.1 N HCl for 30 min to strip the alkali metal cations from the organic phase into aqueous solution for analysis by ion chromatography. Present address: Institute of Inorganic Chemistry and Metallurgy of the Rare Elements, Technical University of Wroclaw, Poland. 0003-2700/87/0359-0494$01.50/0
With this procedure, extractions of alkali metal cations into chloroform solutions of 1 have been conducted by three different co-workers with identical results which demonstrates reproducibility. In the absence of 1, extraction of alkali metal cations into the organic phase was undetectable.
RESULTS AND DISCUSSION Organic phase concentrations of the alkali metal cations as a function of the aqueous phase equilibrium pH for all six organic solvents are shown in Figure 1. The pH range to be examined was determined from preliminary extraction experiments in which the formation of a second oily organic phase was noted for extractions with tetrahydronaphthalene, toluene, and p-xylene at pH values higher than those shown in Figure 1,parts c, e, and f, respectively. For p-xylene this critical pH was 8.7 which was the lowest for these three solvents. For chloroform, l,l,l-trichloroethane, and benzene, the preliminary experiments did not indicate formation of a second oily organic phase even when the aqueous phase was highly alkaline (pH 13). Organic phase loading data (assuming formation of 1:1 complexes) for all six organic solvents at an aqueous phase equilibrium pH of 8.7 are presented in Table I. Selectivity orders for the five alkali metal cations as well as the Na+/Li+ and Na+/K+ concentration ratios in the organic phases are also given. The organic phase loading is highest for chloroform and decreases regularly in the order chloroform > l,l,l-trichloroethane > tetrahydronaphthalene > benzene > toluene > p-xylene. This regular ordering contrasts sharply with the observed extraction selectivities. As would be predicted from the ratio of the cavity size of 1 and the alkdi metal cation diameters (I),Na+ is the best-extracted metal cation for all six organic solvents. However the high selectivity for extraction of Na+ into chloroform is markedly diminished in all five other organic solvents. Since only the identity of organic solvent was varied in these experiments, rationalization of the observed results should involve consideration of solvent properties and solventsolute interactions. Despite considerable interest in the influence of organic solvent variation upon the efficiency of solventsolvent extraction (6, 7), no unifying interpretation has yet emerged. In terms of the theory of regular solutions, Hildebrand and Scott developed the useful concept of the “solubility parameter”, 6, which is the square root of the cohesive energy density of the solvent (i.e., the energy of vaporization per unit molar volume). The solubility parameter contains contributions from three types of interactions. Thus 62 = 6d2 + + 6h2, where &, 6,, and 6 h are dispersion (London), polar (dipole-dipole), and hydrogen-bonding components, respectively, of the solubility parameter. Since 6, characterized by the amount of energy necessary to separate molecules of the liquid, measures the attractive forces between the solvent molecules only, it should not necessarily be a measure of interaction forces between solvent and solute molecules. However, several correlations between 6 and efficiency in solvent-solvent ex0 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 3, FEBRUARY 1, 1987
495
Table I. Organic Phase Loading and Selectivity in Competitive Extraction of Alkali Metal Cations from Aqueous Solutions into Organic Solutions of 24(sym-Dibenzo-l6-crown-5)oxy]decanoicAcid at p H 8.7
concn ratio in organic phase Na+/Li+ Na+/K+
organic phase loading, %
solvent
62 53
chloroform l,l,l-trichloroethane tetrahydronaphthalene benzene to1u ene p-xylene
5.5
34 25
Na+ >> Li+ = K+ > Rb+ > Cs+ Na+ > Li+ = K+ > Rb+ = Cs+ Na+ > Li+ = K+ > Rb+ > Cs+ Na+ > Li+ = K+ > Rb+ = Cs+ Na+ > Li+ = K+ > Rb+ = Cs+ Na+ > Li+ = K+ = Rb+ > Cs+
6.5 1.7 1.6 1.4
1.7 1.6 1.4 1.7
47 42
selectivity order
1.8
1.8
1.4
Table 11. Selected Properties of Organic Solvents Used in Competitive Extractions of Aqueous Alkali Metal Cations by 24(sym-Dibenzo-16-crown-5)oxy]decanoicAcid
solubility parameters solvent
Va
qb
CC
wd
ETe
d
6d
6,
chloroform l,l,l-trichloroethane tetrahydronaphthalene benzene toluene p-xylene
80.7 100.4 136.0 89.4 106.8 123.9
0.596 0.903 2.202 0.603 0.587 0.644
4.81 7.53
1.15 1.57 0.60
39.1 36.2
9.21 8.57 9.758 9.15 8.91 8.80
8.65 8.25 9.60s
1.5 2.1 1.0s
8.95
0.5
1.0 1.48 1.0
8.82 8.65
0.7
1.0
0.5
1.5
2.77 2.27
2.38 2.27
0.00
34.5
0.31 0.02
33.9 33.2h
2.8
"Molar volume in cm3/mol ..om ref 8. *Viscosity at 20 O C in m-. i / m 2 from ref 14. 'Dielectric constant from ref 15. dDipole moment ' / ~ its dispersion component (6d), from ref 15. e Emperical solvent polarity parameter from ref 16. fsolubility parameter in ( c a l / ~ m ~ )with polar component (6J, and hydrogen bonding (6h) components from ref 17. gFrom ref 8. hFor a mixture of isomers.
5
7
9
1
1
5
7
9
1
1
PH Flgure 1. Concentrations of metal cations in the organlc phase vs. the equilibrium pH of the aqueous phase for Competitive extraction of alkali metal cation (0.05 M each) by 0.125 M 2-[(symdibenzo-16crown-5)oxy]decanoic acM In (a) chloroform, (b) l , l ,1-trlchloroethane, (c)tetrahydrona hthalene, (d) benzene, (e)toluene, and (f) p-xylene: (0)Na', (V)Li , (0)K+, (A)Rb', (X) Cs'.
P
traction have been obtained (9-13). In a recent study, the extractability of potassium picrate from aqueous solutions into solutions of the crown ether 18crown-6 in 57 organic solvents was investigated (13). A good correlation was observed between picrate distribution coefficients and the solubility parameters for an assortment of alcohols, ketones, esters, ethers, hydrocarbons, and halogen-
ated hydrocarbons. The hydrocarbons and halogenated hydrocarbons gave a separate correlation line from the other solvents. Good correlation of the picrate distribution coefficients and Dimroth's emperical parameter of solvent polarity, ET, was also noted. Solubility parameters, ET values, and selected physical properties of the organic solvents used in the present study are presented in Table 11. The six solvents are listed in the order of decreasing extraction efficiency. It is immediately apparent from the data contained in Table I1 that no simple relationship exists between the magnitude of the solubility parameters, 6 , and the efficiency of alkali metal cation extraction by 1. However, some correlation is noted for solvents of similar chemical nature. Thus for the two halogenated hydrocarbons and the four aromatic hydrocarbon solvents, the extraction efficiency decreases within the series as the 6 value diminishes. For solvent extraction of aqueous alkali metal cations with organic solutions of 1, the highest organic phase loading is obtained with chloroform followed by l,l,l-trichloroethane. Although both the dielectric constant, e, and the dipole moment, p , are larger for l,l,l-trichloroethane than for chloroform, Dimroth's emperical polarity value E T is larger for the latter. Indeed, the E T values for five of the six solvents which were utilized in this study have the same ordering as the extraction efficiency. Unfortunately, the E T value for the sixth solvent tetrahydronaphthalene has not been determined. Thus the efficiency of aqueous alkali metal cation extraction into an organic solvent by 1 appears to correlate with the emperical solvent polarity value E T but not with such bulk solvent polarity parameters as the dielectric constant and the dipole moment. It is interesting to compare the alkali metal cation extraction efficiencies observed in this study with those reported for four common organic solvents into which potassium picrate was extracted by 18-crown-6 (13). Distribution ratios for extraction of potassium picrate by 18-crown-6 decreased in the order chloroform >> l,l,l-trichloroethane > benzene > toluene. For the competitive extraction of alkali metal cations by 1, the organic phase loading decreased in the order chloroform > l,l,l-trichloroethane > benzene > toluene. Although the
Anal. Chem. 1087, 59, 496-504
498
solvent orderings are identical, the precipitous drop in extraction efficiency noted with 18-crown-6 when the solvent was changed from chloroform to l,l,l-trichloroethane was not observed with the crown ether carboxylic acid 1. Presumably this greater sensitivity of extraction efficiency to the solvent polarity for the neutral crown ether results from the need to transfer an aqueous phase anion as well as the alkali metal cation into the organic phase during extraction. Turning now to the matter of selectivity in competitive alkali metal cation solvent extraction by the lipophlilic crown ether carboxylic acid 1, the high Na+ selectivity which is noted in chloroform is markedly diminished in all five other solvents (Figure 1 and Table I). Neither the solubility parameter 6 nor the emperical solvent polarity ET correlate with the observed extraction selectivity. Of the solvent parameters presented in Table 11, only 6h shows a clear differentiation of chIoroform from the other solvents as a group. This suggests that the hydrogen-bonding ability of the organic solvent may play an important role in determining selectivity for aqueous alkali metal cation extraction into organic solvents by lipophilic crown ether carboxylic acids, such as 1. Registry No. 1, 79519-65-6; Li, 7439-93-2;Na, 7440-23-5; K, 7440-09-7;Rb, 7440-17-7; CS,7440-46-2.
LITERATURE CITED (1) Charewlcz, W.; Heo, G. S.; Bartsch, R. A. Anal. Chem. 1982, 5 4 ,
2094-2097.
Charewicz. W.; Bartsch. R. A. Anal. Chem. 1982, 5 4 , 2300-2303. Strzelblcki, J.; Bartsch, R. A. Anal. Chem. 1981, 53, 1894-1899. Strzelblcki. J.; Bartsch, R. A. Anal. Chem. 1981, 53, 2251-2253. Bartsch, R. A.; Liu, Y.; Kang, S. I.; Son. B.; Heo, G. S.; Hipes, P. G.; Bills, L. J. J. Org. Chem. 1989, 48, 4864-4869. Rozen, A. M. In Solvent Extraction Chemistry; Dyrssen, D., Liijenzin, J.-O., Rydberg, J., Eds.; Wiley: New York, 1967; pp 195-235. Zolotov, Y. A. Extractbn of Chelate Compounds; Ann Arbor-Humphrey Science Publishers: Ann Arbor, MI, 1970; pp 64-74. "Kirk-Othmer Encyclopsdla of Chemlcal Technology", Supplement; Interscience, 1971; pp 889-910; Wiley: New York, 1983; Vol. 21, pp
337-401. Wakahayashi, T.; Oki, S.;Omari, T.; Suzuki, N. J. Inorg. Nucl. Chem. 1964, 26, 2255-2264. Omori, T.; Wakahayashi, T.; Oki, S.; Suzuki, N. J. Inorg. Nucl. Chem. 1984. 26, 2285-2270. Oki, S.; Omori, T.; Wakahayasti, T.; Suzuki, N. J. Inorg. Nucl. Chem. 1965, 27, 1141-1150. Stepnlak-Blenlaklewlcz, D.; Szymanowski, J. J. Chem. Technol. Botechno/. 1979. 29, 686-693. Iwachdo, T.; Mlnami, M.; Nato, H.; Toei, K. Bull. Chem. SOC. Jpn. 1982, 55, 2378-2382. Lange's Handbook of Chemistry; Dean, J. A,, Ed.; McGraw-Hili: New York, 1965. Techniques In Chemistry, Vol. I I , Organlc Solvents; RMdick, J. A,, Bunger, W. B., Eds.; Wiley-Interscience: New York, 1970. Reichart, C. Solvent Effects ln Organic Chemistry; Verlag Chernie: New York, 1979; pp 242-244. Bureli, H. I n P o / y m Handbook, 2nd ed.; Brandrup, J., Irnmergut, E. H., Eds., Wiley-Interscience: New York, 1975; p IV-337ff.
RECEIVED for review June 12, 1986. Accepted October 10, 1986. This research was supported by the Division of Basic Chemical Sciences of the United States Department of Energy (Contract DE-AS05-80ER10604).
Exploitation of Reversed Micelles as a Medium in Analytical Chemiluminescence Measurements with Application to the Determination of Hydrogen Peroxide Using Luminol Hitoshi Hoshino' and Willie L. Hime* Department of Chemistry, Analytical Micellar Institute, Wake Forest University, P.O. Box 7486, Winston-Salem, North Carolina 27109
The lumW (5-amhro-2,3-dlhydro-lYdpMhalrrzkredione)-hydrogen peroxlde-3-ambphhWe (CL) system was characterized In reversed miceWes of hexachkckk (CTAC), fernred in 6 5 (v/v) chloroform-cyclohexane. The relatlve Mecttueness of tMs medlum for analytlaal CL measurements was assessed. Whereas no Ilght emledon was observed from th6 lumlnolh y d r o p peroxlde CL reaction In bulk aqueous s d W n at mHd pH (7.8-9.1) In the absence of any added catalyst or cooxklant, intense CL was observed In the CTAC reversed mkellar system. The effeci of expdmedal varlables (Le., surfactant concentration, CorolWUzed water, luminol concentration) upon the CL InteWy was evaluated. In the CTAC reversed mkdlar medium, the lumtnol CL assay can be applkd to the andysk of hydrogen peroxlde In the 6.4 X lo-' to 6.4 X lod I# concmtratkn r a w . The predakm of the reversedmiodlrrr ~OCWBCI prooedue IS w e satisfactory, renglngfrom 0.9 to 12.3 % In t e r m of the relathre standard d e v k b over the range of peroxkle concentrations examIned. The fe88Mtty of udng the reversed mkellar effect to detemhe enzymes or rwbeQatesincoupled readJon sy$tems that produce hydrogen peroxide is discussed. Present address: D e p a r t m e n t of A p p l i e d Chemistry, T o h o k u University, Aoba, Aramaki, Sendai, J a p a n 980. 0003-2700/87/0359-0496$01.50/0
The development of new or enhanced analytical methodologies based upon the use of surfactant micellar systems is a very active area of current research (for reviews, refer to ref 1-6). For example, their use has been found to be advantageous in such analytical spectroscopic techniques as ultraviolet-visible absorption (1,6, 7), fluorescence (1,5,8, 9), phosphorescence (3,10-12), and atomic absorption (13) as well as in thin-layer (2,14) and high-performance liquid chromatographic separations (2,15,16). The success of such applications is due to the fact that micelle aggregates can be employed to judiciously manipulate the solubility and microenvironment of analytes and reagents and to control the reactivity, equilibrium, and pathway of chemical or photochemical processes among other effects (I, 2, 17). More recently, these unique properties of micelles have been utilized to facilitate analytical chemiluminescence measurements (18-23). Advantages cited include elimination of solubility problems (18, 19), improved sensitivity (19-23), increased selectively (19,22), better precision (22), and a relaxation of the usually strict pH requirements for observation of efficient chemiluminescence (19,22). Surprisingly, almost all of the micellar-enhanced analytical procedures reported to date have utilized normal aqueous micelle systems. Consideration of so-called reversed micelles in chemiluminescence measurements or chemical analysis in general seems to have been overlooked. A reversed micelle 0 1987 American Chemical Society
