Anal. Chem. 1994,66, 1743-1746
Sodium Ion-Selective Electrodes Based on Dibenzo-16-crown-5 Compounds with Pendent Amide Groups Aklra Ohkl and Shlgeru Maeda Department of Applied Chemistry and Chemical Engineering, Faculty of Engineering, Kagoshima Universityl 1-2 1-40 Korimoto, Kagoshima 890, Japan Jian Ping Lu and Richard A. Bartsch' Department of Chemistry and Biochemistry, Texas Tech Universityl Lubbock, Texas 79409
Potentiometric selectivitiesfor alkali-metalcations of dibenzo16-crown-5 compounds with amide-containing side arms attached to the central carbon atom of the three-carbonbridge have been determined in solvent polymeric membrane electrodes. The lariat ethers include N-alkyl- and N,N-dialkyl-sym(R)dibenzo-16-crown-5-oxyacetamides with R = hydrogen or an alkyl group. The presence of an alkyl group on the ring carbon which bears the amide-containingside arm markedly increases the Na+/K+ selectivityof poly(viny1chloride) matix membrane electrodes with o-nitrophenyl octyl ether as the membrane solvent. Lariat ethers with a N,N-dipentyloxyacetamidegroup as the side arm and a geminal alkyl group of two or more carbon atoms exhibit high Na+/K+ selectivities (log KN&K~O* = -2.1) with good selectivities for Na+ over the other alkalimetal cations, H+, NH4+, and alkaline-earth-metal cations. Due to difficulties encountered with theuse of Na+-selective glass electrodes in clinical applications, considerable attention has been focused upon the development of neutral-carrier type Na+-selective electrodes. The earliest Na+-selective electrode of this type with a Na+/K+selectivity greater than 100utilized an acyclic triamide (ETH 227) as the ionophore in a poly(vinyl chloride) (PVC) matrix with o-nitrophenyl octyl ether (NPOE) as the membrane solvent.'v2 More recently Na+selective electrodes with bis( 12-crown-4) compound^,^.^ disubstituted 16-crown-5 compounds,5 a hemispherand,6 and calix[4]arene tetraesters and tetraamide~~-~ as ionophores have been reported. In previous work, we have prepared derivatives of dibenzo16-crown-5, such as sym-(R)dibenzo-16-crown-5-oxyacetacetic acid (lwith R = H or alkyl), in which an oxyacetic acid group (-0CH2C02H) is attached to the central carbon of the (1) Ghggi, M.; Oehme, M.; Pretsch, E.; Simon, W. Helu. Chim. Acta 1976, 59, 24 17-2420. (2) Steiner, R. A.; Oehme, M.; Ammann, D.; Simon, W. Anal. Chem. 1979, 51, 351-353. (3) Shono, T.; Okahara, M.; Ikeda, I.; Kimura, K.; Tamura, H. J. ElecrroanaL Chem. Interfacial Electrochem. 1982, 132, 99-105. (4) Tamura, H.; Kimura, K.; Shono, T. Anal. Chem. 1982,54, 1224-1227. ( 5 ) Suzuki, K.; Hayashi, K.; Tohda, K.; Watanabe, K.; Ouchi, M.; Hakushi, T.; Inoue, Y . Anal. Lett. 1991, 24. 1085-1091. (6) Toner, J. L.; Daniel, D. S.; Geer, S. M. U.S. Patent 4,476,007, 1984. (7) Diamond, D.; Svehla, G.; Seward, E. M.; McKervey, M. A. Anal. Chim. Acto 1988, 204, 223-231. ( 8 ) Kimura, K.; Matsuo, M.; Shono, T. Chem. Letr. 1988, 615-616. (9) Kimura, K.; Miura, T.; Matsuo, M.; Shono, T. Anal. Chem. 1990,62, 15101513.
0003-2700~94/0366-1743$04.50/0 0 1994 American Chemical Society
0
0
II
OCHpCOH
r5
r5
a::D
0 1 L
O
II
OCHpCN(R')(R")
R
J
L
O
J
2
1
0
B
R
B!
3
-H
-H
-C5H11
4
-C3H7
-H
-C5Hll
5
-H
-C5H11
-C5H11
6
C3H7
-C5Hi1
-C5H11
7
C3H7
-CHf2H20CH3
-CH$H20CH3
8
-C3H7
-CH~CHZOCH~CH~OCH~ -CYCHZOCH~CHZOCH~
9
C3H7
IO
-C3H7
-CI+CH2CH2CH2CH2. -C~CH~OCH~CHZ-
Figure 1. Structures of lariat ether derivatives based upon dibenro16-crown-5.
three-carbon bridge in the polyether ring. In their ionized forms, such proton-ionizable lariat ethers1° are efficient and selective agents for the solvent extraction of alkali-metal and alkaline-earth-metal cations and lanthanide ions and their transport across liquid membranes.' 1-20 We have also investigated the influence of nonionizable side arms, such as -0CH2C02H (unionized), -0CHz(IO) Gokel, G. W.; Dishong, D. M.; Diamond, C. J. J . Chem. Soc., Chem. Commun. 1980, 1053-1054. (1 1) Strzelbicki, J.; Bartsch, R. A. Anal. Chem. 1981, 53, 1894-1899. (12) Bartsch, R. A.; Heo, G. S.; Kang, S. I.; Liu, Y.;Strzelbicki, J. J . Org. Chem. 1982,47, 457-460. (13) Charewicz, W. A.; Heo, G. S.; Bartsch, R. A. Anal. Chem. 1982,54,20942097. (14) Bartsch, R. A.; Liu, Y.;Kang, S. I.; Son, B.; Heo, G. S.; Hipes, P. G.; Bills, L. J. Org. Chem. 1983,48,4864-4869. (15)Tang, J.; Wai, C. M.Anal. Chem. 1986, 58, 3233-3235.
AnalyticalChemistty, Vol. 66, No. 10, May 15, 1994
1743
C02C2H5, and -OCH2C(O)N(C2H5)2, upon the complexation of Li+, Na+, and K+ by dibenzo-16-crown-5 compounds in solvent polymeric membrane electrodes.21 Among these lariat ethers,1° those which bear - O C H ~ C ( O ) N ( C ~ H Sside ) ~ arms were found to exhibit the greatest Na+/K+ selectivity. Therefore, dibenzo- 16-crown-5 compounds with pendent amide groups warrant further evaluation as ionophores for Na+-selective membrane electrodes. In the present study we have examined a variety of N-alkyland N,N-dialkyl-sym-( R)dibenzo- 16-crown-5-oxyacetamides (2 with R = H or an alkyl group) in terms of potentiometric selectivity for Na+ in solvent polymeric membrane electrodes. In addition todetermination of the effect of structural variation of the R, R', and R" groups upon the Na+ selectivity, the influence of membrane solvent has been probed. The effect of pH upon the electrode response has also been investigated. EXPERIMENTAL SECTION Chemicals. Poly(viny1 chloride) (PVC) with an average polymerization degree of 1100, dibenzyl ether (DBE), and dioctyl sebacate (DOS) were purchased from Wako Pure Chemical Industries (Osaka, Japan). o-Nitrophenyl octyl ether (NPOE) and potassium tetrakisb-chloropheny1)borate (KTpClPB) were obtained from Dojindo Laboratories (Kumamoto, Japan). Alkali- and alkaline-earth-metal chlorides, ammonium chloride, and tetrahydrofuran (THF) were reagentgrade chemicals. Deionized water was prepared by passing distilled water though three Barnstead D8922 combination cartridges in series. Lariat ether amides 3-10, the series of N,N-dialkyl-symdibenzo-l6-crown-5-oxyacetamides, and the series of N,Ndialkyl-sym-propyldibenzo-16-crown-5-oxyacetamidesare known compounds.22 The series of N,N-dipentyl-sym-alkyldibenzo- 16-crown-5-oxyacetamides with alkyl groups other than propyl were prepared by adaptation of the procedure reported for the synthesis of N,N-dipentyl-sym-propyl-dibenzo16-crown-5-0xyacetemide.~~ Preparation of PVC Membranes. PVC (50 mg), the membrane solvent (100 mg), the lariat ether amide (5.0 mg), and KTpClPB (1 .O mg) were dissolved in 1.5 mL of THF. NPOE was used as the membrane solvent unless noted otherwise. An aliquot of the T H F solution was dropped onto a porous poly(tetrafluoroethy1ene) (PTFE) membrane attached to a PVC tube, and the solvent was allowed to evaporate for 15-20 min. Addition of the T H F solution and evaporation were repeated eight or nine times. The resulting PVC tube with the coated PTFE membrane was fixed on a Denki Kagaku Keiki (DKK, Musashino, Tokyo, Japan) Number 7900 electrode body, and an internal filling solution of 0.10 M NaCl was added to the electrode. The electrode was conditioned by soaking in 0.10 M NaCl solution for 12 h before use. Measurements. Potentiometric measurements with a membrane electrode were carried out at 24-25 O C with a (16) Charewicz, W. A.; Walkowiak, W.; Bartsch, R. A. Anal. Chem. 1987, 59,
494-496. (17) Tang, J.; Wai, C. M. J. Membr. Sci. 1988, 35, 339-345. (18) Bartsch, R. A. Solvent Extr. Ion Exch. 1989, 7, 829-854. (19) Walkowiak, W.; Charewicz, W. A.; Kang, S. 1.; Yang, I. W.; Pugia, M. J.; Bartsch, R. A. Anal. Chem. 1990, 62, 2018-2021. (20) Wai, C. M.; Du, H. S.Anal. Chem. 1990,62, 2412-2414. (21) Ohki, A,; Lu, J. P.; Bartsch, R. A. Anal. Chem. 1994, 66, 651-654. (22) Kasprzyk, S.P.; Bartsch, R. A. J. Heterocycl. Chem. 1993, 30, 119-123.
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Analytical Chemistry, Vol. 66, No. 10,May 15, 1994
voltage meter (Fisher Scientific Accumet 50 pH meter or DKK PHL-40 pH meter), a double junction Ag-AgC1 reference electrode (DKK Number 4083), and a magnetic stirrer to agitate the sample solution. The electrode cell was Ag-AgC1/0. 10 M NaCl/PVC membrane/sample solution/ 0.10 M NH4N03/3.0 M KCl/Ag-AgCl. Singleion activities were obtained as described previously.21 The selectivity coefficients (KN~,M~O') were determined by the fixed interference method.23 The background concentrations of interfering ions were 5.0 X M for Kt, 1.0 X M for the other alkali-metal cations and H+, 1.0 M for Mg2+,and 5.0 X 10-1 M for the other alkaline-earth cations and NH4+. The pH dependence of the electrode response was investigated by adjusting the pH of the sample solutions using 1.0 M ammonium hydroxide or 1.O M hydrochloric acid. For a given solvent polymeric membrane electrode system, the potentiometric selectivity was determined twice for each of two independently prepared membranes. The average value for the potentiometric selectivity and the standard deviation were calculated from thevalues obtained for the four measurements. RESULTS AND DISCUSSION Na+/K+ and Na+/Li+ Selectivities of Lariat Ether Amides 3-10. Lariat ether amides 3-10 were incorporated into polymeric membranes with PVC as the polymer matrix and NPOE as the membrane solvent. For the ion-selective electrodes (ISEs) prepared from these membranes, potentiometric selectivities for Na+ relative to K+ or Li+ were determined by the fixed interference method.23 Nerstian response (59 mV/decade) was observed for these solvent polymeric membrane electrodes. In Table 1 are recorded the Na+/K+ selectivities (log KN~,K'O~)and Na+/Li+ selectivities (log K N ~ , L ~ ~for O ~the ) ISEs based on ionophores 3-10. Lariat ether amides 3 and 5 have N-pentyloxyacetamide and N,N-dipentyloxyacetamideside arms, respectively, attached to the central carbon of the three-carbon bridge of dibenzo-16-crown-5. On the other hand, for lariat ether amides 4 and 6 , the central atoms bear a propyl group as well as the amide-containing side arm. Compared with 3 and 5, which do not possess the geminal propyl group, lariat ether amides 4 and 6, respectively, exhibit Na+/K+ selectivities with log K N ~ ,values K ~ ~which ~ are more negative by 1.O-1.2 units. The presence of the geminal propyl group is proposed to orient the functional side arm over the polyether cavity in a conformation which requires minimal structural reorganization for metal ion complexation.*' The increase in theNa+/ K+ selectivity for lariat ether amides 4 and 6 compared with 3 and 5 is ascribed to such a preorganization of the binding site24by thegeminal propyl group. Lariat ether tertiary amides 5 and 6, which have two pentyl groups attached to the oxyacetamide side arm, exhibit somewhat higher Na+/K+ selectivities than do the corresponding lariat ether secondary amides 3 and 4, respectively, which have a pentyl group and a hydrogen attached to the amide nitrogen. Lariat ether amides 7 and 8 have geminal propyl groups and N,N-bis(3-oxabutyl)oxyacetamide and N,N-bis(3,6-di(23) Recommendations for Nomenclature of Ion-Selective Electrodes. Pure Appl. Chem. 1976, 48, 127-132. (24) Cram, D. J. Angew. Chem.. Inr. Ed. Engl. 1986, 25, 1039-1057.
Table 1. Na+/K+ and Na+/LI+ Selectlvlty Coefflclents Expressed as log K,,.,K~and log KI(.,uw, Rerpectlvely, for ISEs Based on Larlat Ether Amides 3-10
lariat ether amide' 1%
-H -C3H7 -H -C3H7 -C3H7 -C3H7 -C3H7 -C3H7
3 4
5 6
7 8 9 10
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-CsHii
b
b
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45H11 45H11 -CHzCHzOCH3
-(CH2)54H2CH20CHzCHz-
Structures of the lariat ether amides are shown in Figure 1. b R' = R"
-0.89 f 0.05 -1.90 i 0.01 -0.98 i 0.03 -2.13 f 0.01 -2.03 f 0.04 -1.92 f 0.05 -2.12 f 0.04 -2.11 f 0.03
KNa,LiP*
-3.31 f 0.06 -3.21 f 0.02 -3.14 f 0.02 -2.91 f 0.04 -3.02 f 0.03 -2.85 f 0.01 -2.89 f 0.03 -3.01 f 0.01
= -CH&H~OCH~CHZOCH~.
(Figure 2). Another small decrease in theNa+/Li+ selectivity is noted when the methyl group is replaced with ethyl. Further elongation of the geminal alkyl group beyond ethyl yields no significant alteration in the Na+/Li+ selectivity. -l.O Thus the introduction of a geminal methyl group and its replacement by an ethyl group cause an increase in the Na+/ K+ selectivity but a decrease in the Na+/Li+ selectivity. The opposite effects of these structural variations upon the Na+/ K+ and Na+/Li+ selectivities may be rationalized by considering the different types of lariat ether amide-metal ion complexes involved. For dibenzo- 16-crown-5compounds, both Na+ and Li+ should form inclusion or nesting25complexes. On the other hand, K+, which is too large to fit within the -3.51" ' I " . ' ' " I ' 1 polyether cavity, should form a perching complex.25 Therefore, H 2 4 6 8 10 12 14 16 18 the Na+/K+ selectivity results from a competition between Alkyl group chain length (R) Flgure 2. Effect of geminal alkyl group variation on the Na+/K+ and formation of a nesting complex with Na+ and a perching Na+/Li+ selectivities for lariat ether amides 2 with R = H or C,+lHPel, complex with K+. Preorganization of the binding site24by where n = 1-18, and with R' = R" = C5HI1. introduction of a geminal alkyl group apparently favors the nesting complex and enhances the Na+/K+ selectivity. For the Na+/Li+ selectivity, nesting complexes are anoxahepty1)oxyacetamide side arms, respectively, attached to ticipated for both alkali-metal cation species. Preorganization the dibenzo- 16-crown-5 framework. These ionophores exhibit of the binding site24 should enhance complexation of both Na+/K+ selectivitiessimilar to that reported2' for N,N-diethylsym-propyldibenzo- 16-crown-5-oxyacetamide (log K N ~ ,=K ~ ~ Na+ ~ and Li+. It appears that the increase is slightly greater for the harder alkali-metal cation, which produces small -1 -98).Thus it appears that the oxygen atoms in the N-alkyl reductions in the Na+/Li+ selectivity when a geminal methyl groups of 7 and 8 do not interact with the metal ion. Lariat group is introduced and then replaced with an ethyl group. ether amides 9 and 10 have N-piperidinooxyacetamideand Effect of N,N-Dialkyl Group Variation. In Figures 3 and N-morpholinooxyacetamide side arms, respectively. The 4 are shown the effects of varying the length of theN,N-dialkyl Na+/K+ selectivities of these ionophores are the same as that group in N,N-dialkyl-sym-(R)dibenzo16-crown-5-oxyacetaobserved for N,N-dipentyl-sym-propyldibenzo16-crown-5mides (2 with R = H or C3H7 and R' = R" = alkyl) upon oxyacetamide (6). the Na+/K+and Na+/Li+ selectivities,respectively. The alkyl Effect of Geminal Alkyl Group Variation. The effect of group variation includes ethyl, propyl, butyl, pentyl, and hexyl varying the length of the geminal alkyl group in N,N-dipentylgroups. As can be seen, this structural variation does not sym-alkyldibenzo- 16-crown-5-oxyacetamides (2 with R = H produce marked changes in either Na+/K+ or Na+/Li+ or alkyl and R' = R" = C ~ H Iupon ~ ) the Na+/K+ and Na+/ selectivities. However, it is noted that the Na+/K+ selectivities Li+ selectivites is shown in Figure 2. The alkyl groupvariation are largest for the N,N-dipentylamides (Figure 3). Similarly, includes methyl, ethyl, propyl, butyl, hexyl, octyl, decyl, the highest Na+/Li+ selectivity is observed for N,N-dipentyldodecyl, tetradecyl, hexadecyl, and octadecyl groups. Atsym-dibenzo16-crown-5-oxyacetamide (Figure 4). tachment of even the smallest alkyl group (methyl) at the Sodium-Selective Electrode Based on Lariat Ether Amide geminal position in the lariat ether amides results in a marked 6. Since it was found the dibenzo- 16-crown-5 compound with increase in the Na+/K+ selectivity. An additional slight a N,N-dipentyloxyacetamideside arm and a geminal propyl increase in the Na+/K+ selectivity is noted when the methyl group exhibited the highest Na+/K+ selectivity, solvent group is replaced by ethyl. Further elongation of the geminal polymeric membrane electrodes containing lariat ether amide alkyl group from two to 18 carbons produces no significant change in the Na+/K+ selectivity. ( 2 5 ) Cram, D.J.; Trueblood, K.N. Host Guest Complex Chemistry. Macrocycles. Attachment of a methyl group at the geminal position Synthesis, Structures, andApp1ication.s;VBgtlc, F., Weber, E., Eds.; Springerproduces a small diminution for the Na+/Li+ selectivity Verlag: New York, 1985; pp 135-188.
tT
AnalytiMChemistry, Vol. 66, No. 10, May 15, 1994
1745
-o 5m
n z Y
-1.5
-rn
R = C3H7
NPOE
Figure 5. Seiectivii coefficients (log KM,Mpot) for ISEs based on lariat ether amide 6 for Na+ over the other alkali-metal cations, aikaiineearth cations, NH4+, and H+ with NPOE, DBE, and DOS as the membrane solvents.
-2.0
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-
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.-
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6 were evaluated more fully. The electrodes were found to exhibit Nernstian response over a Na+ concentration range of 3.0 x 10-5 to 1.0 x 10-1 M. Selectivity coefficients expressed as log KNa,MPot values for Na+ over the other alkali-metal cations, alkaline-earth metal cations, NH4+, and H+ are presented in Figure 5 for ISEs based on 6. Three different membrane solvents, NPOE (dielectric constant e = 24), DBE (e = 4), and DOS (e =4), were utilized. It has been reported that the Na+ selectivity for neutral-carrier-type solvent polymeric membrane electrodes may be influenced by the choice of membrane s o l ~ e n t . ~ - ~ For ISEs employing lariat ether amide 6 as the ionophore, the use of the lower polarity solvents DBE and DOS results in a decrease in the Na+/K+ selectivity compared with that observed for NPOE. In general, for Na+ selectivity over monovalent ions, such as the other alkali-metal cations and NH4+, NPOE is found to be superior to DBE and DOS. For competition between Na+ and the alkaline-earth-metal cations, variation of the membrane solvent has little effect on the Na+/ Mg2+,Na+/Ca2+, and Na+/Sr2+ selectivities. However the Na+/Ba2+selectivity is considerably lower with NPOE as the membrane solvent compared with DBE and DOS. Measurements of the pH dependence of the electrode response were performed for ISEs prepared with lariat ether 1748
AnalyticalChemistry, Vol. 66, No. 10,May 15, 1994
-150
I
.
,
.
,
.
,
.
,
amide 6 as the ionophore and NPOE as the membrane solvent. The emf remained constant in the pH range 2-9 in the NaCl concentration range 10-2-104 M (Figure 6). In conclusion, we have shown that dibenzo- 16-crown-5 compounds with a N,N-dipentyloxyacetamidegroup attached to the central carbon of the three-carbon bridge and a geminal alkyl group of two or more carbon atoms are excellent ionophores for Na+-selective electrodes. The Na+/K+ selectivities exhibited by these solvent polymeric membrane electrodes approach the best values reported for ISEs employing other neutral-carrier-type ionophores. 1-9 ACKNOWLEDGMENT The portions of this research which were conducted at Texas Tech University were supported by the Division of Chemical Sciences of the Office of Basic Energy Sciences of the U. S. Department of Energy (Grant DE-FG05-88ER13832). Received for review January 3, 1994. Accepted March 3, 1994." ~~~
_________
@Abstractpublished in Aduance ACS Abstrocts, April 1, 1994.
