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Selectivity of C y c k Polyether Type Liquid Membrane Electrodes G . A. Rechnitz and Ehud Eyal Chemistry Department, State University of New York, Bufalo, N . Y. 14214
A NEW DIRECTIONin the development of liquid membrane type ion selective electrodes has been the use of neutral carrier species in the membrane phase. The resulting electrodes respond mainly to univalent cations but electrodes responsive to the alkaline earth ions have recently been reported ( I ) and there is no fundamental limitation to the type of ion which might be measured if suitable carrier molecules can be found. Neutral carrier compounds in popular use at the present time include polyesters (2), antibiotics such as valinomycin (3), and cyclic polyethers ( 4 ) ; the latter are commonly referred to as "Crown" Compounds. In the present study we utilize the crown compounds shown in Figure 1. Previous work on these compounds includes the measurement of formation constants for complex formation with certain metal ions in solution (5, 6) and their effect on ion permeability in bilayer membranes (7). Generally, crown compounds have the ability to solubilize inorganic salts in organic solvents apparently via the formation of complexes with varying stoichiometry (8) in which the metal ion is accommodated within the ring cavity of the crown molecule after partial or complete desolvation. From the point of view of ion selective membrane electrodes, there exists the possibility of utilizing the ion-discriminating ability of the crown molecules to obtain electrochemical selectivity. Furthermore, it appears that the crown compounds may serve as models for the study of neutral carriers in general, and may lead to the design and synthesis of new carrier molecules with selectivity properties for ions not presently accessible to measurement. In a recent paper, we demonstrated (9) that the selectivity coefficient of liquid membrane electrodes using valinomycin as the neutral carrier can be predicted, as a first approximation, from the quotient of the formation constants of the respective metal ions with the carrier. This finding is in agreement with the situation prevailing in thin bilayer membranes (10-12). The present paper extends this concept to the series of crown compounds shown in Figure 1 and demonstrates the validity of the simple relationship between electrochemical selectivity and complex formation constant quotients for alkali metal and some alkaline earth cations. In order to reach these (1) R. J. Levins, ANAL.CHEM.,43, 1045 (1971). (2) R. P. Scholer and W. Simon, Chimiu, 24, 372 (1970). (3) L. A. R. Pioda, V. Stankova, and W. Simon, Anal. Lett., 2 (12), 665 (1969). (4) C. J Pedersen, J. Amer. Chem. SOC.,89,7017 (1967). (5) H. K. Frensdorff, ibid., 93, 600 (1971). (6) R. M. Izatt, P. P. Nelson, J. H. Rytting, B. L. Haymore, and J. J. Christiansen, ibid., p 1619. (7) G. Eisenrnan, Department of Physiology, University of California at Los Angeles, personal communication, June 1971. (8) C. J. Pedersen, J. Amer. Chem. Soc., 92, 386 (1970). (9) Ehud Eyal and G. A. Rechnitz, ANAL.CHEM.,43, 1090 (1971). (10) S. Ciani, G. Eisenman, and G. Szabo, J . Membrane Biol., 1, l(1969). (11) Ibid., p 294. (12) Ibid., p 346. 370
dlbenzo-30-crown-10
benzo-15-crown-5
Figure 1. Schematic representation of cyclic polyethers (Crown Compounds) used in this study objectives it was necessary to construct liquid membrane electrodes using crown compounds in the membrane phase, to measure their electrochemical selectivities, and to determine complex formation constants for those metal ion-crown complexes not yet reported in the literature. EXPERIMENTAL Crown compounds used in this study were the dicyclohexyl-l8-crown-6, dibenzo-18-crown-6, dibenzo-30-crown-10, and benzo-15-crown4 (see Figure 1). These were kindly given to us by Dr. H. K. Frensdorff of E. I. Dupont de Nemours and Co. The compounds were dissolved in nitrobenzene and incorporated into standard liquid membrane electrodes (Orion series 92 or Corning Model 476041) with 0.01M KCl internals. Because of low water solubility the compounds were dissolved in 50 % (volume) tetrahydrofuran-50 water mixtures for formation constant measurements. Metal ion chloride salts used were of reagent grade. Both selectivity and formation constants measurements were carried out over the metal ion concentration range of 10-3M and at room temperature. Formation constant measurements were made, using a Corning Model 476220 cation-sensitive glass electrode us. saturated calomel reference with agar salt bridge, by constructing the appropriate metal ion calibration curve and then adding ligand to the solution without removing the electrodes. The formation constant could thus be evaluated from the total metal ion concentration, the concentration of the added ligand, and the measured equilibrium concentration of the free metal ion. Selectivity coefficients, taken with respect to the potassium ion, were determined by the equipotential and equal concentration methods previously described (13). For studies (13) K . Srinivasan and G. A. Rechnitz, ANAL.CHEM.,41, 1203 (1969).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 2, FEBRUARY 1972
Table I. Potentiometric Selectivity Ratios for the Crown Compounds in Nitrobenzene (All values are taken as K+/M) Selectivity ratios (Orion electrode body) DicycloMetal hexyl-18- DibenzoBenzoDibenzoion crown-6 18-crown-6 15-crown-5 30-crown-10 2.7-2.9 2.7-3.1 3.0-3.2 0.85-0.89 Rb+ NHa+ 5.3-6.5 14-17 18.3-20.5 4.5-5.1 cs+ 7.9-8.6 3.4-4.0 6.9-7.3 2.1 Na+ 21-29 23-25 32-37 48-67 Corning electrode body Rb+ 3.1-3.4 NHd+ 6.1-7.4 cs+ 9.1-14 Na+ 69-72 Sr 2+ 2.2-2.4" a Taken with respect to barium ion: Ba2+/Sr2+. Table 11. Aqueous Formation Constants for Dicyclohexyl-18-Crown-6 Complexes. Metal ion
Log KI
K+
2.02 i 0.03
4"'
1 . 5 2 + 0.01 1 . 3 3 f 0.02
cs+
0.96 i 0.06
Rb' Na+
1.5-1.85*
Ba+'
3.57 f 0.02 3.24 i 0.02
Sr++
Tabulated from Ref. 6. From Ref. 5.
involving the alkaline earths, barium was taken as the reference ion.
RESULTS AND DISCUSSION Table I shows the potentiometric selectivity ratios, taken with respect to K+, for 10-3-10-2Msalt concentrations with the four crown compounds in nitrobenzene as the electrode membrane. We report selectivity ratio ranges rather than single values because of the expected small concentration dependence of this parameter. The Kf selectivity observed for these carriers is much smaller than that found (9) for the valinomycin electrode (-IO4). While the absolute magnitude and sequence of the selectivity values are of no particular importance t o the present study, it is known (7) that selectivity reflects the ability of the cation to accommodate itself within the coordinating cavity of the crown compound and that this
Table 111. Selected Formation Constants for Complexes of the Crown Compounds Crown compound Log Kf (K+) Log K I (Rb+) Dicyclohexyl-182.02 i: 0.03 1.52 i: 0.01 From Ref. 6 crown-6 Dibenzo-18-crown-6 1.87 f 0.10 1.35 + 0.10 This work in Benzo-15-crown-5 0.97 i: 0.10 0.46 f 0.10 50% tetraDibenzo-30-crown-10 1,35 f 0.10 1.56 f 0.10 hydrofuran-50 water
ability is a function of both the cation desolvation energetics and the size of the cation with respect t o cavity size. Thus, it can be seen that potassium ion is preferred by the 15- and 18-membered rings while rubidium is favored in the case of the larger dibenzo-30-crown-10 compound. The small differences in selectivity ratios observed between the Orion and Corning electrode configurations probably result from the difference in the nature and porosity of the supporting membrane--e.g., millipore filter disk and glass frit for the Orion and Corning electrode bodies, respectively. Aqueous formation constants for metal ion-crown compound complexes were available in the literature (5, 6) only for the dicyclohexyl-18-crown-6 case. These are tabulated in Table 11. We, therefore, found it necessary t o measure critical formation constants for the three other crown compounds represented in our selectivity studies and we chose the K+-Rb+ pair for this purpose. Because of the poor water solubility of these crown compounds, we were forced t o make measurements in a 50 tetrahydrofuran-5Ox water mixture. This procedure should be satisfactory for our present-purpose in view of our earlier evidence (9) which showed the quotient of formation constants is essentially constant with changes in solvent composition as long a s sufficient amounts of water are present to meet the hydration requirements of the metal ions involved. The resulting formation constants are summarized in Table 111. From the data of Tables I1 and 111, it is clear that the magnitude of the complex formation constants parallels the selectivity characteristics observed potentiometrically. Indeed, as is shown in Table IV, the quotients of the complex formation constants are in quantitative agreement with the potentiometric selectivity ratios. The only exception to our theoretical expectation is the case of Na+ for the dicyclohexyl-18-crown-6 compound. It should be noted, however, that the sodium K I value was the only value taken from Reference 5 and was determined by a different method than those of Reference 6. A K , value for this complex measured in methanol (5) yields a formation constant quotient in the
Table IV. Comparison of Selectivity Ratios and Formation Constant Quotients (Selectivity ratios and K l quotients with respect to K+) Selectivity ratio Crown compound Metal ion Orion body Corning body Dicyclohexyl-18-crown-6 Rb+ 2.7-2.9 3.1-3.4 NH4+ 5.3-6.5 6.1-7.4 cs+ 7.9-8.6 9.1-14 Na+ 21-29 69-72 Sr 2+ ... 2.2-2. 44 Dibenzo- 18-crown-6 Rb+ 2.7-3.1 ... Benzo-15-crown-5 Rb+ 3.0-3.2 , . . Dibenzo-30-crown-10 Rb+ 0.85-0.89 Taken with respect to barium ion.
Formation constant quotient 3.2
4.9 11.4
3.1 2.10 3.3
3.2 0.62
ANALYTICAL CHEMISTRY, VOL. 44, NO. 2, FEBRUARY 1972
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50-100 range and is in excellent agreement with the electrochemical selectivity ratio. A particularly interesting finding is the excellent agreement for the divalent ions Ba2+and Sr2+. Unfortunately, these are the only multivalent cations for which useful formation constant values have been reported to date. On the basis of our studies on the valinomycin electrode (9) and the present results we propose, for neutral carrier liquid membrane electrodes in general, the useful approximation
where ,2 is the potentiometric selectivity coefficient for the electrode’s response to ion 1 over 2 while K,J and K,,z are the equilibrium formation constants for complexes between the carrier entity and ions l and 2, respectively. In the usual case, where the formation rate constants are independent of the cation, the right hand side of this expression is equivalent to the inverse quotient of the specific dissociation rates and shows that the electrode is selective ,for the ion held longest in the cavity of the carrier molecule. We
expect that this relationship would give an approximate prediction of selectivity coefficients for those electrode systems having no special spatial arrangement of the carrier in the membrane and where the system kinetics are not potential determining. It should, thus, be possible to predict the selectivity properties of suitable neutral carrier electrodes on the basis of homogeneous complex formation data as a first approximation. Similarly, formation constants should be predictable from electrochemical data. We hope that these guidelines will facilitate the selection or synthesis of suitable membrane phase materials and will assist in the practical development of new liquid membrane electrodes.
ACKNOWLEDGMENT We thank H. K. Frensdorff for his gift of the crown compounds and G. Eisenman for some unpublished data. RECEIVED for review August 6, 1971. Accepted September 16, 1971. We gratefully acknowledge the support of the Environmental Protection Agency.
Determination of Gold and Silver in Parts-Per-Billion or Lower Levels in Geological and Metallurgical Samples by Atomic Absorption Spectrometry with a Carbon Rod Atomizer M. P. Bratzel, Jr., C. L. Chakrabarti,’ R. E. Sturgeon, M . W. McIntyre and Haig Agemian Department of Chemistry, Carleton University, Ottawa, Ontario, KIS 5B6, Canada THE RECENT development of simple carbon rod atomization devices (1-4) and their application to the atomic absorption spectrometric determination of various trace metals in biological matrixes ( I , 2 ) and in petroleum matrixes (3) has prompted our investigation of the routine analysis of gold and silver in geological samples, including investigation of sensitivity and detection limit, effect of interferences, and application to samples of different matrixes. Nonflame atomization devices have recently been reviewed by Kirkbright(5).
EXPERIMENTAL Apparatus. The burner assembly of the atomic absorption system employed in this study (Model AA-5, Varian Techtron Pty. Ltd., Melbourne, Victoria, Australia) was replaced with a carbon rod atomizer (CRA) unit (Model 61, Varian Techtron) whose operation has been described elsewhere (3, 4 ) . “Mini-Massmann” type carbon rods were used in this study. The detector was a R-213 photomultiplier tube. Argon was employed as sheath gas. Sampling was accomplished with a hypodermic microliter syringe with a platinum needle (No. 75 SN, The Hamilton Company, Inc., Whittier, Calif.). Gold analyses were performed at the 242.80-nm resonance line and silver analysis at the 328.07nm resonance line. The slit width was 0.100 mm, corresponding to a spectral bandpass of 0.33 nm. 1
To whom all correspondence should be addressed.
(1) T. S. West and X. K. Williams, Anal. Chim. Acta, 45, 27 (1969). (2) M. D. Amos, P. A. Bennett, K. G. Brodie, P. W. Y. Lung, and J. P. Matousek, ANAL.CHEM., 43, 211 (1971). (3) K. G. Brodie and J. P. Matousek, ibid., p 1557. (4) J. P. Matousek and B. J. Stevens, Clin. Chem., 17, 363 (1971). ( 5 ) G. F. Kirkbright, Annlysr, 96, 609 (1971).
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Reagents. Standard analytical-grade reagents were employed for these studies, including standardized 1000 pg/ml gold solution (Hartman-Leddon Company, Philadelphia, Pa., 19143), silver nitrate crystal (Anachemia Chemicals Ltd., Montreal, Quebec), and triisooctyl phosphorothioate (TOTP) (Mobil Chemical, 801 East Main Street, Richmond, Va. 23208). Ammonium pyrrolidine dithiocarbamate (APDC) was prepared by the method discussed by Slavin (6). Standardized rock and mill product samples were provided by Cominco Ltd. (Trail, British Columbia) and the U.S. Geological Survey. Procedure. Slavin (7) and Mallett (8) have reviewed the experimental procedures for preparation of geological samples for trace analysis. For this study an adequate procedure to decompose the sample matrix and to leach the gold and silver from the rock cia acid attack is as follows. The rock sample is leached twice with constant stirring with aliquots of freshly prepared aqua regia. The sample is boiled to dryness after adding each aliquot. For gold a third aliquot of aqua regia is added but for silver the third aliquot is concentrated H N 0 3 . The temperature of the mixture is raised to near boiling, then cooled slowly to room temperature, filtered, washed with 5 % (v/v) HC1 (gold) (v/v) H N 0 3 (silver), and the gold or silver quantitaor tively extracted into methyl isobutyl ketone (MIBK), which had been freshly equilibrated with an aqueous solution containing 5 % (v/v) HC1 (gold) or 5 % (v/v) H N 0 3 (silver). The pH of the aqueous phase is about zero but is not critical
5z
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(6) W. Slavin, At, Absorp. Newslett., 3, 141 (1964). (7) W. Slavin, “Atomic Absorption Spectroscopy,” Interscience, John Wiley & Sons, New York, N.Y., 1968. (8) R. C. Mallett, Minerals Sci. Eng., 2, 28 (1970).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 2, FEBRUARY 1972
