anism; variations in electrode response with variations in bronze composition; and kinetics involved in redox systems, using the bronze electrodes and radio-tracer techniques.
are also grateful for discussions with Paul Hahn, Ames Laboratory, and Harvey Diehl and Dennis Johnson of the Chemistry Department, Iowa State University.
ACKNOWLEDGMENT The authors acknowledge Gordon Danielson, Ames Laboratory, for providing the tungsten bronzes, and Paul Millis, Ames Laboratory, for his technical assistance. We
RECEIVED For review August 20, 1971. Accepted October 26, 1971. Work performed in the Ames Laboratory of the U.S. Atomic Energy Commission.
Role of Solvent Extraction Parameters in Liquid Membrane Ion Sektive Electrodes Helen J. James, Gary P. Carmack, and Henry Freiser Department of Chemistry, University of Arizona, Tucson, Ariz. 85721
THERELATIVELY RECENT development of a new class of electrochemical sensors called ion selective electrodes has quite properly been the subject of widespread attention and study ( I ) . Of these, the type referred to as liquid membrane electrodes offer the exciting prospect that solvent extraction principles provide a guide to their design and operation. Thus, most of the liquid membrane electrodes introduced so far incorporate solvent extraction systems. Metal ion electrodes have made use of their extractable complexes with acid phosphate esters and thiocarboxylic acids. Anion responsive electrodes have employed extractable ion association complexes using ferroin and analogous large cations (2). Recently, we have described a series of anion responsive electrodes employing the extractable complexes formed by the tricaprylmethylammonium cation (Aliquat 3363) and a number of inorganic and organic cations (3-5), in which the qualitative observation of the relation of electrode selectivity to ion pair extractability was made. In this communication we wish to report the results of a quantitative study of the extraction equilibria of a representative series of ion pair complexes undertaken to examine the postulated relationship more closely.
culture tubes (15 X 125 mm), covered with Parafilm, were used as counting containers. Extraction Equilibrium Experiments. DISTRIBUTION OF THAI. Equal volumes of a 3.30 X 10-3M THAI solution in an organic solvent and an aqueous phase containing a sufficient amount of 13II to give good counting statistics were shaken together for 30 minutes, a time adequate for equilibration. The phases were allowed to separate and 5 ml aliquots of each phase were pipetted into separate counting tubes and counted. The ratio of activities observed was taken as a measure of D, the distribution ratio. In order to avoid possible interferences from trace impurities, the organic phase was separated, equilibrated again with distilled water, and the value of D redetermined radiochemically. Agreement was obtained to within lO-15z (0.05-0.07 log unit). COMPETITIVE EXTRACTION EQUILIBRIA.Equal volumes of an aqueous solution containing 3.30 X 10-2M organic anion and a sufficient amount of 1311to give good counting statistics and an organic solution of 3.30 X 10-3M THAI were equilibrated and aliquots of the separated phase counted as previously described. With the amino acid distribution series, the aqueous phases were adjusted to pH 10.5 with NaOH prior to equilibration. The equilibrium pH values of the amino acid solutions were also determined. Agreement in these experiments was 5 or better.
EXPERIMENTAL Reagents. Tetra-n-hexylammonium iodide (THAI) was obtained from Eastman Organic and used as received. All other reagents used in this study were of analytical reagent grade. Distribution studies made use of carrier-free lalI (New England Nuclear Corp.) in the form of NaI. Apparatus. Extractions were performed using 45-ml cylindrical glass vials fitted with polyethylene thimble stoppers and plastic screw caps. Samples were shaken in an Eberbach reciprocating shaker at the high speed setting with temperature control being maintained at 25 f 0.2 “C by circulating water from a Wilkens-Anderson Co. Lo-Temp bath through the jacketed shaker tray. A Nuclear-Chicago Model 186 scaler in conjunction with a Nuclear-Chicago Model DS-55 well-type scintillation detector was employed for radioisotope counting. Kimax lipless (1) “Ion-Selective Electrodes,” R. A. Durst, Ed., Nut. Bur. Stand. (US.)Spec. Publ. 314, Washington, D.C., Nov. 1969. (2) J. W. Ross, ibid.. Chapter 2. (3) C. J. Coetzee and H. Freiser, ANAL.CHEM., 40, 2071 (1968). (4) Zbid.,41, 1128 (1969). ( 5 ) M. Matsui and H. Freiser, Anal. Lett., 3, 161 (1970).
RESULTS AND DISCUSSION
It is relatively simple and convenient to quantitatively determine the participating equilibria of an ion pair complex extraction system containing an easily measured ion such as I3lI. Furthermore, by means of competitive extraction (i.e., a reaction system containing both 1311and another anion capable of forming an extractable ion pair complex), the convenience of radiochemical analysis serves for the determination of the equilibria involved in a whole series of extractable ion pair complexes. The same competitive reaction principle undoubtedly would apply to a series of complexes in which one of the ions has a readily measured characteristic (e.g.,high molar absorptivity, sensitive atomic absorption line, etc.). The distribution of THAI between an organic solvent and water involve two equilibria: ion pair formation in the aqueous phase Q+
+ I- =KIP +=
Q+, I--
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853
Table I. Tetrahexylammonium Iodide Distribution Equilibria log KIPKD [THAI] n-Pentanol n-Octanol Decanol ... 6.01 9.63 X 10e3M 5.63 4.77 X lW3 6.39 6.13 5.97 6.41 6.16 9.08 x 5.78 6.43 6.13 5.79 4.32 x 10-4 6.39 6.30 2.05 x 10-4 5.83 6.90 6.33 3.41 x 10-6 5.76 6.35 6.18 AV log KIPKD 5.80 Table 11. Competitive Extraction Constants for Anions Distributing between Water and Decanol at 25 “C Anion log K Anion log K Formate -1.41 Glycine -2.04 Acetate -1.51 Serine -1.99 Propionate -1.42 Alanine -1.95 Crotonate -0.97 Valine -1.50 Caproate +O, 14 Methionine -1.43 n-Octanoate +0.94 Lysine -1.22 n-Decanoate 1.25 Leucine -0.95 -0.05 Phenylalanine -0.43 Benzoate Salicylate +O. 73 Tryptophan -0.41 p-Toluenesulfonate +O. 27
where Q represents the quaternary ammonium ion, and phase distribution of QI
As-yet-unpublished observations (6) indicate that various KDKIPvalues are of the order of In the low dielectric constants media of the organic solvents, the extent of ion pair dissociation is small enough to neglect (Kdiss 1O-lo) (7). Although it is also possible for ion pairs to aggregate into higher complexes in low dielectric constant organic solvents, the absence of any dependence of calculated equilibrium constants on total ion pair concentration (a necessary consequence of polymolecularity (8)) in this work suggested that such reactions need not be considered. Combination of the equilibrium expressions of reactions 1 and 2 yields
-
rnc
T-I
(3) from which, since electroneutrality demands that [Q+l = [I-], we obtain
(4) Because the distribution ratio of THAI is so large, the activity of the 1311 in the organic phase can be considered to be proportional to the initial THAI concentration in that phase. Further, the proportionality constant thus obtained between activity and concentration can be applied without modification to determining the aqueous phase iodide concentration.
co
= k and A / k = [I-]
+ [Q+,I-] ts [I-]
(5)
(6) G. Colovos, G. P. Carmack, and H. Freiser, University of Arizona, Tucson, Ariz., 1970. (7) C. A. Kraus, J. Phys. Chem., 60, 129 (1956). (8) G. H. Morrison and H. Freiser, “Solvent Extraction in Analytical Chemistry,” John Wiley, New York, N.Y., 1957. 854
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Table 111. log K I ~ K D for Quaternary Hexylammonium Salts of Various Amino Acids in Certain Alcohol-Water Systems at 25 “C Amino acid Pentanol Octanol Decanol Glycine 4.88 4.24 3.75 Serine 4.88 4.32 3.80 Alanine 4.92 4.26 3.84 Valine 5.44 4.79 4.29 Methionine 5.55 4.89 4.36 Lysine 5.38 4.98 4.57 Leucine 5.90 5.29 4.84 Phenylalanine 6.22 5.72 5.36 Tryptophan 6.30 5.77 5.38
where A. and A are the organic and aqueous phase count rates, and Co, the initial organic phase THAI concentration. Hence, Equation 3 becomes
Values of log KIP. KD characterizing THAI distribution between an aqueous phase and a series of alcohols are given in Table I. It is interesting to note that the extraction equilibrium increases with decreasing size of the alkyl group, indicating a preference by the ion pair complex itself highly polar (large dipole moment), for the more polar solvents. This trend would, of course, be opposed by increasing mutual solubility of the alcohol and water. A number of factors must be taken into account in evaluating the effect of the solvent, but probably the solvents giving the highest K I PKD . values would be those whose solubility parameters would be closest to that of the ion pair complex (9). The competitive distribution or solvent extraction exchange taking place between an anion, R-, and iodide ion
(7) can be best described by Equation 3 and its analog for Q+,R-. From this, the equilibrium constant of the exchange reaction 7 can be shown to be equal to the K1pKDratio for the two pairs QI and QR:
K =
(KIPKD)QR - [Q+,R-lo [I-] (KIPKD)QI [R+,I-lo [R-I
(8)
As in the QI distribution experiments, the concentrations of the various species were obtained by monitoring the aqueous and organic phase count rate. This gives [I-] and [Q+,I-]o, respectively, using the proportionality constant of Equation 5, while stoichiometry requires that [Q+,R-]o = [I-] and [R-] = CR(initial) - [Q+,R-Io. In the case of the amino acids, the equilibrium pH measurement was used to correct the concentration for that fraction that was in the zwitterion form since this form did not distribute. The competitive extraction constants as calculated using Equation 8 are listed in Table 11. The reliability of these data is of the order of 510% (0.03-0.05 in logl&. The extractability, as measured by log K, of the ion pair complexes generally increases with the number of carbon atoms in the anion. Although the lower members of the series do not exhibit any appreciable change, the effect is almost linear among the aliphatic acids starting with caproic acid and, using the old solubility rule of thumb that a phenyl group is about equivalent to four acyclic carbon atoms, includes both benzoate and p-toluenesulfonate. (9) H. Freiser, ANAL,CHEM., 41,1354 (1969).
Table IV. Comparison of the Competitive Extraction Constants of Interfering Anions and the Selectivity Coefficient for Various Amino Acid-Responsive Electrodes Phenylalanine Interfering Tryptophan electrode, electrode, Leucine electrode, Methionine electrode, Valine electrode, anion log KiIKG log KiIKa log KilKa log KiIKa 1% KiIKa Glycine - 1.52 (-1.8)” -1.61 (-1.4) -1.11 (-1.2) -0.61 (-0.8) -0.54(-0.9) Alanine - 1.54 (- 1.6) -1.63 (-1.3) -1.13(-1.2) -0.63 (-0.8) -0.56(-0.8) Valine -0.98 .(-0.8) -1.07 (-0.8) -0.57 (-0.6) -0.07 (-0.4) ... ... Leucine -0.41 (-0.4) -0.5 (-0.4) ... ... +0.50 .,. +0.57 , . . Serine -1.47(-1.9) -1.56(-1.4) - 1.06 (- 1.5) -0.56 (- 1.2) -0.49 (-0.9) +0.07(-0.1) Methionine -0.91 (-0.8) -1.0 (-0.7) -0.50 (- 1.5) Phenylalanine +0.09 (-0.1) . . . ... +0.50 ($0.2) + L O (+0:4) +1.07 ($0.4) ... -0.09 (-0.1) +0.41 (+0.2) +0.91(+0.1) +0.98 ($0.7) Tryptophan a (log Ks). Table V. Comparison of the Competitive Extraction Constants of Interfering Anions and the Selectivity Coefficient for Various Organic Anion-Responsive Electrodes pToluene Propionate Benzoate sulfonate Salicylate electrode, electrode, Interfering Formate electrode, Acetate electrode, electrode, electrode, anion log KilKa log KilKa log KilKa log KdIKa log KiIKa log KiIKa 0 . 0 (-0.2) + O . l O (f0.04) Formate Acetate -0.1 (-0.3)a -0.09 (-0.31) -1.46(=-2) .2.24 (-2.4) 0.0 (0.05) 0.09 (0.05) Propionate Benzoate 1.36 (-1 .O) 1.46(~1.4) 1.37(--1.2) -0.32(-0.24) p-Toluene1.78 (1.5) sulfonate 0.78 (0.75) Salicylate ‘(log Ks).
The unusually high extractability of the salicylate probably reflects the influence of H-bonding in the alcohol phase. The distribution behavior of the amino acid salts closely parallels that of the carboxylates. The reduction in extractability that can be expected from the increased polarity of these ions is most clearly seen with valine as a decrease of about 1.4 to 1.6 units of log K from the corresponding carboxylate salt. The effect of chain length of the solvent on the ion pair distribution equilibria may be seen from Table I11 where the log K I P K ~values for the various tetrahexylammoniumamino acid anion pairs in n-pentanol, n-octanol, and ndecanol. The order of extractability for the various amino acid salts is the same in each of the three alcohols. The extraction constant is greatest in the lowest, most polar, of the alcohols by about 0.4-0.6 unit in log K than in octanol which in turn exhibits greater solvent power (by about 0.4-0.5 unit in log K ) than the highest molecular weight alcohol. One of the major points of interest of this study was to test the hypothesis that “liquid membrane” ion-selective electrodes function in a manner predictable from solvent extraction principles by comparing the competitive extraction constants of interfering anions with the corresponding selectivity constant. This constant, Ks, is defined by the Eisenman (IO) equation relating the change in electrode potential, AE, arising from the addition of an interferant of activity ai (10) G. Eisenman, Chapter 1 in Reference (I).
and charge ZI,to a solution of activity aAof the ion to which the electrode responds.
AE
=
0.0591 ~
n
ain/zi
log 1
+ &--aA
(9)
In Tables IV and V are listed values of the competitive extraction constants as log Kex’ where Kex’ is the equilibrium constant of the reaction
+
Ked
+
+
Q+,A-(o) A - . Q+,A-i(o) A(10) A plot of log Ks us. log Kex’for the system shown in both Tables IV and V is essentially linear (correlation coefficient of 0.92) which amply demonstrates the importance of considering extraction parameters in dealing with the electrode response characteristics. It is likely that a non-equilibrium factor, namely the relative ionic mobilities, should be taken into account for an even more quantitative description. Our findings are somewhat at odds with the conclusions of Eyul and Rechnitz (11) whose work with valinomycin-based electrodes concludes that “potentiometric selectivity depends only on the relative formation constants of complexes of the ions in solution. . .and not on any process inside the membrane.” I
RECEIVED for review August 20, 1971. Accepted November 9, 1971. The authors gratefully acknowledge financial support of the Public Health Service. (11) E. Eyul and G. A. Rechnitz, ANAL.CHEM., 43, 1090 (1970).
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