Anal. Chem. 1980, 52, 562-564
562
Response Characteristics of Ion Selective Electrodes Based on Dinonylnaphthalenesulfonic Acid Charles R. Martin and Henry Freiser" Department of Chemistry, University of Arizona, Tucson, Arizona 8572 1
Cation selective electrodes of both the coated wire and the more conventional polymer membrane types based on incorporating the cation as a dinonylnaphthalene sulfonate (DNNS) in a plasticized poly( vinyl chloride) film were prepared and their response characteristicsstudied. These electrodes were found to be very selective for large organic cations, e.g., dodecyttrimethylammoniumion, with respect to smaller organic and all the common inorganic cations. Such selectivity behavior could be readily interpreted In terms of solvent extraction parameters. The detection limits for large cations were found to be about lo-' M. The coated wire and conventional electrodes were shown to have almost identical characteristics. The applicability of the DNNS electrodes for the determination of compounds of pharmaceutical and clinical interest are discussed.
Ion selective electrodes (ISEs) based on ion-exchange resins were investigated a number of years ago (1-5) and extensive investigation by Pungor and his collaborators (6-8) in the mid-1960s led t o the general conclusion that while these electrodes showed some selectivity to ions of different charge, discrimination among ions of the same charge is rather poor. Work on such electrodes in this laboratory (9) aimed a t the development of longer-lived polymer membrane ISEs has corroborated these results. This lack of selectivity is, of course, also observed when cross-linked ion-exchange resins (IERs) are used for chromatographic purposes. Very recently, Yeager and Steck (10) reported improved chromatographic selectivity using a non-cross-linked perfluorinated sulfonic acid cationexchange polymer. They attribute the improved selectivity of this material t o the absence of cross-linking as well as t o improved uniformity of exchange sites in the resin. T h e absence of cross-linking is claimed to give the resin a dynamic structure which can better respond t o changing counterions. If absence of cross-linking and uniformity of sulfonic acid groups does, indeed, account for the improved selectivity of the perfluorinated IER over conventional cross-linked strong acid IERs, then maximum selectivity should be obtained if a solution of a sulfonic acid in an appropriate solvent could be used. For both chromatographic and ISE purposes, a water-immiscible solvent and therefore a lipophilic sulfonic acid compatible with such a solvent would be required. This prompted us to investigate the response characteristics of plasticized polymer membrane ISEs incorporating dinonylnaphthalenesulfonic acid (DNNS) to see if improved selectivity would be obtained. EXPERIMENTAL Electrode Preparation and Handling. DNNS was obtained from Henkel Corporation (Minneapolis, Minn.) and purified using the ion-exchange method of Danesi et al. (11). The purified DNNS was used to make polymer membrane ISEs responsive to Na+, tetrabutylammonium (TBA+),and dodecyltrimethylammonium (DoTA'). The Na+ electrodes prepared were of the conventional polymer membrane type (12) (e.g., with internal reference). The TBA+ electrodes prepared were of the coated-wire (CLVE) type (13) (e.g., without internal reference). DoTA+ electrodes of both 0003-2700/80/0352-0562501 O O / O
the conventional and CWE types were prepared. For all electrodes, polymer membranes 5 w t % in DNNS, 45 wt % in dioctylphthalate (DOP) and 50 wt 7% in poly(viny1 chloride) (PVC) were used. The other components were added to a 5 ( w / v ) 70 solution of PVC in tetrahydrofuran (THF) and the resulting DNNS-DOP-PVC solution in T H F was used to either cast disks or coat copper wires. The DNNS in the membranes thus prepared was converted to the desired cationic form M (TBA+and (Na+,TBA', or DoTA+) by soaking in either a DoTA+) or a lo-' M (Na') solution of the appropriate salt. Soaking times of from 1 to 2 days were employed. When not in use, the electrodes were stored in a lo-, M solution of the appropriate salt. EMF Measurements. All electrode potentials were measured vs. a double junction Ag/AgCl reference electrode; 0.1 M NH4N03 was used in the external junction. Calibration experiments were carried out using the microcomputer-controlled potentiometric analysis system described earlier ( 1 4 ) . Electrodes were repeatedly calibrated over a period of a t least one month. Least squares analysis of the potential vs. log activity data thus obtained was then used to determine average slope and standard deviation values for each electrode. Conventional and CWE DoTA+ electrodes were prepared so that side-by-side comparison of their response characteristics could be made. Ion size parameters for the symmetrical quaternary ammonium ions were obtained from Petrucci (15). Based on these ion size parameters, DoTA+ and decyltrimethylammonium (DTA') were assigned an approximate ion size parameter of 6 X lo-' cm. Selectivity Coefficients. Selectivity coefficients were determined by a modified form of the mixed solutions method described by Srinivasan and Rechnitz ( 1 6 ) . This method is summarized in (13). Reagents. Quaternary ammonium bromides were obtained from Eastman (Rochester, N.Y.). The purity of the quaternary ammonium bromides, determined by potentiometric titration with AgNO, using the automated potentiometric analysis system (141, was found to be between 96-9970. DOP was obtained from Eastman. Chromatographic grade PVC was obtained from Polysciences (Warrington, Pa.). All other reagents were of AR grade. RESULTS AND DISCUSSION T h e critical response characteristics for the various DNNS-based electrodes are summarized in Table I. T h e data indicate that a nearly Nernstian response to TBA' and DoTA+ is obtained over three orders of magnitude in concentration. T h e linear response of the DoTA+ electrode is limited a t higher concentrations by micelle formation. T h e critical micelle concentration (CMC) is about lo-' M. Above this concentration flattening of the calibration curve is observed which does not represent a failure of the electrode t o function properly, but rather represents the complex solution chemistry that results from micelle formation. Such behavior has been observed earlier by Fujinaga e t al. ( 1 7 ) during an investigation of ISEs responsive t o anionic surfactants. Although no evidence for micelle formation was observed with the TBA+ electrode, some minor concave curvature of the calibration curve occurred for concentrations above lo-' M. While this curvature a t higher concentrations is difficult to explain. it does not prohibit the use of the electrode a t these concentrations. Hence, a useable range to lo-' M is reported in Table I. Experiments t o determine why this concave 6 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 3, MARCH 1980
563
Table I. Critical Response Characteristics of DNNS-Based Electrodes ion DoTA'
-
parameter
N af
slope (mVilog a ) standard deviationb (mV/log a ) linear range, M useable range, M
--56.04 ' 0.17' 0.265 10-2-10-4"
TBA' ~-59.99 i 0.30" 0.299 10-'-1 0-5 10-1-10-5.a
CWE
conventional
-59.04 2 0.35" 0.158 10-2-10-5
-59.13 ? O . 3 l a 0.134 10-2-10-s 10- 2 -10- 5.9
10-2-10-6.2
Average of standard deviations obtained from least a Standard deviation in slopes obtained f o r multiple calibrations. sauares analvses of individual calibration curves. ' The Na' electrode was calibrated onlv o v e r this range. Table 11. Selectivity Coefficients for the DNNS-Based Electrodes interferants
TBA+-ISE
DOTA'~
3.3
TBA-'
(1)
DTA+~
0.54 0.02 TEA-f 4 . 7 x 103 -1.4 x 1 0 - 4 TMA+P 30 1.2 N 22 A g' K6.5 NH,' 3.1 N a+ (1) Li' 0.76 H' 0.16 Pb2' Ca2+ Mg2' 0.69
TPA"
DoTA'-ISE conven tional CWE
i1) (1) 0.18 0.21 0.084 0.085 0.031 0.038 1.1 x 10-4 1.5x < 10-4 < 10-4 < 10-4
