Coated Wire Ion Selective Electrodes Helen James, Gary Carmack, and Henry Freiser Department of Chemistry, University of Arizona, Tucson, Ariz. 85721
THERECENT DISCOVERY in this Laboratory ( I ) that a platinum wire coated with a polyvinyl chloride (PVC) solution of calcium didodecylphosphate functioned as a Ca-selective electrode prompted us to attempt to extend this approach in order to learn whether it could be generally used for the production of reliable, miniaturized, inexpensive, and sturdy ion selective electrodes. Inasmuch as such electrodes would be of particular use in problems of organic and biochemical interest, it was decided to examine the "liquid" membrane electrodes for various systems based on the anion association complexes of a large quaternary ammonium ion, Aliquat 336s (233). EXPERIMENTAL
The ion association complex solution was prepared by repeatedly shaking a 60 v/v% solution of Aliquat 3368 in 1-decanol with an aqueous solution (OSM-lM) of the sodium salt of the appropriate anion in order to form the proper quaternary ammonium salt. For the phenylalanine and leucine electrodes a 30 v/v% Aliquat 3368 :decanol solution was used instead. A fine platinum wire (0.01-0.02-in. diameter) was coated with polyvinyl chloride (PVC) by one of two methods. For the phenylalanine and leucine electrodes, the wire was dipped into a 15 w/v% PVC in cyclohexanone solution, allowed to become almost dry (about 30 minutes) and then soaked for 1 to 2 hr in the previously prepared ion association complex solution in decanol. For the other electrodes, the wire was coated with a 1 O : l mixture of 8 w/v% PVC: cyclohexanone and the decanol solution of the complex and the coating was allowed to dry thoroughly (about 1 hr). In either case the remainder of the exposed wire was wrapped tightly with a paraffin film to prevent direct contact of the metal surface with the test solution. The electrode was initially conditioned by soaking it in a 10-lM solution of the anion to be measured for 15 minutes. The electrode was stored in air (1) R. W. Cattrall and H. Freiser, ANAL.CHEM., 43, 1905 (1971). (2) C. J. Coetzee and H. Freiser, ibid., 41, 1128 (1969). (3) M. Matsui and H Freiser, Anal. Lett., 3, 161 (1970).
Electrode Perchlorate Chloride Bromide Iodide Thiocyanate Benzoate Salicylate Oxalate Sulfate Phenylalanine Leucine
a
RESULTS AND DISCUSSION
The electrodes were first tested in pure solutions of the appropriate salts. Equilibrium potentials were achieved within a few seconds and were reproducible to d~0.5mV or better. The potential response was linear with the logarithm of anion activity from 10-l to at least 10-2,eM although the useful concentration range was usually 10-1-10-4M. This information is summarized in Table I. The electrodes required daily restandardization, as was also reported for the liquid-membrane electrodes of similar composition (3). Interferences by other anions were determined by adding a or 4 X sufficient amount of the interfering anion to a
Table I. Response Characteristics of Coated Wire Electrodes Slope, Concn range of Useful concn Electrode mV/log a linear response, M range, M 10-1-10-4 10-1-10-4 Perchlorate 58 10-1-10-4 10-1-10-4 Chloride 55 10-1-10-4 10- 1-10- 3 Bromide 59 10-1-10-4 10-1-10-4 Iodide 60 10- 1-10- 4 10-1-10-3 Thiocyanate 59 10-1-10-4 10-1-10-4 28" Oxalate 10-1-10-3 10- 1-10- 4 50" Acetate 10- 1-10- 3 10-1-10-4 Benzoate 53" 10- 1-10- 4 10- 1-10- 3 Sulfate 28 10-1-1010-1-1 0- 3 53" Salicylate 10- 1-10- 2.6 10-1-10-3 Phenylalanine 54" 10-1-10-3 10-1-10-2.6 52a Leucine a log c.
Table 11. Selectivity Coefficients, K I , for the Coated Wire Electrodes Compared to Those for the Liquid-Membrane Electrodes (2, 3) Interfering Anion -. Chloride Nitrate Sulfate Miscellaneous 0.004 (0.18)" 0.028 (0.12)
