Potassium Ion Responsive Coated Wire Electrode Based on Valinomycin R. W. Cattrall and S. Tribuzio Department of Inorganic and Analytical Chemistry, La Trobe University, Bundoora, Victoria, Australia 3083
Henry Freiser Department of Chemistry, University of Arizona, Tucson, Ariz. 8572 1
The successful application of coated wire ion selective electrodes to a variety of cationic ( 1 ) and anionic (2) species involving ion association and chelate systems encouraged us to investigate the feasibility of this approach in the so-called neutral carrier electrode system. This study is devoted to an examination of the potassium ion selective electrode based on valinomycin and its application to the analysis of potassium in whole blood and in sea water.
EXPERIMENTAL Construction of Coated Wire Electrodes. A fine platinum wire sealed to the core conductor of coaxial cable was coated with polyvinyl chloride (PVC) mixtures (prepared as described below) and allowed to dry for two days in air. The exposed portion of wire was carefully wrapped in paraffin film to prevent direct contact of the metal surface with the test solution. The electrodes were conditioned in 10-5M KCl for about one hour before being used, and stored in air. Preparation of PVC Coating Mixtures. Various coating mixtures of PVC, a plasticizer (either di(2-ethyl hexyl)-2-ethylhexylphosphonate (DOOP) or di-n- decylphthalate (DDP) and valinomycin were prepared by weighing out the materials and dissolving the mixture in the minimum amount of tetrahydrofuran. Potentiometric Measurements. An Orion Model 801 digital pH meter was used for all measurements and double junction calomel electrode with 10%NHdNOz in the outer compartment served as the reference electrode. Calibration curves were obtained using potassium chloride solutions and activities were calculated using the extended Debye-Huckel equation. Standards. I t is essential to use standards which match the sample solution fairly closely and, consequently, for the analysis of the blood samples Versatol (General Diagnostics Division, Warner-Chilcott Labs., Morris Plains, N.J.) standards were used containing 3.1, 4.9, and 7.3 milliequivalents/l. of potassium, respectively. For the analysis of potassium in sea water, standards were used containing 350 g/ml and 430 g/ml potassium and were prepared by dissolving potassium chloride in 0.47M NaCl. Analysis Using Flame Photometry. Comparisons were made of the results obtained using the coated wire electrode with the results obtained using a standard flame photometry method. For the analysis of sample (3),the sample was centrifuged a t 3000 rpm for 15 min to obtain serum, and the serum was diluted using a standard method. The Versatol standards were diluted in the same way. Measurements were made using a Gallenkamp Flame Photometer. For the analysis of sea water, standards were used containing potassium in the range 0-100 g/ml and containing 0.47M NaCI. Sea water samples were diluted 10 times.
RESULTS AND DISCUSSIONS The response characteristics of the coated wire electrodes were tested using solutions of from 0.1M KC1 to 10-5M KCl. Electrodes using DOOP plasticizer give a linear response (slope = 51 mV/log a ) from 0.1M to 10-4M with a usable range down to 10-5M KC1. Those using DDP were significantly better, exhibiting both a longer linear re(1) R. W. Cattrall and H . Freiser, Anal. Cbem., 43, 1905 (1971). (2) H. James, G. Carmack, and H . Freiser, Anal. Cbem., 44, 856 (1972). (3) G. J. Moody and J. D. R. Thomas, "Selective Ion Sensitive Electrodes," Merrow Publishing Co., England, 1971.
sponse range, from 1Ob1M to at least 10-5M KCl, and a greater sensitivity (slope = 57 mV/log a ) . In addition, whereas electrodes using DOOP exhibited a marked hydrogen ion response so that they were not very useful at pH values of 5 or below, the pH profile using DDP is very flat throughout most of the p H range (pH 2 to 10). One disturbing but manageable drawback with these electrodes is the positive drift in the potential for each solution upon the continuous measurement of a series of standards starting with the lowest concentration first. These drifts are of the order of about 3 to 4 mV for a particular solution for each set of measurements. This drift depended on the proportion of plasticizer in the coating. Reducing the plasticizer level (with either DOOP or DDP) from about 70% to about 40% significantly reduced the rate of drift to about half. Also the response time of the electrodes increased with decreasing plasticizer content, however. Whereas with about 70% plasticizer, steady readings for a t least 1 minute were achieved in less than one minute, about 4 to 5 minutes were required a t the lower plasticizer levels. It is our opinion that the advantage of increased reproducibility seen in the latter cases offsets the disadvantage of longer response times. Of course, in practical analytical situations, the reproducibility can be improved dramatically by the simple expedient of running standards along with the unknowns. As an illustration of the analytical utility of these electrodes, they were used in the titration of KCl with sodium tetraphenylborate (TBP). A 2.0-ml solution of 0.1M KCl was diluted to 20 ml with water and then titrated with 0.1M TPB. With the electrode using DOOP as plasticizer, a well-defined titration curve with a symmetrical change of about 45 mV in the vicinity of the equivalence point (between 95 and 105%completion) was obtained. The titration could also be carried out in solutions that were 0.1M in sodium chloride, although the vertical range of titration curve in the vicinity of the end point is significantly reduced. Use of the electrode with DDP plasticizer resulted in a steeper end-point region (about 55 mV in the 95/105% range) which was relatively unchanged by the presence of 0.1M NaCl. The response of the electrodes to pure KC1 solutions was checked after the titrations. For those in which no NaCl was present, the electrode seemed to function even better (more stable and faster response) than before. This is reminiscent of the observation of W. Simon that incorporation of tetraphenylborate into a PVC-valinomycin membrane electrode reduced the "anion effect" ( 4 ) . The response of the electrodes following titration in the presence of sodium chloride was somewhat erratic but could be restored to nor-
Simon, E. Pretsch, W. €. Morf. and D. Ammann, Pittsburg Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 4-8, 1974, Paper 257.
(4) W.
ANALYTICAL CHEMISTRY, V O L . 46, NO. 14, DECEMBER 1974
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Table I. Selectivity Ratios for Various Cations with Coated Wire Potassium-Valinomycin Electrodes
Table 11. Analysis of Whole Blood and Sea Water K+ concentration
S e l e c t i v i t y ratio, I: Sample Interfering ion
H' Li' Na Rb'
+
cs
+
"4'
Be2' Mg2' Ca2+ Sr Ni2' CU2' Zn2+
'+
DOOP-plasticized electrode
2.07 0.005 0.003 2.3 0.42 0.016 0.012
