Effect of using mixed ion-pairs on PVC-membrane electrodes for ephedrine

Mixed ion-pairs based on the use of ephedrinium (EPH)-TPB plus EPH-reineckate (II) and phenylephrine-TPB plus EPH-reineckate (III) were tried for use in plastic membranes. The results were compared to those of an EPH-reineckate (I) single ion-pair electrode. The Nernstian slopes were 50, 49 and 55 mV decade(-1) for membranes I, II and III, respectively. The linear concentration ranges were 10(-5)-10(-1), 4.0 x 10(-5)-10(-1) and 6.3 x 10(-5)-10(-1) M ephedrine. The detection limits were 4 x 10(-6), 10(-5) and 1.2 x 10(-5) M ephedrine for membranes I, II and III, respectively. The pH ranges were 4-9, 3-9 and 2-8 for I, II and III-membranes, respectively. Selectivity coefficient values for membrane II were better than those for membranes I and III. The effects of increasing KCl concentration and temperature changes were explained for the three electrodes. The isothermal temperature coefficients were 0.00145, 0.0007 and 0.00055 V/degrees C for electrodes I, II and III. Electrode III was applied for the determination of ephedrine in its pharmaceutical preparations with an overall relative standard deviation range of 1.3-2.4\% and an overall mean recovery Value of 98.1\%.