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 × 10^–5–10^–1 and 6.3 ×^–5–10^–1 M ephedrine. The detection limits were 4 ×^–6,10^–5 and 1.2 × 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 KC1 concentration and temperature changes were explained for the three electrodes. The isothermal temperature coefficients were 0.00145, 0.0007 and 0.00055 V/ °C for electrodes I, II and III. Electrode III was applied for the determination of ephedrine in its pharmeaceutical preparations with an overall relative standard deviation range of 1.3–2.4% and an overall mean recovery value of 98.1%.