DETERMINATION OF EPHEDRINE IN PHARMACEUTICAL PREPARATIONS WITH A DOUBLE-MEMBRANE SELECTIVE ELECTRODE BASED ON EPHEDRINE-5-NITROBARBITURATE

An ephedrine (EPH) double-membrane ion-selective electrode, based on an internal conducting membrane prepared from tetrabutylammonium bromide in an inert matrix of poly(vinyl chloride) resin (VCR) and another external electroactive membrane formed from the precipitate EPH-5-nitrobarbiturate, has been prepared, with use of VCR as the inert matrix. The electrode exhibited a linear response within the concentration range from 1 X 10(-2) to 5 X 10(-5) mol dm-3 EPH, with a Nernstian response (slope, 55.79 +/- 0.35 mV decade-1) at 25 +/- 0.3-degrees-C. The potentiometric selectivity coefficients k(EPH,X)pot for 14 ions were calculated. The effects of pH, membrane composition and thickness were also studied. The reproducibility (relative standard deviation) was 0.55% (n = 10 determinations) and the stability of its potential was 3.4 mV per 24 h. The electrode was preserved dry because this afforded the best sensitivity, range of linear response and detection limit. The determination of EPH in some pharmaceutical preparations was also attempted with this electrode, by using the interpolation and standard additions methods, with satisfactory results.