Liquid membrane electrode for the direct determination of ephedrine in pharmaceutical preparations

Abstract

The construction and performance characteristics of a liquid membrane electrode responsive to the ephedrine cation are described. The electrode is based on the use of the ephedrine-5-nitrobarbiturate ion-pair complex in nitrobenzene as an ion-exchange site. The electrode shows a stable, near-Nernstian response for 10^–2–10^–5M ephedrine over the pH range 4–7. The lower limit of detection is 4.5 × 10^–6M and the response time 20–90 s, and the selectivity coefficients for ephedrine relative to a number of interfering substances were investigated. Many organic and inorganic cations and pharmaceutical excipients and diluents commonly used in drug formulations do not interfere. The determination of 0.1–2000 µg ml^–1 of ephedrine in aqueous solutions shows an average recovery of 99.2% and a mean relative standard deviation of 1.5%. The direct determination of ephedrine in some pharmaceutical preparations gives results that compare favourably with those obtained by the British Pharmaceutical Codex method.