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

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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 10–2–10–5 M ephedrine over the pH range 4–7. The lower limit of detection is 4.5 × 10–6 M 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.

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