Electrocapillary Curves of Ephedrine and Pseudoephedrine

Abstract

Electrocapillary curves at a series of concentrations of the diastereoisomeric ephedrines in 1 N NaOH and of the corresponding salts in 1 N HCl have been determined. The electrocapillary maximum (e.c.m.) was shifted to more positive potentials with an increase in concentration in each instance. Adsorption of the free bases on mercury was greater than that of the cations. The fractional surface coverage of the electrode by the cations was found to be highly dependent upon potential, with maximum coverage occurring at potentials slightly more negative than the e.c.m. Coverage by the free bases was largely independent of potential over a wide range of potentials. The Gibbs surface excess of the isomeric ephedrinium ions was the same from 0.001 M to 0.2 M . At low coverage for a given reduced concentration, the surface excess of (−)-ephedrine was greater than that of (+)-ψ-ephedrine; whereas at high coverage, the surface excess of the diastereoisomers was the same. These results imply that at high surface coverage, orientation of both isomers at the electrode is the same with planar adsorption of the aromatic nucleus on the electrode surface and the side chain in solution; at low surface coverage, the molecules lie flat on the electrode surface. The extra electrostatic work for bringing (+)-ψ-ephedrine to the surface, compared with (−)-ephedrine, was interpreted in terms of the conformations of the diastereoisomers in the bulk of the solution and at the surface. Polarograms exhibited no discrete catalytic hydrogen wave; however, the limiting currents at potentials preceding discharge appeared to be catalytic currents. The catalytic current of (−)-ephedrine was greater than that of (+)-ψ-ephedrine in borate buffers. A linear dependence of catalytic current on the concentration of each isomer was observed over a narrow concentration range.