Voltammetry of Vitamin B12 on a thin self-assembled monolayer modified electrode

Vitamin B12 showed three reduction waves at a thin self-assembled monolayer of mercaptoacetic acid modified gold electrode at 0.21, 0.16, −0.41 V in a 0.01 mol l−1 HCl solution at a scan rate of 100 mV s−1. The overall electrode reaction followed an ECE mechanism, leading to a total two-electron exchange. The predominant Co(III) form was reduced directly at 0.21 V by a one-electron transfer accomplished by CN− cleavage into the cyanocob(II)alamin. The latter species then equilibrated with the base-off B12r, which was immediately reduced into B12s at 0.16 V. The wave at −0.41 V might be a catalytic hydrogen wave. The cathodic peak currents at 0.21 V were controlled by the diffusion of Vitamin B12. On the base of its diffusion behavior, the semi-derivate voltammetric method for the detection of Vitamin B12 was presented. The semi-derivate voltammetric peak current of the wave at 0.21 V was linear with the content of Vitamin B12 in the range of 4.0 × 10−9 to 4.0 × 10−5 mol l−1. The detection limit was 1.0 × 10−9 mol l−1. The proposed method was applied successfully to determine the content of Vitamin B12 in pharmaceutical preparations.