The chemistry of Mandelin’s test for strychnine

Abstract

Strychnine reacts with Mandelin’s reagent giving a violet-blue colouration which changes to orange-red and finally to yellow. The chromic acid test for strychnine was surpassed by Mandelin’s assay due to better reagent stability, lower detection limits and fewer false positives. The reagent is ammonium metavanadate in sulphuric acid. In this communication we provide the reaction route and the mechanism of each step, from the alkaloid to the complex final oxidation product. Several oxidations take place and the vanadic acid formed in situ is reduced to vanadous acid. Some information related to vanadium and vanadium compounds is given in order to understand the reactivity of the involved compound.