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.

The Heller’s color test for indican in urine is interesting because besides detecting this important biomarker it reveals the dual function of nitric acid since it can accomplish the synthesis of indigo blue and indigo red, as well as it can degrade them to isatin, depending upon the reaction conditions. Moreover, the Heller’s test inspired other tests for indican detection, using other oxidizing reagents. Actually, indican in urine is detected by a colour chart with different tones of blue indicating the concentration of indican via indigo blue formation. In this paper we provide the series of reactions that occur during the test, there are nitrations followed by redox reactions. The organic intermediates are oxidized and the inorganic reagent is reduced to nitrous acid. However, indigo blue and indigo red can be degraded by nitric acid on .heating. The electron flow is given in each step.

The recent interest for new applications of methylene blue as a promising drug for several important ailments prompted us to fill unknown aspects related to this compound. Since there is no mechanism on the Caro synthesis of methylene blue, we provide the electron flow, step by step, from the starting N,N-dimethylphenylenediamine to the dibenzothiazine derivative. It is a free radical generation system due to the oxidizing properties of ferric chloride. We pointed out the alternative reactions that take place with hydrogen sulphide, the reducer from which the thiazine ring is formed.

Although the Froehde reaction is widely used for alkaloid preliminary identification, there is no reaction mechanism in order to explain the reaction course and the end products. We found there can be two redox steps and we provide a reaction mechanism for both of them and give two routes for the second reaction. The chemical deportment of the reagent, molybdic acid and its derivatives is also discussed.

The chemistry of a spot test for identification of a substance with biological activity is important. Besides the academic interest of knowing what is going on in the test, the study reveals the chemical deportment of the active compound, and this is related to biochemistry. This result can be used to enhance, or on the contrary, deter the activity of the compound. In this communication we provide the chemistry involved in the spot test for strychnine identification by means of lead dioxide (Marchand’s test). The electron flow, step by step, is given from the alkaloid to the complex oxidation product. Thus, this toxicological assay has been up-dated.