The chemistry of Heller’s test for urine Indican detection

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.