(for cholesterol) the sample is dissolved in chloroform and an equal volume of strong sulfuric acid is added; if cholesterol is present, the solution becomes bluish red and slowly changes to a violet red, and the sulfuric acid becomes red with a green fluorescence. 2. (for indole) to the solution to be tested, a little nitric acid is added and then slowly a solution of 2 per cent potassium nitrite; a red color shows that indole is present.