ABSTRACT
Visually comparing the color intensity of a sample with a standard of known concentration, after reacting both with a selective reagent, is a very old analytical principle. In the second half of the nineteenth century, it was discovered that several (colorless) alkaloids and other pharmaceutical substances produced characteristic colors when treated with certain inorganic reagents. These reactions made possible semiquantitative determinations of the substances, but there was most often no knowledge about the structure of the colored product formed or the reaction mechanism involved in the reaction.
