Nitrophenol compounds have been used in a number of ways in medicines, explosives, and pesticides (Munnecke 1976). Due to the intense yellow color of the phenolate anion and pH reactivity (p-nitrophenol is colorless at pH 5.6 and the corresponding phenolate anion has a maximum yellow color at pH 7.6), nitrophenols are often used directly in titrations as indicators. Nitrophenols are also used in monitoring the enzyme activity, such as ß-galactosidase activity through detection of nitrophenol moiety and concomitant formation of the yellow color. Picric acid, also called 2,4,6-trinitrophenol, has been extensively used as a military explosive. In addition picric acid has been also used as a yellow dye, as an antiseptic and in the synthesis of chloropicrin, or nitrotrichloromethane, CCl₃NO₂, a powerful insecticide.