Chromotropic acid is a chemical compound with the formula (HO)2C10H4(SO3H)2. Its name is derived from the two ancient Greek words χρωμός (chromos, color) and τροπέιν or τρέπω (verb: tropein, trepo, changing, turning), meaning it changes of color when reacting with some other compounds. This property is put to use in analytical chemistry for the colorimetric assay of various compounds such as formaldehyde, nitrate anions and even some herbicides.