Unstable as the free acid. Occurs in urine of mammals, also in blood plasma. Prepn of potassium salt from potassium indoxyl and potassium bisulfate: Baeyer, Ber. 14, 1745 (1881); from chlorosulfonic acid and N-acetylindoxyl in pyridine: Schwenk, Jolles, Biochem. Z. 69, 467 (1915); by persulfate oxidation of indole: Boyland et al., Biochem. J. 62, 546 (1956).