Usually prepd from iodide ion and p-diazobenzenesulfonic acid, followed by treatment with phosphorus pentachloride. A 5-10-fold excess reacts quantitatively with amino acids as indicated by the disappearance of amino nitrogen. Radioactive pipsyl chloride is used in the analysis of proteins: Keston et al., J. Am. Chem. Soc. 68, 1390 (1946).