Frémy's salt

Frémy's salt is a chemical compound with the formula (K42), sometimes written as (K2). It a bright yellowish-brown solid, but its aqueous solutions are bright violet. The related sodium salt, i.e. disodium nitrosodisulfonate (NDS, Na2ON(SO3)2, CAS RN 29554-37-8) is also referred to as Frémy's salt. Frémy's salt is a chemical compound with the formula (K42), sometimes written as (K2). It a bright yellowish-brown solid, but its aqueous solutions are bright violet. The related sodium salt, i.e. disodium nitrosodisulfonate (NDS, Na2ON(SO3)2, CAS RN 29554-37-8) is also referred to as Frémy's salt. Regardless of the cations, the salts are distinctive because aqueous solutions contain the radical 2-. Fremy's salt, being a long-lived free radical, is used as a standard in electron paramagnetic resonance (EPR) spectroscopy, e.g. for quantitation of radicals. It intense EPR spectrum is dominated by three lines of equal intensity with a spacing of about 13 G (= 1.3 mT). The inorganic aminoxyl group is a persistent radical, akin to TEMPO. It has been used in some oxidation reactions, e.g. for oxidation of some anilines and phenols. It can also be used as a model for peroxyl radicals in studies that examine the antioxidant mechanism of action in a wide range of natural products. Fremy's salt is prepared from hydroxylaminedisulfonic acid. Oxidation of the conjugate base gives the purple dianion: The synthesis can be performed by combining nitrite and bisulfite to give the hydroxylaminedisulfonate. Oxidation is typically conducted at low-temperature, either chemically or by electrolysis.