Roussin's black salt
Roussin's black salt is a chemical compound with the formula KFeS(NO). It consists of the potassium salt of the [FeS(NO)] anion, metal nitrosyl compound. First described by Zacharie Roussin in 1858, it is one of the first synthetic iron-sulfur clusters along with the red salt also bearing his name. Structure. The cluster anion has the geometry of an incomplete cubane-type cluster with "C" symmetry. The dark colour of the complex is attributed to a number of charge-transfer interactions. Synthesis. Roussin’s Black Salt is produced by the reaction of nitrous acid, potassium  hydroxide, potassium sulfide, and iron(II) sulfate in aqueous solution. It can also be formed by the conversion of Roussin's Red Salt in mildly acidic conditions. This reaction is reversible and Roussin’s Red Salt is reformed upon alkalization of the reaction solution. Uses. Roussin’s black salt is a nitric oxide donor. Also, Roussin’s Black Salt exhibits antibacterial activity in some food processing applications.