Potassium Ferricyanide, is typically made from potassium ferrocyanide. The compound has widespread use in blueprint drawing and in photography. The compound is also used to harden iron and steel, in electroplating, dyeing wool, as a laboratory reagent, and as a mild oxidizing agent in organic chemistry. In an aqueous solution it shows some green-yellow fluorescence. It was discovered in 1822 by Leopold Gmelin and was initially used in the production of ultramarine dyes. Potassium ferricyanide is manufactured by passing chlorine through a solution of potassium ferrocyanide.