We offer potassium cyanide. An extremely poisonous white compound, kcn, used in the extraction of gold and silver from ores, in electroplating, and in photography, and as a fumigant and insecticide. (inorganic chemistry) the potassium salt of hydrocyanic acid, kcn; the colourless crystals are soluble in water, and it is extremely poisonous, and is used in electroplating, the extraction of gold and silver from their ores, and in photography.