Potassium Cyanide

Properties

About Potassium Cyanide

Potassium cyanide (KCN, 65.116 g/mol) is the white granular salt that visually resembles sugar and chemically resembles death — it has carried more cultural weight as a poison than almost any other compound on the periodic table, and yet two industries quietly depend on it. The first is gold mining: the MacArthur-Forrest cyanidation process, patented in 1887, dissolves gold from crushed ore by air-oxidation in dilute alkaline cyanide (4 Au + 8 KCN + O2 + 2 H2O -> 4 K[Au(CN)2] + 4 KOH), forming the soluble linear aurocyanide complex [Au(CN)2]^- that is then reduced to gold metal on zinc dust (Merrill-Crowe) or stripped from activated carbon (CIP/CIL). About 13 percent of global cyanide production goes to gold and silver leaching, and most of the world's annual ~3000 tonnes of gold is recovered this way. The second is electroplating: alkaline cyanide baths give the smoothest, most uniform deposits of gold, silver, copper, and zinc on jewelry, electronics connectors, and plumbing fixtures, because the metal-cyanide complex equalizes deposition kinetics across irregular surfaces in a way that simple sulfate baths cannot match. In organic synthesis KCN provides the CN- nucleophile for the Strecker amino-acid synthesis (aldehyde + NH3 + HCN -> amino nitrile -> amino acid) and the Kolbe nitrile synthesis (R-X + KCN -> R-CN, then hydrolysis or reduction). Toxicity is mechanistic: CN- binds the iron(III) center of cytochrome c oxidase (complex IV in the mitochondrial respiratory chain) with sub-micromolar affinity, blocking electron transfer to O2 and shutting down ATP synthesis within minutes.

Where you'll encounter it

If you've worked at a gold mine in Nevada, Western Australia, or Witwatersrand, you've worked downstream of a cyanide leach pad — KCN or NaCN in dilute alkaline solution percolates through crushed ore for weeks, dissolving gold as the [Au(CN)2]^- complex that gets pumped to a recovery circuit. The 2000 Baia Mare disaster in Romania, where a tailings-dam breach released about 100,000 m3 of cyanide-laden process water into the Tisza and Danube rivers, is the standard case study in mining engineering courses for why cyanide containment and the International Cyanide Management Code exist. If you've worked in electronics manufacturing, the gold flash on connector pins from Foxconn or any other contract assembler started as a cyanide bath at a plating shop somewhere in the supply chain — alkaline cyanide gives uniform deposits at 0.1-1 micron thicknesses that no other plating chemistry can match for reliability over a million mating cycles. In a synthesis lab the Strecker reaction with KCN in methanol is still a textbook route to alpha-amino acids, even though most production-scale amino acid is now made by enzymatic resolution or fermentation.

Common Uses

• MacArthur-Forrest cyanidation leaching of gold and silver from crushed ore (most of global gold production)
• Alkaline electroplating baths for gold, silver, copper, and zinc on connectors, jewelry, and fixtures
• Strecker alpha-amino-acid synthesis (aldehyde + NH3 + HCN) and Kolbe R-X to R-CN conversions
• Hydrolysis precursor for carboxylic acids one carbon longer than the starting alkyl halide
• Heat treatment of steel via cyanide case-hardening (Tuftride process and predecessors)
• Analytical reagent for qualitative tests of nickel, cobalt, and other transition metals
• Selective extractant for silver in photographic film recycling
• Historical use in fumigation and pest eradication (largely banned)

Constituent Elements