Sodium Cyanide
Properties
| State | Solid at room temperature |
| Color | White crystalline powder or granules |
| Solubility | Very soluble in water (48 g/100 mL at 10 °C) |
| Melting Point | 564 °C |
| Boiling Point | 1496 °C |
About Sodium Cyanide
Sodium cyanide is an ionic salt (NaCN, MW 49.007) that sits in an awkward place in chemistry: it kills a person at roughly 200 mg orally, yet about 500,000 tonnes get produced every year because nothing else extracts gold from low-grade ore as cheaply or as completely. The cyanidation process invented by John Stewart MacArthur in 1887 leaches gold and silver from crushed ore at 100-500 ppm NaCN solution under aerated, slightly alkaline conditions: 4Au + 8NaCN + O2 + 2H2O -> 4Na[Au(CN)2] + 4NaOH. The dicyanoaurate complex is then stripped onto activated carbon (carbon-in-pulp) and electrowon. The same anion chemistry shows up in hardening steel cases, electroplating gold and silver onto jewelry, and as a nucleophile in organic synthesis to install -CN groups via SN2 displacement (the Kolbe nitrile synthesis). Industrial NaCN is made by the Andrussow process: NH3 + CH4 + air over a Pt-Rh catalyst gives HCN, which is neutralized with NaOH. The lethal mechanism is precise: CN- binds the Fe(III) in cytochrome c oxidase (complex IV), shutting down the terminal step of oxidative phosphorylation so cells cannot use O2 even with normal arterial saturation. Acidifying NaCN releases HCN gas (BP 26 °C), which is why mixed-acid spills in plating shops have killed more workers than the salt itself.
Where you'll encounter it
If you've ever held a gold wedding band, the gold in it almost certainly passed through a cyanide leach circuit at a mine like Newmont's Carlin operations in Nevada or AngloGold's Kibali in the DRC. In an organic synthesis lab, sodium cyanide is the workhorse for one-carbon homologation: an alkyl halide plus NaCN in DMSO gives a nitrile that hydrolyzes to a carboxylic acid one carbon longer than the starting material. The lab protocol that everyone learns is to keep a bottle of sodium thiosulfate and a hydroxocobalamin (Cyanokit) injector nearby, work in a well-monitored fume hood with a HCN detector reading in ppb, and never let the waste stream drop below pH 11. Mining operations destroy spent cyanide with the INCO SO2/air process or alkaline chlorination before tailings discharge, and the Cyanide Code audits compliance internationally after the 2000 Baia Mare spill in Romania.
Common Uses
- Gold and silver leaching from crushed ore via the MacArthur-Forrest cyanidation process
- Carbon-in-pulp adsorption circuits for precious metal recovery from leach solutions
- Cyanide bath for electroplating gold and silver onto jewelry and electronic contacts
- Case hardening of low-carbon steel by liquid carburizing in molten cyanide salts
- Nucleophilic source of -CN for nitrile synthesis (Kolbe synthesis, SN2 on alkyl halides)
- Reagent in the von Richter and benzoin condensation reactions
- Fumigant for shipping containers and museum specimens (largely phased out)
- Source of cyanide for analytical Liebig titration of metal ions
Safety Information
Acute lethal toxin. Oral LD50 in rats is 6.4 mg/kg; estimated human lethal oral dose is 100-200 mg. OSHA PEL for HCN is 10 ppm (8-hour TWA, skin notation); ACGIH TLV-Ceiling is 4.7 ppm. NIOSH IDLH is 25 ppm HCN. GHS pictograms: Skull-and-crossbones, Health hazard, Environment. Codes: H300 (fatal if swallowed), H310 (fatal in skin contact), H330 (fatal if inhaled), H410 (very toxic to aquatic life with long-lasting effects), EUH032 (contact with acids liberates very toxic gas). Never store with or near acids; spills require alkaline hypochlorite destruction. Always work with continuous HCN gas monitoring, double nitrile gloves, full face shield, and a Cyanokit (hydroxocobalamin) and amyl nitrite/sodium thiosulfate antidote kit at the bench. Subject to DEA, OSHA HCS, and DOT class 6.1 packing group I controls.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.