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. Airborne HCN limits carry a skin notation, and the concentration considered immediately dangerous to life sits only a small multiple above the working limit — almost no margin between routine exposure and incapacitation. 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. We deliberately do not publish occupational exposure limits or other regulatory thresholds: those values are revised over time and differ between jurisdictions, so the only correct source is the current SDS and the regulations that apply where you work.