Substance profile
Hydrogen Cyanide
Hydrogen Cyanide is a colorless gas or liquid with a bitter almond odor.
01 · Identification
What Hydrogen Cyanide is
Hydrogen Cyanide is a colorless gas or liquid with a bitter almond odor. Its molecular formula is CHN and its molecular weight is 27.025 g/mol. It boils at 25.6 °C, melts at -13.3 °C, is miscible with water.
Under the GHS it carries the signal word Danger. It is classed as hazardous Air Pollutants (HAPs).
Other names for Hydrogen Cyanide
- hydrocyanic acid
02 · GHS classification
Hazards and label elements
Harmonised classification under CLP Annex VI. This is the legally binding classification in the EU; other jurisdictions may differ.
| Code | Hazard statement |
|---|---|
| H224 | Extremely flammable liquid and vapour. |
| H330 | Fatal if inhaled. |
| H400 | Very toxic to aquatic life. |
| H410 | Very toxic to aquatic life with long lasting effects. |
Precautionary statements: P210, P233, P241, P242, P260, P271, P273, P304, P310, P391, P403, P501
03 · Physical and chemical properties
Hydrogen Cyanide properties
Every value is shown with the conditions it was measured under and the source it came from. Where sources disagree, all values are listed rather than one being picked — the difference is usually a different form of the substance, not an error.
| Property | Value | Conditions | Source |
|---|---|---|---|
| Boiling point | 25.63 °C (78.1 °F) | HSDB | |
| 25.56 °C (78 °F) | NIOSH | ||
| 25.56 °C (78 °F) | at 760 mmHg | CAMEO | |
| Melting point | -13.28 °C (8.1 °F) | HSDB | |
| -13.89 °C (7 °F) | NIOSH | ||
| Density | 0.69 g/cm³ | NIOSH | |
| 0.6875 g/cm³ | at 20 °C | HSDB | |
| Solubility | Miscible with water | HSDB | |
| Miscible with ethanol, ethyl ether | HSDB | ||
| Miscible with alcohol; slightly soluble in ether | HSDB | ||
| Vapor pressure | 630 mmHg | NIOSH | |
| 742 mmHg | at 25 °C | HSDB | |
| Flash point | -17.78 °C (0 °F) | HSDB | |
| Autoignition temperature | 537.78 °C (1000 °F) | HSDB | |
| 540 °C (1004 °F) | CAMEO | ||
| 538 °C (1000.4 °F) | ICSC | ||
| Lower explosive limit (LEL) | 5.6 % by volume | HSDB | |
| Upper explosive limit (UEL) | 40 % by volume | HSDB | |
| Vapor density | 0.932 (air = 1) | HSDB | |
| 0.901 (air = 1) | CAMEO | ||
| 0.94 (air = 1) | ICSC | ||
| Odor threshold | Detection in air of hydrogen cyanide of an unspecified purity or grade: 1X10-3 mg/L. | HSDB | |
| High odor threshold was 5 mg/cu m; Low odor threshold was 0.9 mg/cu m. | HSDB | ||
| Critical temperature and pressure | 183.5 °C (362.3 °F) | HSDB | |
| Corrosivity | Liquid hydrogen cyanide will attack some forms of plastics, rubber, and coatings. | HSDB |
Values compiled from PubChem Laboratory Chemical Safety Summary. Temperatures converted to Fahrenheit for reference.
04 · Source notes
Further notes on Hydrogen Cyanide
These are descriptions rather than measured values, so they are kept out of the table above and quoted here with the source that reported them.
Corrosivity
Although HCN is a weak acid and normally not considered corrosive, it has a corrosive effect under two special conditions: (1) water solutions of HCN cause transcrystalline stress-cracking of carbon steels under stress even at room temperature and in dilute solution; (2) water solutions of HCN containing sulfuric acid as a stabilizer severely corrode steel above 40 °C and stainless steels above 80 °C.
Source: HSDB
Odor
Approximately 60 to 70% of the population can detect the bitter almond odor of hydrogen cyanide. The odor threshold for those sensitive to the odor is estimated to be 1 to 5 ppm in the air. However, even at high toxic concentrations, up to 20% of all individuals are genetically unable to smell hydrogen cyanide.
Source: HSDB
05 · Handling and storage
Storing and handling Hydrogen Cyanide
Storage conditions
Keep cylinders of hydrogen cyanide (HCN) cool and away from open flames. Make certain that HCN cylinders are adequately supported and grounded during storage and emptying. Store cylinders in a vertical position. Do not drop cylinders or damage them by impact. Cylinders must be returned to the supplier within 90 days of the filling date marked on the cylinders, regardless of whether or not the contents have been used. This is due to the possibility of HCN becoming unstable over time. If there is any indication that the HCN is becoming unstable, such as a darkening of the product or an increase in cylinder pressure, contact the supplier immediately for instructions.
Source: Hazardous Substances Data Bank (HSDB)
Incompatible materials
Preparation of alkyliminoformate chlorides by passing hydrogen chloride rapidly into alcoholic hydrogen cyanide proceeds explosively (probably owing to a sudden exotherm), even with strong cooling.
Source: Hazardous Substances Data Bank (HSDB)
Reactivity
HYDROGEN CYANIDE, ANHYDROUS presents a severe explosion hazard when exposed to oxidizing agents. The vapors form explosive mixtures with air. May polymerize explosively at elevated temperature (50-60 °C) or in the presence of traces of alkali [Wohler, L. et al., Chem. Ztg., 1926, 50, p. 761, 781]. In the absence of a stabilizer (such as phosphoric acid), may undergo explosively rapid spontaneous (autocatalytic) polymerization leading to a fire. The reaction is autocatalytic because of ammonia formation. [Bond, J., Loss Prev. Bull., 1991, 101, p.3]. Dissolves in water to give weakly acidic solutions. Reacts violently with acetaldehyde. During the preparation of imidoester hydrochlorides, hydrogen chloride was rapidly passed over alcoholic hydrogen cyanide. An explosion ensued, even with cooling of the process [J. Org. Chem., 1955, 20, 1573].
Source: CAMEO Chemicals
Planning storage for a mix of chemicals? Check Hydrogen Cyanide in the storage compatibility matrix →