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SECTION 1. CHEMICAL IDENTIFICATION

CHEMINFO Record Number: 521
CCOHS Chemical Name: Hydrogen cyanide

Synonyms:
Hydrocyanic acid (non-specific name)
HCN (non-specific name)
Formonitrile
Prussic acid, anhydrous
Carbon hydride nitride (CHN)
Aero liquid HCN
Formic anammonide

Chemical Name French: Cyanure d'hydrogène

Trade Name(s):
Cyclone B
Cyclon

CAS Registry Number: 74-90-8
UN/NA Number(s): 1051, 1614
RTECS Number(s): MW6825000
EU EINECS/ELINCS Number: 200-821-6
Chemical Family: Inorganic gas / inorganic nitrogen compound / inorganic cyanide / hydrogen compound / hydrogen cyanide
Molecular Formula: H-C-N
Structural Formula: H-C#N (# represents a triple bond)

SECTION 2. DESCRIPTION

Appearance and Odour:
Colourless or pale blue liquid below 25.7 deg C; colourless gas above 25.7 deg C; faint, bitter almond odour.(2)

Odour Threshold:
0.58 ppm (5,12); 0.8-4.5 ppm (cited as 0.9-5 mg/m3) (16)

Warning Properties:
POOR - up to 20% of the population is unable to smell hydrogen cyanide and there is wide individual variation in the minimum odour threshold. In addition, the sense of smell easily fatigues.

Composition/Purity:
Hydrogen cyanide is available as a liquid in grades with purity of 96-99.5%. It is also available as a solution of hydrogen cyanide in water, referred to as hydrocyanic acid, in concentrations of 2, 5 and 10%.(12) It is also available absorbed on a porous inert material. This review discusses the hazards and control measures for hydrogen cyanide liquid and gas. For information on hydrocyanic acid solutions, refer to the CHEMINFO review of Hydrocyanic acid solutions. In general, a combination of sulfuric acid or phosphoric acid, and sulfur dioxide are added to stabilize hydrogen cyanide to prevent polymerization. Oxalic acid and acetic acid are also used. The type and quantity of stabilizer used (usually less than 0.5%) depends on storage capacity, temperature and anticipated storage time in the container.(17,18) Hydrogen cyanide is shipped as a liquid in steel cylinders.(19)

Uses and Occurrences:
Hydrogen cyanide is used to manufacture other chemicals, including adiponitrile, acetone cyanohydrin, methyl methacrylate, acrylonitrile, sodium cyanide, cyanuric chloride, chelating agents, such as ethylenediaminetetracetic acid (EDTA) and methionine. Minor uses include the production of ferrocyanides, acrylates, lactic acid, pharmaceuticals and specialty chemicals. It is also used in electroplating, mining, in metallurgical and photographic processes, and in preparative chemistry as a non-aqueous, ionizing solvent.(2,12,17,18)
It was formerly used as a fumigant, rodenticide and nematocide, but this use has been discontinued.(2)
Hydrogen cyanide gas is produced as a by-product in operations such as blast furnaces, gas works and coke ovens, and processes such as electroplating, some metal mining processes, metallurgy, petroleum refining, photography, and photoengraving. It is also a combustion by-product of nitrogen containing materials, such as wool, silk and plastics.(12)
Hydrogen cyanide occurs naturally in many plant materials, such as cassava roots, lima beans and almonds.(12)


SECTION 3. HAZARDS IDENTIFICATION

EMERGENCY OVERVIEW:
Colourless or pale blue liquid below 25.7 deg C or colourlesss gas above 25.7 deg C. It has a faint, bitter almond odour. EXTREMELY FLAMMABLE. Burns in air with a blue flame. Can form explosive mixtures with air over a wide concentration range, and will readily ignite at room temperature. Low ignition energy. Gas can be ignited by a static charge. DANGEROUSLY REACTIVE. Unstabilized liquid or gas may polymerize explosively on contact with alkaline materials, if heated above 50 deg C, if more than 2-5% water is present, if stored longer than 90 days or if impure. During a fire, irritating/toxic nitrogen oxides may be generated. Cylinders or tanks may rupture and explode if heated. VERY TOXIC. May be fatal if inhaled or absorbed through the skin. The early symptoms of cyanide poisoning may include headache, nausea, dizziness, drowsiness, anxiety, rapid breathing, incoordination and confusion. More severe exposures can cause red skin colour, laboured breathing, convulsions, collapse and death.



POTENTIAL HEALTH EFFECTS

Effects of Short-Term (Acute) Exposure

Inhalation:
Hydrogen cyanide is an extremely volatile liquid, which rapidly forms extremely high airborne concentrations at room temperature. It causes death at very low concentrations. It is a rapidly absorbed and a fast-acting poison that poses a very serious inhalation hazard.
The odour threshold is relatively low (0.58-4.5 ppm), but it does not provide a reliable warning of exposure. Some people (up to 20% of the population) are unable to smell cyanide, even at highly toxic concentrations.(12)
The early symptoms of hydrogen cyanide poisoning may include anxiety and excitement, weakness, headache, nausea, vomiting, metallic taste, chest tightness, facial flushing, drowsiness, dizziness, irritation of the eyes, nose and throat, rapid breathing, a rise in blood pressure and a decrease in pulse. Laboured breathing, falling blood pressure, rapid, weak irregular heartbeat, unconsciousness, and convulsions follow these symptoms. In severe cases, cardiovascular collapse, shock, and fluid accumulation in the lungs (pulmonary edema) are followed by death.(13,14,15,35) With massive exposures, many of the signs and symptoms may not be seen, and there is a rapid onset of poisoning with convulsions, collapse and death.(25) A characteristic sign of cyanide poisoning is the bright red colour of the blood, which may result in red skin colour.(15)
There are many reports of cyanide poisoning from accidental, suicidal and homicidal exposure to hydrogen cyanide or its salts (most commonly potassium or sodium cyanide). The majority of people who survive acute cyanide poisoning do not have long-lasting effects. However, depending on the degree of exposure, there may be enduring effects from low oxygen, including impaired memory and mathematical abilities, personality changes, and altered control and coordination of movement.(33)

Skin Contact:
Hydrogen cyanide liquid rapidly evaporates upon contact with human tissue. Any skin contact with the liquid will be brief and will also involve significant inhalation exposure. Evaporative cooling of the skin may result. Airborne and liquid hydrogen cyanide are rapidly absorbed through the skin, causing symptoms similar to those described under "Inhalation" above. There is not enough information available to assess the skin irritation potential of hydrogen cyanide.
A historical case report describes airborne hydrogen cyanide being absorbed through the skin of 3 men wearing respiratory protection. Cyanide toxicity was observed after 8 or 10 minutes of exposure to an extremely high concentration (approximately 20000 ppm).(7)

Eye Contact:
Hydrogen cyanide liquid rapidly evaporates upon contact with human tissue. Any eye contact with the liquid will be very brief and will also involve significant inhalation exposure. Eye contact with hydrogen cyanide can result in absorption and could cause symptoms similar to those described under "Inhalation" above.(6) Moderate to severe eye irritation may occur, based on information available for hydrocyanic acid solutions.
In a case report, a man exposed to 452 ppm hydrogen cyanide for 6 minutes, while cleaning a chemical tank, experienced marked eye irritation, and after 2 weeks was noted to have a slight loss of peripheral vision. However, no baseline vision testing was available for comparison.(35)

Ingestion:
Hydrogen cyanide liquid rapidly evaporates as soon as it comes into contact with human tissue and is unlikely to be ingested. Ingestion is not an applicable route of exposure for gases.

Effects of Long-Term (Chronic) Exposure

Several human population studies have evaluated the potential health effects of long-term cyanide exposure. In general, these studies are limited by factors such as the small number of employees evaluated and the possibility of concurrent exposure to other potentially harmful chemicals (particularly in the electroplating industry). In addition, few studies report reliable measurements of cyanide exposures and even when airborne concentrations are reported, exposure may also have occurred by skin absorption. Despite these limitations, the available evidence suggests that long-term occupational cyanide exposure may be associated with harmful effects on the thyroid gland and the nervous system.
Long-term exposure to cyanide also occurs from smoking, eating foods containing cyanogenic glycosides, and infection with cyanide-producing bacteria.(8) Only studies with occupational exposure are reviewed here.

Nervous System:
Limited information suggests that long-term exposure to cyanides may be associated with harmful effects on the nervous system. Some of the symptoms observed are non-specific (e.g. headaches) and could be associated with many causes. Nevertheless, there does seem to be an association between some nervous system symptoms and cyanide exposure.
Thirty-six male, non-smoking employees were exposed for 5-15 years to 4.2-12.4 ppm cyanide from electroplating baths containing sodium and copper cyanide. Nervous system symptoms were, in order of frequency, headache, weakness, changes in taste and smell, visual difficulties, and nervous instability. Two employees experienced psychotic episodes, which they recovered from in 36-48 hours following removal from the area of exposure.(9)
Fifty-six employees were exposed to hydrogen cyanide (concentrations not reported) while engaged in case hardening and electroplating for 2-20 years. A significant increase in impairment of memory, visual ability, visual learning and psychomotor ability was observed in exposed employees, compared to 34 matched controls. Headaches were more frequently reported in exposed workers.(26)
Thirty-six employees were exposed to hydrogen and sodium cyanide in a silver-reclaiming factory by inhalation (15 ppm, 24-hour average concentration), skin contact and possibly oral exposure. An employee died of acute cyanide poisoning and the plant was closed for 7 months before the study was carried out. An overall exposure index was calculated based on job category, frequency of handling cyanide and ingesting food or drink in the production areas. Nervous system symptoms, which had a significant positive correlation with exposure, were numbness or tingling (paresthesia) of the extremities, easy fatigue and a symptom complex including headache, dizziness, and fainting.(27)
Neuropathies in people living in tropical areas with a diet high in cassava, a root rich in cyanogenic glycosides, have previously been attributed to cyanide.(12,41) However, this diet is also high in scopoletin, a coumarin compound, which is believed to be responsible for some of the neurotoxic effects.(28)

Lungs/Respiratory System:
Two limited studies suggest that long-term cyanide exposure may be associated with laboured breathing.
An increase in effort-induced laboured breathing was observed in 36 male, non-smoking employees exposed for 5-15 years to 4.2-12.4 ppm cyanide from electroplating baths containing sodium and copper cyanide.(9)
An association between laboured breathing and cyanide exposure was also observed in 36 employees exposed to hydrogen and sodium cyanide in a silver-reclaiming factory, by inhalation (15 ppm, 24-hour average concentration), skin contact and possibly oral exposure. An employee had died of acute cyanide poisoning and the plant was closed for 7 months before the study was carried out. An overall exposure index was calculated based on job category, frequency of handling cyanide and ingesting food or drink in the production areas.(27)

Skin:
An association between development of a skin rash and cyanide exposure was also observed in 36 employees exposed to hydrogen and sodium cyanide in a silver-reclaiming factory, by inhalation (15 ppm, 24-hour average concentration), skin contact and possibly oral exposure. An employee had died of acute cyanide poisoning and the plant was closed for 7 months before the study was carried out.(27)

Digestive System:
An increased incidence of nausea and/or vomiting has been reported in two studies that evaluated employees with long-term exposure to cyanide concentrations up to 15 ppm (with possible concurrent ingestion and skin contact).(9,27)

Eyes/Vision:
Eye irritation has been reported in 3 limited studies involving electroplating workers. Exposures, when specified, ranged from 4.2-15 ppm cyanide.(9,26,27) However, it is not possible to draw any specific conclusions about the eye irritation potential of long-term cyanide exposure, because electroplating workers are exposed to a variety of chemicals that are irritating to the eyes.(12)
Degeneration of the optic nerve and part of the retina (the macula) is found in people living in tropical areas with a diet high in cassava, a root rich in cyanogenic glycosides.(8) In some cases, these effects have been attributed to cyanide exposure.(12) However, this diet is also high in scopoletin, a coumarin compound, which is believed to be responsible for some of these effects.(28)

Blood/Blood Forming System:
There is very limited information that long-term exposure to cyanide is associated with harmful effects on the blood.
Blood chemistry changes (increased white blood cells and red blood cell sedimentation rate and decreased hemoglobin levels) were observed in 34 employees exposed to unspecified concentrations of HCN, while engaged in case hardening and electroplating for 2-20 years.(26) Statistical analysis of the results was not conducted.
Blood chemistry changes (increased hemoglobin and lymphocyte counts and red blood cell damage) were observed in 36 male, non-smoking employees exposed for 5-15 years to 4.2-12.4 ppm cyanide during electroplating operations.(9) However, exposure to copper, an agent known to have toxic effects on blood also occurred.
Changes in white blood cell enzyme activity were noted in 43 employees exposed to an average concentration of 0.23 ppm HCN for 0.25-16 years (average 5.4 years) during metal coating operations.(29)

Endocrine System:
Evidence from human and animal studies indicates that long-term exposure to cyanide can result in impaired thyroid function and enlargement of the thyroid (goiter). Thiocyanate, the main metabolite of cyanide, is believed to cause these effects by inhibiting the uptake of iodine by the thyroid.(30)
Findings consistent with impaired thyroid function were observed in 35 male employees, all non-smokers, who were exposed to cyanide salts for at least 5 years, while working with an electroplating process. Cyanide concentrations were not reported.(30)
Mild to moderate thyroid enlargement was observed in 20/36 male electroplating workers, who were exposed to 4.2-12.4 ppm cyanide for 5-15 years. Measurement of radioactive iodine uptake showed a significantly higher iodine uptake in the exposed workers than for the control group.(9)
The health of 36 employees exposed to hydrogen and sodium cyanide in a silver-reclaiming factory was assessed. Inhalation (15 ppm, 24-hour average concentration), skin contact and possibly oral exposure had occurred. An employee died of acute cyanide poisoning and the plant was closed for 7 months before the study was carried out. An overall exposure index was calculated based on job category, frequency of handling cyanide and ingesting food or drink in the production areas. In tests done 7-30 months after the last exposure, the thyroid-stimulating hormone was significantly higher in high exposure index employees, compared to the mean laboratory control value. However, thyroxine levels were normal and no thyroid enlargement was found.(27)
Limited animal studies have also shown Impaired thyroid function.

Carcinogenicity:

Hydrogen cyanide is not known to cause cancer. No human or animal information was located.

The International Agency for Research on Cancer (IARC) has not evaluated the carcinogenicity of this chemical.

The American Conference of Governmental Industrial Hygienists (ACGIH) has not assigned a carcinogenicity designation to this chemical.

The US National Toxicology Program (NTP) has not listed this chemical in its report on carcinogens.

Teratogenicity and Embryotoxicity:
No human information was located. Limited animal information for other cyanide compounds (potassium cyanide) suggests that hydrogen cyanide is not a developmental toxin.

Reproductive Toxicity:
No human information was located. Changes suggestive of reproductive effects were observed in rats and mice in studies with sodium cyanide. However, fertility was not assessed.

Mutagenicity:
No human information was located. The available evidence does not indicate that hydrogen cyanide is mutagenic. Two tests in live mice exposed to potassium cyanide were negative. Hydrogen cyanide has produced positive results in a test using bacteria

Toxicologically Synergistic Materials:
There is no human information available. Co-exposure of rats to hydrogen cyanide and 5% carbon dioxide (not lethal by itself) resulted in an increased in the lethality of hydrogen cyanide gas.(12) Oral pre-treatment of guinea pigs with ascorbate enhanced the toxic effects of oral administration of potassium cyanide. It was suggested that the ascorbate interfered with the reaction to detoxify cyanide.

Potential for Accumulation:
Does not accumulate. The most important route for detoxification of cyanide is by a mitochondrial enzyme, rhodanese, which adds sulfur to the cyanide ion to form thiocyanate. Thiocyanate is less toxic and is excreted in the urine.(11) This enzyme is widely distributed in the tissues but has its greatest activity in the liver. The body has a large capacity to detoxify cyanide but the reaction is dependent on an adequate supply of sulfur.(15) The maximum detoxification rate for humans is 0.6-0.9 micrograms/kg body weight/minute, which is considerably lower than for lab rodents or dogs. Most absorbed cyanide is excreted in the urine as thiocyanate, but small amounts are eliminated in exhaled air and urine as hydrogen cyanide, carbon dioxide and other metabolic products. The average half time for excretion of thiocyanate has been reported to be 2.7 days in healthy volunteers.(14)

Health Comments:
The cyanide ion binds with iron ions in the enzyme cytochrome oxidase, which prevents body cells from using oxygen. Thus, cyanide impairs the body's ability to use oxygen and the primary target organs for acute cyanide poisoning are the central nervous system and the heart.(11,12) Cyanides also inhibit other enzyme systems, especially those containing iron or copper, which contributes to the symptoms observed.(12,13,14)


SECTION 4. FIRST AID MEASURES

Inhalation:
This chemical is very toxic and extremely flammable. Take proper precautions to ensure your own safety before attempting rescue (e.g. remove any sources of ignition, wear appropriate protective equipment, use the buddy system). Remove source of contamination or move victim to fresh air. If breathing is difficult, trained personnel should administer emergency oxygen. DO NOT allow victim to move about unnecessarily. Symptoms of pulmonary edema can be delayed up to 48 hours after exposure. If breathing has stopped, trained personnel should begin artificial respiration (AR) or, if the heart has stopped, cardiopulmonary resuscitation (CPR) or automated external defibrillation (AED) immediately. Avoid mouth-to-mouth contact by using mouth guards or shields. Quickly transport victim to an emergency care facility. See First Aid Comments below for antidote information.

Skin Contact:
Avoid direct contact. Wear chemical protective clothing, if necessary. As quickly as possible, remove contaminated clothing, shoes and leather goods (e.g. watchbands, belts). Flush with lukewarm, gently flowing water for 5 minutes. Quickly transport victim to an emergency care facility. Double bag, seal, label and leave contaminated clothing, shoes and leather goods at the scene for safe disposal. This chemical rapidly evaporates upon contact with human tissue and is rapidly absorbed through the skin. Any skin contact will also involve significant inhalation exposure. See "Inhalation" above for additional procedures. See First Aid Comments below for antidote information.

Eye Contact:
Avoid direct contact. Wear chemical protective gloves, if necessary. Immediately flush the contaminated eye(s) with lukewarm, gently flowing water for 5 minutes, while holding the eyelid(s) open. Quickly transport victim to an emergency care facility. This chemical rapidly evaporates upon contact with human tissue and is rapidly absorbed through the eye. Any eye contact will also involve significant inhalation exposure. See "Inhalation" above for additional procedures. See First Aid Comments below for antidote information.

Ingestion:
Ingestion is not an applicable route of exposure for this chemical.

First Aid Comments:
Provide general supportive measures (comfort, warmth, rest).
Consult a doctor and/or the nearest Poison Control Centre for all exposures.
Some first aid procedures recommended above require advanced first aid training. Protocols for undertaking advanced procedures must be developed in consultation with a doctor and routinely reviewed.
All first aid procedures should be periodically reviewed by a doctor familiar with the material and its conditions of use in the workplace.

ANTIDOTE: Cyanide toxicity can occur following exposure to hydrogen cyanide. Amyl nitrite, which can be used as a first aid measure, may be antidotal to cyanide toxicity. Consult with a doctor familiar with cyanide toxicity and treatment to determine the appropriateness of using amyl nitrite as first aid measure in your workplace and to develop protocols and arrange training for specialized advanced training for first aid providers who may be required to administer amyl nitrite.

Note to Physicians:
There are antidotes available for cyanide toxicity, which can occur following exposure to hydrogen cyanide. Specific information on antidotes which can be used as first aid and therapeutically in a medical setting is available in references 13, 48, 54, 55, and 56.



SECTION 5. FIRE FIGHTING MEASURES

Flash Point:
-17.8 deg C (0 deg F) (closed cup) (17,36)

Lower Flammable (Explosive) Limit (LFL/LEL):
5.6% (37); 6% (17,36)

Upper Flammable (Explosive) Limit (UFL/UEL):
40% (37); 41% (17,36)

Autoignition (Ignition) Temperature:
537.8 deg C (1000 deg F) (17,36)

Sensitivity to Mechanical Impact:
Probably not sensitive.

Sensitivity to Static Charge:
Hydrogen cyanide liquid will not accumulate static charge, since it has a high electrical conductivity.(17,38) Hydrogen cyanide, at concentrations in the flammable range, can be ignited by a static charge.

Electrical Conductivity:
3.3 X 10(8) pS/m at 0 deg C (liquid) (17,38)

Minimum Ignition Energy:
Not available.

Combustion and Thermal Decomposition Products:
Carbon monoxide, carbon dioxide and nitrogen oxides

Fire Hazard Summary:
EXTREMELY FLAMMABLE LIQUID (below 25.7 deg C) or GAS (above 25.7 deg C). Burns in air with a blue flame. Can form explosive mixtures with air over a wide concentration range, and will readily ignite at room temperature. It has a low ignition energy. Hydrogen cyanide can be ignited by a static charge. Hazardous polymerization may occur under fire conditions. Loss of stabilizer or excessive heat from a fire may cause spontaneous polymerization. Heat from a fire can cause hazardous polymerization and/or a rapid build-up of pressure inside cylinders, which may cause explosive rupture and a sudden release of large amounts of flammable, very toxic gas or may cause cylinder to rocket. During a fire, irritating/toxic nitrogen oxides may be generated.

Extinguishing Media:
Carbon dioxide, dry chemical powder or "alcohol resistant" foam.(37) Water will not be effective for extinguishing a fire because it cannot cool hydrogen cyanide below its flash point.(37) Foam manufacturers should be consulted for recommendations regarding types of foams and application rates.

Fire Fighting Instructions:
Use extreme caution. Evacuate all personnel from the fire area. Explosive polymerization and explosive decomposition may occur under fire conditions and may rupture container releasing very toxic, very flammable hydrogen cyanide. Fight fire from a protected, explosion-resistant location or maximum possible distance. Approach fire from upwind to avoid extremely hazardous gas and decomposition products. Wear full protective suit if exposure is possible. See advice in Protection of Firefighters.
If venting or leaking hydrogen cyanide ignites, DO NOT extinguish the flames. Immediately cool containers with water spray from a maximum distance, taking care not to extinguish the flames and allow the fire to burn out. If the flames are accidentally extinguished, keep unprotected personal out of the area as hydrogen cyanide gas is very toxic and explosive reignition may occur.
Protect exposed materials with water spray to prevent ignition of other combustible materials.
Gas clouds may be controlled by water spray or fog. The resulting water solutions of hydrogen cyanide may be flammable and very toxic. Dike fire control water for appropriate disposal.
If a fire occurs in the vicinity of hydrogen cyanide, use unmanned monitors and hoseholders to keep cooling streams of water on fire-exposed containers, cylinders, tanks or pipelines until well after the fire is out, in order to protect their contents from the danger of polymerization. This should begin as soon as possible and should concentrate on any unwetted portions of the container. Take care not to get water inside a container. Reverse flow into a cylinder may cause rupture. No part of a cylinder or other container should be subjected to a temperature higher than 50 deg C (approximately 122 deg F), since polymerization occurs above this temperature.
Stay away from ends of tanks, but be aware that flying material from ruptured cylinders or tanks may travel in any direction. Withdraw immediately in case of rising sound from venting safety device or any discolouration of tank due to fire.
In an advanced or massive fire, the area should be evacuated; use unmanned hoseholders or monitor nozzles. If this is not possible, withdraw from fire area and do not attempt to fight the fire.
Cylinders or other containers should not be approached after they have been involved in a fire until they have completely cooled down.
After the fire has been extinguished, explosive and toxic atmospheres may remain. Before entering such an area, especially confined areas, check the atmosphere with an appropriate monitoring device while wearing full protective suit.

Protection of Fire Fighters:
Hydrogen cyanide is extremely toxic and can be absorbed through the skin. Do not enter without wearing specialized protective equipment suitable for the situation. Firefighter's normal protective clothing (Bunker Gear) will not provide adequate protection. A full-body encapsulating chemical protective suit with positive pressure self-contained breathing apparatus (NIOSH approved or equivalent) may be necessary.



NATIONAL FIRE PROTECTION ASSOCIATION (NFPA) HAZARD IDENTIFICATION

NFPA - Health: 4 - Very short exposure could cause death or major residual injury.
NFPA - Flammability: 4 - Will rapidly or completely vaporize at atmospheric pressure and normal ambient temperature, or readily disperse in air and burn readily.
NFPA - Instability: 2 - Undergoes violent chemical change at elevated temperatures and pressures, or reacts violently with water, or may form explosive mixtures with water.

SECTION 9. PHYSICAL AND CHEMICAL PROPERTIES

Molecular Weight: 27.03

Conversion Factor:
1 ppm = 1.10 mg/m3; 1 mg/m3 = 0.91 ppm at 25 deg C (calculated)

Physical State: Liquid
Melting Point: -13.24 deg C (8.2 deg F) (17,18)
Boiling Point: 25.7 deg C (78.3 deg F) (17,18)
Relative Density (Specific Gravity): 0.688 at 20 deg C (17)
Solubility in Water: Soluble in all proportions (17,36)
Solubility in Other Liquids: Soluble in all proportions in ethanol; slightly soluble in diethyl ether (2)
Coefficient of Oil/Water Distribution (Partition Coefficient): Log P(oct) = -0.25 (experimental) (39)
pH Value: Not applicable (gas); 4.62 (1M solution); 4.12 (10M) (calculated)
Acidity: Very weak acid
Vapour Density: 0.933 at 21 deg C (air = 1) (36)
Vapour Pressure: 83 kPa (622.5 mm Hg) at 20 deg C (18); 98.9 kPa (742 mm Hg) at 25 deg C (40); approximately 170 kPa (1275 mm Hg) at 37.8 deg C (19)
Vapour Pressure at 50 deg C: 250 kPa (2.47 atm; 1875 mm Hg) (19)
Saturation Vapour Concentration: Extremely high; 819000 ppm (81.9%) at 20 deg C; 977000 ppm (97.7%) at 25 deg C (calculated)
Evaporation Rate: Extremely rapid.
Henry's Law Constant: 13.5 Pa.m3/mol (cited as 1.33 X 10(-4) atm.m3/mol) at 25 deg C (experimental) (40); log H = -2.26 (dimensionless constant; calculated)
Critical Temperature: 183.5 deg C (362.3 deg F) (17,36) -17.8 deg C ( deg F)
Critical Pressure: 5391 kPa (53.2 atm) (17,36)

SECTION 10. STABILITY AND REACTIVITY

Stability:
Unstable. However, hydrogen cyanide is stable in the presence of a stabilizer.(17,41) It polymerizes explosively if unstabilized and under alkaline conditions, if heated above 50 deg C, or if contaminants are present.(2,17,37,41)

Hazardous Polymerization:
In the absence of a stabilizer, hydrogen cyanide undergoes polymerization with the generation of heat (exothermic reaction). This polymerization can become explosive, especially if confined. Temperatures greater than 50 deg C and the presence of greater than 2-5% water contribute to the onset of polymerization.(37) Polymerization is accelerated by higher temperatures, the presence of alkaline compounds, water and other contaminants, and sunlight (UV light). Since heat is generated, the polymerization reaction is autocatalytic and when the temperature reaches 184 deg C rapid explosive polymerization occurs.(41,42,43) Hydrogen cyanide is stabilized by a combination of sulfuric, phosphoric, oxalic or acetic acid, and sulfur dioxide to prevent polymerization. The acid stabilizes the liquid phase, while sulfur dioxide stabilizes the gas phase.(17,18) Hydrogen cyanide should not be stored for longer than 90 days, particularly if the acid stabilizer is not maintained at a sufficient concentration. In addition some contaminants and surfaces may deplete the acidic stabilizer and cause polymerization. In such cases, reacidification and cooling is necessary.(17)

Incompatibility - Materials to Avoid:

NOTE: Chemical reactions that could result in a hazardous situation (e.g. generation of flammable or toxic chemicals, fire or detonation) are listed here. Many of these reactions can be done safely if specific control measures (e.g. cooling of the reaction) are in place. Although not intended to be complete, an overview of important reactions involving common chemicals is provided to assist in the development of safe work practices.


ALKALINE MATERIALS (e.g. sodium, potassium or calcium hydroxide, ammonia, sodium carbonate) or AMINES (e.g. triethylamine) - can promote polymerization and cause explosive decomposition.(2,41)
OXIDIZING AGENTS (e.g. chlorates, hypochlorites, nitrates, perchlorates, peroxides or permanganates) - can cause a severe explosion.(2)
STRONG ACIDS (e.g. sulfuric or nitric acid) - explosively violent decomposition (hydrolysis) can occur if an excess of acid is added to hydrogen cyanide.(17)
HYDROCHLORIC ACID - rapid addition of hydrochloric acid into hydrogen cyanide in alcohol caused an explosive reaction.(41,42)
AMMONIUM CHLORIDE - electrolysis of mixtures of ammonium chloride and hydrogen cyanide can form explosive nitrogen trichloride.(41,42)
HEAVY METAL CYANIDES (e.g. mercury cyanide) - hydrogen cyanide detonates explosively in the presence of heavy metal cyanides.(41,42)

Hazardous Decomposition Products:
None reported.

Conditions to Avoid:
Heat, sparks, open flames, static discharge, other ignition sources, depletion of stabilizer, alkaline conditions, moisture, impurities and extended storage.

Corrosivity to Metals:
Hydrogen cyanide is corrosive to copper and copper alloys, such as copper-nickel alloy, silicon bronze, brass, and admiralty, naval and yellow brass at room temperature.(44) t is not corrosive to stainless steel (such as 300 series and Carpenter 20Cb-3), carbon steel (types 1010 and 1020), aluminum (type 3003), nickel-base alloys, such as Hastelloy and Monel, titanium and lead at room temperature.(44) At high temperatures, stainless steel is the material of choice.(36)

Corrosivity to Non-Metals:
Hydrogen cyanide attacks plastics like chlorinated polyether, nylon 11 and polyvinylidene chloride; and elastomers, such as chlorinated polyethylene, polyacrylate, polysulfide and polyurethane.(45,46) It does not attack plastics like polyethylene, polypropylene, polyvinyl chloride and Teflon and other fluorocarbons; and elastomers, such as ethylene propylene, butyl rubber, isoprene, natural rubber, neoprene, Viton and other fluorocarbons.(45,46)


SECTION 11. TOXICOLOGICAL INFORMATION

LC50 (male rat): 71 ppm (4-hour exposure); cited as 158 mg/m3 (1-hour exposure) (whole-body exposure) (4)
LC50 (male rat): 40 ppm (4-hour exposure); cited as 110 ppm (30-minute exposure) (1)
LC50 (male mouse): 59 ppm (4-hour exposure); cited as 166 ppm (30-minute exposure) (3)
Note: This study was done with groups of 4 mice and only includes deaths during the exposure.
LC50 (female rabbit): 71 ppm (4-hour exposure); cited as 208 mg/m3 (35-minute exposure) (nose-only exposure) (4)
Note: Many of the LC50 values commonly cited by secondary sources are for very short exposures. Only studies where the exposures are 30 minutes or greater have been used to more reliably determine LC50s with standardized 4-hour exposures.

Hydrogen cyanide cannot be ingested because it rapidly evaporates upon contact with human tissue and exists as a gas at temperatures higher than 25.7 deg C. Hydrocyanic acid solutions are very toxic.

LD50 (dermal, female rabbit): 6.90 mg/kg (solution administered to intact skin) (9)
LD50 (dermal, female rabbit): 2.34 mg/kg (solution administered to damaged skin) (9)

Skin Irritation:

The skin irritation potential of hydrogen cyanide has not been determined.

No conclusions can be drawn from a study that reported vascular congestion in the skin of guinea pigs after exposure to unspecified concentration of hydrogen cyanide for 15-97 minutes. Air temperatures during the experiment varied from -2 deg C to 6 deg C.(10)

Effects of Short-Term (Acute) Exposure:

Hydrogen cyanide is very toxic by inhalation and can be absorbed through the skin. Toxic effects are primarily attributed to the cyanide ion binding with iron ions in the enzyme cytochrome oxidase, thereby preventing cells from using oxygen. The primary target organs for short-term cyanide poisoning are the central nervous system (CNS) and the heart, because they are the most sensitive to oxygen deprivation.(11,12) Cyanide also inhibits other enzyme systems, especially those containing iron or copper, which contributes to the symptoms observed.(12,13,14) Brain lesions typical of those produced by low oxygen supply have commonly been observed in animals following experimental cyanide poisoning.(15,20) Most of these studies use routes of exposure that are not relevant to occupational exposures and are not reviewed here.

Inhalation:
Exposure to 63 ppm hydrogen cyanide for 30 minutes caused a 50% decrease in the respiration rate of mice due to a depression of the central respiratory center. At 150 ppm, 2/4 mice died at 12-13 minutes, and at 330 ppm, 4/4 mice died within 14-30 minutes.(3) Exposure of individual monkeys to 100-150 ppm for up to 30 minutes (by facemask) caused hyperventilation, reduced activity and slow, irregular heartbeat followed by slowing of breathing and semi-consciousness. At 100 ppm, the time to incapacitation (semi-consciousness) was 16-19 minutes, while at 150 ppm it was 8 minutes. Rapid recovery occurred during the first 10 minutes following exposure.(21) In a study examining the effects of cyanide on the central nervous system (CNS), 6 dogs were exposed for 2-10 minutes to 150-635 ppm hydrogen cyanide (cited as 0.165-0.7 mg/L) resulting in unconsciousness, convulsions, muscular rigidity, coma and 3/6 deaths. Extensive tissue death (necrosis) was seen in the grey matter, but not the white matter, of the CNS.(22) Rats exposed to unspecified concentrations of hydrogen cyanide for 20, 30, 45 or 60 minutes had lesions in both the grey and white matter of the brain.(23) All rats (10/10) died following a whole-body exposure to 6000 ppm for 10 seconds, 2200 ppm for 1 minute, 600 ppm for 5 minutes and 185 ppm for 1 hour.(4)

Skin Contact:
Hydrogen cyanide is readily absorbed through the skin. Application of an unspecified amount of hydrogen cyanide to a small area of the shaved skin of guinea pigs caused death in 7 or 8 minutes.(24) In a similar study with guinea pigs, death occurred at 45-97 minutes.(10)

Effects of Long-Term (Chronic) Exposure:

Most studies investigating the long-term toxicity of cyanide compounds have involved the ingestion of sodium or potassium cyanide. When sodium or potassium cyanide are given orally, they react rapidly with gastric acids to form hydrogen cyanide, which is readily absorbed.(25) In a well-conducted study, significant signs of toxicity were not observed in rats or mice exposed to sodium cyanide in drinking water at doses up to 23.5 mg/kg/day for rats and 50 mg/kg/day for mice. Limited studies suggest that long-term cyanide exposure may cause harmful effects in the thyroid gland.

Inhalation:
In a study, which is not available in English, laboured breathing and vomiting was observed in dogs exposed to 45 ppm hydrogen cyanide for 30 minutes/day at 2-8 day intervals for 28-96 days.(12, unconfirmed)

Teratogenicity, Embryotoxicity and/or Fetotoxicity:
There are no studies available specifically for hydrogen cyanide exposure. Studies for other cyanide compounds indicate that hydrogen cyanide is not a developmental toxin.

Reproductive Toxicity:
There are no studies available specifically for hydrogen cyanide exposure. Some reproductive effects (e.g. reduced sperm motility) were noted in studies with rats and mice exposed by ingestion to sodium cyanide. However, fertility was not assessed in any confirmed study.

Mutagenicity:
The available evidence does not indicate that hydrogen cyanide is mutagenic. In vivo test done with potassium cyanide have given negative results. A positive result was obtained for hydrogen cyanide in bacteria.
There is no specific information for hydrogen cyanide.
A positive result (point mutation) was obtained for hydrogen cyanide in bacteria, with the response weakened by the presence of metabolic activation.(31)

Toxicological Synergisms:
A study with potassium cyanide indicated that ascorbate interferes with the detoxification of cyanide in guinea pigs. When rats were co-exposed to hydrogen cyanide and carbon monoxide for 30 minutes, the effects were additive. Co-exposure of rats to 5% carbon dioxide and hydrogen cyanide caused an increase in the lethality of the hydrogen cyanide.(1) Exposure of male rats to 0, 10, 30 and 50 ppm hydrogen cyanide for 3.5 hours increased permanent noise-induced hearing loss in a dose-dependent manner. A significant enhancement of the hearing loss was noted at 30 ppm.(32)


SECTION 16. OTHER INFORMATION

Selected Bibliography:
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(2) US National Library of Medicine. Hydrogen cyanide. Last revision date: 2003-02-14. In: Hazardous Substances Data Bank (HSDB). CHEMpendium. [CD-ROM]. Canadian Centre for Occupational Health and Safety (CCOHS). Also available at: <ccinfoweb.ccohs.ca/chempendium/search.html>
(3) Matijak-Schaper, M., et al. Toxicity of carbon monoxide, hydrogen cyanide and low oxygen. Journal of Combustion Toxicology. Vol. 9 (Feb. 1982). p. 21-61
(4) Ballantyne, B. Acute inhalation toxicity of hydrogen cyanide vapor to the rat and rabbit. Toxic Substances Journal. Vol. 13, no. 4 (1994). p. 263-282
(5) Amoore, J.E., et al. Odor as an aid to chemical safety: odor thresholds compared with threshold limit values and volatilities for 214 industrial chemicals in air and water dilution. Journal of Applied Toxicology. Vol. 3, no. 6 (1983). p. 278-279
(6) Ballantyne, B. Acute systemic toxicity of cyanides by topical application to the eye. Journal of Toxicology - Cutaneous and Ocular Toxicology. Vol. 2, nos. 2 & 3 (1983). p. 119-129
(7) Drinker, P. Hydrocyanic acid gas poisoning by absorption through the skin. The Journal of Industrial Hygiene. Vol. 14, no. 1 (Jan. 1932). p. 1-2
(8) Wilson, J. Cyanide in human disease. In: Clinical and experimental toxicology of cyanides. Edited by B. Ballantyne et al. Wright, 1987. p. 293-311.
(9) El Ghawabi, S.H., et al. Chronic cyanide exposure: a clinical, radioisotope and laboratory study. British Journal of Industrial Medicine. Vol. 32 (1975). p. 215-219
(10) Fairley, A., et al. The absorption of hydrocyanic acid vapour through the skin with notes on other matters relating to acute cyanide poisoning. Journal of Hygiene. Vol. 34, no. 3 (Oct. 1934). p. 283-294
(11) Basu, D.K., et al. Drinking water criteria document for cyanides (final draft). US Environmental Protection Agency, 1985
(12) Agency for Toxic Substances and Disease Registry (ATSDR). Toxicological profile for cyanide (update). US Department of Health and Human Services, 1997
(13) Beasley, D.M.G., et al. Cyanide poisoning: pathophysiology and treatment recommendations. Occupational Medicine. Vol. 48, no. 7 (1998). p. 427-431
(14) Consensus report for hydrogen cyanide, sodium cyanide and potassium cyanide. Scientific Basis for Swedish Occupational Standards XXII. Edited by J. Montelius. Arbete Och Halsa. No. 20 (2001). p. 43-59
(15) Gosselin, R.E., et al. Clinical toxicology of commercial products. 5th ed. Williams and Wilkins, 1984. p. III-123-III-130
(16) Ruth J.H. Odor thresholds and irritation levels of several chemical substances: a review. American Industrial Hygiene Association Journal. Vol. 47 (Mar. 1986). p A-147
(17) Pesce, L.D. Cyanides: hydrogen cyanide. In: Kirk-Othmer encyclopedia of chemical technology. 4th ed. Vol. 7. John Wiley and Sons, 1993. p. 753-765
(18) Klenk, H, et al. Cyano compounds, inorganic: hydrogen cyanide. In: Ullmann's encyclopedia of industrial chemistry. 5th completely revised ed. Vol. A 8. VCH Publishers, 1987. p. 159-165
(19) Braker, W., et al. Hydrogen cyanide. In: Matheson gas data book. 6th ed. Matheson Gas Products, 1980. p. 385-390
(20) Salkowski, A.A., et al. Cyanide poisoning in animals and humans: a review. Veterinary and Human Toxicology. Vol. 36, no. 5 (Oct. 1994). p. 455-466
(21) Purser, D.A., et al. Intoxication by cyanide in fires: a study in monkeys using polyacrylonitrile. Archives of Environmental Health. Vol. 39 (1984). p. 394-400
(22) Haymaker, W., et al. Residual neuropathological effects of cyanide poisoning: a study of the central nervous system of 23 dogs exposed to cyanide compounds. The Military Surgeon. Vol. 3, no. 4 (Oct. 1952). p. 231-246
(23) Levine, S., et al. Experimental cyanide encephalopathy. Archives of Pathology. Vol. 67 (1959). p. 80-97/306-323
(24) Walton, D.C., et al. Skin absorption of certain gases. Journal of Pharmacology and Experimental Therapeutics. Vol. 26 (1926). p. 315-324
(25) Ballantyne, B. Toxicology of cyanides. In: Clinical and experimental toxicology of cyanides. Edited by B. Ballantyne, et al. Wright, 1987. p. 41-126
(26) Kumar, P., et al. Health status of workers engaged in heat treatment (case hardening) plant and electroplating at cyanide bath. Indian Journal of Environmental Protection. Vol. 12, no. 3 (1992). p. 179-183
(27) Blanc, P., et al. Cyanide intoxication among silver-reclaiming workers. Journal of the American Medical Association. Vol. 253, no. 3 (Jan. 1985). p. 367-371
(28) Obidoa, A., et al. Coumarin compounds in cassava diets: 2 health implications of scopoletin in gari. Plant Foods for Human Nutrition. Vol. 41 (1991). p. 283-289
(29) Dinca, C., et al. Observations on the changes in the activity of leukocytic oxidative enzymes in subjects with long term industrial exposure to hydrocyanic acid. Medicina Interna. Vol. 24, no. 11 (1972). p. 1385-1392
(30) Banerjee, K.K., et al. Evaluation of cyanide exposure and its effects on thyroid function of workers in a cable industry. Journal of Occupational and Environmental Medicine. Vol. 39 (1997). p. 258-260
(31) Kushi A., et al. Mutagen from the gaseous phase of protein pyrolyzate. Agricultural and Biological Chemistry. Vol. 47, no. 9 (1983). p.1979-1982
(32) Fechter, L.D., et al. Potentiation of noise-induced hearing loss by low concentrations of hydrogen cyanide in rats. Toxicological Sciences. Vol. 66 (2002). p. 131-138
(33) Hall, A.H., et al. Clinical toxicology of cyanide. Annals of Emergency Medicine. Vol. 15, no. 9 (Sept. 1986). p. 1067-1074
(34) Kales, S.N., et al. Paranoid psychosis after exposure to cyanide. Archives of Environmental Health. Vol. 52, no. 3 (1997). p. 245-246
(35) Bonsall, J.L. Survival without sequelae following exposure to 500 mg/m3 of hydrogen cyanide. Human Toxicology. Vol. 3 (1984). p. 57-60
(36) Yaws, C.L.. Hydrogen cyanide. In: Matheson gas data book. 7th ed. McGraw-Hill, 2001. p. 453-457
(37) Fire protection guide to hazardous materials. 13th ed. Edited by A.B. Spencer, et al. National Fire Protection Association, 2002. NFPA 49; NFPA 491; NFPA 497
(38) Britton, LG. Using material data in static hazard assessment. Plant/Operations Progress. Vol. 11, no. 2 (Apr. 1992). p. 56-70
(39) Syracuse Research Corporation. Interactive LogKow (KowWin) Database Demo [online]. Date unknown. Available at: <esc-plaza.syrres.com/interkow/kowdemo.htm>
(40) Syracuse Research Corporation. The Physical Properties Database (PHYSPROP). Interactive PhysProp Database Demo. Date unknown. Available at: <esc-plaza.syrres.com/interkow/physdemo.htm>
(41) Urben, P.G., ed. Bretherick's reactive chemical hazards database. [CD-ROM]. 6th ed. Version 3.0. Butterworth-Heinemann Ltd., 1999
(42) Armour, M-A. Hazardous laboratory chemicals disposal guide. 2nd ed. Lewis Publishers, 1996. p. 243-244
(43) Bond, J. Hydrogen cyanide tank explosion. Loss Prevention Bulletin. Vol. 101 (1991). p. 3 -7
(44) Pruett, K.M. Chemical resistance guide to metals and alloys: a guide to chemical resistance of metals and alloys. Compass Publications, 1995. p. 170-181
(45) Pruett, K.M. Chemical resistance guide for elastomers II: a guide to chemical resistance of rubber and elastomeric compounds. Compass Publications, 1994. p. C-194 to C-199
(46) Schweitzer, P.A. Corrosion resistance tables: metals, nonmetals, coatings, mortars, plastics, elastomers and linings, and fabrics. 4th ed. Part B, E-O. Marcel Dekker, Inc., 1995. p. 1541-1544
(47) European Communities. Commission Directive 98/98/EC. Dec. 15, 1998
(48) IPCS/CEC Evaluation of Antidotes Series. Vol. 2. Antidotes for poisoning by cyanide. Edited by T.J. Meredith, et al. Published by Cambridge University Press on behalf of the World Health Organization and of the Commission of European Communities. Cambridge University Press, 1993. Also available at: <www.inchem.org/pages/antidote.html>
(49) Hydrogen cyanide. In: NIOSH pocket guide to chemical hazards. National Institute for Occupational Safety and Health, June, 1997. p. 168-169
(50) Forsberg, K., et al. Quick selection guide to chemical protective clothing. 4th ed. Van Nostrand Reinhold, 2002
(51) Occupational Safety and Health Administration (OSHA). Cyanide in Workplace Atmospheres. In: OSHA Analytical Methods Manual. Revision Date: Oct. 31, 2001. Available at: <www.osha-slc.gov/dts/sltc/methods/toc>
(52) National Institute for Occupational Safety and Health (NIOSH). Hydrogen Cyanide. In: NIOSH Manual of Analytical Methods (NMAM(R)). 4th ed. Edited by M.E. Cassinelli, et al. DHHS (NIOSH) Publication 94-113 (Aug. 1994). Available at: <www.cdc.gov/niosh/nmam/nmammenu.html>
(53) National Institute for Occupational Safety and Health (NIOSH). Cyanides, Aerosol and Gas. In: NIOSH Manual of Analytical Methods (NMAM(R)). 4th ed. Edited by M.E. Cassinelli, et al. DHHS (NIOSH) Publication 94-113 (Aug. 1994). Available at: <www.cdc.gov/niosh/nmam/nmammenu.html>
(54) Agency for Toxic Substances and Disease Registry (ATSDR). Managing hazardous material incidents (MHMI). Vol. III. Medical management guidelines for acute chemical exposures. Medical Management Guidelines (MMGs) for hydrogen cyanide. US Department of Health and Human Services, Public Health Service, ATSDR, 2001. Also available at: <www.atsdr.cdc.gov/MHMI/mmg8.html>
(55) Health and Safety Executive. Cyanide poisoning. New recommendations on first aid treatment. Available at: <www.hse.gov.uk/pubns/misc076.htm>
(56) Cummings, T.F., et al. The treatment of cyanide poisoning. Occupational Medicine. Vol. 54 (2004). p. 82-85

Information on chemicals reviewed in the CHEMINFO database is drawn from a number of publicly available sources. A list of general references used to compile CHEMINFO records is available in the database Help.


Review/Preparation Date: 2003-12-31

Revision Indicators:
Bibliography 2004-04-09
pH 2004-06-03
Dissociation constant 2004-06-03



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