Hydrogen cyanide/hydrocyanic acid guide
Hydrogen Cyanide/Hydrocyanic Acid
Hydrogen cyanide (molecular formula HCN) or hydrocyanic acid is a colorless to slightly yellowish liquid in its pure state. It is also flammable, water soluble and highly volatile. The name prussic acid derives from its former extraction from the inorganic, lightfast pigment “Berlin Blue,” which is characterized by a deep blue color.
Other synonyms for the substance are:
- Hydrogen cyanide
- hydrocyanic acid
- formic acid nitrile
- Acidum borussicum or English prussic acid (a reference to the first extraction from Prussian (or Berlin) blue.
According to prevailing opinion, the smell of hydrogen cyanide reminds one of bitter almonds or even marzipan. Although it should be noted that many people are unable to smell it at all for genetic reasons.
Hydrogen cyanide in its highly pure form can be mixed with water and ethanol in any ratio. Its boiling point is only 25,6 °C. At room temperature, the substance evaporates so quickly. That parts of it can sublimate due to the cold of evaporation.
The aforementioned bitter almond odor usually occurs only at diluted concentrations and is sometimes described as numbing and dull. In highly concentrated form, hydrocyanic acid tends to emit a very unpleasant, barely definable odor that is intensely pungent, biting and scratchy, irritating to the mucous membranes and leaving a bitter taste in the mouth. Smell and taste, however, are perceived only briefly, since the substance, even in small quantities, very quickly leads to paralysis of the olfactory nerves.
Dissolved in water, hydrocyanic acid is a very weak acid that is driven out of its salts – the so-called cyanides – even by carbonic acid. Chemically, it represents a borderline case between organic and inorganic chemistry, because as a nitrile, a component of formic acid, it belongs to the organic substances.
Hydrogen cyanide is highly flammable and explosive in mixtures with air at levels of 5.4 to 46.6 percent by volume. Since the substance is easily mixable with water. Groundwater contamination may occur, for example when extinguishing a fire. For this reason, in case of a fire sometimes controlled burning is preferable, provided that the possibility exists and other hazards can be ruled out.
Hydrogen cyanide and its salts, the cyanides, are extremely toxic. As little as 1 mg to 2 mg per kilogram of body weight may have a lethal effect in humans. In addition to direct oral ingestion, poisoning can occur by inhalation or by absorption through the skin. The latter is strongly favored by sweat due to its high water solubility. The water solubility has also led to hydrogen cyanide being assigned to water hazard class 3 (= highly hazardous to water) by the Federal Environment Agency (UBA).
Primarily, cyanides blocks the binding sites for oxygen in the hemoglobin molecule and thus blocks the respiratory chain.* This blocking is irreversible. As a result of the enzyme deactivation, cellular respiration in the body no longer functions. That means that the cell is no longer able to use oxygen for energy production. The result is the so-called “inner suffocation”.
Symptoms of poisoning
First, a bright red coloring of the skin is noticeable as a typical symptom of cyanide poisoning. The reddish color is caused by the fact that the inhaled oxygen cannot not be transported to the cells. Therefore the blood in the veins is still enriched with oxygen. For this reason dead people have bright red livors on their skin after prussic acid poisoning. This symptom is similar to the symptom seen after carbon monoxide poisoning. The bitter almond odor and the livors are a sure indication of an unnatural death.
If the concentration of hydrogen cyanide in the air is very high, affected individuals will experience hyperventilation within seconds, then respiratory arrest and unconsciousness, followed by cardiac arrest within a few minutes. In these cases, skin coloration usually does not occur because the suffocation process is too fast.
The most important symptoms of hydrogen cyanide poisoning at a glance:
- Shortness of breath
- Smell of bitter almonds in the exhaled air
- Severe cramps
- Light red skin color
Below table relates concentration and typical symptoms:
||TLV, no symptoms
|2 to 5
|20 to 40 ppm
|45 to 54 ppm
||symptoms after exposure > 1 h
|100 – 200 ppm
||fatal after exposure > 30 min
|> 300 ppm
||fatal if not immediately treated
First aid and antidote for hydrogen cyanide poisoning
First responders should first protect themselves with respiratory protection. Exposed persons may exhale hydrogen cyanide.
Quick action is advisable in any case. Exposed persons must be taken out of the danger zone given artificial respiration. If available even with pure oxygen. If respiratory arrest has already occurred, resuscitation should be started immediately. In addition, the patient must receive emergency medical treatment as soon as possible.
Antidotes include sodium thiosulfate, 4-dimethylaminophenol (4-DMAP), hydroxycobalamin, and isoamyl nitrite. Sodium thiosulfate given intravenously facilitates the work of the endogenous enzyme rhodanase, which converts cyanide ions to rhodanide.
All first aid measures at a glance:
- Respiratory protection for the first aider
- Remove the affected person from the danger zone
- Fresh air supply or better given respiration with air or even with pure oxygen
- Flush eyes and skin of poisoned person with plenty of water
- Remove contaminated clothing
- if cyanide salts have been swallowed, give plenty of water, induce vomiting and administer medicinal charcoal
- if the person is unconscious, inject a dose of 5 ml of 4-dimethylaminophenol (4-DMAP), known as hydrogen cyanide, into the thigh
- intravenous administration of sodium thiosulfate, supplemented by other antitoxins
- rapid transfer of the affected person to medical assistance
Hydrogen cyanide occurrence in nature
Hydrogen cyanide is not only produced in the industry, but it also occurs in nature. In small quantities in the kernels of stone fruit such as, cherries, apricots and peaches but also in almonds and bitter almonds. Here the substance probably serves to protect the kernels against being eaten and as a germination inhibitor. The hydrocyanic acid gas cannot escape until the nutshell has rotted. Then the hydrogen cyanide can escape and the germination process can start.
Hydrocyanic acid is also present as a cyanogenic glycoside in cassava, a food widely consumed in the tropics. The root tuber, which is toxic in its raw state and must first be processed before it can be eaten, but it is staple food in some countries. For this purpose, it is thoroughly chopped, ground into flour, heated, fermented or washed out with boiling water.
Other foods containing prussic acid in toxicologically relevant amounts include certain varieties of sweet potato, yam, bamboo, sweet sorghum, lima bean and flaxseed. Especially immature shoots of bamboo, which are considered a delicacy in Asian countries, contain high levels of prussic acid. Cases of poisoning in this context are known. Separation of the poison from the glycosides can be achieved by intensive cooking.
Other plants with prussic acid content that are considered poisonous are the Brazilian gum tree and polypodies. In these, HCN is as an effective antifeedant.
Discovery and production
Hydrogen cyanide was discovered in 1782 by the Swedish chemist Carl Wilhelm Scheele (1742-1786). It was formed during a reaction of diluted sulfuric acid with potassium ferricyanide (II), the so-called red blood leach salt. In a laboratory setting, it can also be prepared by dropping 50 percent sulfuric acid onto potassium cyanide. The HCN gas then escapes from this mixture and can be liquefied with dry ice cooling. But beware: this form of production is not at all recommendable without appropriate protective measures. Hydrogen cyanide is also produced by tobacco smoking and by the combustion of certain plastics.
In industry, the Andrussov process is mainly used, in which a mixture of ammonia and methane is partially oxidized with atmospheric oxygen and platinum-rhodium catalysts.
Use of hydrocyanic acid
In industry, hydrocyanic acid is used in many working processes, for example, for the production of amino acids (especially methionine), cyanuric chloride, cyanogen chloride, sodium cyanide and numerous other derivatives. Large quantities are needed for the production of acetone cyanohydrin and adiponitrile, two intermediates for plastics. The amino acid methionine is produced on an industrial scale in a multi-stage process. It is used as a supplement for animal feed. Cyanuric chloride enables the synthesis of crop protection agents for cereals and sugar cane. To obtain acetone cyanohydrin, hydrocyanic acid is added to acetone by a catalytic route. Adiponitrile is obtained by adding a catalyst of nickel to butadiene. This process is also known as hydrocyanation.
Hydrogen cyanide is also used in mining, for example to leach gold and silver out of ores in which the precious metals are present only in small quantities. To do this, for example, the gold solution is reduced using zinc. Another process uses activated carbon from coconut shells, to which the cyanide complex is adsorbs. The gold is then recovered from the ash.
A third method, which requires modern industrial equipment, is the extraction of the cyanide complex from the activated carbon by means of a hot sodium cyanide solution. The latter allows better handling than hydrocyanic acid. Unfortunately, this method is very problematic for the environment, as is the mercury amalgam process, which is now only rarely used. In the gold-producing countries of the Third World, this process still repeatedly leads to catastrophic poisoning of water bodies and ground water.
Use of cyanides
The salts obtained from hydrogen cyanide are called cyanides. They are widely used for hardening and plating metals and for the synthesis of organic compounds. They also play a big role in the galvanic industry and in the production of pesticides. Cyanides were also used in the coking of coal, so that the soils of former coke oven sites may be contaminated with them today.
Handling, storage and transport
Due to its high toxicity, handling hydrocyanic acid requires extreme caution. Without special protective measures, people are at life-threatening risk when handling it. In addition, between 30 and 50 percent of the population is genetically unable to detect the bitter-almond-like odor of hydrogen cyanide.
This genetic deficiency is taken into account, for example by the Bavarian State Office for Health and Food Safety in aptitude tests for applicants wishing to obtain a certificate of competence for pest control or fumigation.
Anhydrous hydrocyanic acid requires cool storage in all cases. Also, vessels containing the substance may only be opened in a state of strong refrigeration. If this is not taken into account, high pressure will develop in the containers due to the low boiling point of hydrogen cyanide. If opened improperly, this can lead to large quantities suddenly escaping in gaseous form and, in the worst case, even splashing around as a liquid substance.
Pure and anhydrous hydrogen cyanide retains its stability for several months. However, it must be stored properly because hydrogen cyanide can otherwise polymerize explosively after a certain time, forming what is known as azulmic acid, a flake-like brown solid. Polymerization can be accelerated by traces of heavy metal oxides or alkalis. The surface of the (glass) container may also play a role. Conversely, the process can be retarded by adding small amounts of mineral or oxalic acid. The fact that explosive decomposition takes place in the next future can be recognized by an initial coloration of the hydrogen cyanide to yellowish, later to brownish.
By the way, hydrogen cyanide must not be kept in schools or school laboratories in Germany because of its very high toxicity.
Hydrogen cyanide is due to its toxicity only very, very rarely transported. As a rule, the hazardous substance is it ever possible produced at the place of further processing.
Historical information on hydrogen cyanide
The high toxicity of hydrogen cyanide has unfortunately left many dark traces in history. There is evidence of numerous accidents that can be traced back to incorrect handling of the substance. In most cases, the gas escaped into the atmosphere, either in production facilities or during the control of pests such as woodworms, termites and other insects. In other cases, high concentrations in enclosed spaces caused explosions.
In World War I, hydrogen cyanide served as a chemical warfare agent, first used by the French army in 1916 and later by the German army. However, the substance did not prove to be very effective because the gas dissipated too quickly. It is, however, added to modern chemical weapons as a “mask breaker” and, in this function, ensures that the gas mask filter’s resorption capability is nullified more quickly and the actual chemical warfare agent can develop its effect.
In some U.S. states, hydrogen cyanide was sometimes used in gas chamber executions until 1999. The concentrations (around 3,200 ppm) were so high that the condemned usually became unconscious immediately and cardiac arrest occurred after a few minutes. Today, hydrocyanic acid is no longer permitted for executions in the USA.
How can hydrogen cyanide be detected?
A classical method among other procedures is the addition of an iron(II) sulfate solution to an alkaline solution. If cyanide ions are present, Berliner Blau is formed after acidification and hydrogen cyanide is released.
However, such a process is not appropriate in today’s industrial production of hydrogen cyanide. In order to detect hydrogen cyanide in the ambient air, sensitive electronic gas detectors such as the mobile tracer for leak detection or the likewise fixed systems from Compur are clearly the better and above all faster alternative. Due to the high toxicity of the gas fast response is a must. Compur offers the product family Statox with different models: Statox 506 and Starox 560 with self – test.