Cyanide

The ion cyanide CN⁻.
The figure shows:
1. Structure of the Valencia
2. Space model
3. Electrostatic potential surface
4. Pair of ions carbon HOMO

Cyanide is a monovalent anion with the representation CN^-. It contains the cyanide group (:C≡N:), formed by a carbon atom and a nitrogen atom joined by a triple bond.

Organic compounds that have a -C≡N functional group attached to an alkyl residue are called nitriles according to IUPAC nomenclature. It can be found as hydrogen cyanide (or hydrocyanic acid) either in the aqueous phase (HCN(aq)) or as part of gas molecules (HCN(g)), forming compounds such as cyanogen chloride (CNCl) and bromide cyanogen (CNBr), or found in tetrahedral crystalline complexes such as sodium cyanide (NaCN) and potassium cyanide (KCN). It is used in the industrial and mining fields, in the electroplating of zinc, gold, copper and especially silver, and in the production of acrylic-based plastics. It is very toxic, potentially deadly.

Features

It is described as having a strong smell of chestnuts or bitter almonds, but it does not always emit an odor and not all people can detect it, since it has been proven that the ability to detect it is in a recessive gene associated with the female X chromosome. ^{[citation needed]} In addition, the detection limit of the odor is very close to the concentration where it begins to be toxic.^{[citation required]}

Cyanide Chemistry

Hydrogen cyanide [H-C≡N(g)] or hydrocyanic acid [H-C≡N(aq)], prussic acid, methanenitrile or formonitrile is a chemical compound whose formula is: HCN. The solution of hydrogen cyanide in water is called hydrocyanic acid. Pure hydrogen cyanide is a colorless, highly poisonous and highly volatile liquid that boils at 26 °C, is slightly acidic and its salts are known as cyanides; It is miscible with water, giving hydrocyanic acid, which is a weak acid (pKa 9.3), which is a colorless liquid with a characteristic odor of bitter almonds. Table 1 presents physical properties of some cyanides.

Table 1. Physical properties of some cyanides

The term cyanide in general refers to all cyanide compounds that can be determined as the CN⁻ ion. The C≡N^- triple bond is easily hydrolyzed by strong alkalis, formic acid and ammonia, a higher temperature favors these reactions.

Metal-cyanide complexes

Metal cyanides can be represented by the general formula where M is a metal and x is the number of cyano groups, which depends on the valence number of the metal, some cyanide complexes can dissolve in water forming metal ions and cyanide ions according to the following general model:

${displaystyle M(CN)_{x}^{n}-xlongleftrightarrow Mn^{n+}+CN^{-}}$

In solution the metal-cyanide complexes formed can be destroyed to form another complex, substitution depends on the respective formation constants, Table 2 Metal ions in solution can also form complexes with cyanide which can then precipitate generally as complexes insoluble hydroxide.

Table 2. Formation constants of some metal-CN^- complexes

Toxicity

It is potentially lethal, acting as a toxin through the inhibition of the cytochrome c oxidase complex, and therefore blocking the electron transport chain, the central system of the cellular respiration process. Consequently, it causes a drop in the production of intracellular ATP, preventing cell homeostasis. Being negatively charged, it also affects the transfer of electrons through channels, creating a positive environment inside the cell.^{[citation needed]} This produces a large amount of charges that generate enough energy so that cyclic AMP (Adenosine monophosphate) can be converted into ADP (Adenosine diphosphate), creating an overstimulation in various processes.^{[citation needed]}

The main harmful and lethal effect of the various varieties of cyanide is to prevent the oxygen carried by red blood cells from being used as a hydrogen acceptor at the end of the intramitochondrial respiratory chain. In an autopsy, the corpse shows a large amount of oxygen in the veins and a large amount of lactic acid, a product of fermentation carried out by cells devoid of oxygen.

Chemicals found in acetonitrile-based products, used mainly to remove false nails, can release cyanide if accidentally ingested and result in death from cardiorespiratory arrest.^{[citation needed]}

Cyanide is neither persistent nor asphyxiant, since in nature it is destroyed by the action of sunlight (by means of ozone), decomposing by oxidation into COx and NOx-type gases. Creating chlorates and nitrites widely used in the purification of lead-contaminated water.

The US Environmental Protection Agency (EPA) regulates allowable levels of cyanide in drinking water using potassium salts. The maximum level of cyanide allowed in drinking water is 0.2 parts of cyanide per million parts of water (0.2 ppm). The U.E. Occupational Safety and Health Administration. (EU-OSHA) has set a limit for hydrogen cyanide and most cyanide salts of 10 parts cyanide per million parts of air (10 ppm) in work air.

For the industrial destruction of cyanide, four methods are used: natural degradation, chemical oxidation, precipitation and biodegradation. There are reuse or recycling technologies. The industrial and mining use of cyanide must adhere to strict regulations, such as those advised by the International Council on Metals and the Environment, based in Ontario, Canada (2012).

One of the main health and environmental concerns related to synthetic chemicals is that they do not break down quickly and therefore can accumulate in the food chain; however, cyanide is transformed into other less toxic chemicals through natural physical, chemical, and biological processes, since cyanide oxidizes when exposed to air or other oxidants, it breaks down and does not persist. Although it is a deadly poison when ingested in sufficiently high doses, it does not cause chronic health or environmental problems when present in low concentrations. A toxicological summary of CN^- is presented in Table 3.

Table 3. Toxicological summary of CN^-

Cyanide in nature

Treatment of yuca roots in Nigeria to eliminate cyanide.

Hydrogen cyanide formed naturally in the early stages of the development of life on earth. Its effectiveness at low concentrations is sudden and deadly. It is also known by its military designation AN (for hydrogen cyanide) and CK (for cyanogen chloride).

It is a product that is commonly found in nature in various microorganisms, insects and in the growth stage of many plants as a protective mechanism, as a common alkaloid, which makes them an unattractive food source during that period, for certain types of herbivorous animals.

Cyanide is naturally present in some foods such as almonds, walnuts, chestnuts, the inner part of the seeds of fruits such as peaches, plums, apricots, among others, cassava, cassava root and the seeds of many other fruits such as apples, pears or grapes. In them it is found under the name of amygdalin, a compound of glucose, benzaldehyde and cyanide, in concentrations that oscillate between 377 and 2.50 mg per kg, and that under the action of a ferment (emulsin) decomposes, producing hydrocyanic acid. Anthropogenic generation also occurs, as is the case with automobile exhaust, cigarette or tobacco smoke, and industrial salt used to melt ice on roads.

Cyanide also appears in the combustion products of synthetic materials such as cloth and plastics.

Industrial production

It is a by-product of the manufacture of acrylic fibers or generated by the combination of natural gas (prior process of removal of methyl mercaptan) with liquid ammonia. Its primary manufacture is 1.4 million tons and it is produced in the US, Mexico, Singapore, China, England, Spain and Germany. The mining and plastics industry in general consumes 82% of the cyanide produced in the world, of this percentage only 18% is used in mining and the other 64% is used in the industry for the manufacture of plastics and derivatives.

Applications

Cyanide has been used industrially since 1889.

• In the industrial sector, cyanide is used to produce paper, paints, textiles and plastics.
• It is present in the chemicals used to reveal photographs. The cyanide salts are used in the metallurgy for galvanization, metal cleaning and the recovery of gold from the rest of the material eliminated.
• Cyanide gas is used to exterminate pests (rats, mice, lauchas, zarigüeyas etc.) and insects in ships, buildings and other places that need it.
• Mining uses for hydrometalur 6% of the cyanide used in the world, usually in low-concentration solution with water to extract and recover metals such as gold and silver through the process called lixiviation, which replaced the old method of amalgamated extraction of precious metals with mercury. See also: Processes with cyanide in gold mining.
• The pharmaceutical industry also uses it, as in some anticancer drugs and sodium nitroprusiate for high blood pressure.
• Minimum doses of cyanide are used for the manufacture of synthetic glues where acrylic-like compounds exist.
• The products used to remove posture nails made from acetonytrile can create cyanide if swallowed.
• Cyanide is also used in qualitative analytical chemistry to recognize iron ions, copper and other elements.
• The cyanide is widely used in galvanoplasty baths as a complete agent of zinc, silver, gold, copper in order to regulate the entry of ions to the anode due to its relatively low pKa value.
• Potassium ferrocyanide (K₄[Fe(CN)₆]) is used in some food industries such as winemaking, for the elimination of heavy metals found in wine. These metals can come from the very production of grapes (Pesticides, spills, fabrile wastes, etc.) as well as from the machinery that is used causing numbness, since the must and wine attack, percuden, carcomen and dissolve the metals. A high content of metals is precipitated by forming insoluble compounds with certain substances such as potassium ferrocyanide, making it abruptly precipitate in the form of insoluble salts whose sediment is removed by simple sieve. The ferrocianuro develops in the wine a complex chemical action resulting in the insolubilization and precipitation of the metals (Zn, Cu, Pb, Fe and Mn). The wine with the lead forms a salt that cannot be removed by the ferrocianuro, which sweetens the solution.
• It is indispensable in the cementing of steels, in the production of nylon, acrylic, photographic applications, galvanoplasty and the production of synthetic rubber. The Blue Prussian (ferrocianuro férrico) features Hematoxinophils, one of its industrial forms, was discovered by Dipel and Diesbach in 1704.

Military Applications

Hydrogen cyanide was marketed by the German company IG Farben, under the name Zyklon B, and was used as a pesticide in the 1920s. Later, it was used in World War II as a chemical weapon by the Nazis.

According to various reports, ^{[citation needed]} gaseous hydrogen cyanide may have been used in conjunction with other chemical agents against the inhabitants of the Kurdish town from Halabja, northeastern Iraq, during the Iran-Iraq War in the 1980s. There are also allegations against the United States, alleging that it may have been used in Vietnam along with Agent Orange.

Cyanide in the mining industry

Gold mining industries are among the largest consumers of cyanide due to its high affinity for the metal. After the precious metal has been extracted from the ore, the cyanides are discharged as effluents and mine tailings.

Cyanides from gold mill effluents can be broadly classified into 3 categories:

1. soluble cyanides (Example: NaCN, KCN, Ca(CN)₂ and relatively insoluble electrically neutral cyanides (Zn(CN) ₂Cd(CN)₂, CPN₂).
2. Dissociable cyanides in the middle of weak acid (CNWAD, weak acid dissociable cyanides) which are complex transition metal – relatively weak cianide-(includes to the Cd, Cu, Ni and Zn) and are dissociated in neutral or slightly acidic conditions to CN^- and his metal ions.
3. Dissociable cyanide cyanide cyanide (CNSAD, strong acid dissociable cyanide) (strong metals such as Faith, Co, Ag and Au), which are dissociable only under extreme acidity conditions, as they form very stable complexes.

The so-called free cyanide is the one found as HCN or CN^-, and it is these species that are classified as the most toxic due to their high potential for metabolic inhibition, on the other hand the metal-cyanide complexes (eg, and are considered relatively less toxic. The acute toxicity of each metal-cyanide complex is related to the ease with which cyanide can be dissociated to free cyanide, so comparatively weaker complexes they will be more dangerous (toxic) than stronger complexes.

Analytical methods for cyanide determination

Determination of total cyanide by distillation and colorimetry

The method is based on the acid distillation at reflux of the sample in order to cause the volatilization of all forms of cyanide present in it, such as hydrogen cyanide (HCN), to later condense them in an alkaline solution. The cyanide concentration in this solution is determined colorimetrically by UV-VIS spectroscopy, by conversion to CNCl by reaction with chloramine T at pH < 8. After the reaction is complete, CNCl forms a bluish-red compound on addition of barbituric acid and pyridine. The compound formed presents a molecular absorption band between 575 and 582 nm. The colorimetric method is suitable for cyanide concentrations down to a lower limit of 20µg/l (20ppb).

Cyanide determination by direct potentiometry with selective electrode

The determination of cyanides by direct potentiometry with a selective cyanide electrode, which consists of measuring the free cyanide ion concentration with a selective electrode at [CN^-] at a constant ionic strength, to for which a calibration curve is drawn and a regression equation is obtained by applying the Least Squares Method. Finally the concentration of an unknown sample is determined by interpolation, using the determined equation.

Total Cyanide Distillation

The determination of total cyanide ([CN^-]_total) in liquid, semi-liquid and solid samples is a problem for environmental laboratories, since the method involves the distillation in acid medium of gaseous hydrogen cyanide which must be collected in a very concentrated sodium hydroxide solution to impose a pH of at least 11 (due to the acid-base pair HCN/CN^{-< /sup> has a pKa value of 9.3).}

The extraction process involves the destruction of the metal-cyanide complexes, the addition of MgCl₂ and H₂SO₄ to form HCN which is collected in highly concentrated soda according to the following reactions:

Reaction 1. Destruction of the metal-cyanide complexes M(CN)

${displaystyle M(CN)_{x}^{n-x}+MgCl_{2}longleftrightarrow MgCl_{n}+Mg(CN)_{2}}}}}$

Reaction 2. Formation of HCN_(gas) by addition of H₂SO₄

${displaystyle Mg(CN)_{2}+H_{2}SO_{4}longleftrightarrow 2HCN_{(gas)}+MgSO_{4}}}}$

Reaction 3. Formation of NaCN

${displaystyle HCN_{(gas)}+NaOHlongleftrightarrow NaCN+H_{2}O}$

Precautions

Hazard identification

• Dangers for people: Very toxic by inhalation, contact with the skin and by ingestion. It causes burns on the skin and eyes.
• Environmental hazards: In contact with acids releases hydrogen cyanide, very toxic gas.

First Aid

• Inhalation: If inhalation symptoms occur, move the victim to a ventilated place. Stay at rest and warm. Do not breathe mouth to mouth. Apply artificial breathing in case of respiratory failure. Need medical assistance. If there is no recovery to administer amylo nitrite capsules.
• Skin contact: Remove contaminated clothes. Wash the affected area with abundant water. Requiring medical care in case of persistent irritation.
• Eye contact: Wash with plenty of water for 15 minutes, keeping eyelids open. Go immediately to the ophthalmologist.
• Ingestion: Treating the patient as in the case of inhalation, do not cause vomiting and avoid eating food.

Fire-fighting measures

• Suitable means of extinction: Dry chemical powder.
• Special fire protection equipment: Use regular fire fighting equipment. Take autonomous breathing equipment.