THE MOLECULE (click for more)

Perhaps the biggest hindrance to understanding cyanide and the risks of using this chemical is that "cyanide" is a general term. The media and even scientific writings routinely make this mistake by treating all the forms of cyanide as if they were the same. Indeed government environmental regulations have failed to take this into account in some cases. In fact there are many forms of cyanide of varying toxicity and chemical reactivity. Cyanide is the term used to designate chemical with a carbon atom triple bonded to a nitrogen molecule (CN). Cyanide mostly occurs regardless of origin in non-organic form. The main forms of cyanide can be organised as follows:

  • Free Cyanide and Simple Cyanides
  • Metal cyanides
  • Cyanate and Thiocyanate
  • Organocyanides
Simple and Free Cyanides

The free form of cyanide is hydrogen cyanide (HCN) and the cyanide ion CN- that it dissociates to in water. HCN is soluble in water and, above 26 degrees Celsius, is a colourless gas that smells of bitter almonds. At a pH of less than 8 (almost all surface water on earth) HCN volatises, meaning it naturally escapes from the water into air where it is dispersed and rendered harmless. This process is natural and detoxifies HCN solutions. Simple metal cyanide solids include the cyanide salts sodium cyanide (NaCN) and potassium cyanide (KCN). These solid salts readily dissolve in water to produce free cyanide. Cyanogen halides are cyanogen chloride (CNCl) and cyanogen bromide (CNBr) which are colorless gases and commonly found in Municipal sewage treatment facilities as a result of processing the waste. The simple and free forms of cyanide are the most toxic of the cyanide species. The escape (volatility) of dissolved HCN from water and subsequent transformation to benign compounds in air, is an important process. It is a natural mechanism for that removes free cyanide from natural and industrial waste waters. This is why in the plants at gold mines the pH of cyanide solutions at maintained at values near 10.5 in order to prevent volatilization. This preserves cyanide in solution so that it can be used to recover gold and at the same time limits the risk of worker exposure to HCN gas in a confined space.

Metal Cyanides

The cyanide molecule can associate with heavy metals such as copper, nickel, zinc, cadmium, iron, silver, gold to form complexes of variable stability and toxicity. Metal complexed cyanides can be separated into two groups, strong and weak metal cyanide complexes. Both are water soluble. Strong metal cyanide complexes are CN- associated with metals such as iron, gold and cobalt. An example is the iron cyanide dye "Prussian blue". These complexes are chemically very stable, difficult to oxidize and therefore are not toxic or low toxic forms of cyanide. The weak or weak-acid dissociable (WAD) metal cyanides, CN- associated with copper, nickel, silver, cadmium and zinc, are ‘‘toxicologically significant’’ forms of cyanide because they far more easily breakdown into free cyanide.

Cyanate and Thiocyanate

Free cyanide can be oxidized to form cyanate. The oxidiation of CN is hard to acheve so cyanates are rare. CNO-. Cyanate is much less toxic than free cyanide. Thiocyanates are CN combined with sulfur, S, a relatively non-toxic form of cyanide. Both cyanates and thiocyanates can combine with metals to form metal-cyanates and metal-thiocyanates respectively.

Organocyanides

The organocyanide complexes are cyanide associated with organic molecule, such as nitriles, cyanobenzene, cyanogenic glycosides, where the cyanide is associated with the organic molecule. Organic cyanide is produced in nature and in manufacturing.

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IN NATURE (click for more)

HCN is comprised of hydrogen, carbon and nitrogen, three of the four most common atoms that make up life forms on earth. Cyanide compounds and HCN have been around long before humans came on the scene. At the start of earth history cyanide is thought to have played a pivotal role in the development of life on earth by helping form amino acids, peptides, lipids and membranes. More than four billion years later cyanide continues to be an integral part of nature on earth and in the universe. Cyanide gas clouds occur in interstellar space and HCN gas has been found in the atmospheres of other planets included Titan, a moon of Saturn.

Naturally produced cyanide occurs throughout our biosphere. HCN is a naturally produced part of the earth’s atmosphere in concentrations up to 900 parts per trillion (by volume). Research shows that cyanide gets into the atmosphere mostly through forest fires, as the burning of biomass produces HCN gas. Many plants, animals, microbes and fungi organically produce and consume cyanide.

Over 2,600 plants (130 families) are known to produce cyanide. Most cyanide produced by plants is for use as a defense mechanism but almost all fruit-bearing plants release HCN as part of the fruit ripening process. This is why fruit seeds contain such high concentrations of cyanide. Various microorganisms can produce and/or destroy cyanide. They also degrade this same cyanide either to detoxify it, or to use it as a source of nitrogen for growth. Significant amounts of cyanide are formed by a wide range of fungi and a few bacteria. Many forms of bacteria have been identified that use cyanide as a source of nitrogen for growth. Recently it has been discovered that vascular plants like Willow trees, fungi and bacteria can be produced and used to detoxify cyanide. Experiments are underway to remove cyanide from industrial waste using such organisms which convert the cyanide into nitrogen food.

If you listen to your mother and eat vegetables you consume cyanide every day. For example sweet potatoes, corn, lima beans, almonds, radishes, cabbage, kale, brussel sprouts, cauliflower, broccoli, turnips, lettuce and kidney beans and the seeds of cherries, plums, apricots, pears and apples all contain cyanide. The cyanide is produced by the plant to aid in the fruit ripening process. Many fruit pits contain small amounts of cyanohydrins, which slowly release HCN; one hundred grams of crushed apple seeds can yield roughly about 10 mg of HCN. Up to one gram of hydrogen cyanide per kilogram may be obtained from the "bitter" roots of the cassava plant. Cassava is part of the diet of hundreds of millions of people living in equatorial countries of the world. If improperly prepared, cyanide can be released at toxic levels resulting in a chronic paralysis of the arms and legs particularly amongst young people. Several thousand cases of have been documented in African countries.

Due to organic metabolic processes, cyanide occurs naturally in the soil around the root systems of many plants. Since gold and silver naturally dissolve in a weak cyanide solution, these metals have been found concentrated around plant roots where they have been dissolved out of gold rich soil and re-deposited. Typically natural CN in water is in concentrations <0.01 mg/L as it decays rapidly when exposed to UV radiation from sunlight. In tropical soils natural cyanide values up to 0.1 mg/L have been reported.

USES OF CYANIDE (click for more)

Cyanide has been known for over 2,000 years. The word cyanide is derived from the Greek cyanine meaning blue. It has been used as a blue dye in the form of the strong metal cyanide iron ferrocyanide "Prussian Blue" since its discovery in 1704. This cyanide die is still used today make jeans blue. Many other industries use cyanide, and without it our daily lives would not be the same. Over 2 million tonnes of cyanide are produced a year for industrial use. Cyanide is produced industrially in one of two ways: as a by-product of the manufacture of acrylic fibres and certain plastics, or by combining natural gas and ammonia at high temperatures and pressures to produce hydrogen cyanide (HCN) gas. There are three primary producers of solid, liquid and gaseous cyanide in the world: Dupont, in the United States, ICI, in England, and Degussa Corporation, in Germany.

Mining use only accounts for less than 15% of cyanide usage, but mining companies bear the brunt of criticism regarding the use of cyanide. About 85% of cyanide used today is in the manufacture of a wide range of industrial and medical products. Useful items containing cyanide are nylon, plastic, glass, perfume, soaps, fertilizers and animal food supplements, paints and adhesives. Cyanide forms a very stable complex with several metals including gold and silver, copper and zinc. This makes also it very useful in electroplating, to make items such as silver plated dinnerware and brass plated door handles that you likely use every day. Other uses of cyanide include pest control (as a poison), an anti-clumping agent in road salt and fire retardants. HCN vapour has been used to fumigate buildings and ships, to exterminate rabbits, rodents, and large predators, and in horticultural practice to control insect pests that have developed resistance to other pesticides. Sodium cyanide was used for about 50 years by the US Fish and Wildlife Service to control coyote numbers and still used by the New Zealand government to control possums where cyanide-laced pellets are dropped from the air.

While these industries use cyanide as a part of the manufacturing or mining process, other human activities produce cyanide as a byproduct. Everyday airborne sources of cyanide arise from fires and cigarette smoke. Coffee and table salt also contain cyanide. Manufactured gas plants (MGP) produced cyanide wastes and there are thousands of past MGP sites in the US and Europe. Chemical manufacturing, iron and steel production, petroleum refining and aluminum smelting all produce cyanide waste.

Medical Uses

The cyanide compound sodium nitroprusside is occasionally used in emergency medical situations to produce a rapid decrease in blood pressure in humans; it is also used as a vasodilator in vascular research. The cobalt in artificial Vitamin B12 contains a cyanide ligand as an artifact of the purification process. During World War I, a copper cyanide compound was briefly used by Japanese physicians for the treatment of tuberculosis and leprosy. Cyanide is used in pharmaceuticals such as the anti cancer drug laetrile and in specialised surgical dressings that foster healing and reduce scarring. Laetrile (an anti-cancer preparation made from apricot kernels) and sodium nitroprusside (a drug used to reduce high blood pressure), release cyanide upon metabolism.