A cyanide is any chemical compound that contains the group C≡N, with the carbon atom triple bonded to the nitrogen atom. Inorganic cyanides contain the highly toxic cyanide ion CN- and are the salts of the acid hydrogen cyanide (HCN). Organic cyanides contain the CN group single-bonded to another carbon atom and are also known as nitriles. Two cyanide ions can bond to each other via their carbon atoms, forming the gas cyanogen.
Appearance
Hydrogen cyanide is a colorless gas with a faint, bitter, almond-like odor. Some people are unable to smell cyanide at all, due to a genetic trait. Sodium cyanide (NaCN) and potassium cyanide (KCN) are both white solids or powder with a bitter, almond-like odor in damp air.
Occurrence and use
Cyanides can be produced by certain bacteria, fungi, and algae, and are found in a number of foods and plants. Cyanide occurs naturally in cassava roots, which are potato-like tubers of cassava plants grown in tropical countries; these must be processed prior to consumption. Fruits which have a pit, such as cherries or apricots, often contain either cyanides or cyanogenic glycosides in the pit. Bitter almonds, from which almond oil and flavoring is made, also contain cyanide.
Hydrogen cyanide is contained in vehicle exhaust and in tobacco smoke. The smoke of burning plastics contains hydrogen cyanide, and house fires often result in cyanide poisonings. A deep blue pigment called Prussian blue, used in the making of blueprints, is iron (III) ferrocyanide. (Hence the name cyanide, from cyan, a shade of blue.) It produces hydrogen cyanide when exposed to acids.
Cyanides are one of the very few soluble compounds of gold, and thus are used in electroplating, gold mining, metallurgy and jewellery for chemical gilding, buffing , and extraction of gold.
Cyanides and hydrogen cyanide are used in production of chemicals, photographic development, making plastics, fumigating ships, and some mining processes.
Effects on the human body
To deal with the cyanides contained in many foods, the body has an enzyme (rhodanide synthetase) which can convert small amounts of cyanides to the harmless sulfur-containing thiocyanate (SCN−). Cyanides are also essential components of vitamin B12.
In larger amounts, cyanides are harmful. Symptoms of moderate poisoning include vomiting, convulsions, deep breathing, shortness of breath and anxiety; more serious cases result in convulsions, loss of consciousness, and death after apnea and heart arrest. The lethal dose for adults is 200–300 mg of potassium or sodium cyanide, or 50 mg of hydrogen cyanide.
Exposure to lower levels of cyanide over a long period (e.g. after use of cassava roots as a primary food source in tropical Africa) results in increased blood cyanide levels. These may result in weakness of the fingers and toes, difficulty walking, dimness of vision, deafness, and decreased thyroid gland function, but chemicals other than cyanide may contribute to these effects. Skin contact with cyanide can produce irritation and sores.
It is not known whether cyanides can directly cause birth defects in people. Birth defects were seen in rats that ate diets of cassava roots. Effects on the reproductive system were seen in rats and mice that drank water containing sodium cyanide.
There are medical tests to measure blood and urine levels of cyanide; however, small amounts of cyanide are not always detectable in blood and urine. Tissue levels of cyanide can be measured if cyanide poisoning is suspected, but cyanide is rapidly cleared from the body, so the tests must be done soon after the exposure. An almond-like odor in the breath may alert a doctor that a person was exposed to cyanide.
Mechanism of toxicity and treatment
The cyanide ion kills all aerobic organisms by shutting down the respiration in cells. It interrupts the electron transport chain in the inner membrane of the mitochondrion because it binds more strongly than oxygen to the Fe3+ in cytochrome a3, preventing this cytochrome from combining electrons with oxygen.
Contrary to popular belief, cyanide does not bind well to ferrous hemoproteins, such as hemoglobin, the mechanism which makes carbon monoxide toxic. One of the therapies for cyanide poisoning is to convert part of the hemoglobin of the blood from ferrous hemoglobin to ferric; this creates a pool of binding potential that can divert cyanide from the cytochromes it poisons. This is done with the compound 4-Dimethylaminophenyl, or with sodium nitrite.
The standard cyanide antidote kit uses a small inhaled dose of amyl nitrite followed by intravenous sodium nitrite. This converts a portion of the hemoglobin's iron from ferrous iron to ferric iron, converting the hemoglobin into methemoglobin. Cyanide is more strongly drawn to methemoglobin than to the cytochrome oxidase of the cells, effectively pulling the cyanide off the cells and onto the methemoglobin. Once bound with the cyanide, the methemoglobin becomes cyanmethemoglobin.
The next part of the cyanide antidote kit is sodium thiosulfate, which is administered intravenously. The sodium thiosulfate and cyanmethemoglobin becomes thiocyanate, releasing the hemoglobin, and the thiocyanate is excreted by the kidneys.
Use as a poison
The cyanide ion, if used as poison, is generally delivered in the form of gaseous hydrogen cyanide or in the form of potassium cyanide (KCN) or sodium cyanide (NaCN).
Zyklon B, the poison gas used in Nazi gas chambers during the Holocaust, works by delivering hydrogen cyanide gas. Cyanide is also the compound used in U.S. execution chambers.
Cyanide salts are sometimes used as fast acting "suicide pills". When they reach the stomach acids, cyanide ions are released; therefore they work faster on an empty stomach. Famous cyanide salt suicides include:
Poisoning by cyanide also figures prominently in crime fiction, for example Agatha Christie's Sparkling Cyanide (also titled Remembered Death); cyanide is the instrument of one murder in The Big Sleep by Raymond Chandler. See also: Victims of poisoning
Cyanides were stockpiled in both the Soviet and the United States chemical weapons arsenals in the 1950s and 1960s. During the Cold War, the Soviet Union was thought to be planning to use hydrogen cyanide as a "blitzkrieg" weapon to clear a path through the opposing front line, knowing that the harmful gas itself would evaporate and allow unprotected access to the captured zone.
Mining
Cyanide salts are used in silver and gold mining, called the cyanide process. The ore is finely ground and mixed or sprayed with cyanide solution. The precious metal cations bind to the cyanide anions and form a soluble cyanide. The left-over dirt is discarded and the metal is recovered from the clear solution with zinc. This process can result in environmental and health problems.
Fishing
Cyanides are used to capture live fish near coral reefs for the aquarium and seafood market. In this method, a diver uses a large, needleless syringe to squirt a cyanide solution into areas where the fish are hiding, stunning them so that they can be easily gathered. Environmental organizations decry the practice, as do responsible aquarists and aquarium dealers. Many fish caught in this fashion die either immediately or in shipping. Those that survive to find their way into pet stores often die from shock, or from massive digestive damage. The high concentrations of cyanide on reefs so harvested has also resulted in cases of cyanide poisioning among local fishermen and their families.
Last updated: 10-24-2005 03:58:35
