Hydrogen sulfide
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- For an alternative meaning for H2S, see H2S radar.
The chemical compound hydrogen sulfide (hydrogen sulphide in British English), H2S, is a colorless, toxic, flammable gas that is responsible for the foul odor of rotten eggs. It often results when bacteria break down organic matter in the absence of oxygen, such as in swamps and sewers. It also occurs in volcanic gases and some well waters.
Hydrogen sulfide is also known as sulfane, sulfur hydride, dihydrogen monosulfide, sulfurated hydrogen, sewer gas and stink damp. IUPAC accepts the names "hydrogen sulfide" and "sulfane"; the latter one is used exclusively in more complicated compounds.
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Chemistry
Hydrogen sulfide is a covalent hydride chemically similar to water (H2O) since oxygen and sulfur occur in the same periodic table group.
The gas is weakly acidic; in solution it partially dissociates into hydrogen cations H+ and the sulfide anion S2-. It reacts with alkali and metals to produce sulfides, the salts of hydrogen sulfide (not to be confused with sulfites, which are certain compound that contain sulfur and oxygen.)
Metal sulfides are often black; silver jewelry turns black over time because hydrogen sulfide from the air reacts with the silver to produce silver sulfide.
Hydrogen sulfide penetrates the lattice of some steels and makes them brittle, leading to sulphide stress cracking - a concern especially for handling sour crude in the oil industry.
Small amounts of hydrogen sulfide can be disposed by burning it to sulfur dioxide, which is also corrosive but less toxic.
Occurrence
Hydrogen sulfide occurs naturally in crude petroleum, natural gas (even up to 28%), volcanic gases, and some hot springs.
Sulfate-reducing bacteria obtain their energy by using sulfates to oxidize organic matter or hydrogen, thereby reducing the sulfates to hydrogen sulfate. They are especially efficient in low-oxygen environments, such as in swamps and standing waters. Some other anaerobic bacteria liberate hydrogen sulfide when they digest sulfur-containing amino acids, for instance during the decay of organic matter. Hydrogren sulfide producing bacteria also operate in the human colon, and the odor of flatulence is largely due to trace amounts of the gas. They can also be found in the mouth and contribute to bad breath.
Hydrogen sulfide can also result from industrial activities, such as food processing, sewage treatment, coke ovens, paper mills (using the sulphate method), tanneries, and petroleum refineries, in coal mines (as iron sulfides such as pyrite decompose) and anywhere where sulfur comes in contact with organic material at high temperatures. Anthropogenic emissions of hydrogen sulfide are however just 10% of total global emissions.
Normal average concentration in clean air is about 0.0001-0.0002 ppm.
Hydrogen sulfide can be present naturally in well water. In such cases, ozone is often used for its removal. An alternative method uses a filter with manganese dioxide. Both methods oxidize sulfides to fairly non-toxic sulfates.
Manufacture and use
Hydrogen sulfide used to have importance in analytical chemistry for well over a century, in the qualitative chemical analysis of metal ions. For such small-scale laboratory use the gas is made as needed in a Kipp generator (also known as Kipp's aparatus, named after its inventor Petrus Johannes Kipp) by reaction of sulfuric acid with ferrous sulfide FeS.
Industrial production focuses on separation of hydrogen sulfide from sour gas - natural gas with high content of H2S.
The most important industrial use of hydrogen sulfide is as a source of about 25% of the world production of elemental sulphur. The manufacturing process is based on burning about 1/3 of hydrogen sulfide to sulphur dioxide, then letting the resulting SO2 react with H2S.
Other uses are in metallurgy for the preparation of metallic sulfides. It also finds use in preparation of phosphors and oil additives, in separation of metals, removal of metallic impurities, and in organic chemical synthesis. Hydrogen sulfide is also used in nuclear engineering, in the Girdler Sulfide process of manufacturing heavy water.
Dangers
The gas is highly toxic and can kill or seriously injure sewer workers. It is heavier than air, so tends to concentrate at the bottom of poorly ventilated spaces - deep wells, sewers, underground tanks. It is also highly flammable, forming explosive mixture with air over wide range of concentrations (4.3-46%, or 43000-460000 ppm).
Hydrogen sulfide created in sewage has an insidious behavior. When the sewage is allowed to stand for long time, hydrogen sulfide can build up in high concentration - up to 6000 ppm, and then gets quickly released when the liquid is disturbed (like when a soda can gets shaked), rapidly building up fatal concentration. This can happen even in open spaces, when opening manhole covers; a stream of escaping gas can be - and often is - deadly. Due to the fast action of the gas in high concentration, loss of consciousness is possible even after a single breath. Attempts to rescue unconscious people from spaces with high concentration of hydrogen sulfide often lead to the death of rescuers (so called "second man fatalities").
Health effects
Hydrogen sulfide is considered a broad-spectrum poison, meaning it can poison several different systems in the body, in particular the nervous system. Its toxicity is comparable with hydrogen cyanide. It forms a complex bond with iron in mitochondrial cytochrome enzymes, thereby blocking oxygen from binding and stopping cellular respiration.
Poisoning can happen by inhalation of hydrogen sulfide or ingestion of soluble sulfides; absorption through skin is low. Breathing high levels of hydrogen sulfide may paralyze the olfactory nerve (making it impossible to smell the gas) and can cause death within just a few breaths. There could be loss of consciousness after one or more breaths. Cases of acute hydrogen sulfide poisonings are rare, occurring mostly in industrial settings; however, emergency physicians should be aware of its symptoms, as quick identification and treatment is critical. An interesting diagnostic clue is discoloration of copper coins in the pockets of the patient. Treatment involves immediate inhalation of amyl nitrite, injections of sodium nitrite, inhalation of pure oxygen, administration of bronchodilators to overcome eventual bronchospasm, and in some cases hyperbaric oxygen therapy.
Exposure to lower concentrations can result in eye irritation (because of the high alkality of the SH- anion), a sore throat and cough, shortness of breath, and fluid in the lungs. These symptoms usually go away in a few weeks. Long-term, low-level exposure may result in fatigue, loss of appetite, headaches, irritability, poor memory, and dizziness. Higher concentrations of 700-800 ppm tend to be fatal.
The recognition threshold - the lowest concentration at which 50% of humans can perceive the characteristic rotten egg odor of hydrogen sulfide - is about 0.002 ppm. 10-20 ppm is the borderline concentration for eye irritation. 50-100 ppm leads to eye damage. At 150-250 ppm the olfactory nerve is paralyzed after a few inhalations, and the sense of smell disappears, often together with awareness of danger. 320-530 ppm leads to pulmonary edema with the possibility of death. 530-1000 ppm causes strong stimulation of CNS and rapid breathing leading to loss of breathing; 800 ppm is the lethal concentration for 50% of humans for 5 minutes exposition. Concentrations over 1000 ppm cause immediate collapse with loss of breathing, even after inhalation of a single breath.
Animal studies showed that pigs that ate food containing hydrogen sulfide had diarrhea after a few days and weight loss after about 105 days.
According to Lee R. Kump, a geoscientist from Penn State University, a buildup of hydrogen sulfide in the atmosphere could have caused the Permian-Triassic extinction event 252 million years ago.
There is some evidence that hydrogen sulfide produced by sulfate reducing bacteria in the colon may cause or contribute to ulcerative colitis.
Function in the body
Hydrogen sulfide is produced in small amounts by some cells of the mammalian body and has a number of biological functions. (Only two other such gases are currently known: nitric oxide NO and carbon monoxide CO.) It is produced from cysteine by various enzymes. It acts as a vasodilator and is also active in the brain, where it increases the response of the NMDA receptor and facilitates long term potentiation, which is involved in the formation of memory. Eventually the gas is converted to sulfites and further oxidized to thiosulfate and sulfate.
In Alzheimer's disease, the concentration of hydrogen sulfide in the brain is abnormally low; in trisomy 21 the body produces an excess of hydrogen sulfide.
Induced hibernation
In 2005, Mark Roth and other scientists from the University of Washington and the Fred Hutchinson Cancer Research Center in Seattle demonstrated that mice can be put into a state of suspended animation by applying a low dosage of hydrogen sulfide (80 ppm H2S) in the air. The heartbeat of the animals sank from 120 to 10 beats per minute and their temperature fell from 37 degrees Celsius to 2 degrees above room temperature (in effect, they had become cold-blooded). The mice survived this procedure for 6 hours and afterwards showed no negative health consequences.
Such a hibernation occurs naturally in many mammals and also in toads, but not in mice. (Mice can fall into a state called clinical torpor when food shortage occurs). If the H2S-induced hibernation can be made to work in humans, it could be useful in the emergency management of severely injured patients, and in the conservation of donated organs.
As mentioned above, hydrogen sulfide binds to cytochrome oxidase and thereby prevents oxygen from binding, which apparently leads to the dramatic slowdown of metabolism. Animals and humans naturally produce some hydrogen sulfide in their body; researchers have proposed that the gas is used to regulate metabolic activity and body temperature, which would explain the above findings.
Participant in the sulfur cycle
Hydrogen sulfide is a major participant in the sulfur cycle, the biogeochemical cycle of sulfur on Earth. As mentioned above, sulfate-reducing bacteria derive energy from converting sulfur or sulfate into hydrogen sulfide by oxidizing hydrogen or organic molecules. Other bacteria liberate hydrogen sulfide from sulfur-containing amino-acids. Several groups of bacteria can use hydrogen sulfide as fuel, oxidizing it to elemental sulfur or to sulfate by using oxygen or nitrate as oxidant. The purple sulfur bacteria and the green sulfur bacteria use hydrogen sulfide as electron donor in photosynthesis, thereby producing elemental sulfur. (In fact, this mode of photosynthesis is older than the mode of cyanobacteria, algae and plants which uses water as electron donor and liberates oxygen.)
External links
- MSDS safety data sheet (http://ptcl.chem.ox.ac.uk/MSDS/HY/hydrogen_sulfide.html)
- Abstract of survey article on H2S as used by the body (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=15329822&query_hl=7), by P. Kamoun
- BBC about suspended animation by H2S (http://news.bbc.co.uk/go/pr/fr/-/2/hi/science/nature/4469793.stm)cs:Sirovodík
de:Schwefelwasserstoff es:Sulfuro de hidrógeno fr:Sulfure d'hydrogène it:Acido solfidrico nl:Zwavelwaterstof ja:硫化水素 pl:Siarkowodór ru:Сероводород zh:硫化氫