Hydrogen sulfide

Hydrogen sulfide
General
Systematic name	Hydrogen sulfide
Other names	Sulfurated hydrogen sulfane
Molecular formula	H₂S
Molar mass	34.0758 g/mol
Appearance	Colorless gas.
CAS number
Properties
Density and phase	1.363 g/L, gas.
Solubility in water	? g/100 ml (?°C)
Melting point	-82.30°C (190.85 K)
Boiling point	-60.28°C (212.87 K)
Acidity (pK_a)	6.89 19±2 (See Text)
Structure
Molecular shape	Bent.
Dipole moment	0.97 D
Hazards
MSDS	External MSDS
Main hazards	Highly toxic, highly flammable.
NFPA 704
Flash point	-82.4°C
R/S statement	R: , , S: , , , , ,
RTECS number	MX1225000
Supplementary data page
Structure and properties	n, ε_r, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Related compounds
Other anions	?
Other cations	?
Related thioethers	Dimethyl sulfide
Related compounds	Water hydrogen selenide sodium sulfide
Except where noted otherwise, data are given for materials in their standard state (at 25°C, 100 kPa) Chemical infobox

Hydrogen sulfide (hydrogen sulphide in British English), H₂S, is a colorless, toxic, flammable gas that is responsible for the foul odor of rotten eggs and flatulence. It often results when bacteria break down organic matter in the absence of oxygen, such as in swamps, and sewers (alongside the process of anaerobic digestion). It also occurs in volcanic gases, natural gas and some well waters.

Hydrogen sulfide is also known as sulfane, sulfur hydride, sour gas, sulfurated hydrogen, sewer gas and stink damp. IUPAC accepts the names "hydrogen sulfide" and "sulfane"; the latter one is used exclusively when naming more complicated compounds.

Chemistry

Hydrogen sulfide is a covalent hydride chemically related to water (H₂O) since oxygen and sulfur occur in the same periodic table group.

Hydrogen sulfide is weakly acidic, dissociating in aqueous solution into hydrogen cations H⁺ and the hydrosulfide anion HS⁻:

H₂S → HS⁻ + H⁺

K_a = 1.3×10⁻⁷ mol/L; pK_a = 6.89.

The sulfide ion, S²⁻, is known in the solid state but not in aqueous solution (c.f. oxide). The second dissociation constant of hydrogen sulfide is often stated to be around 10⁻¹³, but it is now clear that this is an error caused by oxidation of the sulfur in alkaline solution. The current best estimate for pK_a2 is 19±2.Giggenbach, W. (1971). Inorg. Chem. 10:1333. Meyer, B.; Ward, K.; Koshlap, K.; & Peter, L. (1983). Inorg. Chem. 22:2345. Myers, R. J. (1986). J. Chem. Educ. 63:687.

Hydrogen sulfide reacts with many metals to produce the corresponding metal sulfides. Well-known examples are silver sulfide (Ag₂S), the tarnish that forms on silver when exposed to the hydrogen sulfide of the atmosphere, and cadmium sulfide (CdS), a pigment known as cadmium yellow. Transition metal sulfides are characteristically insoluble, thus H₂S is commonly used to separate metal ions from aqueous solutions.

Hydrogen sulfide is corrosive and renders some steels brittle, leading to sulphide stress cracking - a concern especially for handling acid gas and sour crude in the oil industry.

Metal sulfides should not to be confused with sulfites, which are derived from the sulfite ion SO₃²⁻.

Hydrogen sulfide burns to give the gas sulfur dioxide, which is more familiar to people as the odor of a burnt match.

Occurrence

Small amounts of hydrogen sulfide occur in crude petroleum but natural gas can contain up to 28%. Volcanoes and hot springs emit some H₂S, where it probably arises via the hydrolysis of sulfide minerals, i.e. MS + H₂O to give MO + H₂S.

Normal average concentration in clean air is about 0.0001-0.0002 ppm.

Sulfate-reducing bacteria obtain their energy by oxidizing organic matter or hydrogen with sulfates, producing H₂S. These microorganisms are prevalent in low-oxygen environments, such as in swamps and standing waters. Sulfur-reducing bacteria and some archaea obtain their energy by oxidizing organic matter or hydrogen with elemental sulfur, also producing H₂S. Other anaerobic bacteria liberate hydrogen sulfide when they digest sulfur-containing amino acids, for instance during the decay of organic matter. H₂S-producing bacteria also operate in the human colon, and the odor of flatulence is largely due to trace amounts of the gas. Such bacterial action in the mouth may contribute to bad breath.

About 10% of total global emissions of H₂S are due to human activity. By far the largest industrial route to H₂S occurs in petroleum refineries: the hydrodesulfurization process liberates sulfur from petroleum by the action of hydrogen. The resulting H₂S is converted to elemental sulfur by partial combustion via the Claus process, which is a major source of elemental sulfur. Other anthropogenic sources of hydrogen sulfide include coke ovens, paper mills (using the sulphate method), and tanneries. H₂S arises from virtually anywhere where elemental sulfur comes into contact with organic material, especially at high temperatures.

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 inorganic analysis of metal ions. For such small-scale laboratory use, H₂S was made as needed in a Kipp generator by reaction of sulfuric acid (H₂SO₄) with ferrous sulfide FeS. Kipp generators were superseded by the use of thioacetamide, an organic solid that converts in water to H₂S. In these analyses, heavy metal (and nonmetal) ions (e.g. Pb(II), Cu(I), Hg(II), As(III)) are precipitated from solution upon exposure to H₂S. The components of the resulting precipitate redissolve with some selectivity.

Industrial production focuses on separation of hydrogen sulfide from sour gas - natural gas with high content of H₂S.

It is used 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 the separation of deuterium oxide, i.e. heavy water, from normal water via the Girdler Sulfide process.

Dangers

The gas is highly toxic. It is slightly heavier than air, so tends to concentrate at the bottom of poorly ventilated spaces - deep wells, sewers, and underground tanks. It is also highly flammable, forming an explosive mixture with air over a wide range of concentrations (4.3-46%, or 43000-460000 ppm).

Breathing hydrogen sulfide may paralyze the olfactory nerve making it impossible to smell the gas after an initial strong exposure. This paralysis can result in carelessness on the part of the victim, as sensory indications of the gas disappear, while the danger may not. The effect appears reversible, i.e. people who survive intense exposure to H₂S regain their ability to smell the gas. In exceptional circumstances where the gas is in high concentration, loss of consciousness is possible. Attempts to rescue unconscious people from spaces with high concentration of hydrogen sulfide can lead to the death of rescuers (so called "second man fatalities"). For example, 4 people died in an accident at the Sullivan mine in May 2006. A contractor succumbed to the gas in an enclosed shed; a mine employee and two paramedics successively attempted rescue and also died.

Hydrogen sulfide generated by sewage has an insidious behavior. When the sewage is allowed to stand for a long time, hydrogen sulfide can build up to a high concentration - up to 6000 ppm - and then gets quickly released when the liquid is disturbed, rapidly building up to a fatal concentration.

In aquariums, compacted substrate can breed bacteria that generate this gas due to the low level of oxygen available. Normally the level of released gas is very low as the substrate traps it and the fish or owner is not aware of it. However if the owner decides to rearrange objects in the tank, the substrate is disturbed and so large amounts of the gas can be released and prove to be toxic or even deadly to the fish in the tank. Aquarium owners often either regularly disturb the substrate to prevent the buildup or they add Malaysian Trumpet Snails which burrow into the substrate and thereby aerate it to prevent the production of the gas.

Health effects

Hydrogen sulfide is considered a broad-spectrum poison, meaning that it can poison several different systems in the body, although the nervous system is most affected. The toxicity of H₂S is comparable with that of hydrogen cyanide. It forms a complex bond with iron in the mitochondrial cytochrome enzymes, thereby blocking oxygen from binding and stopping cellular respiration. Loss of consciousness can result 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. Since hydrogen sulfide occurs naturally in the environment and the gut, there are enzymes in the body capable of detoxifying it by oxidation to (harmless) sulfate. Hence low levels of sulfide may be tolerated indefintely. However, at some threshold level, the oxidative enzymes will be overwhelmed and then the free sulfide can coordinate significant amounts of Iron II. This threshold level is believed to average around 300-350 PPM. Personal safety gas dectectors are set to start alarming at 10 PPM and to go into high alarm at 15 PPM; (Utility, sewage & petrochemical workers). An interesting diagnostic clue of extreme poisoning by H₂S is the discoloration of copper coins in the pockets of the victim. 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.

Poisoning can occur by ingestion of soluble sulfides, although absorption through skin is low.

Exposure to lower concentrations can result in eye irritation, 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.

0.0047 ppm is the recognition threshold, the concentration at which 50% of humans can detect the characteristic rotten egg odor of hydrogen sulfide ^*
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 the central nervous system and rapid breathing, leading to loss of breathing;
- 800 ppm is the lethal concentration for 50% of humans for 5 minutes exposition (LC50).
Concentrations over 1000 ppm cause immediate collapse with loss of breathing, even after inhalation of a single breath.

A practical test used in the oilfield industry to determine whether someone requires overnight observation for pulmonary edema is the knee test: if a worker that gets "gassed" loses his balance and at least one knee touches the ground, the dose was high enough to cause pulmonary edema. This is important as the worker may feel fine after some fresh air, and not think medical attention is needed, but the onset of pulmonary edema may occur many hours later when the worker is asleep: the worker's lungs could fill with fluid, and the sedative effects of the gas may prevent the worker from waking up.

Animal studies showed that pigs that ate food containing hydrogen sulfide had diarrhea after a few days and weight loss after about 105 days.

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 trisomy 21 (the most common form of Down syndrome) 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 breathing rate of the animals sank from 120 to 10 breaths per minute and their temperature fell from 37°C to 2°C above ambient 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 central participant in the sulfur cycle, the biogeochemical cycle of sulfur on Earth. As mentioned above, sulfur-reducing and 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.)

Reference

"Hydrogen Sulfide", Committee on Medical and Biological Effects of Environmental Pollutants, University Park Press, 1979, Baltimore. ISBN 0-8391-0127-9

External links

Hydrogen compounds | Sulfides | Foul-smelling chemicals

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