Hydrogen sulfide

Sulfur, S   Hydrogen, H
Names
Systematic IUPAC name Hydrogen sulfide[1]
Other names Dihydrogen monosulfide Sour gas Dihydrogen sulfide Sewer gas Egg gas Sulfane Sulfurated hydrogen Sulfureted hydrogen Sulfuretted hydrogen Sulfur hydride Hydrosulfuric acid Hydrothionic acid Thiohydroxic acid Sulfhydric acid
Identifiers
CAS Number	7783-06-4 Y
3D model (JSmol)	Interactive image
3DMet	B01206
Beilstein Reference	3535004
ChEBI	CHEBI:16136 Y
ChEMBL	ChEMBL1200739 N
ChemSpider	391 Y
ECHA InfoCard	100.029.070
EC Number	231-977-3
Gmelin Reference	303
KEGG	C00283 Y
MeSH	Hydrogen+sulfide
PubChem CID	402
RTECS number	MX1225000
UNII	YY9FVM7NSN Y
UN number	1053
CompTox Dashboard (EPA)	DTXSID4024149
InChI InChI=1S/H2S/h1H2 Y Key: RWSOTUBLDIXVET-UHFFFAOYSA-N Y InChI=1/H2S/h1H2 Key: RWSOTUBLDIXVET-UHFFFAOYAJ
SMILES S
Properties
Chemical formula	H2S
Molar mass	34.08 g·mol−1
Appearance	Colorless gas
Odor	Foul, pungent, like that of rotten eggs
Density	1.539 g.L−1 (0°C)[2]
Melting point	−85.5[3] °C (−121.9 °F; 187.7 K)
Boiling point	−59.55[3] °C (−75.19 °F; 213.60 K)
Solubility in water	3.980 g dm−3 (at 20 °C) [4]
Vapor pressure	1740 kPa (at 21 °C)
Acidity (pKa)	7.0[5][6]
Conjugate acid	Sulfonium
Conjugate base	Bisulfide
Magnetic susceptibility (χ)	−25.5·10−6 cm3/mol
Refractive index (nD)	1.000644 (0 °C)[2]
Structure
Point group	C2v
Molecular shape	Bent
Dipole moment	0.97 D
Thermochemistry
Heat capacity (C)	1.003 J K−1 g−1
Std molar entropy (S⦵298)	206 J mol−1 K−1[7]
Std enthalpy of formation (ΔfH⦵298)	−21 kJ mol−1[7]
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	Flammable and highly toxic
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H220, H330, H400
Precautionary statements	P210, P260, P271, P273, P284, P304+P340, P310, P320, P377, P381, P391, P403, P403+P233, P405, P501
NFPA 704 (fire diamond)	4 4 0
Flash point	−82.4 °C (−116.3 °F; 190.8 K)[10]
Autoignition temperature	232 °C (450 °F; 505 K)
Explosive limits	4.3–46%
Lethal dose or concentration (LD, LC):
LC50 (median concentration)	713 ppm (rat, 1 hr) 673 ppm (mouse, 1 hr) 634 ppm (mouse, 1 hr) 444 ppm (rat, 4 hr)[9]
LCLo (lowest published)	600 ppm (human, 30 min) 800 ppm (human, 5 min)[9]
NIOSH (US health exposure limits):
PEL (Permissible)	C 20 ppm; 50 ppm [8]
REL (Recommended)	C 10 ppm (15 mg/m3) [8]
IDLH (Immediate danger)	100 ppm[8]
Related compounds
Related hydrogen chalcogenides	Water Hydrogen selenide Hydrogen telluride Hydrogen polonide Hydrogen disulfide Sulfanyl
Related compounds	Phosphine
Except where otherwise noted, data are given for materials in their standard state (at 25 °C , 100 kPa). N verify (what is YN ?) Infobox references

Hydrogen sulfide is a chemical compound with the formula H₂S. It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs.^[11] Swedish chemist Carl Wilhelm Scheele is credited with having discovered the chemical composition of purified hydrogen sulfide in 1777.^[12]

Hydrogen sulfide is toxic to humans and most other animals by inhibiting cellular respiration in a manner similar to hydrogen cyanide. When it is inhaled or its salts are ingested in high amounts,^{[clarification needed]} damage to organs occurs rapidly with symptoms ranging from breathing difficulties to convulsions and death.^[13][14] Despite this, the human body produces small amounts of this sulfide and its mineral salts, and uses it as a signalling molecule.^[15]

Hydrogen sulfide is often produced from the microbial breakdown of organic matter in the absence of oxygen, such as in swamps and sewers; this process is commonly known as anaerobic digestion, which is done by sulfate-reducing microorganisms. It also occurs in volcanic gases, natural gas deposits, and sometimes in well-drawn water.

Properties

Hydrogen sulfide is slightly denser than air. A mixture of H₂S and air can be explosive.

Oxidation

In general, hydrogen sulfide acts as a reducing agent, as indicated by its ability to reduce sulfur dioxide in the Claus process. Hydrogen sulfide burns in oxygen with a blue flame to form sulfur dioxide (SO₂) and water:

2 H₂S + 3 O₂ → 2 SO₂ + 2 H₂O

If an excess of oxygen is present, sulfur trioxide (SO₃) is formed, which quickly hydrates to sulfuric acid:

H₂S + 2 O₂ → H₂SO₄

Acid-base properties

It is slightly soluble in water and acts as a weak acid (pK_a = 6.9 in 0.01–0.1 mol/litre solutions at 18 °C), giving the hydrosulfide ion HS⁻. Hydrogen sulfide and its solutions are colorless. When exposed to air, it slowly oxidizes to form elemental sulfur, which is not soluble in water. The sulfide anion S²⁻ is not formed in aqueous solution.^[16]

Extreme temperatures and pressures

At pressures above 90 GPa (gigapascal), hydrogen sulfide becomes a metallic conductor of electricity. When cooled below a critical temperature this high-pressure phase exhibits superconductivity. The critical temperature increases with pressure, ranging from 23 K at 100 GPa to 150 K at 200 GPa.^[17] If hydrogen sulfide is pressurized at higher temperatures, then cooled, the critical temperature reaches 203 K (−70 °C), the highest accepted superconducting critical temperature as of 2015. By substituting a small part of sulfur with phosphorus and using even higher pressures, it has been predicted that it may be possible to raise the critical temperature to above 0 °C (273 K) and achieve room-temperature superconductivity.^[18]

Hydrogen sulfide decomposes without a presence of a catalyst under atmospheric pressure around 1200 °C into hydrogen and sulfur.^[19]

Tarnishing

Hydrogen sulfide reacts with metal ions to form metal sulfides, which are insoluble, often dark colored solids. Lead(II) acetate paper is used to detect hydrogen sulfide because it readily converts to lead(II) sulfide, which is black. Treating metal sulfides with strong acid or electrolysis often liberates hydrogen sulfide. Hydrogen sulfide is also responsible for tarnishing on various metals including copper and silver; the chemical responsible for black toning found on silver coins is silver sulfide (Ag₂S), which is produced when the silver on the surface of the coin reacts with atmospheric hydrogen sulfide.^[20] Coins that have been subject to toning by hydrogen sulfide and other sulfur-containing compounds may have the toning add to the numismatic value of a coin based on aesthetics, as the toning may produce thin-film interference, resulting in the coin taking on an attractive coloration.^[21] Coins can also be intentionally treated with hydrogen sulfide to induce toning, though artificial toning can be distinguished from natural toning, and is generally criticised among collectors.^[22]

Production

Hydrogen sulfide is most commonly obtained by its separation from sour gas, which is natural gas with a high content of H₂S. It can also be produced by treating hydrogen with molten elemental sulfur at about 450 °C. Hydrocarbons can serve as a source of hydrogen in this process.^[23]

S + H₂ → H₂S

The very favorable thermodynamics for the hydrogenation of sulfur implies that the dehydrogenation (or cracking) of hydrogen sulfide would require very high temperatures.^[24]

A standard lab preparation is to treat ferrous sulfide with a strong acid in a Kipp generator:

FeS + 2 HCl → FeCl₂ + H₂S

For use in qualitative inorganic analysis, thioacetamide is used to generate H₂S:

CH₃C(S)NH₂ + H₂O → CH₃C(O)NH₂ + H₂S

Many metal and nonmetal sulfides, e.g. aluminium sulfide, phosphorus pentasulfide, silicon disulfide liberate hydrogen sulfide upon exposure to water:^[25]

6 H₂O + Al₂S₃ → 3 H₂S + 2 Al(OH)₃

This gas is also produced by heating sulfur with solid organic compounds and by reducing sulfurated organic compounds with hydrogen. It can also be produced by mixing ammonium thiocyanate to concentrated sulphuric acid and adding water to it.

Biosynthesis

Hydrogen sulfide can be generated in cells via enzymatic or non-enzymatic pathways. Three enzymes catalyze formation of H
₂S: cystathionine γ-lyase (CSE), cystathionine β-synthetase (CBS), and 3-mercaptopyruvate sulfurtransferase (3-MST).^[26] CBS and CSE are the main proponents of H₂S biogenesis, which follows the trans-sulfuration pathway.^[27] These enzymes have been identified in a breadth of biological cells and tissues, and their activity is induced by a number of disease states.^[28] These enzymes are characterized by the transfer of a sulfur atom from methionine to serine to form a cysteine molecule.^[27] 3-MST also contributes to hydrogen sulfide production by way of the cysteine catabolic pathway.^[28][27] Dietary amino acids, such as methionine and cysteine serve as the primary substrates for the transulfuration pathways and in the production of hydrogen sulfide. Hydrogen sulfide can also be derived from proteins such as ferredoxins and Rieske proteins.^[28]

Sulfate-reducing (resp. sulfur-reducing) bacteria generate usable energy under low-oxygen conditions by using sulfates (resp. elemental sulfur) to oxidize organic compounds or hydrogen; this produces hydrogen sulfide as a waste product.

Water heaters can aid the conversion of sulfate in water to hydrogen sulfide gas. This is due to providing a warm environment sustainable for sulfur bacteria and maintaining the reaction which interacts between sulfate in the water and the water heater anode, which is usually made from magnesium metal.^[29]

Signalling role

H₂S in the body acts as a gaseous signaling molecule with implications for health and in diseases.^{[26] [30][31]}

Hydrogen sulfide is involved in vasodilation in animals, as well as in increasing seed germination and stress responses in plants.^[32] Hydrogen sulfide signaling is moderated by reactive oxygen species (ROS) and reactive nitrogen species (RNS).^[32] H₂S has been shown to interact with NO resulting in several different cellular effects, as well as the formation of another signal called nitrosothiol.^[32] Hydrogen sulfide is also known to increase the levels of glutathione, which acts to reduce or disrupt ROS levels in cells.^[32]

The field of H₂S biology has advanced from environmental toxicology to investigate the roles of endogenously produced H₂S in physiological conditions and in various pathophysiological states.^[33] H₂S has been implicated in cancer and Down syndrome and vascular disease.^{[34][35][36][37]}

It inhibits Complex IV of the mitochondrial electron transport chain, which effectively reduces ATP generation and biochemical activity within cells.^[32]

Uses

Production of sulfur

Hydrogen sulfide is mainly consumed as a precursor to elemental sulfur. This conversion, called the Claus process, involves partial oxidation to sulfur dioxide. The latter reacts with hydrogen sulfide to give elemental sulfur. The conversion is catalyzed by alumina.^[38]

2H₂S + SO₂→ 3S + 2H₂O

Production of thioorganic compounds

Many fundamental organosulfur compounds are produced using hydrogen sulfide. These include methanethiol, ethanethiol, and thioglycolic acid.^[23] Hydrosulfides can be used in the production of thiophenol.^[39]

Production of metal sulfides

Upon combining with alkali metal bases, hydrogen sulfide converts to alkali hydrosulfides such as sodium hydrosulfide and sodium sulfide:

H₂S + NaOH → NaSH + H₂O

NaSH + NaOH → Na₂S + H₂O

Sodium sulfides are used in the paper making industry. Specifically, salts of SH⁻ break bonds between lignin and cellulose components of pulp in the Kraft process.^[23]

As indicated above, many metal ions react with hydrogen sulfide to give the corresponding metal sulfides. Oxidic ores are sometimes treated with hydrogen sulfide to give the corresponding metal sulfides which are more readily purified byy flotation.^[23] Metal parts are sometimes passivated with hydrogen sulfide. Catalysts used in hydrodesulfurization are routinely activated with hydrogen sulfide.

Hydrogen sulfide was a reagent in the qualitative inorganic analysis of metal ions. In these analyses, heavy metal (and nonmetal) ions (e.g., Pb(II), Cu(II), Hg(II), As(III)) are precipitated from solution upon exposure to H₂S. The components of the resulting solid are then identified by their reactivity.

Miscellaneous applications

Hydrogen sulfide is used to separate deuterium oxide, or heavy water, from normal water via the Girdler sulfide process.

A suspended animation-like state has been induced in rodents with the use of hydrogen sulfide, resulting in hypothermia with a concomitant reduction in metabolic rate. Oxygen demand was also reduced, thereby protecting against hypoxia. In addition, hydrogen sulfide has been shown to reduce inflammation in various situations.^[40]

Occurrence

Volcanoes and some hot springs (as well as cold springs) emit some H₂S. Hydrogen sulfide can be present naturally in well water, often as a result of the action of sulfate-reducing bacteria.^{[41][better source needed]} Hydrogen sulfide is produced by the human body in small quantities through bacterial breakdown of proteins containing sulfur in the intestinal tract, therefore it contributes to the characteristic odor of flatulence. It is also produced in the mouth (halitosis).^[42]

A portion of global H₂S emissions are due to human activity. By far the largest industrial source of H₂S is 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 Kraft process), tanneries and sewerage. H₂S arises from virtually anywhere where elemental sulfur comes in contact with organic material, especially at high temperatures. Depending on environmental conditions, it is responsible for deterioration of material through the action of some sulfur oxidizing microorganisms. It is called biogenic sulfide corrosion.

In 2011 it was reported that increased concentrations of H₂S were observed in the Bakken formation crude, possibly due to oil field practices, and presented challenges such as "health and environmental risks, corrosion of wellbore, added expense with regard to materials handling and pipeline equipment, and additional refinement requirements".^[43]

Besides living near gas and oil drilling operations, ordinary citizens can be exposed to hydrogen sulfide by being near waste water treatment facilities, landfills and farms with manure storage. Exposure occurs through breathing contaminated air or drinking contaminated water.^[44]

In municipal waste landfill sites, the burial of organic material rapidly leads to the production of anaerobic digestion within the waste mass and, with the humid atmosphere and relatively high temperature that accompanies biodegradation, biogas is produced as soon as the air within the waste mass has been reduced. If there is a source of sulfate bearing material, such as plasterboard or natural gypsum (calcium sulfate dihydrate), under anaerobic conditions sulfate reducing bacteria converts this to hydrogen sulfide. These bacteria cannot survive in air but the moist, warm, anaerobic conditions of buried waste that contains a high source of carbon – in inert landfills, paper and glue used in the fabrication of products such as plasterboard can provide a rich source of carbon^[45] – is an excellent environment for the formation of hydrogen sulfide.

In industrial anaerobic digestion processes, such as waste water treatment or the digestion of organic waste from agriculture, hydrogen sulfide can be formed from the reduction of sulfate and the degradation of amino acids and proteins within organic compounds.^[46] Sulfates are relatively non-inhibitory to methane forming bacteria but can be reduced to H₂S by sulfate reducing bacteria, of which there are several genera.^[47]

Removal from water

A number of processes have been designed to remove hydrogen sulfide from drinking water.^[48]

Continuous chlorination

For levels up to 75 mg/L chlorine is used in the purification process as an oxidizing chemical to react with hydrogen sulfide. This reaction yields insoluble solid sulfur. Usually the chlorine used is in the form of sodium hypochlorite.^[49]

Aeration

For concentrations of hydrogen sulfide less than 2 mg/L aeration is an ideal treatment process. Oxygen is added to water and a reaction between oxygen and hydrogen sulfide react to produce odorless sulfate.^[50]

Nitrate addition

Calcium nitrate can be used to prevent hydrogen sulfide formation in wastewater streams.

Removal from fuel gases

Hydrogen sulfide is commonly found in raw natural gas and biogas. It is typically removed by amine gas treating technologies. In such processes, the hydrogen sulfide is first converted to an ammonium salt, whereas the natural gas is unaffected.

RNH₂ + H₂S ⇌ [RNH₃⁺ + SH⁻

The bisulfide anion is subsequently regenerated by heating of the amine sulfide solution. Hydrogen sulfide generated in this process is typically converted to elemental sulfur using the Claus Process.

Safety

The underground mine gas term for foul-smelling hydrogen sulfide-rich gas mixtures is stinkdamp. Hydrogen sulfide is a highly toxic and flammable gas (flammable range: 4.3–46%). It can poison several systems in the body, although the nervous system is most affected.^{[citation needed]} The toxicity of H₂S is comparable with that of carbon monoxide.^[51] It binds with iron in the mitochondrial cytochrome enzymes, thus preventing cellular respiration. Its toxic properties were described in detail in 1843 by Justus von Liebig.^[52]

Even before hydrogen sulfide was discovered, Italian physician Bernardino Ramazzini hypothesized in his 1713 book De Morbis Artificum Diatriba that occupational diseases of sewer-workers and blackening of coins in their clothes may be caused by an unknown invisible volatile acid (moreover, in late 18th century toxic gas emanation from Paris sewers became a problem for the citizens and authorities).^[53]

Although very pungent at first (it smells like rotten eggs^[54]), it quickly deadens the sense of smell, creating temporary anosmia,^[55] so victims may be unaware of its presence until it is too late. Safe handling procedures are provided by its safety data sheet (SDS).^[56]

Low-level exposure

Since hydrogen sulfide occurs naturally in the body, the environment, and the gut, enzymes exist to metabolize it. At some threshold level, believed to average around 300–350 ppm, the oxidative enzymes become overwhelmed. Many personal safety gas detectors, such as those used by utility, sewage and petrochemical workers, are set to alarm at as low as 5 to 10 ppm and to go into high alarm at 15 ppm. Metabolism causes oxidation to sulfate, which is harmless.^[57] Hence, low levels of hydrogen sulfide may be tolerated indefinitely.

Exposure to lower concentrations can result in eye irritation, a sore throat and cough, nausea, shortness of breath, and fluid in the lungs.^[51] These effects are believed to be due to hydrogen sulfide combining with alkali present in moist surface tissues to form sodium sulfide, a caustic.^[58] These symptoms usually subside in a few weeks.

Long-term, low-level exposure may result in fatigue, loss of appetite, headaches, irritability, poor memory, and dizziness. Chronic exposure to low level H₂S (around 2 ppm) has been implicated in increased miscarriage and reproductive health issues among Russian and Finnish wood pulp workers,^[59] but the reports have not (as of 1995) been replicated.

High-level exposure

Short-term, high-level exposure can induce immediate collapse, with loss of breathing and a high probability of death. If death does not occur, high exposure to hydrogen sulfide can lead to cortical pseudolaminar necrosis, degeneration of the basal ganglia and cerebral edema.^[51] Although respiratory paralysis may be immediate, it can also be delayed up to 72 hours.^[60] Diagnostic of extreme poisoning by H₂S is the discolouration of copper coins in the pockets of the victim.

Inhalation of H₂S resulted in about 7 workplace deaths per year in the U.S. (2011–2017 data), second only to carbon monoxide (17 deaths per year) for workplace chemical inhalation deaths.^[61] Zdroj:https://en.wikipedia.org?pojem=Hydrogen_sulfide
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