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Hydrogen Sulfide

Hydrogen sulfide (British English: hydrogen sulphide) is the chemical compound with the formula H2S. It is a colorless, very poisonous, flammable gas with a characteristic foul odor. It often results from the bacterial breakdown of organic matter in the absence of oxygen, such as in swamps and sewers (anaerobic digestion). It also occurs in volcanic gases, natural gas, and some well waters.

Properties
Hydrogen sulfide is a highly toxic and flammable gas (Flammability range: 4.3 % - 46 % volume % in air). Being heavier than air, it tends to accumulate at the bottom of poorly ventilated spaces. Although very pungent at first, it quickly deadens the sense of smell, so potential victims may be unaware of its presence until it is too late. For safe handling procedures, a hydrogen sulfide material safety data sheet (MSDS) should be consulted.

Hydrogen sulfide and oxygen burn with a blue flame to form sulfur dioxide (SO2) and [|water]]. In general, hydrogen sulfide acts as a reducing agent.

At high temperature and in the presence of catalysts, sulfur dioxide can be made to react with hydrogen sulfide to form elemental sulfur and water. This is exploited in the Claus process, the main way to convert hydrogen sulfide into elemental sulfur.

Hydrogen sulfide is slightly soluble in water and acts as a weak acid, giving the hydrosulfide ion HS&minus; (pKa=6.9 in 0.01-0.1 mol/litre solutions at 18°C) and the sulfide ion S2&minus; (pKa=11.96).

Hydrogen sulfide reacts with metal ions to form metal sulfides, which may be considered the salts of hydrogen sulfide. Some ores are sulfides. Metal sulfides often have a dark color.

If gaseous hydrogen sulfide is put into contact with concentrated nitric acid, it explodes.

Hydrogen sulfide reacts with alcohols to form thiols.

Toxicity
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.


 * 0.00047 ppm is the recognition threshold, the concentration at which 50% of humans can detect the characteristic foul odor of hydrogen sulfide.
 * At 100–150 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 pulmonarration for 50% of humans for 5 minutes exposure ([[LC50).
 * Concentrations over 1000 ppm cause immediate collapse with loss of breathing, even after inhalation of a single breath

Occurrence
Small amounts of hydrogen sulfide occur in crude petroleum, but natural gas can contain up to XX %. Volcanoes and some hot springs emit some H2S, where it probably arises via the hydrolysis of sulfide minerals

About 10% (???) of total global emissions of H2S is 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.