Hydrogen sulfide (or hydrogen sulphide) is the chemical compound with the formula H₂S. It is a colorless, very poisonous, flammable

gas with the characteristic foul odor of rotten eggs. 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. The human body produces small amounts of H₂S and uses it as a signaling molecule.

Hydrogen sulfide is slightly heavier than air; a mixture of H₂S and air is explosive. Hydrogen sulfide and oxygen burn with blue flame to form sulfur dioxide (SO₂) and water. In general, it acts as a reducing agent.

At high heat or 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.

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Hydrogen sulfide is somewhat soluble in water and acts as a weak acid, giving the hydrosulfide ion                   HS^- (pK_a=6.9 in 0.01-0.1 mol/litre solutions at 18°C) and the sulfide ion S^2- (pK_a=11.96). A solution of hydrogen sulfide in water is initially clear but over time turns cloudy. This is due to the slow reaction of hydrogen sulfide with the oxygen dissolved in water, yielding elemental sulfur, which precipitates out.           

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. Lead(II) acetate paper is used to detect hydrogen sulfide because it turns grey in the presence of the gas as lead(II) sulfide is produced. Reacting metal sulfides with strong acid liberates hydrogen sulfide.

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

It reacts with alcohols to form mercaptanes.

Production

Hydrogen sulfide is most commonly obtained by its separation from sour gas, which is natural gas with high content of H₂S. It can also be produced by reacting hydrogen gas with molten elemental sulfur at about 450 °C. Hydrocarbons can replace hydrogen in this process.^[2]

Sulfate-reducing bacteria (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.

The standard lab preparation is to gently heat ferrous sulfide (FeS) with a strong acid in a Kipp generator.

FeS + 2 HCl = FeCl₂ + H₂S

A less well-known and more convenient alternative is to react aluminium sulfide with water:

3 H₂O + Al₂S₃ ? 3 H₂S + Al₂O₃.

This gas is also produced by heating sulfur with solid organic compounds and by reducing sulfurated organic compounds with hydrogen.

Hydrogen sulfide is also a byproduct of some reactions and caution should be used when production is likely as exposure can be fatal.

Occurrence

Deposit of sulfur on a rock, caused by volcanic gases

Small amounts of hydrogen sulfide occur in crude petroleum, but natural gas can contain up to 90%.^[3] Volcanoes and some hot springs (as well as cold springs) emit some H₂S, where it probably arises via the hydrolysis of sulfide minerals, i.e. MS + H₂O ? MO + H₂S.

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. Other anthropogenic sources of hydrogen sulfide include coke ovens, paper mills (using the sulfate method), and tanneries. H₂S arises from virtually anywhere where elemental sulfur comes in 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 less toxic sulfates