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Hydrogen Sulfide

Hydrogen sulfide (British English: hydrogen sulphide) is the chemical compound with the formula H₂S. It is a colorless, 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, petroleum crude oil and some well waters.

Chemical properties

Properties Property Value Common name hydrogen sulfide IUPAC name hydrogen sulfide CAS number 7783-06-04 Molecular formula H2S Molecular mass 134.0809 g/mol Atmospheric boiling point -60.2 °C at 1 atm Liquid density 0.9149 g/ml at -60.2 °C and 1 atm Gas density 0.00193 g/ml at -60.2 °C and 1 atm, 0.00145 g/ml at 15 °C and 1 atm Critical point 100 °C at 88.162 atm Melting point -86 °C) Gas specific heat, cp 4.184 kJ/(mol·K) at 25 °C and 1 atm Heat of vaporization Heat of fusion Viscosity Refractive index

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⁻ (pK_a=6.9 in 0.01-0.1 mol/litre solutions at 18°C) and the sulfide ion S²⁻ (pK_a=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 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 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.