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Hydrogen Sulfide

Hydrogen sulfide (or hydrogen sulphide) is a chemical compound with the formula H₂S. It is a colorless, highly toxic, flammable gas with a characteristic foul odor.

Hydrogen sulfide is present in very large amounts in raw natural gas and also occurs in large amounts during the refining of petroleum crude oil. In fact, the vast majority of the 66,000,000 metric tons of elemental sulfur produced worldwide in 2006 was by-product sulfur from petroleum refining and natural gas processing plants. It is also present in volcanic gases and some water well sources

It also occurs in swamps and sewers as a result of the bacterial breakdown of organic matter in the absence of oxygen, which is known as anaerobic digestion.

Chemical properties

Properties Property Value Common name hydrogen sulfide Other names hydrogen sulphide, dihydrogen sulfide, sulfur hydride, sewer gas IUPAC name hydrogen sulfide CAS number 7783-06-04 Molecular formula H2S Molecular mass 34.082 g/mol Melting point -85.47 °C Normal boiling point -60.35 °C and 1 atm Critical point 100.38 °C at 88.46 atm Heat of fusion 69.74 kJ/kg at -85.47 °C and 1 atm Heat of vaporization 517.87 kJ/kg at -60.35 °C and 1 atm Liquid density 914.9 kg/m3 at -60.35 °C and 1 atm Gas density 1.93 kg/m3 at -60.35 °C and 1 atm, 1.411kg/m3 at 20°C and 1 atm Gas specific heat, cp 1.005 kJ/(kg·K) at 25 °C and 1 atm Gas specific heat ratio, cp/cv 1.32 at 25 °C and 1 atm Gas Viscosity 126.02 micropoise at 25 °C

Hydrogen sulfide is a highly toxic and flammable gas (flammability range: 4.3 to 46 volume % in air). It is heavier than air and tends to accumulate at the bottom of poorly ventilated spaces.

Hydrogen sulfide and oxygen burn to form sulfur dioxide (SO₂) and water. In general, hydrogen sulfide acts as a reducing agent.

At high temperature (about 1000 °C) and with the use of a catalyst, sulfur dioxide will react with hydrogen sulfide to form elemental sulfur and water (in the form of steam). This is commonly accomplished by the Claus process, the primary method of converting hydrogen sulfide into elemental sulfur.

Hydrogen sulfide is slightly soluble in water and, in solution, 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 mineralss are sulfides.

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.