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
Hydrogen sulfide is a highly toxic and flammable gas (flammability range: 4.0 to 46 volume % in air). It is heavier than air and tends to accumulate at the bottom of poorly ventilated, confined 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 readily soluble in water, amounting to 4.1 grams per litre of water at 20 °C which is equivalent to 0.41 weight %. When it is in solution in water, the H2S forms a weak acid (sometimes referred to as hydrosulfuric acid) containing hydrosulfide and sulfide ions (HS− and S2−). The solubility is further enhanced in alkaline solutions due to increased ionization.

Hydrogen sulfide reacts with metal ions to form metal sulfides, which may be considered the salts of hydrogen sulfide. Some minerals 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 ppmv is the recognition threshold, the concentration at which 50% of humans can detect the characteristic foul odor of hydrogen sulfide.
 * At 100 – 150 ppmv, the olfactory nerve is paralyzed after a few inhalations, and the sense of smell disappears, often together with awareness of danger.
 * At 200 ppmv, the central nervous system is depressed.
 * At 500 ppmv, the cardiovascular system is depressed.
 * At 1000 ppmv, the central nervous system is paralyzed and death occurs.