Hydrogen sulfide

Hydrogen sulfide (hydrogen sulphide in British English) is the sulfur, which is in fact odorless. Hydrogen sulfide has numerous names, some of which are archaic (see Table).

General properties

Hydrogen sulfide is a periodic table group.

Hydrogen sulfide is weakly acidic, dissociating in aqueous solution into hydrogen cations H⁺ and the hydrosulfide anion HS⁻:

H₂S → HS⁻ + H⁺

K_a = 1.3×10⁻⁷ mol/L; pK_a = 6.89.

The sulfide ion, S²⁻, is known in the solid state but not in aqueous solution (c.f. oxidation of the sulfur in alkaline solution. The current best estimate for pK_a2 is 19±2.^[1]

Hydrogen sulfide reacts with many sulfates, which contain the sulfite ion SO₃²⁻ and the sulfate ion SO₄²⁻, respectively.)

Hydrogen sulfide is sulfur dioxide, which is more familiar as the odor of a burnt match.

Occurrence

Small amounts of hydrogen sulfide occur in crude petroleum but natural gas can contain up to 90%. ppm.^{[citation needed]}

Sulfate-reducing bacteria obtain energy by amino acids, for instance during the decay of organic matter. H₂S-producing bacteria also operate in the human colon, and the odor of flatulence is largely due to trace amounts of the gas. Such bacterial action in the mouth may contribute to bad breath. Evidence exists that hydrogen sulfide produced by sulfate-reducing bacteria in the colon may cause or contribute to ulcerative colitis.

About 10% of total global emissions of H₂S are due to human activity. By far the largest industrial route to H₂S occurs in coke ovens, paper mills (using the sulphate method), and tanneries. H₂S arises from virtually anywhere where elemental sulfur comes into contact with organic material, especially at high temperatures.

Hydrogen sulfide can be present naturally in well water. In such cases, sulfates.

A buildup of hydrogen sulfide in the atmosphere could have caused the Permian-Triassic extinction event 252 million years ago.^[2]

Production

Hydrogen sulfide is obtained by its separation from sour gas, that is natural gas with high content of H₂S. It can be produced by reacting hydrogen gas with molten elemental sulfur at about 450 °C. Hydrocarbons can replace hydrogen in this process.^[3] Sulfate-reducing bacteria produce hydrogen sulfide under ambient conditions by the reduction of sulfate or from elemental sulfur. The standard lab preparation is to gently heat iron sulfide with a strong acid; see below. A less well known and more convenient alternative is to react aluminum sulfide with water:

H₂O + Al₂S₃ → H₂S + Al₂O₃.

Al₂S₃ is readily prepared by ignition of the elements.

Uses

Production of thioorganic compounds

Several organosulfur compounds are produced using hydrogen sulfide. These include thioglycolic acid.

Alkali metal sulfides

Upon combining with alkali metal bases, hydrogen sulfide converts to alkali hydrosulfides such as sodium hydrosulfide and sodium sulfide, which are used in the degradation of biopolymers. The depilation of hides and the delignification of pulp by the Kraft process both are effected by alkali sulfides.

In analytical chemistry

Hydrogen sulfide used to have importance in nonmetal) ions (e.g. Pb(II), Cu(II), Hg(II), As(III)) are precipitated from solution upon exposure to H₂S. The components of the resulting precipitate redissolve with some selectivity.

A precursor to metal sulfides

As indicated above, many metal ions react with hydrogen sulfide to give the corresponding metal sulfides. This conversion is widely exploited. In the purification of metal ores by flotation, mineral powders are often treated with hydrogen sulfide to enhance the separation. Metal parts are sometimes passivated with hydrogen sulfide. Catalysts used in refinery is also modified using hydrogen sulfide.

Miscellaneous applications

Hydrogen sulfide is also used in the separation of deuterium oxide, i.e. heavy water, from normal water via the Girdler Sulfide process.

Safety

Hydrogen sulfide is a highly toxic and flammable gas. 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 more information see an MSDS.

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. The toxicity of H₂S is comparable with that of cellular respiration. Since hydrogen sulfide occurs naturally in the environment and the gut, enzymes exist in the body capable of detoxifying it by oxidation to (harmless) sulfate.^[4] Hence low levels of sulfide may be tolerated indefinitely. However, at some threshold level, the oxidative enzymes will be overwhelmed. This threshold level is believed to average around 300-350 ppm. Many personal safety gas detectors are set to alarm at 10 PPM and to go into high alarm at 15 PPM (Utility, sewage & petrochemical workers).

An interesting diagnostic clue of extreme poisoning by H₂S is the discoloration of bronchodilators to overcome eventual bronchospasm, and in some cases hyperbaric oxygen therapy.

Exposure to lower concentrations can result in eye irritation, a sore throat and cough, nausea, shortness of breath, and fluid in the lungs. These symptoms usually go away in a few weeks. Long-term, low-level exposure may result in fatigue, loss of appetite, headaches, irritability, poor memory, and dizziness. Chronic exposures to low level H₂S (around 2 ppm tend to be fatal.

• 0.0047 ppm is the recognition threshold, the concentration at which 50% of humans can detect the characteristic odor of hydrogen sulfide [1], normally described as resembling "a rotten egg".
• 10-20 ppm is the borderline concentration for eye irritation.
• 50-100 ppm leads to eye damage.
• At 150-250 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.

A practical test used in the oilfield industry to determine whether someone requires overnight observation for pulmonary edema is the knee test: if a worker that gets "gassed" loses his balance and at least one knee touches the ground, the dose was high enough to cause pulmonary edema.

Function in the body

Hydrogen sulfide is produced in small amounts by some cells of the mammalian body and has a number of biological functions. (sulfate.^{[citation needed]} Due to its effects similar to NO (without it's potential to form glutathione.^[6] In trisomy 21 (the most common form of Down syndrome) the body produces an excess of hydrogen sulfide.

Induced hibernation

In 2005 it was shown that mice can be put into a state of suspended animation by applying a low dosage of hydrogen sulfide (80 ppm H₂S) in the air. The breathing rate of the animals sank from 120 to 10 breaths per minute and their temperature fell from 37 °C to 2 °C above ambient temperature (in effect, they had become cold-blooded). The mice survived this procedure for 6 hours and afterwards showed no negative health consequences.^[7]

Such a hibernation occurs naturally in many mammals and also in toads, but not in mice. (Mice can fall into a state called clinical torpor when food shortage occurs). If the H₂S-induced hibernation can be made to work in humans, it could be useful in the emergency management of severely injured patients, and in the conservation of donated organs.

As mentioned above, hydrogen sulfide binds to metabolism. Animals and humans naturally produce some hydrogen sulfide in their body; researchers have proposed that the gas is used to regulate metabolic activity and body temperature, which would explain the above findings.^[8]

In 2006 it was shown that the blood pressure of mice treated in this fashion with hydrogen sulfide did not significantly decrease.^[9]

Participant in the sulfur cycle

Hydrogen sulfide is a central participant in the algae and plants which uses water as electron donor and liberates oxygen.)

H₂S implicated in mass extinctions

Hydrogen sulfide has been implicated in some of the five mass extinctions that have occurred in the Earth's past. Although asteroid impacts are thought to have caused some extinctions, the Permian mass extinction (sometimes known as the "Great Dying") may have been caused by hydrogen sulfide. Organic residues from these extinction boundaries indicate that the oceans were anoxic (oxygen depleted) and had species of shallow plankton that metabolized H₂S. The formation of H₂S may have been initiated by massive volcanic eruptions, which emitted CO₂ and methane into the atmosphere which warmed the oceans, lowering their capacity to absorb oxygen which would otherwise oxidize H₂S. The increased levels of hydrogen sulfide could have killed oxygen-generating plants as well as depleted the ozone layer causing further stress. Small H₂S blooms have been detected in modern times in the Dead sea and in the Atlantic ocean off the coast of Namibia.^[2]

See also

• Amine gas treating
• Claus process

References

• "Hydrogen Sulfide", Committee on Medical and Biological Effects of Environmental Pollutants, University Park Press, 1979, Baltimore. ISBN 0-8391-0127-9

• NACE (National Association of Corrosion Epal) http://www.nace.org/