Hydrogen sulfide

Hydrogen sulfide in aqueous solution (H₂S_(aq)), is a hydrocid with the formula H₂S. This gas, heavier than air, is flammable, colorless, toxic, odoriferous: its odor is that of decomposing organic matter, similar to the smell of rotten eggs. Despite this, in the human body it performs essential functions.

In chemical nomenclature, the ending -ide corresponds to halides, that is, to salts that do not contain oxygen. Since salt "is an ionic compound formed by a cation other than H⁺ and an anion other than OH^- or O^2-: acid + base --> salt + water", the expression "hydrogen sulfide" is strange, since it implies that hydrogen replaces itself.

To avoid this inconsistency in its name, H₂S could be considered as an aqueous solution (aq in the formula), that is, hydrogen sulfide .

With strong bases it generates salts: sulfides. Its boiling point is 212.86 K. Hydrogen sulfide is held together by bonds

General information

Hydrogen sulfide occurs naturally in oil, natural gas, gases emitted in processes associated with volcanic phenomena, and hot springs.

It can be generated by decomposition of existing organic matter under anaerobic conditions (hydrogen sulfide comes from proteins that contain amino acids that have sulfur, such as methionine, cystine and cysteine, which are more abundant in horns, nails and hair), by the action of bacteria also in anaerobic conditions that reduce the sulfates present in the water, by contact of a sulfide or a sulfhydrate with an acid, with acidic or even neutral water, or by combustion of a hydrocarbon or hydrogen in contact with sulfur.

It can exist in swampy waters, lagoons or stagnant waters, drains, fishmeal or fish oil ponds, slurry pits, hoppers for dispossession of organic waste (meat, etc.), decanters, tanks and ponds of treatment plants wastewater (both biological and physical-chemical), places where sulfides or sulfhydrates are worked, oil refineries, basements with stagnant water or remains of biological materials, industrial facilities where sulfur compounds are worked, fishing boat holds and sewer networks.

Hydrogen sulfide (H₂S) is a colorless, flammable gas with a characteristic odor of rotten eggs, perceptible at very low levels. This odor comes from H₂S generated by bacterial breakdown of sulfur-containing proteins. It is commonly known as hydrosulfuric acid or sewer gas. It is one of the compounds that stands out as causing discomfort due to bad odors.

The presence of hydrogen sulfide gives rise to the following harmful effects:

• annoying smells
• health risks due to high toxicity of sulfhydric acid

It is common for workers in the port sector to be affected, with fatal results, when they enter warehouses that have transported products derived from fishing.

Deaths have been produced in lakes or stagnant lagoons when hydrogen sulfide bubbles up from the depths, where it originates (by any of the phenomena mentioned above, of natural origin, by decomposition of organic matter or by any other cause), and reaches people on the surface. As this acid is denser than air, fractionation is generated due to differences in density, leaving hydrogen sulfide on the surface of the water, which displaces the air towards higher layers.

For this reason, various deodorization processes have been developed that eliminate it from polluted streams. Examples: gas treatment with amines in industry and the use of calcium nitrate in wastewater.

Hydrogen sulfide is extremely harmful to health. Just 20-50 parts per million (ppm) in air is enough to cause acute discomfort leading to suffocation and death from overexposure. Due to its high degree of toxicity, it is compared to hydrocyanic acid (HCN), which has been the gas commonly used in capital punishment by means of a gas chamber that was used in the concentration camps of World War II.

Summary

In the laboratory, hydrogen sulfide can be conveniently generated by reacting hydrochloric acid (HCl) with ferrous sulfide (FeS). Another method consists of heating a mixture of paraffin with elemental sulfur (S).

In industry, hydrogen sulfide is a by-product of cleaning natural gas or biogas, in which it usually exists with contents of up to 10%.

Applications

Traditionally, hydrogen sulfide has been used in qualitative analysis, in the cationic process to precipitate the heavy metal cations of group II of the analytical process, which are then separated by other methods. Amorphous precipitates can be black (mostly) or white.

A conjugated salt, sodium sulfide (Na₂S), is used as an antiquity agent (simulation of antiquity) of bronzes (locks, goldsmiths). Also Na₂S is used for example in leather preparation (tannery).

Recent studies have managed to induce hibernation in mice by applying hydrogen sulfide.

Hydrogen sulfide is responsible for the blackening of some paints based on lead carbonate, by formation of lead sulfide (lead II sulfide), (PbS), black in color. In some cases this can be remedied by applying hydrogen peroxide (hydrogen peroxide): (H₂O₂), which oxidizes the PbS to lead(II) sulphate. (PbSO₄), white. It is the starting compound in some organic syntheses.

Recent studies seem to show that although in certain doses, dissolved in water, hydrogen sulfide is poisonous to animals and plants, in small doses (1 part per 1000 million) it can increase their growth speed. This is confirmed by a study by professors Fredeirck D. Dooley, Suven p. Nair and Peter D. Ward of the University of Washington.^{[citation needed]}

Analytics

Hydrogen sulfide is commonly detected by its foul, rotten-egg odor. Chemically, it is manifested by the appearance of a black PbS stain on wet paper impregnated with lead acetate: Pb(CH₃COO)₂. The following reaction occurs:

Pb(CH₃COO)₂ + H₂S → PbS (ppto) + 2 CH₃COOH

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This acid is commonly found in:

• Confined chambers of served water.
• Fish oil storage ponds.
• Seabeds of fishing vessels.
• Cure treated and degraded in tanning.
• Galvanotecnia plants.
• Degradation of floating reagents based on sulphides used in mining.
• Water chambers of peach lavatory.
• Waters stagnated or with laminar flow of marshes, cenotes and swamp lagoons with putrefaction processes in the background.
• Certain regions of the marine depths where there are subaquatic volcanic sources, in which there are geysers that emanate sulphate materials that combine with carbonic acid (per density, the sulfur part goes to the bottom).
• Wastewater treatment stations (E.D.A.R.).
• It's the smell of rotten eggs.

Toxicity

The toxicity of hydrogen sulfide is high, being able to cause death in humans at very low concentrations in the environment, of the same order as hydrocyanic acid (used in executions with a gas chamber in the United States), or much lower than those of carbon monoxide to cause death (which is the source of many fatal poisonings with combustion heaters in poorly ventilated places). Just taking into account that there is a much higher probability of exposure to hydrogen sulfide than to hydrocyanic acid, the death rate for hydrogen sulfide is lower than for hydrogen cyanide, probably because it is provided with a "warning property.";, which is its strong smell.

However, the knowledge that it has a foul odor even at a concentration well below that which causes death, can lead to false security situations, since when the concentration of the gas increases, the sense of smell quickly becomes saturated or narcotized, with the odor disappearing, so potential victims of exposure may go unnoticed by its airborne presence until its possibly fatal effects are manifested.

Thus, 2 ppm in the air is enough to perceive it olfactorily, when it is considered deadly from 20 or 25 ppm in the air. But starting at 5 ppm, it causes a narcotic effect on the olfactory receptor cells, so that affected people no longer perceive its stench.

As the density of hydrogen sulfide is greater than that of air, it usually accumulates in low and poorly ventilated places, such as wells, sewage networks, etc., which on the other hand are places also prone to accumulate organic matter that, by decomposition, generates usually this gas, so they are places where victims usually occur. In these places, technically called "confined spaces", when a poisoning accident occurs, it usually ends up affecting several people successively (a first person is affected, falling unconscious, and then all the others are also affected, without proper respiratory protection equipment, they come to your rescue) with usually fatal results for all of them.

There is no adequate filter for this gas, so respiratory protection must be done with equipment provided with an independent air supply from the environment contaminated with this gas (autonomous, semi-autonomous or assisted breathing apparatus).

As with hydrocyanic acid salts, cyanides, hydrogen sulfide salts, and sulfides must be handled with great care, preventing them from coming into contact with acids or acid solutions (even slightly acid or neutral)., which would give rise to the release of toxic hydrogen sulfide. Manually dumping a bag of sodium sulfide onto a slightly acidic solution causes the sudden generation of a cloud of hydrogen sulfide that quickly covers the breathing zone of the person performing the operation, causing death.

Hydrogen sulfide seems to act above all on the metal centers of the enzymes, blocking them, thus preventing their functioning. For treatment it is recommended to take the affected person as quickly as possible to fresh air and apply pure oxygen.

In addition, the sulfide ion S^2- combines with hemoglobin in the same way as oxygen, and accelerates the suffocation of the organism.

Exposure to low levels of hydrogen sulfide may cause eye, nose, or throat irritation. It is also likely to cause breathing difficulties in people with asthma. Brief exposures to high contents (greater than 500 ppm) of hydrogen sulfide can cause loss of consciousness and even death.

In most cases, those who lose consciousness seem to recover without further effects. However, some people seem to suffer permanent or long-term effects, such as headache, poor concentration, poor memory, and poor motor function.

No health effects have been detected in people exposed to hydrogen sulfide in the contents commonly found in the environment (0.00011-0.00033 ppm). Scientists have no information showing deaths of people poisoned by ingesting hydrogen sulfide.

Scientists have little information about what happens when a person's skin is exposed to hydrogen sulfide. However, caution is known to be necessary with compressed liquid hydrogen sulfide as it can cause frostbite to the skin.

Despite the high toxicity of hydrogen sulfide gas in mammals, there are microorganisms that have an evolutionary adaptation capacity that tolerate high contents of this gas, or even feed on it. There are theories in which the metabolization of hydrogen sulfide gas —as it exists, for example, near underwater volcanic sources— is related to the initial development of life on Earth with environmental conditions greatly affected by the numerous volcanic eruptions that would be expected on a planet in the process of formation.

In 2012, there were reports of at least two cases in which several people appear to have been poisoned by hydrogen sulfide. In one of both cases the results were fatal.

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• Rui Wang, “The Dual Function of Hydrogen Sulphide”, Research and Science, 404, May 2010, pp. 56-61.