Hydrogen sulfide (preferred IUPAC name and American English) or hydrogen sulphide (Commonwealth English) is a chemical compound with the formula H2S. It is a colorless, toxic, and flammable hydrogen chalcogenide gas. Trace amounts in the ambient atmosphere have a characteristic foul odor of rotten eggs. Swedish chemist Carl Wilhelm Scheele is credited with having discovered the chemical composition of purified hydrogen sulfide in 1777. Hydrogen sulfide is toxic to humans and most other animals by inhibiting cellular respiration in a manner similar to hydrogen cyanide. When it is inhaled or its salts are ingested in high amounts, damage to organs occurs rapidly with symptoms ranging from breathing difficulties to convulsions and death. Despite this, the human body produces small amounts of this sulfide and its mineral salts, and uses it as a signalling molecule. Hydrogen sulfide is often produced from the microbial breakdown of organic matter in the absence of oxygen, such as in swamps and sewers; this process is commonly known as anaerobic digestion, carried out by sulfate-reducing microorganisms. It also occurs in volcanic gases, natural gas deposits, and sometimes in well-drawn water.
Properties Hydrogen sulfide is slightly denser than air. A mixture of H2S and air can be explosive.
Oxidation In general, hydrogen sulfide acts as a reducing agent, as indicated by its ability to reduce sulfur dioxide in the Claus process. Hydrogen sulfide burns in oxygen with a blue flame to form sulfur dioxide (SO2) and water:
2 H2S + 3 O2 → 2 SO2 + 2 H2O If an excess of oxygen is present, sulfur trioxide (SO3) is formed, which quickly hydrates to sulfuric acid:
SO3 + H2O → H2SO4
Acid-base properties It is slightly soluble in water and acts as a weak acid (pKa = 6.9 in 0.01–0.1 mol/litre solutions at 18 °C), giving the hydrosulfide ion HS−. Hydrogen sulfide and its solutions are colorless. When exposed to air, it slowly oxidizes to form elemental sulfur, which is not soluble in water. The sulfide anion S2− is not formed in aqueous solution. H2S and H2O exchange protons rapidly. This behavior is the basis of the Girdler sulfide process which is used in the purification of deuterium oxide ("heavy water" or D2O) and exploits the easy distillation of these compounds.
Extreme temperatures and pressures At pressures above 90 GPa (gigapascal – 900,000 atmospheres), hydrogen sulfide becomes a metallic conductor of electricity. When cooled below a critical temperature this high-pressure phase exhibits superconductivity. The critical temperature increases with pressure, ranging from 23 K at 100 GPa to 150 K at 200 GPa. If hydrogen sulfide is pressurized at higher temperatures, then cooled, the critical temperature reaches 203 K (−70 °C), which was the highest accepted superconducting critical temperature until the discovery of lanthanum decahydride in 2019. By substituting a small part of sulfur with phosphorus and using even higher pressures, it has been predicted that it may be possible to raise the critical temperature to above 0 °C (273 K) and achieve room-temperature superconductivity. Under atmospheric pressure and in the absence of a catalyst, hydrogen sulfide decomposes around 1200 °C into hydrogen and sulfur.
Reactions with metals Hydrogen sulfide reacts with metal ions to form metal sulfides, which are insoluble, often dark colored solids. This behavior is the basis of the use of hydrogen sulfide as a reagent in the qualitative inorganic analysis of metal ions. In these analyses, heavy metal (and 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 solid are then identified by their reactivity. Lead(II) acetate paper is used to detect hydrogen sulfide because it readily converts to lead(II) sulfide, which is black. Hydrogen sulfide is also responsible for tarnishing on various metals including copper and silver; the chemical responsible for black toning found on silver coins is silver sulfide (Ag2S), which is produced when the silver on the surface of the coin reacts with atmospheric hydrogen sulfide. Coins that have been subject to toning by hydrogen sulfide and other sulfur-containing compounds may have the toning add to the numismatic value of a coin based on aesthetics, as the toning may produce thin-film interference, resulting in the coin taking on an attractive coloration. Coins can also be intentionally treated with hydrogen sulfide to induce toning, though artificial toning can be distinguished from natural toning, and is generally criticised among collectors.
Production Hydrogen sulfide is most commonly obtained by its separation from sour gas, which is natural gas with a high content of H2S. It can also be produced by treating hydrogen with molten elemental sulfur at about 450 °C. Hydrocarbons can serve as a source of hydrogen in this process.
S + H2 → H2S The very favorable thermodynamics for the hydrogenation of sulfur implies that the dehydrogenation (or cracking) of hydrogen sulfide would require very high temperatures. A standard lab preparation is to treat ferrous sulfide with a strong acid in a Kipp generator:
FeS + 2 HCl → FeCl2 + H2S For use in qualitative inorganic analysis, thioacetamide is used to generate H2S:
CH3C(S)NH2 + H2O → CH3C(O)NH2 + H2S Many metal and nonmetal sulfides, e.g. aluminium sulfide, phosphorus pentasulfide, silicon disulfide liberate hydrogen sulfide upon exposure to water:
6 H2O + Al2S3 → 3 H2S + 2 Al(OH)3 This gas is also produced by heating sulfur with solid organic compounds and by reducing sulfurated organic compounds with hydrogen. It can also be produced by mixing ammonium thiocyanate to concentrated sulfuric acid and adding water to it.
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