2. General physicochemical properties of hydrogen sulfide
Hydrogen sulfide is a colorless and highly toxic flammable gas, with its unique odor being the smell of rotten eggs or the smell of decaying sewage. Its molecular weight is 34.08, and its vapor density is heavier than air, making it easier to diffuse at lower points[17, 18]. As a binary weak acid, hydrosulfuric acid is an aqueous solution of H2S that can reach dissociation equilibrium at room temperature (25 ℃). The solution concentration in a saturated state is 0.11mol.dm-3, and its pH value is approximately 4.0. At 37 ℃ and pH 7.4, pK α 1=6.76, there is about 20% H2S, 80% HS - and a small amount of negligible S2- in extracellular fluid[19]. At the same time, H2S is also a small gas molecule with high lipophilicity, which allows it to freely pass through the Lipid bilayer structure of the cell membrane[20]. Hydrogen sulfide is a compound similar to water molecules that can be oxidized into sulfur dioxide, sulfate, thiosulfate, and elemental sulfur. In the body, H2S can be oxidized to sulfates and thiosulfates, which can be excreted in the urine. Some studies suggest that urinary thiosulfates can serve as one of the biomarkers of hydrogen sulfide poisoning[17, 21].