Hydrosulfuric acid has the simple formula \(\text{H}_2\text{S}\). This substance is most commonly known by its chemical name, hydrogen sulfide. It is a highly relevant compound for public health and industrial safety due to its frequent presence in nature and its significant toxicity, even at low concentrations.
The Formula and Nomenclature
The formula \(\text{H}_2\text{S}\) indicates the compound is composed of two hydrogen atoms (\(\text{H}\)) bonded to one sulfur atom (\(\text{S}\)). In chemical nomenclature, “hydrogen sulfide” refers to the compound in its gaseous state, which is its most common and hazardous form. When this gas is dissolved in water, the resulting solution is the weak acid traditionally named “hydrosulfuric acid.”
The IUPAC naming convention for binary acids, which are compounds of hydrogen and one other element, uses the prefix hydro- and the suffix -ic acid. This system dictates the name hydrosulfuric acid for the aqueous solution of \(\text{H}_2\text{S}\), following the pattern of hydrochloric acid (\(\text{HCl}\)). Although the term “hydrosulfuric acid” is chemically precise for the aqueous form, “hydrogen sulfide” is more frequently used in industrial and safety contexts regardless of its state. It is important to distinguish this binary acid from sulfuric acid (\(\text{H}_2\text{SO}_4\)), which is a much stronger oxyacid containing oxygen.
Physical State and Characteristics
Hydrogen sulfide is a colorless, flammable gas that is slightly denser than air, meaning it tends to settle in low-lying or confined spaces. This property makes it a significant hazard in trenches, manholes, and tanks where it can accumulate to dangerous concentrations. The most recognizable characteristic of \(\text{H}_2\text{S}\) is its pungent, foul odor, which is often described as smelling like rotten eggs.
The gas is slightly soluble in water. The odor can be detected by humans at extremely low concentrations, sometimes as low as 0.13 parts per million (ppm). Relying on this smell for safety is dangerous because the gas rapidly deadens the sense of smell at higher concentrations, a phenomenon known as olfactory fatigue. This olfactory paralysis can occur at concentrations around 100 to 150 ppm, eliminating the body’s natural warning system.
Natural and Industrial Sources
Hydrogen sulfide is a naturally occurring compound released through geological and biological processes. It is a component of volcanic gases and is found in geothermal hot springs and natural gas deposits, where it is often referred to as “sour gas.” A major source is the anaerobic decomposition of organic matter by sulfate-reducing bacteria in environments without oxygen.
This biological process explains its presence in sewage systems, swamps, bogs, and poorly ventilated manure pits on farms. Industrially, \(\text{H}_2\text{S}\) is a common by-product in several major sectors. Petroleum refining and natural gas extraction are principal sources, as the gas is removed from crude oil and natural gas to reduce corrosion and improve quality. Other anthropogenic sources include pulp and paper mills, tanneries, and landfills where construction debris containing gypsum is broken down.
Toxicity and Exposure Risks
Hydrogen sulfide is a fast-acting, broad-spectrum poison that primarily targets the nervous system and the respiratory system. Its mechanism of action is similar to cyanide, as it inhibits cellular respiration by interfering with the mitochondrial cytochrome enzymes that allow cells to use oxygen. This effect starves the body’s cells of oxygen, starting with the most sensitive organs like the brain and heart.
Low-level exposure, between 5 and 50 ppm, can cause irritation to the eyes, nose, and throat, along with headaches and nausea. Exposure to high concentrations can be immediately life-threatening. Concentrations above 500 ppm can lead to rapid unconsciousness, known as “knockdown,” and death from respiratory failure within minutes. High-level exposure that does not result in immediate death can still lead to long-term neurological damage, including poor memory and poor attention span.