Prussic acid, widely known as hydrogen cyanide (HCN), is a compound with rich history and unique properties, and one of the most notorious compounds of the chemical world. From its discovery to modern applications, prussic acid has within it both potential and peril.
Discovery of Prussic Acid
Hydrogen cyanide is first discovered in 1782 by the Swedish chemist Karl Wilhelm Scheele, who has a relentless interest in acids. Scheele isolates the compound by heating potassium ferrocyanide with sulfuric acid, which produces the lethal gas.
Although Scheele is credited with its discovery, it the term "prussic acid" was officially adopted later. The name comes from "Prussian blue," a pigment containing iron and cyanide anions.
Before his discovery, people already recognize cyanide compounds in sources such as bitter almonds and certain plants. The average bitter almond contains 6 to 12% amygdalin. Amygdalin is a cyanogenic compound, producing hydrogen cyanide when metabolized.
Forms and Properties of Hydrogen Cyanide
Hydrogen cyanide is a colorless, highly toxic liquid or gas at room temperature. In its pure form, it has a faint, bitter almond-like odor, which is distinctive yet not easily detectable by all individuals due to genetic factors.
Physical Properties:
Molecular Formula: HCN
Boiling Point: 25.7 °C (78.3 °F)
Density: 0.687 g/cm³
Solubility: Fully miscible with water, which can lead to significant risks in aquatic environments.
Forms:
Gas: At room temperature, hydrogen cyanide is typically gaseous, particularly in enclosed spaces.
Liquid: At low temperatures, it can exist as a liquid and can be easily condensed under pressure.
Water Solution: Hydrogen cyanide can also form solutions when dissolved in water, a state that can be particularly hazardous if ingested.
Occurrence in Nature
While associated with artificial production and use, hydrogen cyanide does occur naturally in small quantities. The yellow-spotted millipede (Harpaphe haydeniana) or cyanide millipede of the Pacific Northwest and varieties in the Appalachians use cyanide in self-defense.
Prussic acid is naturally present in various plants, although typically in small amounts. Many species produce cyanogenic compounds that remain non-toxic until the plant's cells are damaged. For example,:
Bitter Almonds: These nuts can contain up to 2.5 mg of prussic acid per almond and must be properly processed to reduce toxicity levels.
Peach Pits: Each pit can contain around 0.5 to 1 mg of cyanogenic glucosides, which convert to hydrogen cyanide.
Cassava: This root vegetable can have anywhere from 10 to 100 mg of cyanide per kilogram if not processed correctly, underscoring the importance of proper preparation.
Apricot Kernels: Boil for 30 minutes to remove cyanide. Even then avoid excess consumption.
While these plants host inherent dangers, they also provide nutritional benefits when correctly handled, such as using bitter almonds in almond oil production, which eliminates harmful compounds.
Historical and Modern Uses of Hydrogen Cyanide
Historically, prussic acid has had a storied past, serving various applications primarily driven by its potent properties.
Historical Uses:
Pesticide: During the early 20th century, hydrogen cyanide was utilized as a fumigant for buildings, ships, and agricultural products, effective in exterminating pests and rodents.
Gas Chamber Executions: During World War I, hydrogen cyanide emerges as a chemical weapon, causing severe casualties. Later, it's infamous as Zyklon B, widely known for its use in gas chambers during World War II.
Modern Uses:
Chemical Synthesis: Today, hydrogen cyanide is predominantly used in the manufacture of certain synthetic fibers, plastics, and elastomers. It is a key precursor in the production of polyacrylonitrile, which is used for products like acrylic fibers and as a precursor for carbon fiber.
Mining: It is also used in the gold mining industry to extract gold from ore through a cyanidation process.
Historically, hydrogen cyanide has been linked to lethal applications. Hydrogen cyanide has also made its mark in photography and metal processing. In history it acts as a critical reducing agent in extraction of gold from ores, first used in the 19th century.
Facts about Hydrogen Cyanide
Lethal Dose: Hydrogen cyanide is highly toxic, and as little as 50 mg, or 0.5 to 3 mg/kg can be lethal to humans.
Cyanide Antidotes: There are specific antidotes for hydrogen cyanide poisoning, including hydroxocobalamin, which binds to cyanide ions and facilitates their excretion.
Industrial Safety: Due to its extreme toxicity, maintaining stringent safety protocols during the handling and transport of hydrogen cyanide is crucial in industries that utilize it.
Biochemical Impact: Hydrogen cyanide can disrupt cellular respiration, blocking oxygen usage in cells, leading to rapid asphyxiation in high concentrations.
Synthetic Preferences: Today, hydrogen cyanide is primarily produced synthetically, with modern methods yielding around 10 million tons globally each year to meet industrial and research demands.
Odor Detection Variance: Interestingly, not all individuals can smell hydrogen cyanide. Genetic differences mean that some people may lack the ability to detect its almond-like scent, posing additional risks.
Government Regulation: Hydrogen cyanide's toxicity makes it subject to strict regulations worldwide. Safety protocols are essential in any environment where it is used.
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