Hydrochloric AcidEdit

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Hydrochloric acid is a colorless, highly pungent aqueous solution of hydrogen chloride (HCl). In aqueous solution it behaves as a strong acid, dissociating to yield hydronium ions (H3O+) and chloride ions (Cl−). The gas hydrogen chloride is hygroscopic and forms the familiar hydrochloric acid when dissolved in water. In the human body, hydrochloric acid is a natural component of the stomach’s gastric juice, where it contributes to digestion and defense against ingested pathogens Gastric acid.

Hydrochloric acid is a cornerstone of modern chemistry and industry due to its combination of strong acidity, high solubility in water, and robust chemical compatibility in many environments. It is also a familiar chemical in laboratory work and in industrial processes where precise pH control or chloride provisioning is required.

Chemical properties

Acidity: In water, HCl is a strong Brønsted–Lowry acid, meaning it dissociates nearly completely to form hydronium (H3O+) and chloride (Cl−) ions. The concept of strong acids is framed in part by the Arrhenius and Brønsted–Lowry models Arrhenius acid Brønsted–Lowry acid.
Dissociation: HCl → H3O+ + Cl− in aqueous solution; the pKa is well below zero, reflecting near-complete ionization in typical conditions pKa.
Physical state: Concentrated hydrochloric acid is a colorless liquid with a strong, characteristic odor; the gas HCl is highly soluble in water and rapidly forms hydrochloric acid when dissolved.
Reactivity: The solution is highly corrosive and reacts readily with many metals and organic materials. It is compatible with certain materials of construction (e.g., glass-lined steel, certain plastics) but is corrosive to many metals and rubber Corrosion.
Safety in handling: Contact with skin or eyes can cause severe burns; inhalation of fumes can irritate respiratory tissues and, at sufficient concentrations, cause acute injury. Safe handling relies on appropriate ventilation, containment, and personal protective equipment Occupational safety.

Production and handling

Industrial production routes: The most common large-scale routes involve direct synthesis of hydrogen chloride from chlorine (Cl2) and hydrogen (H2), followed by dissolution in water, producing HCl in water as hydrochloric acid. Hydrogen chloride is also recovered as a byproduct from various chlorination processes and, in some cases, from vinyl chloride and related chemical production streams; these streams are treated and concentrated to form usable hydrochloric acid Chlorine Hydrogen Chlor-alkali process.
Byproduct streams: In chlor-alkali and related industrial settings, HCl can be captured and recycled or sold as a concentrated acid, contributing to overall process efficiency and waste minimization. This makes hydrochloric acid a common commodity in chemical supply chains Industrial chemistry.
Storage and transport: Concentrated hydrochloric acid is typically stored in corrosion-resistant containers, often with secondary containment and equipped with ventilation for any fugitive fumes. Pipelines, totes, and drums used for transport are selected to withstand corrosive exposure to the liquid and vapor form Material compatibility.

Uses and applications

Metal processing: Hydrochloric acid is widely used in metal pickling to remove rust, scale, and oxides from steel and other metals, preparing surfaces for subsequent coating or treatment. This application requires strict control of concentration and exposure to prevent damage to the substrate and equipment Metalworking.
Chloride synthesis and chemical manufacturing: It serves as a source of chloride ions in the production of various inorganic and organic chlorides, catalysts, and polymers. It is also used in the production of polyvinyl chloride (PVC) and related materials, where chloride chemistry is central Vinyl chloride.
pH control and chemical neutralization: In laboratories and industrial processes, hydrochloric acid is employed to adjust acidity and to neutralize basic streams in a controlled manner. Its predictable, strong acid behavior makes it a reliable reagent for precise acidification pH.
Food processing and digestion aids: Food-grade hydrochloric acid is used in some processing steps to adjust acidity and as a processing aid in certain products, though this use is tightly regulated to ensure safety and labeling compliance Food safety.
Water treatment and environmental management: HCl is used to adjust pH in water treatment and to facilitate certain chemical reactions in environmental remediation processes, under applicable environmental safeguards Environmental protection.

Safety, regulation, and environmental considerations

Human health: Exposure to hydrochloric acid can cause irritation to the eyes, skin, and respiratory tract. Severe exposure can lead to chemical burns and tissue damage; appropriate emergency response and first aid are standards in workplaces that handle the chemical Safety data sheet.
Environmental impact: Releases of hydrochloric acid can lower pH in water and soil, affecting aquatic life and ecosystems. Regulatory frameworks govern permissible release limits, handling practices, and wastewater treatment to minimize ecological impact Environmental regulation.
Occupational safety: Standards and guidelines from labor and health agencies guide the use, storage, and transport of hydrochloric acid, including ventilation requirements, containment, and protective equipment to reduce risk to workers Occupational safety.

History and naming

The term muriatic acid is a historical name for hydrochloric acid, deriving from early uses in mining and chemistry. The recognition of hydrochloric acid as a distinct chemical species and its systematic study progressed through the 18th and 19th centuries, paralleling advances in acid-base theory and industrial chemistry. The modern understanding ties the substance to the compound hydrogen chloride, the simple diatomic molecule that forms hydrochloric acid upon dissolution in water Hydrogen chloride.