Hydrochloric acid is the aqueous solution of hydrogen chloride gas (HCl). It is a strong acid, and the major component of gastric acid.^[1] It is also widely used in industry. Hydrochloric acid must be handled with appropriate safety precautions because it is a highly corrosive solution.

Hydrochloric acid, or Muriatic acid by its historical but still occasionally used name, has been an important and frequently-used chemical from early history, and was discovered by the alchemist Jabir ibn Hayyan in the 8th century. It was used throughout the Middle Ages by alchemists in the quest for the philosopher's stone, and later by several European scientists including Glauber, Priestley, and Davy in order to help establish modern chemical knowledge.

Chemistry

Hydrogen chloride (HCl) is a monoprotic acid, which means it can dissociate (i.e., ionize) only once to give up one H⁺ ion (a single proton). In aqueous hydrochloric acid, the H⁺ joins a water molecule to form a hydronium ion, H₃O⁺:

HCl + H₂O ⇌ H₃O⁺ + Cl⁻

The other ion formed is Cl⁻, the chloride ion. Hydrochloric acid can therefore be used to prepare salts called chlorides, such as sodium chloride. Hydrochloric acid is a strong acid, since it is fully dissociated in water.

Physiology and pathology

Hydrochloric acid constitutes the majority of gastric acid, the human digestive fluid. In a complex process and at a large energy burden, it is secreted by parietal cells (also known as oxyntic cells). These cells contain an extensive secretory network (called canaliculi) from which the HCl is secreted into the lumen of the stomach. They are part of the fundic glands (also known as oxyntic glands) in the stomach.

Safety mechanisms that prevent the damage of the epithelium of digestive tract by hydrochloric acid are the following:

• Negative regulators of its release
• A thick mucus layer covering the epithelium
• Sodium bicarbonate secreted by gastric epithelial cells and pancreas
• The structure of epithelium (tight junctions)
• Adequate blood supply
• Prostaglandins (many different effects: they stimulate mucus and bicarbonate secretion, maintain epithelial barrier integrity, enable adequate blood supply, stimulate the healing of the damaged mucous membrane)

When, due to different reasons, these mechanisms fail, heartburn or peptic ulcers can develop. Drugs called proton pump inhibitors prevent the body from making excess acid in the stomach, while antacids neutralize existing acid.

In some instances, the stomach does not produce enough hydrochloric acid. These pathologic states are denoted by the terms hypochlorhydria and achlorhydria. They have the potential to lead to gastroenteritis.