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Hydrochloric Acid

What is hydrochloric acid?

Hydrochloric acid or muriatic acid is an aqueous solution of hydrogen chloride gas with the chemical formula HCl. It was historically produced from rock salt. Hydrochloric acid is a strong acid which used widely in various laboratory reagents, cleaning products, and the production of many industrial chemicals.

Hydrochloric acid or muriatic acid or aqueous solution of hydrogen chloride gas chemical formula HCl and structure with properties and uses

Hydrochloric acid is a component of gastric acid formed in the digestive system or stomach of many animals. Hydrochloric acid is a colorless corrosive, strong mineral acid that affects our eyes, skin, and mucous membranes.

Hydrogen chloride gas is formed by burning hydrogen in chlorine. Industrially, hydrochloric acid is prepared by dissolving hydrogen chloride gas in water.

Hydrochloric acid uses

Common uses

  • Hydrochloric acid uses commonly in many household cleaners due to its corrosive nature. It is very useful for cleaning tough stains. Hence we used HCl widely in toilet bowl cleaners, bathroom tile cleaners, and porcelain cleaners.
  • Concentrated hydrochloric acid is used for the dissolution of geological samples.
  • It is an excellent solvent for many carbonates, phosphates, metal oxides, and metals. HCl is a better solvent for dissolving iron and manganese oxides than nitric acid due to its reducing properties and the complexing ability of Cl.
  • It is used for the production of many inorganic and organic compounds.
  • It is also used in the regeneration of ion-exchange resins.
  • It is used widely in oil production processes.
  • Muriatic acid is a good bleaching agent which used widely in the food, textile, metal, and rubber industries.
  • HCl is used for the recovery of magnesium from seawater.
  • We use HCl for the production of batteries, photoflash bulbs, and fireworks.

Industrial application

Hydrochloric acid is a versatile chemical used in the chemical industries for the large-scale production of vinyl chloride. The reagent vinyl chloride is used widely for the production of PVC plastic, and polyurethane. The most common industrial applications of hydrochloric acid are,


Hydrochloric acid in the hydrometallurgical process is used for the production of many industrial metal halides such as TiCl4, ZrCl4, AlCl3, etc.

Chlorine dioxide synthesis

It is used widely for the synthesis of chlorine dioxide. Hydrochloric acid (HCl) or chlorine is brought together with sodium chlorite for the synthesis of chlorine dioxide gas.

Chlorine dioxide is a useful bleaching agent for wood pulp and textile bleaching. It is also a disinfectant used in the food industry and municipal drinking water.

Regulating pH level

Hydrochloric acid is used for regulating the pH level of various industrial manufacturing processes such as drinking water, foods, and pharmaceutical industries.
OH + HCl → H2O + Cl

Metal cleaning

It is a corrosive acid used in the chemical industry for cleaning metal and pickling steel. It is also used to remove stains or rust from metals like iron, copper, etc.

Properties of hydrochloric acid

Physical properties

Hydrochloric acid or muriatic acid is a colorless, corrosive, strong mineral acid available in different concentrations of water. Many physical properties such as boiling and melting points, density, pH depend on the concentration or molarity of aqueous HCl solution. The colurless solution of HCl has a highly pungent odor.

Chemical formula HCl
Molecular weight 36.458 g/mol
Appearance Colorless transparent, liquid concentrated HCl fumes in the air with a pungent smell
Acidity (pka) −5.9
Melting point Depending on the concentration of HCl
Boiling point Depending on the concentration of HCl
Density Depending on the concentration of HCl

Concentration of hydrochloric acid

Hydrochloric acid is produced at a concentration between 10% and 38% (kg HCl/kg). Higher concentrations of HCl over 40% are chemically possible but it has an extremely high evaporation rate. Therefore, the storage and handling of forty percent or above forty percent HCl required extra precautions.

When the concentration of HCl is above 30%, the boiling point of the solution decreases rapidly due to the increased evaporation rate. Due to corrosive behavior, EPA suggested the concentration of HCl above 37% is a toxic substance. Short-term inhalation affects our mucous membranes, skin, and eyes.

Bulk industrial-grade HCl has a concentration between 30% to 35%. Solutions of between 20% and 32% are sold as muriatic acid.

The United Kingdom and US household cleaning solutions contain approximately 10% to 12% of HCl. Further dilution is also needed before using HCl solution in household products.

Chemical properties

Hydrochloric acid is a strong monoprotic mineral acid that is fully dissociated in water to form hydronium and chloride ions. It is commonly used in laboratories for the neutralization of alkaline agents.

It is a very good solvent for carbonates, phosphates, many metal oxides, and metals.
CaCO3 + 2 HCl → CaCl2 + CO2 + H2O
NiO + 2 HCl → NiCl2 + H2O
Zn + 2 HCl → ZnCl2 + H2

Due to reducing properties and the complexing ability of Cl, HCl is a better solvent for dissolving iron and manganese oxides than nitric acid (HNO3). The gaseous phase or hydrogen chloride gas significantly increases the steel corrosion rate.

Production of hydrochloric acid

It may be prepared by several chemical and industrial processes. It should be produced industrially by dissolving hydrogen chloride in water.

Most of the hydrogen chloride in the United States is produced as a by-product of the chlorination reaction. During the chlorination process, the gas stream contains hydrogen chloride vapour. Here concentrated liquid hydrochloric acid is produced by absorption of hydrogen chloride vapors on a weak solution of HCl.

In the chloralkali process, electrolysis of a mixture of sodium chloride and water (brine solution) should be produced chlorine (Cl2), sodium hydroxide, and hydrogen (H2).

Pure chlorine gas can be combined with hydrogen to produce hydrogen chloride gas.
H2 + Cl2 → 2 HCl (gas)

The produced hydrogen chloride gas is dissolved in deionized water to produce chemically pure hydrochloric acid.

Hazards of HCl

According to EPA, the concentration of hydrogen chloride or muriatic acid in water greater than 37 percent is toxic. Acute or short-term inhalation causes skin and eyes irritation and damaging mucous membranes.

Exposure to HCl is dangerous for humans and animals. Long-term inhalation of HCl by humans may cause coughing, hoarseness, inflammation, ulceration of the respiratory tract, chest pain, and pulmonary edema.

The contact may cause burns, ulceration, and scarring. HCl may change the pH level of our body and denatures proteins. It leads to the formation of edema and tissue necrosis.

Function of hydrochloric acid in the stomach

Hydrochloric acid is one of the major components of gastric juice in the stomach. The most important function of HCl in the stomach is to activate the enzyme pepsin. It is used to digest proteins in the stomach.

The acidity or pH level of the stomach by secretions of HCl is exceedingly low. Such a pH level is very useful for the destruction of some dangerous germs consumed with food. Therefore, it is helpful to destroy bacteria and other harmful species.

It is also necessary for the absorption of vitamin B12 which is needed for the proper development of red blood cells (RBCs).

The stomach should be protected from the strong hydrochloric acid by the secretion of a thick mucus layer and buffering by sodium bicarbonate. When such formation fails, heartburn or peptic ulcers can develop.

Several types of drugs or antacids are used to inhibit the production or neutralization of hydrochloric acid or hydrogen chloride in the stomach.