HYDROCHLORIC ACID (HCL): STRUCTURE, PROPERTIES AND USES - CHEMISTRY - 2024

The hydrochloric acid (HCl) or muriatic acid is an inorganic compound formed by dissolution in water of hydrogen chloride, resulting hydronium ion (H ₃ O ⁺) and chloride ion (Cl ^-). More specifically, it is the hydracid of the halogen chlorine with the hydrogen.

HCl is a strong acid that completely ionizes in water and its ionization products are stable. The complete ionization of HCl is corroborated by the fact that the pH of a 0.1 M HCl solution is 1.

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The main method for the industrial production of HCl is the chlorination of organic compounds to produce, for example, dichloromethane, trichlorethylene, perchlorethylene or vinyl chloride. HCl is a by-product of the chlorination reaction.

It is used in the titration of bases in numerous chemical reactions, in the chemical digestion of organic compounds, etc.

Hydrochloric acid (hydrogen chloride) vapors can seriously injure the eyes. In addition, they can cause irritation and severe problems in the respiratory tract.

The gastric lumen has an acidic pH (1-3) with a high concentration of HCl. The acid presence favors the sterilization of the gastric content, inactivating numerous bacteria present in food. This would explain the gastroenteritis associated with the condition of achlorhydria.

In addition, HCl facilitates the digestion of proteins by activating the proteolytic enzyme pepsin.

It is used in the cleaning of swimming pools, usually a common detergent is enough but there are stains that adhere between the tiles, requiring in these cases the use of hydrochloric acid.

It is used in the control of pH in pharmaceuticals, food and drinking water. It is also used in the neutralization of waste streams containing alkaline material.

Hydrochloric acid is used in the regeneration of ion exchange resins, used to sequester metal ions or other types of ions in industry, in research laboratories and in the purification of drinking water.

On the other hand, it can also be said that hydrogen chloride, a gaseous compound, is a diatomic molecule and the atoms that form it are joined by a covalent bond. Meanwhile, hydrochloric acid is an ionic compound that in aqueous solution dissociates into H ⁺ and Cl ^-. The interaction between these ions is electrostatic.

Chemical structure

Figure 1: Hydrochloric acid is formed by dissolving HCl in water

Each HCl molecule is made up of a hydrogen atom and a chlorine atom. Although at room temperature HCl is poisonous and a colorless gas, if it dissolves in water, it gives hydrochloric acid.

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Figure 2: appearance of hydrochloric acid.

-It can be produced by electrolysis of NaCl (sodium chloride) which originates H ₂ (g), Cl ₂ (g), 2Na (aq) and OH ^- (aq). Then:

H ₂ + Cl ₂ => 2 HCl

This is an exothermic reaction.

-HCl is produced by reacting sodium chloride with sulfuric acid. Process that can be outlined in the following way:

NaCl + H ₂ SO ₄ => NaHSO ₄ + HCl

The hydrogen chloride is then collected and the sodium chloride is reacted with the sodium bisulfite according to the following reaction:

NaCl + NaHSO ₄ => Na ₂ SO ₄ + HCl

This reaction was introduced by Johan Glauber in the 17th century to produce hydrochloric acid. Currently it is used mainly in laboratories, since the importance of its industrial use has declined.

-Hydrochloric acid can be produced as a by-product of the chlorination of organic compounds, for example: in the production of dichloromethane.

C ₂ H ₄ + Cl ₂ => C ₂ H ₄ Cl ₂

C ₂ H ₄ Cl ₂ => C ₂ H ₃ Cl + HCl

This method of producing HCl is more used industrially, calculating that 90% of the HCl produced in the United States is by this methodology.

-And finally, HCl is produced in the incineration of chlorinated organic waste:

C ₄ H ₆ Cl ₂ + 5 O ₂ => 4 CO ₂ + 2 H ₂ O + 2 HCl

Where is it located?

Hydrochloric acid is concentrated in the gastric lumen where a pH of 1 is reached. The existence of a mucus barrier, rich in bicarbonate, prevents gastric cells from being damaged due to low gastric pH.

There are three main physiological stimuli for the secretion of H ⁺ by the parietal cells of the gastric body: gastrin, histamine, and acetylcholine.

Gastrin

Gastrin is a hormone that is secreted in the region of the gastric antrum that acts by increasing the intracellular concentration of Ca, an intermediary in the activation of the active transport of H ⁺ to the gastric lumen.

Active transport is performed by an ATPase enzyme that uses the energy contained in ATP to carry H ⁺ to the gastric lumen and introduce K ⁺.

Histamine

It is secreted by so-called enterochromaffin-like cells (SEC) of the gastric body. Its action is mediated by an increase in the concentration of cyclic AMP and acts by increasing, like gastrin, the active transport of H ⁺ to the gastric lumen mediated by an H ⁺ -K ⁺ pump.

Acetylcholine

It is secreted by the vagal nervous terminals, just as gastrin mediates its action by an increase in intracellular Ca, activating the action of the H ⁺ -K ⁺ pump.

The H ⁺ of the parietal cells comes from the reaction of CO ₂ with H ₂ O to form H ₂ CO ₃ (carbonic acid). This subsequently decomposes into H ⁺ and HCO ₃ ^-. H ⁺ is actively transported to the gastric lumen through the gastric apical membrane. Meanwhile, HCO ₃ ^- is brought into the blood coupled with the entry of Cl ^-.

The counter-transport or anti-transport mechanism Cl ^- HCO ₃ ^- that occurs in the basement membrane of parietal cells produces the intracellular accumulation of Cl ^-. Subsequently, the ion passes into the gastric lumen accompanying the H ⁺. Gastric HCl secretion is estimated to have a concentration of 0.15 M.

Other sources of biological HCl

There are other stimuli for HCl secretion by parietal cells such as caffeine and alcohol.

Gastric and duodenal ulcers occur when the barrier that protects gastric cells from the damaging action of HCl is disrupted.

By eliminating the protective action mentioned by the Helicobacter pylori bacteria, acetylsalicylic acid and non-steroidal anti-inflammatory drugs (NSAIDs) contribute to the production of ulcers.

The acid secretion has the function of eliminating microbes present in food and initiating the digestion of proteins, through the action of pepsin. The main cells of the gastric body secrete pepsinogen, a proenzyme that is transformed into pepsin by the low pH of the gastric lumen.

Physical and chemical properties

Molecular weight

36.458 g / mol.

Color

It is a colorless or slightly yellowish liquid.

Odor

It is an irritating acrid odor.

Taste

The threshold for tasting in pure water is a concentration of 1.3 x 10 ^-4 mol / l.

Boiling point

-121º F to 760 mmHg. -85.05 ° C to 760 mmHg.

Melting point

-174º F (-13.7º F) for a 39.7% w / w HCl solution in water), -114.22º C.

Water solubility

The HCl solution can be 67% w / p at 86 ° F; 82.3 g / 100 g of water at 0 ° C; 67.3 g / 100 g of water at 30º C and 63.3 g / 100 g of water at 40º C.

Solubility in methanol

51.3 g / 100 g of solution at 0 ° C and 47 g / 100 of solution at 20 ° C

Solubility in ethanol

41.0 / 100 g of solution at 20º C

Solubility in ether

24.9 g / 100 of solution at 20ºC.

Density

1.059 g / ml at 59 ° F in a 10.17% w / w solution.

Gas density

1,00045 g / L

Vapor density

1,268 (in relation to air taken as 1)

Vapor pressure

32,452 mmHg at 70 ° F; 760 mmHg at -120.6º F

Stability

It has a high thermal stability.

Autoignition

It is not flammable.

Decomposition

It decomposes on heating emitting toxic chlorine smoke.

Viscosity: 0.405 cPoise (liquid at 118.6 º K), 0.0131 cPoise (vapor at 273.06 º K).

Corrosiveness

It is highly corrosive to aluminum, copper, and stainless steel. Attacks all metals (mercury, gold, platinum, silver, tantalum except for certain alloys).

Surface tension

23 mN / cm at 118.6º K.

Polymerization

Aldehydes and epoxides undergo violent polymerization in the presence of hydrochloric acid.

Physical properties such as viscosity, vapor pressure, boiling point, and melting point are influenced by the percent concentration w / w of the HCl.

Applications

Hydrochloric acid has numerous uses at home, in different industries, in teaching and research laboratories, etc.

Industrial and home

-Hydrochloric acid is used in hydrometallurgical processing, for example in the production of alumina and titanium dioxide. It is used in the activation of the production of oil wells.

The injection of the acid increases the porosity around the oil, thus favoring its extraction.

-It is used to eliminate CaCO ₃ (calcium carbonate) deposits by transforming it into CaCl ₂ (calcium chloride) which is more soluble and easier to remove. Likewise, it is used industrially in the processing of steel, a material with numerous uses and applications, both in industry, in construction and in the home.

-The masons use HCl solutions to wash and clean the bricks. It is used in the home for cleaning and disinfection of bathrooms and their drains. Additionally, hydrochloric acid is used in engravings including metal cleaning operations.

-Hydrochloric acid has application in the elimination of the moldy iron oxide layer that accumulates on steel, prior to its subsequent processing in extrusion, rolling, galvanization, etc.

Fe ₂ O ₃ + Fe + 6 HCl => 3 FeCl ₂ + H ₂ O

-Despite being highly corrosive, it is used to remove metal stains present in iron, copper and brass, using a 1:10 dilution in water.

Synthesis and chemical reactions

-Hydrochloric acid is used in the titration reactions of bases or alkalis, as well as in adjusting the pH of solutions. In addition, it is used in numerous chemical reactions, for example in the digestion of proteins, a procedure prior to studies of amino acid content and their identification.

-A main use of hydrochloric acid is the production of organic compounds, such as vinyl chloride and dichloromethane. Acid is an intermediate in the production of polycarbonates, activated carbon, and ascorbic acid.

-It is used in the manufacture of glue. While in the textile industry it is used in the bleaching of fabrics. It is used in the leather tanning industry, intervening in its processing. It also finds use as a fertilizer and in the production of chloride, colorants, etc. It is also used in electroplating, photography, and the rubber industry.

-It is used in the production of artificial silk, in the refining of oils, fats and soaps. In addition, it is used in polymerization, isomerization and alkylation reactions.

Risks and Toxicity

It has a corrosive action on the skin and mucous membranes, causing burns. These, if serious, can cause ulcerations, leaving keloid and retractile scars. Contact with the eyes can cause reduction or total loss of vision due to damage to the cornea.

When the acid reaches the face it can cause serious cycles that disfigure the face. Frequent contact with acid can also cause dermatitis.

Ingestion of hydrochloric acid burns the mouth, throat, esophagus, and gastrointestinal tract, causing nausea, vomiting, and diarrhea. In extreme cases, perforation of the esophagus and intestine can occur, with cardiac arrest and death.

On the other hand, acid vapors, depending on their concentration, can cause irritation of the respiratory tract, causing pharyngitis, edema of the glottis, narrowing of the bronchi with bronchitis, cyanosis and pulmonary edema (excessive accumulation of fluid in the lungs) and in extreme cases, death.

Exposure to high levels of acid fumes can cause throat swelling and spasm with consequent suffocation.

Dental necroses that appear in the teeth with loss of shine are also frequent; they turn yellowish and soft, and eventually break apart.

Hydrochloric acid damage prevention

There is a set of rules for the safety of people working with hydrochloric acid:

-People with a history of respiratory and digestive diseases should not work in environments with the presence of acid.

-Workers have to wear acid resistant clothing, even with hoods; eye protection glasses, arm protectors, acid resistant gloves and shoes with the same characteristics. They should also wear gas masks and in cases of severe exposure to hydrochloric acid vapors, the use of self-contained breathing apparatus is recommended.

-The work environment must also have emergency showers and fountains for washing the eyes.

-In addition, there are standards for work environments, such as the type of floor, closed circuits, protection of electrical equipment, etc.

References

1. StudiousGuy. (2018). Hydrochloric Acid (HCl): Important Uses & Applications. Taken from: studiousguy.com
2. Ganong, WF (2003). Review of Medical Physiology. Twenty-first edition. The McGraw-Hill Companies INC.
3. PubChem. (2018). Hydrochloric Acid. Taken from: pubchem.ncbi.nlm.nih.gov
4. Weebly. Hydrochloric acid. Taken from: psa-hydrochloric-acid.weebly.com
5. CTR. Hydrochloric Acid Safety Data Sheet.. Taken from: uacj.mx