The Science Behind Hydrochloric Acid

by Lucy Bell-Young

Hydrochloric acid (HCl) is a naturally occuring acid that’s produced in the stomachs of mammals to aid digestion. In humans, it’s found in the gastric juice, a.k.a. stomach acid, where it helps to break down food. But HCl is also produced industrially by dissolving gaseous hydrogen chlorine in water. As a corrosive and acidic solution, hydrochloric acid has several applications in manufacturing and processing various products.

Brown glass bottle of HCl in lab

What Does Hydrochloric Acid Do?

Biologically, hydrochloric acid plays an important role in digestion. A component of gastric juice in the mammalian stomach, it gives our stomach acid a normal volume range of between 20 and 100 mL with acidic pH of between 1.5 and 3.5.

In the body, HCl is produced by the parietal cells, which release hydrogen and chlorine ions. Hydrochloric acid is particularly important in maintaining the necessary pH required to activate pepsin, the main digestive enzyme that’s crucial in digesting proteins.

Industrially, HCl is produced by dissolving hydrogen chloride gas in water. When it comes to industrial applications, hydrochloric acid has various essential roles that it plays, from keeping swimming pools at the optimum pH to producing inorganic and organic compounds. Here are some of the practical uses of hydrochloric acid in industries:

  • Processing or manufacturing organics: Many organic compounds, like dichloroethane and vinyl chloride, require HCl as a reagent. For instance, the chlorine in polymer plastics, like poly vinyl chloride (PVC), comes from the polymerisation of vinyl chloride monomer. One pathway of producing vinyl chloride monomers is through the exothermic reaction of acetylene with anhydrous hydrogen chloride gas in the presence of mercuric chloride as a catalyst. The balanced chemical equation is: C2H2 + HCl → CH2=CHCl. 
  • Chemical buffering: Many types of manufacturing processes require pH levels to be maintained at a certain range. HCl serves as a buffer in these instances. Pharmaceutical products, processed foods, and clean drinking water need hydrochloric acid in one or more stages of processing. 
  • Maintaining pH level of swimming pools: Hydrochloric acid is also used to regulate the pH in swimming pools, where the pH level of the water needs to be kept at a slightly alkaline range of between 7.2 and 7.6. Sodium hypochlorite, an alkali, is regularly added to swimming pools to disinfect them. But its alkalinity means that it can easily become excessive and irritating to the skin of pool users. The pH level of the water can thus be lowered by adding HCl.
  • Purifying table salt: Sodium chloride, the common table salt, is abundant in nature. Salt is about 2.68% concentration in ocean water but can also be mined on land as thick deposits from the evaporation of prehistoric oceans. However, there are several alkaline substance impurities that need to be washed away, and this is done with the use of HCl.
  • Pickling steel: Before metals can be processed, they must undergo a surface treatment called pickling, which removes any impurities like rust, iron oxide scale, stains, and other contaminants. HCl is a common and effective pickling agent because of how corrosive it is – but this also means that it can only be used on carbon steel grades, since it could stain or corrode stainless steel grades.
  • Leather processing: The leather tanning industry uses HCl to prevent the growth of bacteria on leather, as well as to maintain its pH. Doing this helps to prevent decomposition from occurring. 
  • Producing inorganic compounds: Hydrochloric acid easily reacts with alkaline substances and other chemicals. These reactions form compounds, such as ferric acid and aluminium chlorohydrate, that can then be used in water treatment.
  • Assisting oil well production: HCl is also used in large quantities in the oil mining industry, where it’s applied in bore drilling to dissolve rock minerals, making it easier to drill.
  • Processing food additives: Hydrochloric acid is used in processing various food additives such as fructose, citric acid, and hydrolysed vegetable protein.
Underneath a swimming pool
Hydrochloric acid is used to maintain the pH of swimming pools

What is the Chemical Formula for Hydrochloric Acid?

Hydrochloric acid is an inorganic acid composed of only two elements: hydrogen and chlorine. It has the simple chemical formula HCl. It only becomes an acid when dissolved in water, technically known as an aqueous solution. When dissolved in water, the formula can be written as HCl (aq).

If it’s not dissolved in water, it’s known simply as hydrogen chloride. In this state, it’s a colourless gas at room temperature. The official IUPAC name of hydrochloric acid is chlorane, but it’s also known by many other names, including muriatic acid, spirits of salt, hydronium chloride, and chlorhydric acid.

In an aqueous solution, the negative chlorine ion dissociates with the positive hydrogen ion. The extra hydrogen then combines with the water molecule, forming a positive ion group consisting of a central oxygen atom that’s surrounded by three hydrogens. This forms a tetrahedral ionic structure known as protonated water, with the ionic formula H3O+

A study that combined infrared, Raman, X-ray, and neutron diffraction revealed that concentrated hydrochloric acid forms H5O2+ as the dominant cation. This cation and the chloride anions are hydrogen-bonded to the neighboring water molecules in a variety of ways.

What is the pH of Hydrochloric Acid?

Just like other types of acids, the pH of hydrochloric acid varies depending on the concentration. For instance, commercially available muriatic acid has a pH level of between 1 and 2 at the concentration of 31.45%.

The true measure of the strength of an acid is the dissociation constant, or pKa. This is the ratio between the number of ions and the number of molecules of a substance in a water solution. The formula is written as:

The formula for finding the dissassociation content, or strength of an acid

  • Ka = Acid dissociation constant
  • [A] = Concentration of the conjugate base of the acid
  • [H+] = Concentration of hydrogen ions
  • [HA] = Concentration of chemical species HA

The lower the pKa, the easier it is for an acid solution to give up or donate protons. This means that acid is basically a proton donor. The acidity level of hydrochloric acid is relatively high. When hydrogen chloride is dissolved in water, the acidity constant or pKa is −5.9.

When hydrogen chloride gas becomes hydrochloric acid, it has a greater tendency to completely dissociate into ions compared to weaker acids. As a strong acid, HCl can easily lose its hydrogen ions, which are then replaced by other elements. It dissolves many types of metals, including iron, by liberating the hydrogen gas from the solution. This process is exothermic.

Here’s a list of other strong acids:

  • Perchloric acid (HClO4)
  • Hydrobromic acid (HBr)
  • Hydroiodic acid (Hl)
  • Nitric acid (HNO3)
  • Sulphuric acid (H2SO4)


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