5 Uses of Hydrochloric Acid

An indoor swimming pool

Hydrochloric acid (HCl) may be the simplest chlorine-based acid, but it is still strongly corrosive. Last week we looked at the history behind and properties of hydrochloric acid, so this week we thought we’d take a look into its 5 most popular uses.

But limiting the uses of hydrochloric acid to 5 is not an easy task. Known as a ‘workhorse’ chemical, this compound has an enormous scope of applications across many different industries.

  1. Swimming Pools

The water chemistry in swimming pools plays a very important role, and maintaining the correct pH balance is one of the crucial elements in water chemistry. The pH of your swimming pool:

  • Should be slightly alkaline (7.2 – 7.6) because it provides the optimum chlorine levels, is comfortable for your eyes and skin and isn’t corrosive
  • Should not be below 7 because the water becomes acidic. This can cause eye and skin irritation, general corrosion, staining and the liner may begin to wrinkle
  • Should not be above 8 because the alkalinity can slow down chlorine activity, cause scale and discolouration, turn the water cloudy and overwork the filter

Therefore, it is vital for health and safety to maintain optimum water chemistry. When alkalinity is too high in a pool, hydrochloric acid is used to adjust the pH and bring the water back within its optimum range.

Hydrochloric acid is the preferred chemical to use here because of its cleaning properties. Not only will it save your pH, it will also effectively remove any dark stains that have adhered to the grout or tiles of the pool. When used in this way, a 10% solution of HCl is recommended to prevent against any corrosion.

  1. Steel Pickling

Another industry that makes use of the cleaning properties of hydrochloric acid is steel pickling. This is a surface treatment process that is used to remove impurities on metals. Pickling removes things like rust, iron oxide scale, stains, inorganic contaminants, and other general impurities. This is an essential step that is carried out before the metal is processed i.e. before extrusion, rolling or galvanising.

Hydrochloric acid is used as a pickling agent because its corrosive nature means that it can effectively dissolve the metal oxides that accumulate on the surface. When compared to other pickling agents, like sulphuric acid, HCl has several advantages:

  • It requires less pickling time
  • It works well at lower temperatures
  • It gives a better surface quality

As a pickling agent, hydrochloric acid is mostly used for carbon steel grades. It isn’t used for stainless steel grades because its acidity could cause staining or corrosion.

A close-up of a welder working on steel

Before metal can be processed or worked with, it is pickled with hydrochloric acid. This removes any surface impurities like iron oxide scale or rust.

  1. Production of Inorganic & Organic Compounds

There are a variety of products that can be obtained using hydrochloric acid. From wastewater treatment and electroplating to a range of pharmaceuticals, this compound is used in the manufacturing of many different compounds.

Inorganic Compounds

These are the products of an acid-base reaction that uses hydrochloric acid. Many inorganic compounds can be manufactured this way, including:

  • Iron (III) Chloride and Polyaluminium Chloride (PAC): these are used in wastewater treatment, drinking water production and paper production as flocculation and coagulation agents
  • Calcium Chloride: this is used in road application salt because it is able to melt through the ice must faster and more efficiently than common salt
  • Nickel (II) Chloride: this is used in electroplating to transfer a layer of decorative or wear-resistant nickel onto a metal object
  • Zinc Chloride: in the galvanising industry, this inorganic compound is used to coat an iron or steel object with a protective layer of zinc

Organic Compounds

One of the largest industries for the consumption of hydrochloric acid is in the manufacturing of organic compounds:

  • Vinyl Chloride (PVC): this is one of the most widely produced synthetic plastic polymers
  • TDI and MDI: HCl is used to manufacture toluene diisocyanate (TDI) and methylene diphenyl diisocyanate (MDI) which are commonly used in polyutherane
  • Bisphenol-A (BPA): this is used in the synthesis of plastics, and can be found in plastic bottles, soda cans and even epoxy resins that line water pipes
  1. Regenerating Cation Exchange Resins

Ion exchange resins are used during the deionisation process to remove cations (positively charged molecules like calcium and magnesium) and anions (negatively charged molecules like sulphates and nitrates) from an aqueous solution. This method produces deionised or demineralised water.

High-quality hydrochloric acid has a crucial role in the deionisation process, which can be summarised in 10 simple steps:

  1. Water passes through an ion exchange bed that contains positively and negatively charged resins. These are used to remove their respective ions.
  2. Cation resin has a negative functional group in order to attract the positive ions present in the water.
  3. Anion resin has a positive functional group in order to attract the negative ions present in the water.
  4. Before the process begins, the cation resin is regenerated with hydrochloric acid and becomes loaded with hydrogen (H+) ions.
  5. Anion resin is similarly regenerated with sodium hydroxide (NaOH) and becomes loaded with hydroxyl (OH-) ions.
  6. Electrical neutrality must be maintained throughout the deionisation process because ions are electrically charged. This is why the ion resins must be regenerated.
  7. Regeneration achieves electrical neutrality because every ion in the water that is attracted to the resin bead is replaced by another ion that is leaving the resin bead.
  8. Therefore, positively charged molecules that are attracted to the cation resin are replaced by the H+ ions obtained from the hydrochloric acid.
  9. Similarly, negatively charged molecules that are attracted to the anion resin are replaced by the OH- ions obtained from the sodium hydroxide. This is called ion exchange.
  10. Finally, the solution will be comprised of H+ and OH- ions. These combine to form pure H2O, producing water that has been entirely deionised.

There are a number of acids that can be used to regenerate cation exchange resins, like sulphuric acid. Battling against HCl in the steel pickling and now the ion exchange industry, sulphuric acid is preferred by many manufacturers because it is less expensive.

However, hydrochloric acid still takes the edge because it has a better overall operating capacity. It also isn’t at risk of causing the calcium levels in the feed water to precipitate in the resin bed, which a high concentration of sulphuric acid is likely to do.

Underwater

Hydrochloric acid is used to regenerate cation exchange resin in the deionisation process. This produces deionised water.

  1. Chemical Warfare

Although hydrochloric acid has many brilliant applications that are useful to many industries, there’s no denying that it also has a dark history.

Phosgene

We know that hydrochloric acid can be found in the body where it is produced by our stomach glands to form gastric acid. While this poses no danger to us, there are other areas of the body that HCl was specifically designed to target and destroy.

In World War I, phosgene (COCl2) was a popular chemical used in warfare. Phosgene is a colourless gas that is a valuable industrial reagent and has an odour that resembles hay. It is produced by a reaction between purified carbon monoxide and chlorine gas in the presence of activated carbon.

Phosgene was used as a chemical weapon instead of chlorine gas, which was easily identified by its green colour. As a colourless gas that smelt like familiar hay, phosgene was an insidious chemical that would deeply permeate the mucous membranes in the lungs. Here in the body, it would undergo hydrolysis and convert into carbonic acid and hydrochloric acid which would begin destroying the internal organs. It is estimated that 85% of approximately 100,000 deaths in WW1 were caused by phosgene gas.

Mustard Gas

Chemically known as sulphur mustard, this toxic compound has been used as a chemical warfare agent throughout history. An incredibly powerful vesicant, mustard gas causes severe blistering on those who are exposed to it. It is also a strong mutagen and carcinogen.

One of the reasons why mustard gas causes such extreme blistering is because of how it breaks down. When it comes into contact with water, mustard gas breaks down and forms hydrochloric acid. Not a lot of water is required for this, either; the moist surface of the eyes or lungs are enough to cause the compound to break down. This is why mustard gas doesn’t only cause blistering to your skin – it can also blister many of your organs if inhaled.

Other Uses

The applications of hydrochloric acid are virtually limitless. It is also used:

  • In the food industry to process additives like fructose and citric acid
  • In the tanning industry to process leathers
  • In the North Sea oil production industry to facilitate oil-well acidizing
  • In pH control and neutralisation across many treatment processes and industries
  • In the purification of common table salt
  • In cleaning brickwork from stains and mould growth
  • In masonry for neutralising alkalinity

An old sepia photo of a soldier walking through ruins

Whether you’re using hydrochloric acid in the manufacture of organic compounds or as a cation exchange resin regenerator, ReAgent hase the grade that is suited to your needs. Our online shop stocks HCl in a range of different pack sizes, and every product is backed by a 100% quality guarantee so that you can buy with confidence. Order yours today for nationwide next-day delivery, or call a member of our customer services team for more information. 

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