Hydrochloric acid (IUPAC name: chlorane), also known as hydrochloric acid (English: hydrochloric acid), is the aqueous solution of hydrogen chloride (chemical formula: HCl), which belongs to inorganic strong acid, and is widely used in industry. Hydrochloric acid is a colorless transparent liquid with strong pungent odor and high corrosiveness. Concentrated hydrochloric acid (mass fraction is about 37%) has very strong volatility, so when the container containing concentrated hydrochloric acid is opened, the hydrogen chloride gas will volatilize and combine with water vapor in the air to produce small droplets of hydrochloric acid, which causes acid mist above the bottle mouth. Hydrochloric acid is the main component of gastric acid, which can promote food digestion and resist microbial infection.

In the 16th century, Libafius formally recorded the preparation method of pure hydrochloric acid: heating concentrated sulfuric acid with salt. Later, Glauber, Priestley, David and other scientists used hydrochloric acid in their research.

During the Industrial Revolution, hydrochloric acid began to be produced in large quantities. Hydrochloric acid has many important applications in chemical industry and plays a decisive role in the quality of products. Hydrochloric acid can be used for pickling steel products, and is also a chemical reagent for large-scale preparation of many inorganic and organic compounds, such as vinyl chloride, the precursor of PVC plastics. Hydrochloric acid also has many small-scale uses, such as household cleaning, production of gelatin and other food additives, scale removal agents, leather processing. The world produces about 20 million tons of hydrochloric acid annually.

application area

Human use

There is a special gland on the stomach wall of humans and other animals that can turn the salt eaten into hydrochloric acid. Hydrochloric acid is a component of gastric juice (concentration is about 0.5%). It can keep the most suitable pH value for activating pepsin in gastric juice. It can also denaturate and hydrolyze proteins in salt, and kill bacteria that enter the stomach with food. In addition, after hydrochloric acid enters the small intestine, it can promote the secretion of pancreatic and intestinal juice, as well as the secretion and discharge of bile. The acidic environment also helps the absorption of iron and calcium in the small intestine.

Daily use

Making toilet cleaning, rust remover and other daily necessities.

Industrial use

Hydrochloric acid is a strong inorganic acid, which is widely used in industrial processing, such as metal refining. Hydrochloric acid often determines the quality of products.

analytical chemistry

In analytical chemistry, when acid is used to determine the concentration of alkali, hydrochloric acid is usually used to titrate. The strong acid titration can make the end point more obvious and the result more accurate. At 1 standard atmospheric pressure, 20.2% hydrochloric acid can form an azeotropic solution, which is often used as a benchmark for quantitative analysis at certain atmospheric pressure. The concentration at azeotropic boiling varies with the change of air pressure.

Pickling steel products

One of the most important uses of hydrochloric acid is pickling steel. Before subsequent treatment of iron or steel (extrusion, rolling, galvanizing, etc.), the rust or iron oxide on the surface can be reacted with hydrochloric acid. Carbon steel is usually cleaned with 18% hydrochloric acid solution as a pickling agent.

The remaining waste acid is often used as ferrous chloride solution, but the content of heavy metals is higher, so this method has gradually become less.

Acid pickling steel industry has developed hydrochloric acid regeneration process, such as spray roaster or fluidized bed hydrochloric acid regeneration process. These processes allow hydrogen chloride gas to regenerate from the pickling solution. The most common one is the high temperature hydrolysis process. The reaction equation is as follows:

Hydrochloric acid is obtained by dissolving the prepared hydrogen chloride gas in water. Through the recovery of waste acid, a closed acid cycle has been established. By-product iron oxide is also widely used in various industrial processes.

Preparation of Organic Compounds

Another major use of hydrochloric acid is to prepare organic compounds, such as vinyl chloride, dichloroethane, polycarbonate precursor bisphenol A, catalytic adhesive polyvinyl alcohol formaldehyde, ascorbic acid and so on. When synthesizing PVC, enterprises usually use hydrochloric acid prepared internally instead of being marketed. Hydrochloric acid also has a great use in pharmaceuticals.

Preparation of inorganic compounds

Hydrochloric acid can react with acid and base, so it can prepare many inorganic compounds, such as iron chloride and polyaluminium chloride (PAC): (preparing iron chloride from hematite)

Ferric chloride and polyaluminium play the role of flocculant and coagulant in sewage treatment, paper and drinking water production.

Hydrochloric acid can also be used to prepare other inorganic substances, including road salt, calcium chloride, nickel chloride for electroplating, zinc chloride for galvanizing industry and zinc chloride for battery manufacturing. In addition, activated carbon is usually prepared from charcoal by zinc chloride activation method. (preparation of calcium chloride from limestone)

Controlling pH and neutralizing lye

Hydrochloric acid can be used to adjust the pH of the solution:

When the purity requirement is very high in industry (such as for food, pharmaceutical and drinking water), pure hydrochloric acid is often used to regulate the pH of water flow; when the requirement is relatively low, industrial pure hydrochloric acid is enough to neutralize wastewater or treat water in swimming pools.

REGENERATION OF CATIONIC EXCHANGE RESIN

High quality hydrochloric acid is often used in the regeneration of cation exchange resins. Cation exchange is widely used in the production of mineral water, removing the plasma contained in the solution, and hydrochloric acid can wash out these ions in the resin after reaction. Replacing one requires two

Ion exchange resins and softened water are used in almost all chemical industries, especially in drinking water production and food industry.

Other applications

Hydrochloric acid also has many small-scale uses, such as leather processing and food.