Hydrochloric acid

Industrial Uses

Hydrochloric acid is an inorganic strong acid solution that has a wide range of applications in industrial processes, such as the refining of metals. Hydrochloric acid often determines the quality of the product.

Analytical chemistry

In analytical chemistry, when acid is used to determine the concentration of a base, hydrochloric acid is generally used for titration. Titration with a strong acid solution can make the end point more obvious, resulting in more accurate results. At 1 standard atmosphere, 20.2% hydrochloric acid can form a constant boiling solution, which is often used as a benchmark in quantitative analysis under a certain atmospheric pressure. Its constant boiling concentration changes with changes in air pressure.

Hydrochloric acid is often used to dissolve solid samples for further analysis, including dissolving some metals and calcium carbonate or copper oxide to form easily soluble substances to facilitate analysis.

Pickled steel

One of the most important Uses of hydrochloric acid is for pickling steel. Before subsequent processing of iron or steel (extrusion, rolling, galvanizing, etc.), hydrochloric acid can be used to react off the rust or iron oxide on the surface. Hydrochloric acid solution with a concentration of 18% is usually used as a pickling agent to clean carbon steel:

The remaining waste acid is often used as ferrous chloride solution, but the heavy metal content is high, so this practice has gradually become less common.

The pickling steel industry has developed hydrochloric acid regeneration processes, such as spray roasting furnaces or fluidized bed hydrochloric acid regeneration processes. These processes allow hydrogen chloride gas to be regenerated from the pickling liquor. The most common one is the high-temperature hydrolysis process, whose reaction equation is as follows:

Dissolve the hydrogen chloride gas produced in water to obtain hydrochloric acid. By recycling waste acid, a closed acid cycle has been established. The by-product iron oxide is also widely used in various industrial processing processes [3] [28].

Control pH and neutralize alkaline solution

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

When the purity requirements in industry are extremely high (such as for food, pharmaceuticals, drinking water, etc.), high-purity hydrochloric acid is often used to adjust the pH of the water flow; when the requirements are relatively low, industrial pure hydrochloric acid is enough to neutralize wastewater. Or treat water in swimming pools.

Flame reaction

Flame reaction is often used to test metals or their compounds. The platinum wire used for testing needs to be washed with dilute hydrochloric acid to remove the influence of impurity elements.

Before testing the substance, the platinum wire should be cleaned with hydrochloric acid, and then burned on a flame until the flame takes on its original color.

Regeneration of cation exchange resin

High quality hydrochloric acid is often used to regenerate cation exchange resins. Cation exchange is widely used in the production of pure water to remove Na+, Ca2+ and other ions contained in the solution, and hydrochloric acid can wash away these ions in the resin after the reaction. One H+ replaces one Na+, and Ca2+ requires two H+.

Ion exchange resins and softened water find applications in almost all chemical industries, especially drinking water production and the food industry.

Other applications

Hydrochloric acid also has many small-scale Uses, such as leather processing, salt production, and in the construction industry [28]. Hydrochloric acid is also commonly used in the oil industry: it is injected into oil wells to dissolve rock, creating a giant cavity. This method is often used in the oil extraction industry in North Sea oil fields.

Hydrochloric acid can dissolve calcium carbonate and its applications include descaling or lime mortar used in bricklaying, but hydrochloric acid is more dangerous and should be used with caution. It reacts with calcium carbonate in lime mortar to produce calcium chloride, carbon dioxide and water:

Hydrochloric acid is commonly used in the production of gelatin, foods, food ingredients and food additives. Typical examples include aspartame, fructose, citric acid, lysine, acid-hydrolyzed vegetable protein, etc. These processes all use food grade (very pure) hydrochloric acid.