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Hydrogen chloride and hydrochloric acid

By Thomas Huang December 8th, 2023 100 views

I, What is Hydrogen Chloride?

1.1 Hydrogen chloride gas

Hydrogen chloride, chemical formula HCl. Its gas is a colorless gas with an irritating odor.
 Its molecule is made up of one chlorine atom and one hydrogen atom photo of HCl molecule

1.2 Aqueous solutions of hydrogen chloride

1.2.1 Overview

Its aqueous solution is called hydrochloric acid. Hydrogen chloride is extremely soluble in water. One volume of water can dissolve around 500 volumes of hydrogen chloride at 0°C.

1.2.2 Hydrochloric Acid Concentrations and Applications

 Chemical Laboratory  Industrial production  Pharmaceuticals  Others
 37%  20% to 32%  0.1% to 1%   38% to 40%, or 0.01% to 0.1%
Hydrochloric acid is available in a wide range of concentrations and commonly used concentrations are 37%, 20% to 32%, and 0.1% to 1%. Different concentrations of hydrochloric acid have different applications in different fields. When using hydrochloric acid, it is necessary to select the appropriate concentration according to specific needs and pay attention to corrosivity and operational safety. Chemical Laboratory
Commonly used hydrochloric acid with a concentration of 37%, often called concentrated hydrochloric acid, is one of the most commonly used acid solutions in laboratories.
Concentrated hydrochloric acid has strong acidity and can react with many substances, and is commonly used in acid-base neutralization reactions, metal corrosion experiments, and other laboratory operations. When using concentrated hydrochloric acid in the laboratory, you need to pay attention to its corrosiveness and volatility to avoid harming the human body and the environment. Industrial production
Hydrochloric acid is generally available in concentrations ranging from 20% to 32%. This concentration of hydrochloric acid is commonly used in industrial processes such as metal cleaning, pickling steel, and preparing other chemicals. The concentration of hydrochloric acid used in industry is lower than that of concentrated hydrochloric acid in laboratories because lower concentrations are safer and more cost-effective. Pharmaceuticals
The concentration of hydrochloric acid is very demanding. Hydrochloric acid is often used in the pharmaceutical process to regulate the pH and solubility of drugs. The concentration of hydrochloric acid used in pharmaceuticals is very low, generally in the range of 0.1% to 1%. This low concentration of hydrochloric acid acts as an aid in the drug to help it dissolve or stabilize better. Other applications:
Hydrochloric acid in concentrations as high as 38% to 40% is required in some specialized chemical synthesis reactions.
In pickling optical instruments, hydrochloric acid at concentrations of 0.01% to 0.1% is used.

II, Properties of hydrogen chloride

2.1 Physical Properties

Appearance: Colorless, hygroscopic, asphyxiating gas.
 Density  Melting point  Boiling point  Solubility (water)  Saturated vapor pressure (kPa)
 1.477g/L (gas)  -114.2°C  -85.1℃  72g/100mL (20℃) (standard pressure)  4225.6 (20℃)

2.2 Chemical properties


Non-combustible in air, thermally stable, does not decompose until about 1500℃.


Corrosive, non-reactive with water but soluble in water, often in the form of hydrochloric acid mist in the air. 1 volume of water can dissolve 503 volumes of hydrogen chloride gas at 25℃ and 1 atmospheric pressure.


Soluble in ethanol and ether, also soluble in a variety of other organic substances.


The chemical properties of dry hydrogen chloride are very inactive. Alkali metals and alkaline earth metals can be burned in hydrogen chloride, and sodium burns with a bright yellow flame.


The aqueous solution of hydrogen chloride is hydrochloric acid.  The presence of hydrochloric acid is often tested with ammonia, which reacts with hydrogen chloride to form white ammonium chloride particles. Hydrogen chloride has a strong dipole and hydrogen bonds with other dipoles.
photo of Industrial hydrochloric acid  
 Industrial hydrochloric acid is often slightly yellow, mainly because of the presence of ferric chloride. 

III, Methods of Preparation

3.1 Laboratory Preparation

Generally, solid sodium chloride reacts with concentrated sulfuric acid without heating or with a little heating to form sodium bisulfate and hydrogen chloride, respectively.
The reaction is then continued at 500°C to 600°C to form hydrogen chloride and sodium sulfate.
The total chemical equation can be expressed as follows:
2NaCl+H2SO4=Na2SO4+2HCl↑ (Note: HCl is added ↑ only when heated and in the absence of water).

3.2 Industrial Preparation

3.3 Industrial high-purity hydrogen chloride production process and design

3.3.1 Co-production of hydrogen chloride gas from potassium sulfate by Mannheim method

Concentrated sulfuric acid is used to react with dried potassium chloride to produce high-purity hydrogen chloride gas, which is pressed into steel cylinders with a compressor.
photo of  steel cylinders

3.3.2 Hydrochloric acid decalcification method

Concentrated hydrochloric acid is heated and decalcified in a decalcification tower to produce hydrogen chloride gas.
The hydrochloric acid decalcification method of high-purity hydrogen chloride is widely used in the production of PVC, chloroprene, and high-purity hydrochloric acid.

3.3.3 Synthesis

The raw materials hydrogen and chlorine gas undergo a combustion reaction in a synthesis furnace to produce hydrogen chloride gas. The hydrogen chloride gas coming out of the synthesis furnace is purified at room temperature and low-temperature drying and adsorption, cooled under low temperature and low pressure to exclude non-condensable gases and impurities, and bottled in a liquid state.

3.3.4 Industrial by-product acid desalination method

The hydrogen chloride is a by-product of the production by using dilute acid to absorb organic chlorides in the adiabatic absorption tower.  After thickening, HCl enters the resolution tower to desorb the highly concentrated gas.

3.3.5 Petrochemical by-production of hydrogen chloride purification method

Use petrochemical by-product hydrogen chloride as raw material to prepare high-purity hydrogen chloride.
 In the presence of a hydrogen chloride reaction catalyst, the acetylene and hydrogen chloride react into halogenated hydrocarbons, which have a higher boiling point than hydrogen chloride. Then separation and removal of reaction products.

3.4 Methods of Industrial Drying of HCl

Drying HCl can effectively remove the water from HCl gas, improve its purity, and ensure the quality stability of its products.

3.4.1 Sulfuric acid drying method

Sulfuric acid drying method is a commonly used method for industrial drying of HCl. It makes use of the strong hygroscopicity of sulfuric acid and passes the HCl gas through the sulfuric acid bed to make it in contact with sulfuric acid and adsorb the water therein, thus realizing the purpose of drying. 
This method has the advantages of simple operation and low investment in equipment and is widely used in industrial production.

3.4.2 Molecular sieve drying

The molecular sieve drying method is another commonly used method to industrially dry HCl. A molecular sieve is a material with a microporous structure and has strong adsorption capacity. Bypassing the HCl gas through the molecular sieve bed, the water in it is adsorbed and removed by utilizing the adsorption effect of the molecular sieve. This method is characterized by high efficiency and reliability and is widely used in the production of high-purity HCl.
photo of Molecular sieve dryer

IV, Uses

4.1 Uses of hydrogen chloride

Hydrogen chloride is mainly used in the production of dyes, spices, drugs, various chlorides, and corrosion inhibitors.

4.2 Uses of hydrochloric acid

4.2.1 Used in hydrometallurgy of rare metals.

When smelting tungsten, scheelite (calcium tungstate ore) is first mixed with sodium carbonate and roasted in the air (800℃~900℃) to produce sodium tungstate.
The sintered block is immersed in water at 90°C to dissolve sodium tungsten and acidified with hydrochloric acid, the precipitated tungstic acid is filtered out and then scorched to produce tungsten oxide.
H2WO4==high temperature==WO3+H2O↑
Finally, tungsten oxide is cauterized in a stream of hydrogen to obtain tungsten metal.
WO3+3H2==high temperature==W+3H2O(g)

4.2.2 Organic synthesis.

At a temperature of 180℃~200℃ and under the condition of mercury salt (e.g. HgCl2) as the catalyst, hydrogen chloride, and acetylene undergo an addition reaction to produce vinyl chloride, and then polymerize into polyvinyl chloride under the action of the initiator.

4.2.3 Bleaching and Dyeing Industry.

Hydrochloric acid is used in the pickling of cotton fabric after bleaching and in the neutralization of residual alkali after mercerization of cotton fabric. In the process of printing and dyeing, some dyes are insoluble in water and need to be treated with hydrochloric acid to make them into soluble hydrochloric acid before they can be applied. 

4.2.4 Metal processing.

photo of HCl using in metal processing
Pre-plating treatment of steel parts, first washed with caustic soda solution to remove oil, and then soaked in hydrochloric acid; before metal welding, the need to apply a little hydrochloric acid in the weld joints, etc., the use of hydrochloric acid can dissolve metal oxides of this property, to remove rust. In this way, the metal surface can be plated and welded firmly. It is also possible to achieve the purpose of removing materials by corrosion through a dissolution reaction with some metals (e.g., aluminum).

4.2.5 Food Industry.

When making chemical soy sauce, the boiled bean cake, and other raw materials are soaked) soaked in a solution containing a certain amount of hydrochloric acid, maintaining a certain temperature, hydrochloric acid has a catalytic effect, which can promote the hydrolysis of complex proteins, after a certain period, the generation of amino acids with a fresh taste, and then neutralize the caustic soda (or soda ash), that is, the sodium amino acid. The principle of manufacturing monosodium glutamate is similar to this.

4.2.6 Production of Inorganic Drugs.

Hydrochloric acid is a strong acid that reacts with certain metals, metal oxides, metal hydroxides, and most metal salts (e.g., carbonates, sulfites, etc.), to produce hydrochlorid. Therefore, hydrochloric acid is used in the production of several inorganic drugs.

4.2.7 Production of Organic Drugs.

Many organic drugs, such as novocaine and thiamine hydrochloride (a preparation of vitamin B1), are also made from hydrochloric acid.

V, Storage, and Transportation

photo of storage tanks and vehicle

5.1 Dry HCl

5.1.1 Operation notice

Operators should wear rubber gloves, high-waisted rubber boots, a rubber apron, protective eyewear, and a gas supply mask when using hydrogen chloride.
The workplace should be adequately ventilated to keep the ambient air fresh and dry.
Dry hydrogen chloride should be stored in steel cylinders.
Cylinders should be stored in a cool, dry, well-ventilated outdoor area or forced ventilation indoors, away from oxides and sources of ignition. Prevent collision of containers.
Soap or ammonia can be used to detect leaks regularly.

5.1.2 Equipment, piping, and valves:

Keeping hydrogen chloride dry is the key to avoiding corrosion because wet hydrogen chloride accelerates the corrosion of the metal. External materials
Stainless steel is a common choice for dry hydrogen chloride in equipment and piping design. Stainless steel has good corrosion resistance and can effectively resist the corrosive effects of drying hydrogen chloride. 
In addition, high-alloy steels and titanium alloys are some common choices for high-temperature and high-pressure environments with excellent corrosion resistance and strength.
 Temperature  Material / Metal
 T < 1200 ° C  Platinum
 400 ° C < T < 425 ° C  High nickel alloys such as Inconel 600, Hastelloy B, and C
 400 ° C < T < 425 ° C   Chromium-nickel alloys
 T < 760 ° C  Cast iron
 260 ° C < T < 315 ° C   Carbon steel Internal coatings
The selection of appropriate anti-corrosion coatings is necessary for internal coatings on equipment and piping. We need to consider the coating's resistance to high temperatures and chemicals.
Polymer coatings are a common choice and provide good corrosion resistance and chemical stability. Polymer coatings can delay the onset of corrosion by forming a protective layer that prevents dry hydrogen chloride from directly contacting metal surfaces. 
photo of internal coating Sealing
The sealing of equipment and piping is important. Seals need to use good corrosion-resistance material, such as fluoroelastomer or polytetrafluoroethylene (PTFE).

5.1.3 Operating Procedures Before using
Bake and evacuate to keep them in a water-free and sealed condition. When not in use
The HCl should be released and filled with dry inert gas to equipment. If the gas leaks, cover the leak with a polyethylene cover, nylon hose, etc. Introduce the gas into water to absorb it and then neutralize it with slaked lime. In case of liquid leakage, absorb it fully with a large amount of water. When a large amount of gas is spewed out, we can spray water in the distance to absorb the gas. Wastewater is absorbed by water and then neutralized by alkaline substances such as caustic soda, sodium bicarbonate, soda ash, and slaked lime.

5.2 Storage and transportation of hydrochloric acid

5.2.1 Storage

Store in a cool, ventilated warehouse. The storage temperature should not exceed 30°C and the relative humidity should not exceed 85%. Keep the container sealed. Mixed storage is not allowed. It should be stored separately from alkalis, amines, alkali metals, and flammable (combustible) materials. The storage area should be equipped with leakage emergency treatment equipment and suitable shelter materials.

5.2.2 Waste

Waste disposal method: Neutralize with lye-lime water to generate sodium chloride and calcium chloride, dilute with water, and discharge into the wastewater system.

5.2.3 Transportation

Dangerous Goods No.: 81013
UN No.: 1789
Packing category: O52
Packing Method: Acid-resistant altar or ceramic bottle outside the ordinary wooden case or half lattice wooden case; glass bottle or plastic barrel (can) outside the ordinary wooden case or half lattice wooden case; frosted mouth glass bottle or threaded mouth glass bottle outside the ordinary wooden case; threaded mouth glass bottles, iron cap compression-mouth glass bottles, plastic bottles, or metal barrels (cans) outside ordinary wooden case.

5.2.4 Notes

This product should be shipped in a steel tank car with rubber lining or a special plastic enterprise's tank car for railroad transportation.
Packing should be complete and loading should be stable.
During transportation, make sure the container does not leak, collapse, fall, or get damaged. It is strictly prohibited to mix and transport with alkali, amine, alkali metal, flammable or combustible materials, edible chemicals, and so on.
Vehicles should be equipped with leakage emergency treatment equipment. During transportation, it should be protected from exposure to sunlight, rain, and high temperatures.
When transported by road, it should be driven according to the specified routes, and should not stop in residential areas and densely populated areas.
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