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hydrogen chloride

hydrogen chloride, chemical compound, HCl, a colorless, poisonous gas with an unpleasant, acrid odor. It is very soluble in water and readily soluble in alcohol and ether. It fumes in moist air. It is not flammable, and the liquid is a poor conductor of electricity. Hydrogen chloride is prepared commercially by the reaction of sulfuric acid with sodium chloride (common salt); niter cake, a mixture of sodium bisulfite and sulfuric acid that is a byproduct of nitric acid manufacture, is sometimes used in place of sulfuric acid. Hydrogen chloride is also produced as a byproduct of the manufacture of chlorinated organic chemicals. It can be prepared directly by reaction of hydrogen and chlorine gases; the reaction is very exothermic and takes place readily in sunlight or at elevated temperatures. Although anhydrous (water-free) hydrogen chloride is commercially available as a high-pressure compressed gas in steel cylinders, most of the gas produced is dissolved in water to form hydrochloric acid (see acids and bases), a commercially important chemical. Pure grades of hydrochloric acid are colorless, but technical grades, commonly called muriatic acid, are often yellow-colored because of impurities such as dissolved metals. Most hydrochloric acid produced has a concentration of 30% to 35% hydrogen chloride by weight. The major use of hydrochloric acid is in the manufacture of other chemicals. It is also used in large amounts in pickling (cleaning) metal surfaces, e.g., iron before galvanizing. It reacts with most common metals, releasing hydrogen and forming the metal chloride; with most metal oxides and hydroxides it reacts to form water and the metal chloride. Hydrochloric acid is also used in small amounts in processing glucose and other foods and for various other uses. Concentrated solutions are strong acids and highly corrosive. Hydrochloric acid is not an oxidizing agent but can be oxidized by very strong oxidizing agents, liberating chlorine gas. In dilute solutions of the acid the hydrogen chloride is almost completely dissociated into hydrogen and chloride ions. A solution containing 20.24% hydrogen chloride by weight is azeotropic, boiling at a constant temperature of 110°C at atmospheric pressure. Hydrogen chloride also forms monohydrates, dihydrates, and trihydrates that are liquids at room temperature.

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Hydrogen Chloride

Hydrogen Chloride

Properties

Early discovery of hydrogen chloride

Preparation and uses

Resources

Hydrogen chloride is a chemical compound composed of the elements hydrogen and chlorine. It dissolves readily in water to produce a solution called hydrochloric acid. Both substances have many important industrial applications, including those in metallurgy, and the manufacture of pharmaceuticals, dyes, and synthetic rubber. Hydrochloric acid is found in most laboratories, since its strong acidic nature makes it an extremely useful substance in analyses and as a general acid. Because hydrogen chloride and hydrochloric acid are so closely related, they are usually discussed together.

Properties

Hydrogen chloride is represented by the chemical formula HCl. This means that a molecule of hydrogen chloride contains one atom of hydrogen and one atom of chlorine. At room temperature (about 77°F [25°C]) and at a pressure of one atmosphere, hydrogen chloride exists as a gas. Consequently it is generally stored under pressure in metal containers.

A much more convenient way to use hydrogen chloride is by dissolving it in water to form a solution. Hydrogen chloride is very soluble in water, the latter dissolving hundreds of times its own volume of hydrogen chloride gas. The resulting solution is known as hydrochloric acid and this also is generally given the chemical formula HCl. Commercial hydrochloric acid usually contains 28-35% hydrogen chloride by weight, and is generally referred to as concentrated hydrochloric acid. When smaller amounts of hydrogen chloride are dissolved in water, the solution is known as dilute hydrochloric acid.

Hydrogen chloride is a colorless, nonflammable gas with an acrid odor. The gas condenses to a liquid at -121°F (-85°C) and freezes into a solid at 173.2°F (114°C). Hydrochloric acid is a colorless, fuming liquid with an irritating odor. Both hydrogen chloride and hydrochloric acid are corrosive, and so must be treated with great care. Both substances strongly irritate the eyes and are highly toxic if inhaled or ingested. Exposure to hydrogen chloride vapor can damage nasal passages and produce coughing, pneumonia, headaches and rapid throbbing of the heart; death can occur from exposure to levels in air greater than about 0.2%. Concentrated hydrochloric acid solutions cause burns and skin inflammation. Chemists always wear protective gloves and safety glasses when using either hydrogen chloride or hydrochloric acid, and generally work in a well ventilated area to reduce exposure to fumes.

While dry hydrogen chloride gas is fairly unreactive, moist hydrogen chloride gas (and hydrochloric acid solutions) react with many metals. Consequently, dry hydrogen chloride gas can be stored in metal containers, whereas solutions of highly corrosive hydrochloric acid must be handled in acid-proof materials such as ceramics or glass. When hydrochloric acid reacts with metals, hydrogen gas and compounds known as metal chlorides are usually generated. Metal chlorides are formed when a metal displaces the hydrogen from the hydrogen chloride. For example, zinc metal dissolves in hydrochloric acid to form hydrogen gas and zinc chloride. Both moist hydrogen chloride and hydrochloric acid also react with many compounds including metal oxides, hydroxides, and carbonates. These are all examples of basic compounds, which neutralize hydrochloric acid, and form metal chlorides.

Like most acids, hydrogen chloride forms hydrogen ions in water. These are positively charged atoms of hydrogen that are very reactive and are responsible for all acids behaving in much the same way. Because all the hydrogen atoms in hydrogen chloride are converted into hydrogen ions, hydrochloric acid is called a strong acid. Nitric and sulfuric acids are other examples of strong acids.

Early discovery of hydrogen chloride

The alchemists of medieval times first prepared hydrogen chloride by heating ordinary salt (sodium chloride) with iron sulfate. The German chemist Johann Glauber (1604-1668) made hydrogen chloride by the reaction of salt with sulfuric acid, and this became the common method for conveniently preparing hydrogen chloride in the laboratory. By passing hydrogen chloride gas into water, hydrochloric acid is produced.

Because hydrogen chloride was first prepared from salt, hydrochloric acid was originally referred to as spirits of salt. Commercially, it was also commonly called muriatic acid, from the Latin muria, meaning brine, or salt water. Hydrochloric acid dissolves many substances, and alchemists found the acid very useful in their work. For example, it was used to dissolve insoluble ores, thereby simplifying the methods of chemical analysis to determine the metal content of the ores. A mixture of hydrochloric acid and nitric acid (known as aqua regia) also became very useful since it was the only acid that will dissolve gold.

Preparation and uses

Hydrogen chloride can be prepared on an industrial scale from the reaction of salt with sulfuric acid. It is also formed rapidly above 482°F (250°C) by direct combination of the elements hydrogen and chlorine, and it is generated as a byproduct during the manufacture of chlorinated hydrocarbons. Hydrochloric acid is obtained by passing hydrogen chloride gas into water.

Both hydrogen chloride and hydrochloric acid have many important practical applications. They are used in the manufacture of pharmaceutical hydrochlorides (water soluble drugs that dissolve when ingested), chlorine, and various metal chlorides, in numerous reactions of organic (carbon containing) compounds, and in the plastics and textiles industries. Hydrochloric acid is used for the production of fertilizers, dyes, artificial silk, and paint pigments; in the refining of edible oils and fats; in electroplating, leather tanning, refining, and concentration of ores, soap production, petroleum extraction, cleaning of metals, and in the photographic and rubber industries.

Small quantities of hydrochloric acid occur in nature in emissions from active volcanos and in waters from volcanic mountain sources. The acid is also present in digestive juices secreted by glands in the stomach wall and is therefore an important component in gastric digestion. When too much hydrochloric acid is produced in the digestive system, gastric ulcers may form. Insufficient secretion of stomach acid can also lead to digestion problems.

See also Acids and bases.

Resources

BOOKS

Emsley, John. Natures Building Blocks: An A-Z Guide to the Elements. Oxford: Oxford University Press, 2002.

Heiserman, D.L. Exploring the Chemical Elements and Their Compounds. Blue Ridge Summit, PA: Tab Publications, 1992.

Mahn, W.J. Academic Laboratory Chemical Hazards Guidebook. New York: Van Nostrand Rheinhold, 1991.

Salzberg, H.W. From Caveman to Chemist. Washington, DC: American Chemical Society, 1991.

Sittig, M. Handbook of Toxic and Hazardous Chemicals and Carcinogens. 3rd ed. Park Ridge, NJ: Noyes Publications, 1991.

OTHER

U.S. Environmental Protection Agency: Technology Transfer Network, Air Toxics Website. Hydrochloric Acid (Hydrogen Chloride) <http://www.epa.gov/ttn/atw/hlthef/hydrochl.html> (accessed November 29, 2006).

University College Cork, Department of Chemistry. Hydrogen Chloride <http://www.ucc.ie/academic/chem/dolchem/html/comp/hcl.html> (accessed November 29, 2006).

Nicholas C. Thomas

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Hydrogen Chloride

Hydrogen chloride

Hydrogen chloride is a chemical compound composed of the elements hydrogen and chlorine . It readily dissolves in water to produce a solution called hydrochloric acid. Both substances have many important industrial applications, including those in metallurgy , and the manufacture of pharmaceuticals, dyes, and synthetic rubber. Hydrochloric acid is found in most laboratories since its strong acidic nature makes it an extremely useful substance in analyses and as a general acid. Because hydrogen chloride and hydrochloric acid are so closely related, they are usually discussed together.


Properties

Hydrogen chloride is represented by the chemical formula HCl. This means that a molecule of hydrogen chloride contains one atom of hydrogen and one atom of chlorine. At room temperature (about 77°F [25°C]) and at a pressure of one atmosphere, hydrogen chloride exists as a gas. Consequently it is generally stored under pressure in metal containers. A much more convenient way to use hydrogen chloride is by dissolving it in water to form a solution. Hydrogen chloride is very soluble in water, the latter dissolving hundreds of times its own volume of hydrogen chloride gas. The resulting solution is known as hydrochloric acid and this also is generally given the chemical formula HCl. Commercial hydrochloric acid usually contains 28-35% hydrogen chloride by weight, and is generally referred to as concentrated hydrochloric acid. When smaller amounts of hydrogen chloride are dissolved in water, the solution is known as dilute hydrochloric acid.

Hydrogen chloride is a colorless, nonflammable gas with an acrid odor. The gas condenses to a liquid at -121°F (-85°C) and freezes into a solid at -173.2°F (-114°C). Hydrochloric acid is a colorless, fuming liquid having an irritating odor. Both hydrogen chloride and hydrochloric acid are corrosive, and so must be treated with great care. Both substances strongly irritate the eyes and are highly toxic if inhaled or ingested. Exposure to hydrogen chloride vapor can damage the nasal passages and produce coughing, pneumonia , headaches and rapid throbbing of the heart , and death can occur from exposure to levels in air greater than about 0.2%. Concentrated hydrochloric acid solutions cause burns and inflammation of the skin. Chemists always wear protective rubber gloves and safety glasses when using either hydrogen chloride or hydrochloric acid, and generally work in a well ventilated area to reduce exposure to fumes.

While dry hydrogen chloride gas is fairly unreactive, moist hydrogen chloride gas (and hydrochloric acid solutions) react with many metals. Consequently, dry hydrogen chloride gas can be stored in metal containers, whereas solutions of highly corrosive hydrochloric acid must be handled in acid-proof materials such as ceramics or glass . When hydrochloric acid reacts with metals, hydrogen gas and compounds known as metal chlorides are usually generated. Metal chlorides are formed when a metal displaces the hydrogen from the hydrogen chloride. For example, zinc metal dissolves in hydrochloric acid to form hydrogen gas and zinc chloride. Both moist hydrogen chloride and hydrochloric acid also react with many compounds including metal oxides, hydroxides, and carbonates. These are all examples of basic compounds, which neutralize hydrochloric acid, and form metal chlorides.

Obviously, hydrochloric acid is acidic. Like most acids, hydrogen chloride forms hydrogen ions in water. These are positively charged atoms of hydrogen that are very reactive and are responsible for all acids behaving in much the same way. Because all the hydrogen atoms in hydrogen chloride are converted into hydrogen ions, hydrochloric acid is called a strong acid. Nitric and sulfuric acids are other examples of strong acids.


Early discovery of hydrogen chloride

The alchemists of medieval times first prepared hydrogen chloride by heating ordinary salt ( sodium chloride ) with iron sulfate. The German chemist Johann Glauber (1604-1668) made hydrogen chloride by the reaction of salt with sulfuric acid , and this became the common method for conveniently preparing hydrogen chloride in the laboratory. By passing hydrogen chloride gas into water, hydrochloric acid is produced. Because hydrogen chloride was first prepared from salt, hydrochloric acid was originally referred to as spirits of salt. Commercially, it was also commonly called muriatic acid, from the Latin muria, meaning brine, or salt water. Hydrochloric acid dissolves many substances, and the alchemists found this acid to be very useful in their work. For example, it was used to dissolve insoluble ores thereby simplifying the methods of chemical analysis to determine the metal content of the ores. A mixture of hydrochloric acid and nitric acid (known as aqua regia) also became very useful since it was the only acid that will dissolve gold.


Preparation and uses

Hydrogen chloride can be prepared on an industrial scale from the reaction of salt with sulfuric acid. It is also formed rapidly above 482°F (250°C) by direct combination of the elements hydrogen and chlorine, and it is generated as a by-product during the manufacture ofchlorinated hydrocarbons . Hydrochloric acid is obtained by passing hydrogen chloride gas into water.

Both hydrogen chloride and hydrochloric acid have many important practical applications. They are used in the manufacture of pharmaceutical hydrochlorides (water soluble drugs that dissolve when ingested), chlorine, and various metal chlorides, in numerous reactions of organic (carbon containing) compounds, and in the plastics and textiles industries. Hydrochloric acid is used for the production of fertilizers , dyes, artificial silk, and paint pigments; in the refining of edible oils and fats; in electro-plating, leather tanning, refining, and concentration of ores, soap production, petroleum extraction, cleaning of metals, and in the photographic and rubber industries.

Small quantities of hydrochloric acid occur in nature in emissions from active volcanos and in waters from volcanic mountain sources. The acid is also present in digestive juices secreted by glands in the stomach wall and is therefore an important component in gastric digestion. When too much hydrochloric acid is produced in the digestive system , gastric ulcers may form. Insufficient secretion of stomach acid can also lead to digestion problems.

See also Acids and bases.


Resources

books

Emsley, John. Nature's Building Blocks: An A-Z Guide to theElements. Oxford: Oxford University Press, 2002.

Heiserman, D.L. Exploring the Chemical Elements and TheirCompounds. Blue Ridge Summit, PA: Tab Publications, 1992.

Mahn, W.J. Academic Laboratory Chemical Hazards Guidebook. New York: Van Nostrand Rheinhold, 1991.

Salzberg, H.W. From Caveman to Chemist. Washington, DC: American Chemical Society, 1991.

Sittig, M. Handbook of Toxic and Hazardous Chemicals andCarcinogens. 3rd ed. Park Ridge, NJ: Noyes Publications, 1991.


Nicholas C. Thomas

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Hydrogen Chloride

Hydrogen Chloride

OVERVIEW

Hydrogen chloride (HY-druh-jin KLOR-ide) is a colorless gas with a strong, suffocating odor. The gas is not flammable, but is corrosive, that is, capable of attacking and reacting with a large variety of other compounds and elements. Hydrogen chloride is most commonly available as an aqueous solution known as hydrochloric acid. It is one of the most important industrial chemicals in the world. In 2004, just over 5 million metric tons (5.5 million short tons) of hydrogen chloride were produced in the United States, making it the eighteenth most important chemical in the nation for that year.

KEY FACTS

OTHER NAMES:

Anhydrous hydrochloric acid

FORMULA:

HCl

ELEMENTS:

Hydrogen; chlorine

COMPOUND TYPE:

Inorganic acid

STATE:

Gas

MOLECULAR WEIGHT:

36.46 g/mol

MELTING POINT:

−114.17°C (−173.51°F)

BOILING POINT:

−85°C (−121°F)

SOLUBILITY:

Very soluble in water; soluble in alcohol and ether

Hydrogen chloride has probably been known as far back as the eighth century, when the Arabian chemist Jabir ibn Hayyan (c. 721–c. 815; also known by his Latinized name of Geber) described the production of a gas from common table salt (sodium chloride; NaCl) and sulfuric acid (H2SO4). The compound was mentioned in the writings of a number of alchemists during the Middle Ages and was probably first produced in a reasonably pure form by the German chemist Johann Rudolf Glauber (1604–1670) in about 1625. The first modern chemist to prepare hydrogen chloride and describe its properties was the English chemist Joseph Priestley (1733–1804) in 1772. Forty years later, in 1818, the English chemistry and physicist Humphry Davy (1778–1829) showed that the compound consisted of hydrogen and chlorine, giving it the correct formula of HCl.

Commercial production of hydrogen chloride had its beginning in Great Britain in 1823. The method of production most popular there and, later, throughout Europe was one originally developed by the French chemist Nicholas Leblanc (1742–1806) in 1783. Leblanc had invented the process as a method for producing sodium hydroxide and sodium carbonate, two very important industrial chemicals. Hydrogen chloride was produced as a byproduct of the Leblanc process, a byproduct for which there was at first no use. The gas was simply allowed to escape into the air. The suffocating and hazardous release of hydrogen chloride prompted governments to pass legislation requiring some other means of disposal for the gas. In England, that law was called the Alkali Act and was adopted by the parliament in 1863. Unable to release hydrogen chloride into the air, manufacturers began dissolving it in water and producing hydrochloric acid. Before long, a number of important commercial and industrial uses for the acid itself were discovered. The "useless" byproduct of the Leblanc process soon became as important as the primary products of the process, sodium hydroxide and sodium carbonate.

HOW IT IS MADE

Hydrogen chloride is still sometimes made today by the traditional process of reacting sodium chloride (NaCl) with a sulfate, such as sulfuric acid or iron(II) sulfate (FeSO4). However, more than 90 percent of the hydrogen chloride produced throughout the world today comes as the byproduct of the chlorination of organic compounds. Chlorination is the process by which chlorine gas reacts with an organic compound, usually replacing some of the hydrogen present in the compound. Since a large number of important chlorinated organic compounds are produced each year, large amounts of hydrogen chloride gas are produced as a byproduct. That gas is simply removed from the reaction and stored in cylinders for future use. Other methods of producing hydrogen chloride include the direct synthesis of hydrogen gas and chlorine gas (producing a very pure product) and the reaction of sodium chloride, sulfur dioxide, oxygen, and water with each other at high temperatures (the Hargreaves process).

Interesting Facts

  • Hydrogen chloride was studied by the famous alchemist Basil Valentine (c. 1394–?), who gave it the name spiritus salis ("the spirit of salt") by which it was known to most alchemists.
  • Hydrochloric acid has traditionally been known as muriatic acid, a name that is still sometimes used by workers in fields in which it is used.

COMMON USES AND POTENTIAL HAZARDS

Hydrogen chloride and hydrochloric acid have some uses in common, and some that are different from each other. In both dry and liquid form, the largest single use of hydrogen chloride is in the synthesis of organic and inorganic chlorides. A large number of compounds important in commerce and industry contain chlorine, including most pesticides, many pharmaceuticals, and a number of polymeric products. Hydrochloric acid is also used widely in the processing of metallic ores and the pickling of metals. Pickling is the process by which a metal is cleaned, usually with an acid, to remove rust and other impurities that have collected on the metal. Some additional uses of hydrogen chloride and hydrochloric acid include the following:

  • In the brining of foods and other materials. Brining is the process by which a material is soaked in a salt solution, usually in order to preserve the material;
  • In the treatment of swimming pool water;
  • As a catalyst in industrial chemical reactions;
  • In the manufacture of semiconductors and other electronic components;
  • To maintain the proper acidity in oil wells (to keep oil flowing smoothly);
  • For the etching of concrete surfaces;
  • In the production of aluminum, titanium, and a number of other important metals.

Words to Know

ALCHEMY
An ancient field of study from which the modern science of chemistry evolved.
ANHYDROUS
Without water or moisture.
AQUEOUS
A solution is one that consists of some material dissolved in water.
CATALYST
A material that increases the rate of a chemical reaction without undergoing any change in its own chemical structure.
POLYMER
A compound consisting of very large molecules made of one or two small repeated units called monomers.

Both hydrogen chloride and hydrochloric acid pose serious health risks to humans and other animals. The gas is an irritant to the eyes and respiratory system, causing coughing, choking, and tearing, as well as more serious damage to tissues. Hydrochloric acid can burn the skin and mucous membranes. Exposure of only five parts per million of the gas can produce noticeable symptoms of distress, and exposure of more than 2,000 parts per million can be fatal. If hydrochloric acid gets into the eyes, blindness may result. Since hydrochloric acid is present in many household products, users should exercise great care when working with such materials.

FOR FURTHER INFORMATION

"Hydrochloric Acid." National Safety Council. http://www.nsc.org/library/chemical/Hydrochl.htm (accessed on October 12, 2005).

"Hydrogen Chloride." Agency for Toxic Substances and Disease Registry. http://www.atsdr.cdc.gov/tfacts173.pdf (accessed on October 12, 2005).

"Hydrogen Chloride." http://www.ucc.ie/ucc/depts/chem/dolchem/html/comp/hcl.html (accessed on October 12, 2005).

"Hydrogen Chloride, HCl." Defense Service Center. http://www.c-f-c.com/gaslink/pure/hydrogen-chloride.htm (accessed on October 12, 2005).

See AlsoSodium Chloride

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