Hydrogen Chloride
Hydrogen Chloride
Early discovery of hydrogen chloride
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. Nature’s 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