Hydrogen Chloride

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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.