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.
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.
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).
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.
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.
