Hydrochloric Acid

CHEMICAL NAME = hydrochloric acid CAS NUMBER = 7647–01–0 MOLECULAR FORMULA = HCl_(aq) MOLAR MASS = 36.5 g/mol

COMPOSITION = H(2.8%) Cl(97.2%)

MELTING POINT = −59°C (20%), −30°C (36%) BOILING POINT = 108°C (20%), 61°C (36%) DENSITY = 1.10 g/cm³ (20%), 1.18 g/cm³ (36%)

Hydrochloric acid is a strong, corrosive acid that results when the gas hydrogen chloride dis-solves in water. Ancient alchemists prepared hydrochloric acid and Jabbar ibn Hayyan, known in Latin as Geber (721–815), is credited with its discovery around the year 800. Th e original method of preparation involved reacting salt with sulfuric acid, producing sodium hydrogen sulfate and hydrogen chloride gas. Th e hydrogen chloride gas is captured and dissolved in water to produce hydrochloric acid. Hydrochloric acid was formerly called muriatic acid.

Terms such as muriatic and muriate were used in association with chloride substances before the discovery and nature of chlorine were fully understood. Th e Latin term muriaticus means pickled from muri, which is the Latin term for brine. Chlorides were naturally associated with seawater salt solutions, as chloride is the principal ion in seawater.

Hydrochloric acid is used in numerous applications, but it is generally obtained indirectly as a by-product in other chemical processes. Th e fi rst large-scale production of hydrochloric acid resulted from the mass production of alkalis such as sodium carbonate (Na₂CO₃) and potassium carbonate (potash, K₂CO₃). Th e depletion of European forests and international disputes made the availability of alkali salts increasingly uncertain during the latter part of the 18th century.

Th is prompted the French Academy of Science to off er a reward to anyone who could fi nd a method to produce soda ash from common salt (NaCl). Nicholas LeBlanc (1743–1806) was credited with solving the problem. LeBlanc proposed a procedure in 1783 and a plant based on LeBlanc’s method was opened in 1791. LeBlanc’s method uses sulfuric acid and common salt

142 ^| Th e 100 Most Important Chemical Compounds

to initially produce sodium sulfate (Na₂SO₄). Sodium sulfate is then reacted with charcoal and lime to produce sodium carbonate and calcium sulfi de:

H₂SO_4(l) + 2NaCl_(aq) → Na₂SO_4(s) + 2HCl_(g) Na₂SO_4(s) + 2C_(s) + CaCO_3(s) → Na₂CO_3(s) + CaS_(s)

Plants using the LeBlanc process were situated in areas associated with salt mines, and this naturally created locales for other industries that depended on soda ash. Th e alkali indus-try using the LeBlanc process created environmental problems near the alkali plants. Th e hydrogen chloride gas produced killed vegetation in the immediate vicinity of the plants. To decrease air pollution the gas was dissolved in water, creating hydrochloric acid, which was then discharged to streams, and transforming the air pollution problem into a water pollution problem. Th e LeBlanc process produced an oversupply of hydrochloric acid until the Solvay process supplanted it in the late 19th century. Because the Solvay process did not produce hydrochloric acid and the increase demand for hydrochloric acid, it became necessary to develop chemical plants solely for its production.

Th e traditional method of preparation of hydrochloric acid is the reaction of metal chlorides, especially sodium chloride with sulfuric acid (see the fi rst reaction described). Hydrochloric acid is also produced by direct synthesis from its elements. In the chlorine-alkali industry, elec-trochemical reactions produce elemental chlorine and hydrogen, which can then be combined to give hydrogen chloride: Cl_2(g) + H_2(g) → 2HCl_(g). Hydrogen chloride is then dissolved in water to produce hydrochloric acid. By far, the most common method of producing hydro-chloric acid involves its production as a by-product in chlorination reactions. Th e production of chlorofl uorocarbons was a large source of hydrochloric acid, but the Montreal Protocol, signed in 1987 and amended in the early 1990s, restricts the production of these compounds owing to their infl uence on the ozone layer. Th is has curtailed this source of hydrochloric acid.

Th e production of other common industrial organic chemicals such as Tefl on, perchloroethy-lene, and polyvinyl chloride result in the production of hydrogen chloride. Th e production of hydrochloric acid in polyvinyl chloride production takes place when ethylene is chlorinated:

C₂H_4(g) + Cl_2(g) → C₂H₄Cl_2(g) C₂H₄Cl_2(g) → C₂H₃Cl_(g) + HCl_(g)

Approximately 30% of hydrochloric acid production in the United States is used in the production of other chemicals. Th e second most common use is in the pickling of steel (20%), followed by oil well acidizing (19%) and food processing (17%). Pickling is a metal treat-ment process used to prepare metal surfaces for subsequent processing such as galvanizing or extrusion. In the iron industry, pickling involves immersing iron and steel products in vats of diluted hydrochloric acid. Th is removes oxides, dirt, and grease. Oil well acidizing involves injecting hydrochloric acid down well holes to dissolve limestone and carbonate formations.

Th is expands existing fi ssures and creates new fi ssures to open channels for oil extraction.

Stomach acid is hydrochloric acid with a pH of about 1.5. Th e hydrochloric acid present in gastric juices helps digest food and activates specifi c enzymes in the digestive process. Th e stomach and digestive tract are protected from the hydrochloric acid secreted during diges-tion by a protective mucus lining and gastric buff ers. Cells lining the digestive system con-stantly regenerate the protective mucus layer. Heartburn and stomach ulcers develop when the

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protective mucus lining is weakened or when excess stomach acid is generated as a result of health problems. Th e use of antacids are a common method to neutralize excess stomach acid.

An antacid is nothing more than a base or salt that produces a basic solution when ingested.

Hydrochloric acid is also used extensively in pharmaceuticals and the food industry. When it is listed after a drug name, the drug was produced by combining a free base and hydrochlo-ric acid to produce a hydrochloride salt. Drugs delivered as hydrochloride salts rather than free bases are more soluble in water than free forms of the drugs, tend to be more stable, are solids, and are often more compatible with the chemistry of the digestive system. In the food industry it is used in the production of gelatin and sodium glutamate, to convert cornstarch to syrup, to refi ne sugar, and as an acidulant.