Hydrochloric acid

• 1. A Water That Cleaves Stone

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  "There will distil over a strong water, which will cleave stone (sakhr) instantly." That line comes from a recipe in Kitāb al-Asrār, "The Book of Secrets", by the Persian alchemist Abu Bakr al-Razi. The liquid it describes may have been hydrochloric acid, a solution of hydrogen chloride in water. Today the acid is colorless, has a distinctive pungent smell, and counts as a strong acid.

  It is also a working chemical of enormous reach. Laboratories rely on it as a reagent, factories use it as an industrial chemical, and it is a component of the gastric acid in the digestive systems of most animal species, including humans. So how did a substance that alchemists could barely recognize end up in every steel mill and every stomach? Why did it take so long for anyone to say clearly that they had made it? And what does a liquid this useful cost the people who handle it?

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• 2. Spirits Of Salt

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  Salzsäure remains the everyday German word for hydrochloric acid, and it carries a very old idea inside it. European alchemists called the acid spirits of salt, or acidum salis, meaning salt acid. The names fit because the acid was produced from rock salt, following the methods of Johann Rudolph Glauber.

  The gas itself was known as marine acid air. Muriatic acid, another surviving name, grows from the same salty root. Muriatic means pertaining to brine or salt, and a muriate is simply a hydrochloride.

  The modern name arrived in 1814, when the French chemist Joseph Louis Gay-Lussac coined hydrochloric acid. By then the substance had been forming in alchemists' flasks for centuries, usually without anyone giving it a name at all.

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• 3. Almost Discovered

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  Al-Razi lived from about 865 to 925 and is known in Latin as Rhazes. He distilled sal ammoniac, which is ammonium chloride, together with vitriol, a term for the hydrated sulfates of various metals. The result was the gas hydrogen chloride, and he may have stumbled onto a primitive route to the acid. Yet in most of his experiments he disregarded the gaseous products. He cared more about the color changes in the residue left behind. The historian Robert P. Multhauf concluded that hydrogen chloride was made many times without anyone clearly seeing that dissolving it in water produces hydrochloric acid.

  An Arabic text called De aluminibus et salibus, "On Alums and Salts", dates to the eleventh or twelfth century. It was falsely attributed to al-Razi, and Gerard of Cremona, who lived from 1144 to 1187, translated it into Latin. It described heating metals with various salts. With mercury, the result was mercury(II) chloride, known as corrosive sublimate. Hydrochloric acid actually began to form in that process, but it reacted at once with the mercury.

  Thirteenth-century Latin alchemists treated that text as one of their main reference works. They were fascinated by the chlorinating properties of corrosive sublimate. Soon they found that if the metals were left out when heating vitriols, alums and salts, strong mineral acids could be distilled directly.

  Aqua regia was one invention to come from this work. It is a mixture of nitric acid and hydrochloric acid in a 1:3 proportion, and it can dissolve gold. A work credited to pseudo-Geber, De inventione veritatis, "On the Discovery of Truth", first described it after about 1300. There it was prepared by adding ammonium chloride to nitric acid.

  That does not mean hydrochloric acid was found first. Isolating it happened about 300 years later. A pure, separate acid needed more efficient cooling apparatus, and that equipment only developed in the centuries that followed.

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• 4. The Recipe Hunters

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  From the point of view of Western chemistry, hydrochloric acid was the last of the three well-known mineral acids whose production method reached the printed literature. Recipes began to appear in the late sixteenth century. The earliest are found in Giovanni Battista Della Porta's Magiae naturalis, "Natural Magic", and in the works of contemporaries such as Andreas Libavius, Jean Beguin and Oswald Croll.

  Historians have worked hard to untangle who came first. They include the German chemists Hermann Franz Moritz Kopp in 1845 and Edmund Oscar von Lippmann in 1938. Mining engineer Herbert Hoover, a future U.S. president, wrote on the subject in 1912 with his wife, the geologist Lou Henry Hoover. Later came Robert Jacobus Forbes in 1948, Mary Elvira Weeks in 1956, F. Sherwood Taylor in 1957 and J. R. Partington in 1960.

  The Italian chemist Ladislao Reti summed up their findings. In his account, the first clear preparations appear with Della Porta in 1589 and 1608 and Libavius in 1597. They continue with pseudo-Basil in 1604, van Helmont in 1646 and Glauber in 1648. Less convincing hints turn up earlier, in Rosetti's Plichto of 1540 and in Agricola in 1558.

  Credit for the first practical method, from vitriol and common salt, goes without doubt to pseudo-Basil, who precedes Glauber. Reti still gave Glauber the unquestionable merit of pointing to the route that the chemical industry would later adopt for large-scale work.

  Meanwhile, the acid's power over metals made it a teaching tool. Daniel Sennert, who lived from 1572 to 1637, and Robert Boyle, who lived from 1627 to 1691, used its ability to dissolve metals rapidly. They did so in demonstrations that bodies are composite. The coming of the Industrial Revolution would give the acid a far less academic role.

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• 5. From Waste To Market

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  Nicolas Leblanc of Issoudun, France, developed a process that made sodium carbonate, or soda ash, cheaply and at scale. Demand for alkaline substances was rising during the Industrial Revolution in Europe. The Leblanc process converted common salt using sulfuric acid, limestone and coal, and it released hydrogen chloride as a by-product.

  For decades that gas was a problem rather than a product. Until the British Alkali Act 1863 and similar legislation in other countries, the excess HCl was often vented into the air. The Bonnington Chemical Works was an early exception. In 1830 it began capturing the HCl and used the resulting acid to make sal ammoniac. After the 1863 act, soda ash producers were obliged to absorb the waste gas in water, which created hydrochloric acid on an industrial scale.

  In the 20th century the Solvay process effectively replaced Leblanc, and it gave off no hydrochloric acid. The acid was by then fully settled as an important chemical in numerous applications. Commercial interest therefore led to other production methods, some still in use. Since 2000, most hydrochloric acid has been made by absorbing by-product hydrogen chloride from the manufacture of industrial organic compounds.

  Direct synthesis is the other route. Chlorine and hydrogen combine to give hydrogen chloride, and the reaction gives off heat, so the equipment is called an HCl oven or HCl burner. The gas is absorbed in deionized water. The result is chemically pure acid, clean enough for the food industry. Large-scale production is almost always tied to other chemicals, as in the chloralkali process, whose hydrogen and chlorine can be combined into HCl.

  Strength varies widely at the point of sale. The concentrated grade reaches 38% HCl. A little over 40% is chemically possible, but evaporation then becomes so fierce that storage needs pressurization and cooling. Bulk industrial grade is therefore 30% to 35%.

  In the United States, solutions between 20% and 32% are sold as muriatic acid. Household cleaning versions there are typically 10% to 12%, with strong advice to dilute them before use. In the United Kingdom the domestic cleaner called "Spirits of Salt" has the same potency as the US industrial grade. In Italy, "Acido Muriatico" ranges from 5% to 32%.

  Dow Chemical leads the producers at 2 million tonnes a year, counted as HCl gas. Georgia Gulf Corporation, Tosoh Corporation, Akzo Nobel and Tessenderlo each make 0.5 to 1.5 million tonnes. World production is estimated at 20 million tonnes a year, with 3 million from direct synthesis and the rest a secondary product of organic and similar syntheses. Most of it is consumed by the producer itself, and the open market is estimated at 5 million tonnes. One of its most important customers is the steel industry.

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• 6. The Workhorse Acid

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  Technical quality hydrochloric acid at typically 18% is the most common agent for pickling carbon steel. Pickling removes rust or iron oxide scale before later processing such as extrusion, rolling and galvanizing. The spent acid was long reused as iron(II) chloride solutions, also called ferrous chloride. High heavy-metal levels in the used liquor have reduced that practice.

  The steel industry has answered with regeneration processes, including the spray roaster and the fluidized bed. The most common is pyrohydrolysis, which recovers HCl from spent pickling liquor and creates a closed acid loop. Its iron(III) oxide by-product is valuable and goes to a variety of secondary industries.

  Elsewhere, the acid dissolves many metals, metal oxides and metal carbonates. Zinc, nickel oxide and calcium carbonate each give a metal chloride. Those chlorides are used for analysis or further production.

  It also controls acidity. Industries demanding purity, such as food, pharmaceuticals and drinking water, use high-quality acid to adjust the pH of process water. Less demanding users rely on technical grade for neutralizing waste streams and for swimming pool pH control. Ion exchange resins that strip Na+ and Ca2+ from water to make demineralized water are rinsed with high-quality acid, which swaps in H+ for the captured cations.

  In the laboratory, hydrochloric acid has a reputation as the friendly strong acid. Among the common strong mineral acids, it is the monoprotic one least likely to cause an interfering oxidation-reduction reaction. Its chloride ion is non-reactive and non-toxic, so it is one of the least hazardous strong acids to handle. Intermediate-strength solutions keep their concentrations well in storage, and the acid is available as a pure, inexpensive reagent.

  Theory puts its pKa in water at about -5.9. Dissociation in water is complete, so the concentration of intact HCl molecules is effectively zero. That makes it the preferred acid for titrating bases, since a strong acid titrant gives a more distinct endpoint.

  A solution of about 20.2% HCl boils at a constant 108.6 degrees Celsius, forming an azeotrope. This constant-boiling acid can serve as a primary standard in quantitative analysis, although its exact concentration depends on the atmospheric pressure when it is prepared.

  Smaller jobs fill the rest of the list. They include leather processing, household cleaning and building construction. In oil production, injecting the acid into the rock formation of a well dissolves part of the rock and opens up a large-pore structure. Oil well acidizing is a common process in North Sea production. The acid also de-scales kettles and cleans mortar off brickwork, and that reaction stops once the acid has all been converted to calcium chloride, carbon dioxide and water.

  Food manufacturing uses it too. Reactions involving the acid help make aspartame, fructose, citric acid, lysine, hydrolyzed vegetable protein as a food enhancer, and gelatin. Food-grade, extra-pure acid is available when the final product needs it.

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• 7. The Acid Inside Us

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  A stomach holds its contents at a pH of 1 to 2, and gastric acid consists mainly of hydrochloric acid. Chloride and hydrogen ions are secreted separately by parietal cells of the gastric mucosa, in the fundus region at the top of the stomach. They pass into a secretory network called canaliculi before entering the stomach lumen.

  The acid has two jobs. It acts as a barrier against microorganisms, helping prevent infections. It is also important for digesting food. The low pH denatures proteins, which makes them easier for enzymes such as pepsin to break down. It also activates the enzyme precursor pepsinogen, which cleaves itself into active pepsin.

  When the chyme leaves the stomach, bicarbonate neutralizes its hydrochloric acid in the duodenum. The stomach wall itself is shielded by a thick mucus layer and by buffering with sodium bicarbonate, induced by secretin. When those defenses fail, heartburn or peptic ulcers can develop. Drugs of the antihistaminic and proton pump inhibitor classes can reduce acid production, and antacids neutralize acid already present.

  The body has one more use for the acid. Osteoclasts, the cells that break down bone, use hydrochloric acid alongside proteases for bone resorption.

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• 8. Handle With Care

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  Rubber is the notable exception to a long list of materials that hydrochloric acid attacks. Being a strong acid, it is corrosive to living tissue and to many materials, but not to rubber. For that reason rubber protective gloves and related gear are typical when handling concentrated solutions.

  Vapors and mists add a respiratory hazard. A respirator fitted with cartridges designed to capture hydrochloric acid can partly reduce that risk. Airborne acid also irritates the eyes, and protective goggles or a facemask may be needed.

  The hazard labels follow concentration. Solutions from 10% up to just under 25% cause skin irritation and serious eye irritation. At 10% and above the acid may also cause respiratory irritation. At 25% and above it causes severe skin burns and eye damage.

  The law treats the acid with caution as well. The 1988 United Nations Convention Against Illicit Traffic in Narcotic Drugs and Psychotropic Substances lists it as a Table II precursor. The reason is its use in the production of heroin, cocaine and methamphetamine. That listing puts a chemical used for cleaning brickwork and processing food under the same international convention as narcotics.

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