Hydrochloric acid

Hydrochloric acid, or muriatic acid by its historical but still occasionally used name, has been an important and frequently-used chemical from early history, and was discovered by the Islamic alchemist Jabir around 800. It was used throughout the Middle Ages by alchemists in the quest for the philosopher's stone, and later by several European scientists including Glauber, Priestley, and Davy, helping establish modern chemical knowledge.

During the Industrial Revolution, it became an important industrial chemical for many applications. Applications include large scale production of organic compounds such as vinyl chloride for PVC and MDI/TDI for PUR and smaller scale applications, such as production of gelatin and other ingredients in food, and leather processing. Currently, production is approximately 20 million metric tonnes annually (20 Mt/year) of HCl gas.

History

In the Middle Ages, hydrochloric acid was known to European alchemists as spirit of salt or acidum salis. Gaseous HCl was called marine acid air. The old (pre-systematic) name muriatic acid has the same origin (muriatic means "pertaining to brine or salt"), and this name is still sometimes used. Notable production was recorded by Basilius Valentinus, the alchemist-abbott of the Augustinians abbey in Erfurt, Germany in the 15th century.

In the 17th century, Johann Rudolf Glauber from Karlstadt, Germany used salt (sodium chloride) and sulfuric acid for the preparation of sodium sulfate, releasing hydrogen chloride gas. Joseph Priestley from Leeds, England prepared pure hydrogen chloride in 1772, and in 1818 Humphry Davy from Penzance, England proved that the chemical composition included hydrogen and chlorine.

During the Industrial Revolution in Europe, demand for alkaline substances, such as soda ash increased, and the new industrial soda-process by Nicolas Leblanc (Issoundun, France) enabled cheap large-scale production. In the Leblanc process, salt is converted to soda ash, using sulfuric acid, limestone, and coal, releasing hydrogen chloride as a by-product. Until the Alkali Act of 1863 prohibited it, excess HCl was was vented to the air. After the passage of the act, soda ash producers were obliged to absorb the waste gas in water, producing hydrochloric acid on an industrial scale.

When early in the 20th century the Leblanc process was effectively replaced by the Solvay process without hydrochloric acid by-product, hydrochloric acid was already fully settled as an important chemical in numerous applications. The commercial interest initiated other production methods which are still used today, as described below. Today, most hydrochloric acid is made by absorbing hydrogen chloride from industrial organic compounds production.

Chemistry

Hydrogen chloride (HCl) is a monoprotic acid, which can dissociate (i.e., ionize) only once to give up one H⁺ ion (a single proton). In aqueous hydrochloric acid, the H⁺ joins a water molecule to form a hydronium ion.

HCl + H₂O → H₃O⁺ + Cl^-

Monoprotic acids have one acid dissociation constant, K_a, which indicates the level of dissociation in water. For a strong acid like HCl, K_a is large. Theoretical attempts to assign a K_a to HCl have been made; see [1]. When chloride salts such as NaCl are added to aqueous HCl they have practically no effect on pH, indicating that Cl^- is an exceedingly weak conjugate base and that HCl is fully dissociated in aqueous solution. For intermediate to strong solutions of hydrochloric acid, the assumption that H⁺ molarity (a unit of concentration) equals HCl molarity is excellent, agreeing to four significant digits.
Of the seven common strong acids in chemistry, all of them inorganic, hydrochloric acid is the monoprotic acid least likely to undergo an interfering oxidation-reduction reaction. It is one of the least hazardous strong acids to handle; despite its acidity, it produces the less reactive and non-toxic chloride ion. Intermediate strength hydrochloric acid solutions are quite stable, maintaining their concentrations over time. These attributes, plus the fact that it is available as a pure reagent, means that hydrochloric acid makes an excellent acidifying reagent and acid titrant (for determining the amount of an unknown quantity of base in titration). Strong acid titrants are useful because they give more distinct endpoints in a titration, making the titration more precise. Hydrochloric acid is frequently used in chemical analysis and to digest samples for analysis. Concentrated hydrochloric acid will dissolve some metals to form oxidized metal chlorides and hydrogen gas. It will produce metal chlorides from basic metal compounds such as calcium carbonate or copper(II) oxide. It is also used as a simple acid catalyst for some chemical reactions.

Physical properties

The physical properties of hydrochloric acid, such as boiling and melting points, density, and pH depend on the concentration or molarity of HCl in the acid solution. They can range from those of water at 0% HCl to values for fuming hydrochloric acid at over 40% HCl.

Hydrochloric acid as the binary (two-component) mixture of HCl and H₂O has a constant-boiling azeotrope at 20.2% HCl and 108.6 °C (227 °F). There are four constant-crystallization eutectic points for hydrochloric acid, between the crystal form of HCl·H₂O (68% HCl), HCl·2H₂O (51% HCl), HCl·3H₂O (41% HCl), HCl·6H₂O (25% HCl), and of course ice (0% HCl). There is also a metastable eutectic at 24.8% between ice and the HCl·3H₂O crystallization.

Production

Direct synthesis

The large scale production of hydrochloric acid is always integrated in other industrial scale chemical industry. In the chlor-alkali industry, salt solution is electrolyzed producing chlorine, caustic soda, and hydrogen. The pure chlorine gas can be re-combined with the hydrogen gas, forming chemically pure HCl gas. As the reaction is exothermic, the installation is called a HCl oven.

Cl₂ + H₂ → 2HCl

Organic synthesis

The largest production of hydrochloric acid is integrated with the formation of chlorinated and fluorinated organic compounds, e.g., Teflon, Freon and other CFCs, chloro-acetic acid, and PVC. Often this production of hydrochloric acid is integrated with captive use of it on-site. In the chemical reactions, hydrogen atoms are replaced by chlorine atoms, whereupon the released hydrogen atom re-combines with the spare atom from the chlorine molecule, forming hydrogen chloride. Fluorination is a subsequent chlorine-replacement reaction, producing again hydrogen chloride.

R-H + Cl₂ → R-Cl + HCl
:: R-Cl + HF → R-F + HCl

Industrial market

Hydrochloric acid is produced in solutions up to 38% HCl (concentrated grade). Higher concentrations up to just over 40% are chemically possible, but the evaporation rate is then so high, that storage and handling need extra precautions, such as pressure and low temperature. Bulk industrial grade is therefore 30% to 34%, optimized for effective transport and limited product loss by HCl vapours. Solutions for household purposes, mostly cleaning, are typically 10% to 12%, with strong recommendations to dilute before use.

Major producers worldwide include Dow Chemical at 2 million metric tonnes annually (2 Mt/year), calculated as HCl gas, and FMC, Georgia Gulf Corporation, Tosoh Corporation, Akzo Nobel, and Tessenderlo at 0.5 to 1.5 Mt/year each. Total world production, for comparison purposes expressed as HCl, is estimated at 20 Mt/year, with 3 Mt/year from direct synthesis, and the rest as secondary product from organic and similar syntheses. By far most of all hydrochloric acid is consumed captively by the producer. The open world market size is estimated at 5 Mt/year.

Applications

Hydrochloric acid is a strong inorganic acid that is used in many industrial processes. The application often determines the required product quality.

Regeneration of ion exchangers

An important application of high-quality hydrochloric acid is the regeneration of ion exchange resins. Cation exchange is widely used to remove ions such as Na⁺ and Ca²⁺ from aqueous solutions, producing demineralized water.

Na⁺ is replaced by H₃O⁺
:: Ca²⁺ is replaced by 2H₃O⁺

pH control and neutralization

A very common application of hydrochloric acid is to regulate the basicity (pH) of solutions.

OH^- + HCl → H₂O + Cl^-

Pickling of steel

Pickling is an essential step in metal surface treatment, to remove rust or iron oxide scale from iron or steel before subsequent processing, such as extrusion, rolling, galvanizing, and other techniques.

Fe₂O₃ + Fe + 6HCl → 3FeCl₂ + 9H₂O

Production of inorganic compounds

Fe₂O₃ + 6 HCl → 2 FeCl₃ + 3 H₂O

Other inorganic compounds produced with hydrochloric acid include road application salt calcium chloride, nickel(II) chloride for electroplating, and zinc chloride for the galvanizing industry and battery production.

Production of organic compounds

The largest hydrochloric acid consumption is in the production of organic compounds such as vinyl chloride for PVC, and MDI and TDI for polyurethane. This is often captive use, consuming locally produced hydrochloric acid that never actually reaches the open market. Other organic compounds produced with hydrochloric acid include bisphenol A for polycarbonate, activated carbon, and ascorbic acid, as well as numerous pharmaceutical products.

Other applications

Hydrochloric acid is a fundamental chemical, and as such it is used for a large number of small scale applications, such as leather processing, household cleaning, and building construction. In addition, a way of stimulating oil production is by injecting hydrochloric acid into the rock formation of an oil well, dissolving a portion of the rock, and creating a large pore structure. Oil well acidizing is a common process in the North Sea oil production industry.

Many chemical reactions involving hydrochloric acid are applied in the production of food, food ingredients, and food additives. Typical products include aspartame, fructose, citric acid, lysine, hydrolyzed protein, and gelatin. Food grade (extra pure) hydrochloric acid can be applied when needed for the final product.

Hydrochloric acid and living organisms

Physiology

Over-acidity in the stomach can lead to heartburn or peptic ulcers. Drugs called proton pump inhibitors prevent the body from making excess acid in the stomach, while antacids neutralize existing acid. Achlorhydria is the inability of parietal cells to produce the required amount of hydrochloric acid in the stomach, potentially leading to gastroenteritis.

In the life cycle of living bone material, specialized bone resorbing cells called osteoclasts also produce hydrochloric acid to dissolve bone. Hydrogen ions react with carbonated hydroxyapatite, the main mineral in bone, and break it down into Ca²⁺, H₃PO₄, H₂CO₃, and water.

Chemical weapons

Hydrochloric acid is also partly responsible for the harmful or blistering effects of mustard gas. In the presence of water, such as on the moist surface of the eyes or lungs, mustard gas breaks down forming hydrochloric acid.

Safety

Hydrochloric acid in high concentrations forms acidic mists. Both the mist and the solution have a corrosive effect on human tissue, potentially damaging respiratory organs, eyes, skin and intestines. Upon mixing hydrochloric acid with common oxidizing chemicals such as bleach (NaOCl) or permanganate (KMnO₄), the toxic gas chlorine is produced. To minimize the risks while working with hydrochloric acid, appropriate precautions should be taken. See references for details.

Dangerous goods labels for hydrochloric acid:


The following Risk and Safety Statements for labeling apply:

See also

Related chemical substances

• Chloride, inorganic salts of hydrochloric acid
• Hydrochloride, organic salts of hydrochloric acid
• Hydrogen chloride, the pure gas, of which hydrochloric acid is the solution
• Hypochlorous acid, and its salt hypochlorite
• Chlorous acid, and its salt chlorite
• Chloric acid, and its salt chlorate
• Perchloric acid, and its salt perchlorate

  Related topics

• Acid
• Chemical industry
• Chemical engineering
• List of chemistry topics
• List of compounds
• Inorganic chemistry

  External links

  General safety information

• EPA Hazard Summary
• NIH Description and Hazard Summary
• Hydrochloric acid MSDS by Jones-Hamilton
• Hydrochloric acid MSDS by Hampton Research
• Hydrochloric acid MSDS by American Bioanalytical
• Hydrochloric acid MSDS by Georgia Institute of Technology
• Hydrochloric acid MSDS by Akzo Nobel

  Manufacturer information

• Hydrochloric acid product information of Akzo Nobel
• Hydrochloric acid product information of Tessenderlo
• Hydrochloric acid product information of Solvay
• Dow Chemical
• Chlor-Alkali information of Tosoh

  References

• Chemicals Economics Handbook, Hydrochloric Acid, SRI International, 2001, p. 733.4000A-733.3003F
• Van Dorst, W.C.A., et al., technical product brochure Hydrochloric Acid, Akzo Nobel Base Chemicals, 2004 (public document)
• Van Dorst, W.C.A., various technical papers, Akzo Nobel Base Chemicals, 1996-2002 (not for open publication)
• Lide, David, NIST, CRC Handbook of Chemistry and Physics, CRC Press, 61st edition, 1980-1981
• Aspen Technology, Aspen Properties, binary mixtures modelling software, calculations by Akzo Nobel Engineering, 2002-2003
• Evison D, Hinsley D, Rice P. Chemical weapons. BMJ 2002;324(7333):332-5. PMID 11834561