Hydrochloric Acid
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Hydrochloric Acid
Structure of Hydrochloric acid as dissociated chloride and hydronium ions
3D model of hydrogen chloride
3D model of water
3D model of the chloride anion
3D model of the hydronium cation
Sample of hydrochloric acid in a bottle
IUPAC name
Other names
  • Muriatic acid[1]
  • Spirits of salt[2]
    Hydronium chloride
    Chlorhydric Acid
  • 7647-01-0 checkY
ECHA InfoCard 100.210.665 Edit this at Wikidata
EC Number
  • 231-595-7
E number E507 (acidity regulators, ...)
UN number 1789
Appearance Colorless, transparent liquid, fumes in air if concentrated
Odor Pungent characteristic
Melting point Concentration-dependent - see table
Boiling point Concentration-dependent - see table
log P 0.00[4]
Acidity (pKa) -5.9 (HCl gas)[5]
A09AB03 (WHO) B05XA13 (WHO)
Safety data sheet See: data page
GHS pictograms GHS07: Exclamation markGHS05: Corrosive
GHS Signal word Danger[6]
H290, H314, H335[6]
P260, P280, P303+P361+P353, P305+P351+P338[6]
NFPA 704 (fire diamond)
Related compounds
Related compounds
Supplementary data page
Refractive index (n),
Dielectric constant (?r), etc.
Phase behaviour
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
?N verify (what is checkY?N ?)
Infobox references

Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride (chemical formula: HCl). It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid. It is a component of the gastric acid in the digestive systems of most animal species, including humans. Hydrochloric acid is an important laboratory reagent and industrial chemical.[7][8]


In the early tenth century, the Persian physician and alchemist Abu Bakr al-Razi (c. 865-925, Latin: Rhazes) conducted experiments with sal ammoniac (ammonium chloride) and vitriol (hydrated sulfates of various metals), which he distilled together, thus producing the gas hydrogen chloride. In doing so, al-Razi came very close to discovering hydrochloric acid, but it appears that he disregarded the gaseous products of his experiments, concentrating instead on the color changes that could be effected in the residue.[9] Drawing on al-Razi's experiments, the De aluminibus et salibus ("On Alums and Salts", an eleventh- or twelfth century Arabic text falsely attributed to al-Razi and translated into Latin in the second half of the twelfth century by Gerard of Cremona, 1144-1187) described the heating of metals with various salts, which in the case of mercury resulted in the production of mercury(II) chloride (corrosive sublimate).[10] In this process, hydrochloric acid actually started to form, but it immediately reacted with the mercury to produce corrosive sublimate. Thirteenth-century Latin alchemists, for whom the De aluminibus et salibus was one of the main reference works, were fascinated by the chlorinating properties of corrosive sublimate, and they soon discovered that when the metals are eliminated from the process of heating vitriols, alums, and salts, strong mineral acids can directly be distilled.[11] One important invention that resulted from the discovery of the mineral acids is aqua regia, a mixture of nitric acid and hydrochloric acid in a 1:3 proportion, capable of dissolving gold. This was first described in pseudo-Geber's De inventione veritatis ("On the Discovery of Truth", after c. 1300), where aqua regia was prepared by adding ammonium chloride to nitric acid.[12] However, the production of hydrochloric acid itself (i.e., as an isolated substance rather than as already mixed with nitric acid) depended on the use of more efficient cooling apparatus, which would only develop in subsequent centuries.[13] Thus, recipes for the production of hydrochloric acid only appear in the late sixteenth century, the earliest being found in Giovanni Battista Della Porta's (1535-1615) Magiae naturalis ("Natural Magic") and in the works of other contemporary chemists like Andreas Libavius (c. 1550-1616), Jean Beguin (1550-1620), and Oswald Croll (c. 1563- 1609).[14] The knowledge of mineral acids such as hydrochloric acid would be of key importance to seventeenth-century chemists like Daniel Sennert (1572-1637) and Robert Boyle (1627-1691), who used their capability to rapidly dissolve metals in their demonstrations of the composite nature of bodies.[15]


Because it was produced from rock salt according to the methods of Johann Rudolph Glauber, hydrochloric acid was historically called by European alchemists spirits of salt or acidum salis (salt acid). Both names are still used, especially in other languages, such as German: Salzsäure, Dutch: Zoutzuur, Swedish: Saltsyra, Spanish: Salfumán, Turkish: Tuz Ruhu, Polish: kwas solny, Hungarian: sósav and Czech: kyselina solná

Gaseous HCl was called marine acid air. The name muriatic acid has the same origin (muriatic means "pertaining to brine or salt", hence muriate means hydrochloride), and this name is still sometimes used.[1][16] The name hydrochloric acid was coined by the French chemist Joseph Louis Gay-Lussac in 1814.[17]

Industrial developments

During the Industrial Revolution in Europe, demand for alkaline substances increased. A new industrial process developed by Nicolas Leblanc of Issoudun, France enabled cheap large-scale production of sodium carbonate (soda ash). In this Leblanc process, common salt is converted to soda ash, using sulfuric acid, limestone, and coal, releasing hydrogen chloride as a by-product. Until the British Alkali Act 1863 and similar legislation in other countries, the excess HCl was often vented into the air. An early exception was the Bonnington Chemical Works where, in 1830, the HCl began to be captured and the hydrochloric acid produced was used in making sal ammoniac (ammonium chloride).[18] 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.[19][20]

In the 20th century, the Leblanc process was effectively replaced by the Solvay process without a hydrochloric acid by-product. Since hydrochloric acid was already fully settled as an important chemical in numerous applications, the commercial interest initiated other production methods, some of which are still used today. After the year 2000, hydrochloric acid is mostly made by absorbing by-product hydrogen chloride from industrial organic compounds production.[19][20][7]

Structure and reactions

Hydrochloric acid is the salt of the protonated water and chloride. Its ions are often written as H3O+ Cl-,[21] although the cation is in fact often bonded to other water molecules. A combined IR, Raman, X-ray, and neutron diffraction study of concentrated hydrochloric acid revealed that the primary form of H+(aq) in these solutions is H5O2+, which, along with the chloride anion, is hydrogen-bonded to neighboring water molecules in several ways.[22] (See Hydronium for further discussion of this issue.)


As a strong acid, hydrogen chloride has a large Ka. Theoretical estimates suggest that the pKa of hydrogen chloride is -5.9.[5] However, it is important to distinguish between hydrogen chloride gas and hydrochloric acid. Due to the leveling effect, except when highly concentrated and behavior deviates from ideality, hydrochloric acid (aqueous HCl) is only as acidic as the strongest proton donor available in water, the aquated proton (popularly known as "hydronium ion"). When chloride salts such as NaCl are added to aqueous HCl, they have only a minor effect on pH, indicating that Cl- is a very weak conjugate base and that HCl is fully dissociated. Dilute solutions of HCl have a pH close to that predicted by assuming full dissociation into hydrated H+ and Cl-.[23]

Physical properties

Concentration Density Molarity pH Viscosity Specific
kg HCl/kg  kg HCl/m3 Baumé kg/L mol/L mPa·s kJ/(kg·K) kPa °C °C
10% 104.80 6.6 1.048 2.87 -0.5 1.16 3.47 1.95 103 -18
20% 219.60 13 1.098 6.02 -0.8 1.37 2.99 1.40 108 -59
30% 344.70 19 1.149 9.45 -1.0 1.70 2.60 2.13 90 -52
32% 370.88 20 1.159 10.17 -1.0 1.80 2.55 3.73 84 -43
34% 397.46 21 1.169 10.90 -1.0 1.90 2.50 7.24 71 -36
36% 424.44 22 1.179 11.81 -1.1 1.99 2.46 14.5 61 -30
38% 451.82 23 1.189 12.39 -1.1 2.10 2.43 28.3 48 -26
The reference temperature and pressure for the above table are 20 °C and 1 atmosphere (101.325 kPa).
Vapour pressure values are taken from the International Critical Tables and refer to the total vapour pressure of the solution.
Melting temperature as a function of HCl concentration in water[24][25]

Physical properties of hydrochloric acid, such as boiling and melting points, density, and pH, depend on the concentration or molarity of HCl in the aqueous solution. They range from those of water at very low concentrations approaching 0% HCl to values for fuming hydrochloric acid at over 40% HCl.[26][27][28]

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 [H3O]Cl (68% HCl), [H5O2]Cl (51% HCl), [H7O3]Cl (41% HCl), [H3O]Cl·5H2O (25% HCl), and ice (0% HCl). There is also a metastable eutectic point at 24.8% between ice and the [H7O3]Cl crystallization.[28] They are all Hydronium salts.


Hydrochloric acid is usually prepared industrially by dissolving hydrogen chloride in water. Hydrogen chloride can be generated in many ways, and thus several precursors to hydrochloric acid exist. The large-scale production of hydrochloric acid is almost always integrated with the industrial scale production of other chemicals, such as in the chloralkali process which produces hydroxide, hydrogen, and chlorine, the latter of which can be combined to produce HCl.[26][27]

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 require extra precautions, such as pressurization and cooling. Bulk industrial-grade is therefore 30% to 35%, optimized to balance transport efficiency and product loss through evaporation. In the United States, solutions of between 20% and 32% are sold as muriatic acid. Solutions for household purposes in the US, mostly cleaning, are typically 10% to 12%, with strong recommendations to dilute before use. In the United Kingdom, where it is sold as "Spirits of Salt" for domestic cleaning, the potency is the same as the US industrial grade.[19] In other countries, such as Italy, hydrochloric acid for domestic or industrial cleaning is sold as "Acido Muriatico", and its concentration ranges from 5% to 32%.

Major producers worldwide include Dow Chemical at 2 million metric tons annually (2 Mt/year), calculated as HCl gas, 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 hydrochloric acid is consumed captively by the producer. The open world market size is estimated at 5 Mt/year.[19]


Hydrochloric acid is a strong inorganic acid that is used in many industrial processes such as refining metal. The application often determines the required product quality.[19] Hydrogen chloride, not hydrochloric acid, is used more widely in industrial organic chemistry, e.g. for vinyl chloride and dichloroethane.[8]

Pickling of steel

One of the most important applications of hydrochloric acid is in the pickling of steel, to remove rust or iron oxide scale from iron or steel before subsequent processing, such as extrusion, rolling, galvanizing, and other techniques.[19][7] Technical quality HCl at typically 18% concentration is the most commonly used pickling agent for the pickling of carbon steel grades.

The spent acid has long been reused as iron(II) chloride (also known as ferrous chloride) solutions, but high heavy-metal levels in the pickling liquor have decreased this practice.

The steel pickling industry has developed hydrochloric acid regeneration processes, such as the spray roaster or the fluidized bed HCl regeneration process, which allow the recovery of HCl from spent pickling liquor. The most common regeneration process is the pyrohydrolysis process, applying the following formula:[19]

By recuperation of the spent acid, a closed acid loop is established.[7] The iron(III) oxide by-product of the regeneration process is valuable, used in a variety of secondary industries.[19]

Production of inorganic compounds

Akin to its use for pickling, hydrochloric acid is used to dissolve many metals, metal oxides and metal carbonates. The conversion are often depicted in simplified equations:

Zn + 2 HCl -> ZnCl2 + H2
NiO + 2 HCl -> NiCl2 + H2O
CaCO3 + 2 HCl -> CaCl2 + CO2 + H2O

These processes are used to produce metal chlorides for analysis or further production.[26][27][7]

pH control and neutralization

Hydrochloric acid can be used to regulate the acidity (pH) of solutions.

In industry demanding purity (food, pharmaceutical, drinking water), high-quality hydrochloric acid is used to control the pH of process water streams. In less-demanding industry, technical quality hydrochloric acid suffices for neutralizing waste streams and swimming pool pH control.[7]

Regeneration of ion exchangers

High-quality hydrochloric acid is used in 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. The acid is used to rinse the cations from the resins.[19] Na+ is replaced with H+ and Ca2+ with 2 H+.

Ion exchangers and demineralized water are used in all chemical industries, drinking water production, and many food industries.[19]

Laboratory use

Of the six common strong mineral acids in chemistry, 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 consists of the non-reactive and non-toxic chloride ion. Intermediate-strength hydrochloric acid solutions are quite stable upon storage, maintaining their concentrations over time. These attributes, plus the fact that it is available as a pure reagent, make hydrochloric acid an excellent acidifying reagent. It is also inexpensive.

Hydrochloric acid is the preferred acid in titration for determining the amount of bases. Strong acid titrants give more precise results due to a more distinct endpoint. Azeotropic, or "constant-boiling", hydrochloric acid (roughly 20.2%) can be used as a primary standard in quantitative analysis, although its exact concentration depends on the atmospheric pressure when it is prepared.[29]


Hydrochloric acid is used for a large number of small-scale applications, such as leather processing, household cleaning,[30] and building construction.[7] Oil production may be stimulated 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.[19]

Hydrochloric acid has been used for dissolving calcium carbonate, e.g. such things as de-scaling kettles and for cleaning mortar off brickwork. When used on brickwork the reaction with the mortar only continues until the acid has all been converted, producing calcium chloride, carbon dioxide, and water:

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 vegetable protein as food enhancer, and in gelatin production. Food-grade (extra-pure) hydrochloric acid can be applied when needed for the final product.[19][7]

Presence in living organisms

Diagram of alkaline mucous layer in stomach with mucosal defense mechanisms

Gastric acid is one of the main secretions of the stomach. It consists mainly of hydrochloric acid and acidifies the stomach content to a pH of 1 to 2.[31][32] Chloride (Cl-) and hydrogen (H+) ions are secreted separately in the stomach fundus region at the top of the stomach by parietal cells of the gastric mucosa into a secretory network called canaliculi before it enters the stomach lumen.[33]

Gastric acid acts as a barrier against microorganisms to prevent infections and is important for the digestion of food. Its low pH denatures proteins and thereby makes them susceptible to degradation by digestive enzymes such as pepsin. The low pH also activates the enzyme precursor pepsinogen into the active enzyme pepsin by self-cleavage. After leaving the stomach, the hydrochloric acid of the chyme is neutralized in the duodenum by bicarbonate.[31]

The stomach itself is protected from the strong acid by the secretion of a thick mucus layer, and by secretin induced buffering with sodium bicarbonate. Heartburn or peptic ulcers can develop when these mechanisms fail. Drugs of the antihistaminic and proton pump inhibitor classes can inhibit the production of acid in the stomach, and antacids are used to neutralize excessive existing acid.[31][34]


A rhombic-shaped label with letters 8 and "corrosive", indicating that drops of a liquid corrode materials and human hands. Causes severe skin burns and eye damage. May cause respiratory irritation.

Being a strong acid, hydrochloric acid is corrosive to living tissue and to many materials, but not to rubber. Typically, rubber protective gloves and related protective gear are used when handling concentrated solutions.[8]

Classification[35] List of
10% Causes skin irritation, Causes serious eye irritation, H315, H319
C >= 10% May cause respiratory irritation H335
C >= 25% Causes severe skin burns and eye damage H314

Hydrochloric acid has been listed as a Table II precursor under the 1988 United Nations Convention Against Illicit Traffic in Narcotic Drugs and Psychotropic Substances because of its use in the production of heroin, cocaine, and methamphetamine.[36]

See also


  1. ^ a b "Hydrochloric Acid". Archived from the original on 15 October 2010. Retrieved 2010.
  2. ^ "spirits of salt". Retrieved 2012.
  3. ^ Favre HA, Powell WH, eds. (2014). Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013. Cambridge: The Royal Society of Chemistry. p. 131.
  4. ^ "Hydrochloric acid". www.chemsrc.com.
  5. ^ a b Trummal A, Lipping L, Kaljurand I, Koppel IA, Leito I (May 2016). "Acidity of Strong Acids in Water and Dimethyl Sulfoxide". The Journal of Physical Chemistry A. 120 (20): 3663-9. Bibcode:2016JPCA..120.3663T. doi:10.1021/acs.jpca.6b02253. PMID 27115918.
  6. ^ a b c Sigma-Aldrich Co., Hydrochloric acid.
  7. ^ a b c d e f g h Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. pp. 946-48. ISBN 978-0-08-037941-8.
  8. ^ a b c Austin, Severin; Glowacki, Arndt (2000). Hydrochloric Acid. doi:10.1002/14356007.a13_283. ISBN 3527306730.
  9. ^ Multhauf, Robert P. (1966). The Origins of Chemistry. London: Oldbourne. OCLC 977570829. pp. 141-142.
  10. ^ Multhauf 1966, pp. 160-162.
  11. ^ Multhauf 1966, pp. 162-163.
  12. ^ Karpenko, Vladimír; Norris, John A. (2002). "Vitriol in the History of Chemistry". Chemické listy. 96 (12): 997-1005. p. 1002.
  13. ^ Multhauf 1966, p. 204.
  14. ^ Multhauf 1966, p. 208, note 29; cf. p. 142, note 79.
  15. ^ Newman, William R. (2006). Atoms and Alchemy: Chymistry and the Experimental Origins of the Scientific Revolution. Chicago: University of Chicago Press. p. 98.
  16. ^ "Muriatic Acid" (PDF). PPG Industries. 2005. Archived from the original (PDF) on 2 July 2015. Retrieved 2010.
  17. ^ Gay-Lussac (1814) "Mémoire sur l'iode" (Memoir on iodine), Annales de Chemie, 91 : 5-160. From page 9: " ... mais pour les distinguer, je propose d'ajouter au mot spécifique de l'acide que l'on considère, le mot générique de hydro; de sorte que le combinaisons acide de hydrogène avec le chlore, l'iode, et le soufre porteraient le nom d'acide hydrochlorique, d'acide hydroiodique, et d'acide hydrosulfurique; ... " (... but in order to distinguish them, I propose to add to the specific suffix of the acid being considered, the general prefix hydro, so that the acidic combinations of hydrogen with chlorine, iodine, and sulfur will bear the name hydrochloric acid, hydroiodic acid, and hydrosulfuric acid; ...)
  18. ^ Ronalds BF (2019). "Bonnington Chemical Works (1822-1878): Pioneer Coal Tar Company". International Journal for the History of Engineering & Technology. 89 (1-2): 73-91. doi:10.1080/17581206.2020.1787807. S2CID 221115202.
  19. ^ a b c d e f g h i j k l "Hydrochloric Acid". Chemicals Economics Handbook. SRI International. 2001. pp. 733.4000A-733.3003F.
  20. ^ a b Aftalion F (1991). A History of the International Chemical Industry. Philadelphia: University of Pennsylvania Press. ISBN 978-0-8122-1297-6.
  21. ^ Petrucci, Ralph H.; Harwood, William S.; Herring, F. Geoffrey (2002). General chemistry: principles and modern applications. Prentice Hall. pp. 668-669. ISBN 978-0-13-014329-7.
  22. ^ Agmon N (January 1998). "Structure of Concentrated HCl Solutions". The Journal of Physical Chemistry A. 102 (1): 192-199. Bibcode:1998JPCA..102..192A. CiteSeerX doi:10.1021/jp970836x. ISSN 1089-5639.
  23. ^ McCarty CG, Vitz E (May 2006). "pH Paradoxes: Demonstrating That It Is Not True That pH ? -log[H+]". Journal of Chemical Education. 83 (5): 752. Bibcode:2006JChEd..83..752M. doi:10.1021/ed083p752. ISSN 0021-9584.
  24. ^ "Systemnummer 6 Chlor". Gmelins Handbuch der Anorganischen Chemie. Chemie Berlin. 1927.
  25. ^ "Systemnummer 6 Chlor, Ergänzungsband Teil B - Lieferung 1". Gmelins Handbuch der Anorganischen Chemie. Chemie Weinheim. 1968.
  26. ^ a b c Lide D (2000). CRC Handbook of Chemistry and Physics (81st ed.). CRC Press. ISBN 978-0-8493-0481-1.
  27. ^ a b c Perry R, Green D, Maloney J (1984). Perry's Chemical Engineers' Handbook (6th ed.). McGraw-Hill Book Company. ISBN 978-0-07-049479-4.
  28. ^ a b Aspen Properties. binary mixtures modeling software (calculations by Akzo Nobel Engineering ed.). Aspen Technology. 2002-2003.
  29. ^ Mendham J, Denney RC, Barnes JD, Thomas MJ, Denney RC, Thomas MJ (2000). Vogel's Quantitative Chemical Analysis (6th ed.). New York: Prentice Hall. ISBN 978-0-582-22628-9.
  30. ^ Simhon R (13 September 2003). "Household plc: really filthy bathroom". The Daily Telegraph. London. Archived from the original on 23 February 2009. Retrieved 2010.
  31. ^ a b c Maton A, Hopkins J, McLaughlin CW, Johnson S, Warner MQ, LaHart D, Wright JD (1993). Human Biology and Health. Englewood Cliffs, New Jersey, USA: Prentice Hall. ISBN 978-0-13-981176-0.
  32. ^ Haas E (6 December 2000). "Digestive Aids: Hydrochloric acid". healthy.net.
  33. ^ Arthur C, Guyton MD, Hall JE (2000). Textbook of Medical Physiology (10th ed.). W.B. Saunders Company. ISBN 978-0-7216-8677-6.
  34. ^ Bowen R (18 March 2003). "Control and Physiologic Effects of Secretin". Colorado State University. Retrieved 2009.
  35. ^ "Regulation (EC) No 1272/2008 of the European Parliament and of Council of 16 December 2008 on classification, labelling and packaging of substances and mixtures, amending and repealing Directives 67/548/EEC and 1999/45/EC, and amending Regulation (EC) No 1907/2006". EUR-lex. Retrieved 2008.
  36. ^ List of precursors and chemicals frequently used in the illicit manufacture of narcotic drugs and psychotropic substances under international control (PDF) (Eleventh ed.). International Narcotics Control Board. January 2007. Archived from the original (PDF) on 2008-02-27.

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