View source for Hydrochloric acid

{{short description|Aqueous solution of hydrogen chloride}}
{{about|the solution|the gas|hydrogen chloride}}
{{pp-move}}
{{Chembox
| Verifiedfields = changed
| Watchedfields  = changed
| verifiedrevid  = 477002675
| ImageFile      = Hydrochloric acid 30 percent.jpg
| ImageSize      = 150px
| ImageFile1     = Hydrochloric acid dissociated.svg
| ImageClass1    = skin-invert
| ImageName1     = Structure of Hydrochloric acid as dissociated chloride and hydronium ions
| ImageFileL1    = Hydrogen-chloride-3D-vdW.svg
| ImageNameL1    = 3D model of hydrogen chloride
| ImageFileR1    = Water molecule 3D.svg
| ImageNameR1    = 3D model of water
| ImageFileL2    = Chloride-ion-3D-vdW.png
| ImageNameL2    = 3D model of the chloride anion
| ImageFileR2    = Hydronium-3D-vdW.svg
| ImageNameR2    = 3D model of the hydronium cation
| OtherNames     = {{Unbulleted list|Muriatic acid<ref name="muriatic_acid" />|Spirits of salt<ref>{{Cite news |url=http://www.thefreedictionary.com/spirits+of+salt |title=spirits of salt |newspaper=Thefreedictionary.com |access-date=29 May 2012}}</ref>|Hydronium chloride|Chlorhydric acid
}}
| IUPACName      = Chlorane<ref>{{cite book|title=Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013| veditors = Favre HA, Powell WH |date=2014|publisher=The [[Royal Society of Chemistry]]|place=Cambridge|page=131}}</ref>
| Section1       = {{Chembox Identifiers
|UNII_Ref = {{fdacite|correct|FDA}}
|UNII = QTT17582CB
|EINECS = 231-595-7
|ChEMBL_Ref = {{ebicite|changed|EBI}}
|ChEMBL = 1231821
|CASNo_Ref = {{cascite|correct|CAS}}
|CASNo = 7647-01-0
|PubChem = 313
|SMILES = [OH3+].[Cl-]
|ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
|ChemSpiderID = 307
|UNNumber = 1789
}}
| Section2       = {{Chembox Properties
|Formula = HCl(''aq'')
|H=1|Cl=1
|Appearance = Colorless, transparent liquid, fumes in air if concentrated
|Odor = Pungent characteristic
|Density = 1.18 g/cm<sup>3</sup>
|MeltingPt = Concentration-dependent – see [[#Physical properties|table]]
|BoilingPt = Concentration-dependent – see [[#Physical properties|table]]
|pKa = −5.9 (HCl gas)<ref name="Trummal 3663–3669">{{cite journal | vauthors = Trummal A, Lipping L, Kaljurand I, Koppel IA, Leito I | title = Acidity of Strong Acids in Water and Dimethyl Sulfoxide | language = EN | journal = The Journal of Physical Chemistry A | volume = 120 | issue = 20 | pages = 3663–9 |date = May 2016 | pmid = 27115918 | doi = 10.1021/acs.jpca.6b02253 | bibcode = 2016JPCA..120.3663T | s2cid = 29697201}}</ref>
|LogP = 0.00<ref name="chemsrc">{{Cite web|url=https://www.chemsrc.com/en/cas/7647-01-0_81412.html|title=Hydrochloric acid|website=www.chemsrc.com}}</ref>
}}
| Section3       = {{Chembox Pharmacology
|ATCCode_prefix = A09
|ATCCode_suffix = AB03
|ATC_Supplemental = {{ATC|B05|XA13}}
}}
| Section4       = {{Chembox Hazards
|MainHazards = Corrosive, reactive,<br> toxic fumes
|FlashPt = Non-flammable
|GHSPictograms ={{GHS05}}{{GHS07}}<ref name="sigma">{{Sigma-Aldrich|sigald|id=258148|name=Hydrochloric acid|access-date=2022-03-16}}</ref>
|GHSSignalWord = Danger<ref name="sigma" />
|HPhrases = {{H-phrases|290|314|335}}<ref name="sigma" />
|PPhrases = {{P-phrases|234|261|271|280|303+361+353|305+351+338}}<ref name="sigma" />
|NFPA-H = 3
|NFPA-F = 0
|NFPA-R = 1
|NFPA-S = ACID
}}
| Section5       = {{Chembox Related
|OtherAnions = {{Unbulleted list|[[Hydrofluoric acid]]|[[Hydrobromic acid]]|[[Hydroiodic acid]]}}
|OtherCompounds = [[Hydrogen chloride]]
 }}
}}

'''Hydrochloric acid''', also known as '''muriatic acid''' or '''spirits of salt''', is an [[aqueous solution]] of [[hydrogen chloride]] (HCl). It is a colorless solution with a distinctive [[pungency|pungent]] smell. It is classified as a [[acid strength|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.<ref name="G&E"/><ref name=Ullmann/>

== Etymology ==
Because it was produced from [[halite|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 {{langx|de|Salzsäure}}, {{langx|nl|zoutzuur}}, {{langx|af|soutsuur}}, {{langx|sv|saltsyra}}, {{langx|fi|suolahappo}}, {{langx|es|salfumán}}, {{langx|tr|tuz ruhu}}, {{langx|pl|kwas solny}}, {{langx|hu|sósav}}, {{langx|cs|kyselina solná}}, {{langx|ja|塩酸}} (''ensan''), {{lang-zh|盐酸}} (''yánsuān''), and {{langx|ko|염산}} (''yeomsan'').

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.<ref name="muriatic_acid">{{Cite web |title=Hydrochloric Acid |url=http://www.bayermaterialsciencenafta.com/products/index.cfm?mode=grades&pp_num=EB7C4476-A4F6-7AE6-7CD78F4E6C60AA44&o_num=3 |url-status=dead |archive-url=https://web.archive.org/web/20101015233337/http://bayermaterialsciencenafta.com/products/index.cfm?mode=grades&pp_num=EB7C4476-A4F6-7AE6-7CD78F4E6C60AA44&o_num=3 |archive-date=15 October 2010 |access-date=16 September 2010}}</ref><ref>{{Cite web |year=2005 |title=Muriatic Acid |url=http://www.ppg.com/chemicals/chloralkali/Documents/english/MuriaticAcid.pdf |url-status=dead |archive-url=https://web.archive.org/web/20150702030125/http://www.ppg.com/chemicals/chloralkali/documents/english/muriaticacid.pdf |archive-date=2 July 2015 |access-date=10 September 2010 |publisher=[[PPG Industries]] |df=dmy-all}}</ref> The name ''hydrochloric acid'' was coined by the French chemist [[Joseph Louis Gay-Lussac]] in 1814.<ref>Gay-Lussac (1814) "Mémoire sur l'iode" (Memoir on iodine), ''Annales de Chemie'', '''91''' : 5–160. [https://books.google.com/books?id=Tao9AQAAMAAJ&pg=PA9 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; ...)</ref>

==History==

=== 9th–10th century ===
In the early tenth century, the Persian physician and alchemist [[Abu Bakr al-Razi]] ({{Circa|865}}–925, Latin: Rhazes) conducted experiments with [[sal ammoniac]] ([[ammonium chloride]]) and [[vitriol]] (hydrated [[sulfates]] of various metals), which he [[distillation|distilled]] together, thus producing the gas [[hydrogen chloride]].<ref>{{harvnb|Multhauf|1966|pp=141–142}}.</ref> In doing so, al-Razi may have stumbled upon a primitive method for producing hydrochloric acid,<ref>{{cite journal <!-- Citation bot bypass-->|last1=Stapleton|first1=Henry E.|author1-link=Henry Ernest Stapleton|last2=Azo|first2=R.F.|last3=Hidayat Husain|first3=M.|year=1927|title=Chemistry in Iraq and Persia in the Tenth Century A.D.|journal=Memoirs of the Asiatic Society of Bengal|volume=VIII|issue=6|pages=317–418|oclc=706947607|url=http://www.southasiaarchive.com/Content/sarf.100203/231270}} p. 333.</ref> as perhaps manifested in the following recipe from his {{Transliteration|ar|Kitāb al-Asrār}} ("The Book of Secrets"):
{{blockquote|Take equal parts of sweet salt, Bitter salt, {{Transliteration|ar|Ṭabarzad}} salt, {{Transliteration|ar|Andarānī}} salt, Indian salt, salt of {{Transliteration|ar|Al-Qilī}}, and salt of Urine. After adding an equal weight of good crystallised Sal-ammoniac, dissolve by moisture, and distil (the mixture). There will distil over a strong water, which will cleave stone ({{Transliteration|ar|sakhr}}) instantly.<ref>{{harvnb|Stapleton|Azo|Hidayat Husain|1927|p=333}} (for a glossary of the terms used in this recipe, see p. 322). German translation of the same passage in {{cite book|last1=Ruska|first1=Julius|author1-link=Julius Ruska|date=1937|title=Al-Rāzī's Buch Geheimnis der Geheimnisse. Mit Einleitung und Erläuterungen in deutscher Übersetzung|series=Quellen und Studien zur Geschichte der Naturwissenschaften und der Medizin|volume=VI|location=Berlin|publisher=Springer}} p. 182, §5. An English translation of Ruska 1937's translation can be found in {{cite book|last1=Taylor|first1=Gail Marlow|date=2015|title=The Alchemy of Al-Razi: A Translation of the "Book of Secrets"|publisher=CreateSpace Independent Publishing Platform|isbn=9781507778791}} pp. 139–140.</ref>}}
However, it appears that in most of his experiments al-Razi disregarded the gaseous products, concentrating instead on the color changes that could be effected in the residue.<ref>{{cite book|last=Multhauf|first=Robert P.|author-link=Robert P. Multhauf|year=1966|title=The Origins of Chemistry|location=London|publisher=Oldbourne|oclc=977570829}} pp. 141–142.</ref> According to [[Robert P. Multhauf]], hydrogen chloride was produced many times without clear recognition that, by dissolving it in water, hydrochloric acid may be produced.<ref>{{harvnb|Multhauf|1966|p=142}}. Multhauf refers to a number of recipes from the {{Transliteration|ar|Kitāb al-Asrār}} translated by {{harvnb|Ruska|1937|pp=103–110}}, but does not seem to have noted the existence of the recipe in {{harvnb|Ruska|1937|p=182, §5}} quoted above.</ref>

=== 11th–13th century ===
Drawing on al-Razi's experiments, the {{lang|la|De aluminibus et salibus}} ("On Alums and Salts"), an eleventh- or twelfth-century Arabic text falsely attributed to al-Razi and [[Latin translations of the 12th century|translated into Latin]] by [[Gerard of Cremona]] (1144{{Ndash}}1187), described the heating of metals with various salts, which in the case of [[mercury (element)|mercury]] resulted in the production of [[mercury(II) chloride]] (corrosive sublimate).<ref>{{harvnb|Multhauf|1966|pp=160–162}}.</ref> 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.<ref>{{harvnb|Multhauf|1966|pp=162–163}}.</ref>

=== 14th–15th century ===

==== ''Aqua regia'' ====
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 {{Circa|1300}}), where ''aqua regia'' was prepared by adding ammonium chloride to nitric acid.<ref>{{Cite journal|last1=Karpenko|first1=Vladimír|last2=Norris|first2=John A.|year=2002|title=Vitriol in the History of Chemistry|journal=Chemické listy|volume=96|issue=12|pages=997–1005|url=http://www.chemicke-listy.cz/ojs3/index.php/chemicke-listy/article/view/2266}} p. 1002.</ref> The fact that ''aqua regia'' typically is defined as a mixture of nitric acid and hydrochloric acid does not mean that hydrochloric acid was discovered before or simultaneously with ''aqua regia''. The isolation of hydrochloric acid happened about 300 years later. 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.<ref>{{harvnb|Multhauf|1966|p=204}}.</ref>

=== 16th–17th century ===

==== Isolation of hydrochloric acid ====
From the point of view of Western history of chemistry, hydrochloric acid was the last of the three well-known mineral acids for which the method of its production appeared in the literature.{{Sfn|Reti|1965|p=11}} Recipes for its production started to appear in the late sixteenth century. The earliest recipes for the production of hydrochloric acid are found in [[Giambattista della Porta|Giovanni Battista Della Porta]]'s (1535–1615) [[Magia Naturalis|''Magiae naturalis'']] ("Natural Magic") and in the works of other contemporary chemists like [[Andreas Libavius]] ({{Circa|1550}}–1616), [[Jean Beguin]] (1550–1620), and [[Oswald Croll]] ({{Circa|1563}}–1609).<ref>{{harvnb|Multhauf|1966|loc=p. 208, note 29; cf. p. 142, note 79}}.</ref> Among the historians who have written about this are German chemists [[Hermann Franz Moritz Kopp]] (1845) and [[Edmund Oscar von Lippmann]] (1938), mining engineer (and future U.S. president) [[Herbert Hoover]] with his wife geologist [[Lou Henry Hoover]] (1912), Dutch chemist [[Robert Jacobus Forbes]] (1948), American chemist [[Mary Elvira Weeks]] (1956), and British chemists [[F. Sherwood Taylor]] (1957) and [[J. R. Partington]] (1960). Italian chemist Ladislao Reti have summarized the result of their efforts thus:<ref>{{Cite journal |last=Reti |first=L. |year=1965 |title=How Old Is Hydrochloric Acid? |url=https://www.jstor.org/stable/d72c84aa-5ade-303c-87f0-621140e50237 |journal=Chymia |volume=10 |pages=11–23|doi=10.2307/27757245 |jstor=27757245 |url-access=subscription }}</ref>

{{Blockquote|text=The first clear instance of the preparation of hydrochloric acid appears in the writings of Della Porta, (1589 and 1608), Libavius (1597), pseudo-Basil (1604), van Helmont (1646) and Glauber (1648). Less convincing earlier references are found in the ''Plichto'' of Rosetti (1540) and in Agricola (1558). As for the first practical method of preparation from vitriol and common salt, there is no doubt that pseudo-Basil precedes Glauber, but the latter has the unquestionable merit of having indicated the way of producing the acid later to be adopted by the chemical industry for large-scale operations.|author=Ladislao Reti|title=How Old Is Hydrochloric Acid?}}

==== Dissolving metals ====
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.<ref>{{cite book|last=Newman|first=William R.|author-link=William R. Newman|year=2006|title=Atoms and Alchemy: Chymistry and the Experimental Origins of the Scientific Revolution|location=Chicago|publisher=University of Chicago Press|isbn=978-0226576961}} p. 98.</ref>

===Industrial developments===
During the [[Industrial Revolution]] in Europe, demand for [[alkaline]] substances increased. A new industrial process developed by [[Nicolas Leblanc]] of [[Issoudun|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]]).<ref>{{cite journal| vauthors = Ronalds BF |date=2019|title=Bonnington Chemical Works (1822-1878): Pioneer Coal Tar Company|journal=International Journal for the History of Engineering & Technology|volume=89|issue=1–2|pages=73–91|doi=10.1080/17581206.2020.1787807|s2cid=221115202}}</ref> 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.<ref name="ceh">{{Cite book |title=Chemicals Economics Handbook |chapter=Hydrochloric Acid |publisher=[[SRI International]] |year=2001 |pages=733.4000A–733.3003F}}</ref><ref name="aftalion">{{cite book | vauthors = Aftalion F |title=A History of the International Chemical Industry |location=Philadelphia |publisher=University of Pennsylvania Press |year=1991 |isbn=978-0-8122-1297-6}}</ref>

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 2000, hydrochloric acid is mostly made by absorbing by-product hydrogen chloride from [[Hydrochloric acid#Production|industrial organic compounds production]].<ref name="ceh" /><ref name="aftalion" /><ref name="G&E">{{Greenwood&Earnshaw |pages=946–48}}</ref>

==Chemical properties==
Gaseous hydrogen chloride is a molecular compound with a covalent bond between the hydrogen and chlorine atoms. In aqueous solutions dissociation is complete, with the formation of chloride ions and hydrated hydrogen ions ([[hydronium ion]]s).<ref>{{cite book |last1 = Petrucci |first1 = Ralph H. |last2 = Harwood |first2 = William S. |last3 = Herring |first3 = F. Geoffrey |date=2002 |title = General chemistry: principles and modern applications |publisher=Prentice Hall |isbn = 978-0-13-014329-7 |pages=668–669}}</ref> A combined IR, Raman, X-ray, and neutron diffraction study of concentrated hydrochloric acid showed that the hydronium ion forms [[hydrogen bond]]ed complexes with other water molecules.<ref>{{cite journal | vauthors = Agmon N |date=January 1998 |title=Structure of Concentrated HCl Solutions |journal=The Journal of Physical Chemistry A |volume=102 |issue=1 |pages=192–199 |doi=10.1021/jp970836x |issn=1089-5639 |citeseerx=10.1.1.78.3695 |bibcode=1998JPCA..102..192A }}</ref>  (See [[Hydronium]] for further discussion of this issue.)

The p''K''<sub>a</sub> value of hydrochloric acid in aqueous solution is estimated theoretically to be −5.9.<ref name="Trummal 3663–3669"/> A solution of hydrogen chloride in water behaves as a [[strong acid]]: the concentration of HCl molecules is effectively zero.

==Physical properties==
{| class="wikitable" style="margin: 0 auto; text-align: center;"
|-
! Mass <br/>fraction
! colspan=2| [[Concentration]]
! [[Density]]
! [[Molarity]]
! [[pH]]
! [[Viscosity]]
! [[Specific heat capacity|Specific<br />heat]]
! [[Vapor pressure|Vapour<br />pressure]]
! [[Boiling point|Boiling<br />point]]
! [[Melting point|Melting<br />point]]
|-
! kg&nbsp;HCl/kg&nbsp;
! kg&nbsp;HCl/m<sup>3</sup>
! [[Baumé scale|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 || 11.64 || −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
|-
| colspan=11|The reference temperature and [[pressure]] for the above table are 20&nbsp;°C and 1 atmosphere (101.325&nbsp;kPa).<br />Vapour pressure values are taken from the ''International Critical Tables'' and refer to the total vapour pressure of the solution.
|}

[[File:Phase diagram HCl H2O s l.PNG|thumb|Melting temperature as a function of HCl concentration in water<ref>{{Cite book |title=Gmelins Handbuch der Anorganischen Chemie |chapter=Systemnummer 6 Chlor |publisher=Chemie Berlin |year=1927}}</ref><ref>{{Cite book |title=Gmelins Handbuch der Anorganischen Chemie |chapter=Systemnummer 6 Chlor, Ergänzungsband Teil B&nbsp;– Lieferung 1 |publisher=Chemie Weinheim |year=1968}}</ref>]]
[[physical property|Physical properties]] of hydrochloric acid, such as [[boiling point|boiling]] and [[melting point]]s, [[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.<ref name="crc" /><ref name="perry" /><ref name="aspen">{{Cite book |publisher=Aspen Technology |title=Aspen Properties |series=binary mixtures modeling software |edition=calculations by Akzo Nobel Engineering |year = 2002–2003}}</ref>

Hydrochloric acid as the binary (two-component) mixture of HCl and H<sub>2</sub>O has a constant-boiling [[azeotrope]] at 20.2% HCl and {{Convert|108.6|C|K F}}. There are four constant-[[crystallization]] [[eutectic|eutectic points]] for hydrochloric acid, between the [[crystal]] form of [H<sub>3</sub>O]Cl (68% HCl), [H<sub>5</sub>O<sub>2</sub>]Cl (51% HCl), [H<sub>7</sub>O<sub>3</sub>]Cl (41% HCl), [H<sub>3</sub>O]Cl·5H<sub>2</sub>O (25% HCl), and ice (0% HCl). There is also a metastable eutectic point at 24.8% between ice and the [H<sub>7</sub>O<sub>3</sub>]Cl crystallization.<ref name="aspen" /> They are all [[Hydronium#Solid hydronium salts|hydronium salts]].

==Production==
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 [[chemical industry|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.<ref name="crc">{{Cite book | vauthors = Lide D |title = CRC Handbook of Chemistry and Physics |publisher=[[CRC Press]] |edition=81st |year=2000 |isbn=978-0-8493-0481-1}}</ref><ref name="perry">{{cite book | vauthors = Perry R, Green D, Maloney J |title=Perry's Chemical Engineers' Handbook |publisher=[[McGraw-Hill]] Book Company |edition=6th |year=1984 |isbn=978-0-07-049479-4 |title-link=Perry's Chemical Engineers' Handbook }}</ref>

Hydrogen chloride is produced by combining [[chlorine]] and [[hydrogen]]:

:{{chem2|Cl2 + H2 -> 2 HCl}}

As the reaction is [[Exothermic reaction|exothermic]], the installation is called an HCl [[oven]] or HCl burner. The resulting hydrogen chloride gas is [[Absorption (chemistry)|absorbed]] in [[deionized water]], resulting in chemically pure hydrochloric acid. This reaction can give a very pure product, e.g. for use in the food industry.

===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.<ref name="ceh" /> 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 Company|Dow Chemical]] at 2&nbsp;million tonnes annually (Mt/year), calculated as HCl gas, [[Georgia Gulf|Georgia Gulf Corporation]], [[Tosoh|Tosoh Corporation]], [[Akzo Nobel]], and [[Tessenderlo Group|Tessenderlo]] at 0.5 to 1.5&nbsp;Mt/year each. Total world production, for comparison purposes expressed as HCl, is estimated at 20&nbsp;Mt/year, with 3&nbsp;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&nbsp;Mt/year.<ref name="ceh" />

==Applications==
{{main|Hydrogen chloride}}
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.<ref name="ceh" /> Hydrogen chloride, not hydrochloric acid, is used more widely in industrial organic chemistry, e.g. for [[vinyl chloride]] and [[1,2-Dichloroethane|dichloroethane]].<ref name=Ullmann>{{cite book |doi=10.1002/14356007.a13_283|title=Hydrochloric Acid|year=2000|last1=Austin|first1=Severin|last2=Glowacki|first2=Arndt|isbn=3527306730}}</ref>

===Pickling of steel===
One of the most important applications of hydrochloric acid is in the [[pickling (metal)|pickling]] of steel, to remove rust or [[iron oxide]] scale from iron or steel before subsequent processing, such as [[extrusion]], [[rolling (metalworking)|rolling]], [[galvanization|galvanizing]], and other techniques.<ref name="ceh"/><ref name="G&E" /> Technical quality HCl at typically 18% concentration is the most commonly used pickling agent for the pickling of [[carbon steel]] grades.
:{{chem2|Fe3O4 + Fe + 8 HCl -> 4 FeCl2 + 4 H2O}}

The [[spent acid]] has long been reused as [[iron(II) chloride]] (also known as ferrous chloride) solutions, but high [[heavy metals|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:<ref name="ceh"/>
:{{chem2|4 FeCl2 + 4 H2O + O2 -> 8 HCl + 2 Fe2O3}}

By recuperation of the spent acid, a closed acid loop is established.<ref name="G&E" /> The [[iron(III) oxide]] by-product of the regeneration process is valuable, used in a variety of secondary industries.<ref name="ceh"/>

===Production of inorganic compounds===
Akin to its use for pickling, hydrochloric acid is used to dissolve many metals, metal oxides and metal carbonates.  The conversions are often depicted in simplified equations:
:{{chem2|Zn + 2 HCl -> ZnCl2 + H2}}
:{{chem2|NiO + 2 HCl -> NiCl2 + H2O}}
:{{chem2|CaCO3 + 2 HCl -> CaCl2 + CO2 + H2O}}
These processes are used to produce metal chlorides for analysis or further production.<ref name="crc" /><ref name="perry" /><ref name="G&E" />

===pH control and neutralization===
Hydrochloric acid can be used to regulate the [[acidity]] ([[pH]]) of solutions.
:{{chem2|HO- + HCl -> H2O + Cl-}}

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 [[acid-base reaction theories|neutralizing]] waste streams and swimming pool pH control.<ref name="G&E" />

===Regeneration of ion exchangers===
High-quality hydrochloric acid is used in the regeneration of [[ion exchange resin]]s. [[Ion exchange|Cation exchange]] is widely used to remove [[ion]]s such as Na<sup>+</sup> and Ca<sup>2+</sup> from [[aqueous]] solutions, producing [[demineralized water]]. The acid is used to rinse the cations from the resins.<ref name="ceh"/> Na<sup>+</sup> is replaced with H<sup>+</sup> and Ca<sup>2+</sup> with 2 H<sup>+</sup>.

Ion exchangers and demineralized water are used in all chemical industries, drinking water production, and many food industries.<ref name="ceh"/>

===Laboratory use===
[[File:Hydrochloric acid 30-33% by Danny S. - 001.jpg|thumb|125px|30% hydrochloric acid]]
Of the common strong [[mineral acid]]s in chemistry, hydrochloric acid is the [[Monobasic acid|monoprotic]] acid least [[oxidizing acid|likely to undergo]] an interfering [[redox|oxidation-reduction]] reaction. It is one of the least hazardous strong acids to handle; despite its acidity, it contains 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 [[base (chemistry)|bases]]. Strong acid titrants give more precise results due to a more distinct endpoint. [[Azeotrope|Azeotropic]], or "constant-boiling", hydrochloric acid (roughly 20.2%) can be used as a [[primary standard]] in [[chemical analysis|quantitative analysis]], although its exact concentration depends on the [[atmospheric pressure]] when it is prepared.<ref>{{Cite book | vauthors = Mendham J, Denney RC, Barnes JD, Thomas MJ, Denney RC, Thomas MJ |year = 2000 |title = Vogel's Quantitative Chemical Analysis |edition = 6th |location = New York |publisher = Prentice Hall |isbn = 978-0-582-22628-9}}</ref>

===Other===
Hydrochloric acid is used for a large number of small-scale applications, such as leather processing, household cleaning,<ref>{{Cite news |url=https://www.telegraph.co.uk/property/3317144/Household-plc-really-filthy-bathrooms.html |archive-url=https://web.archive.org/web/20090223230226/http://www.telegraph.co.uk/property/3317144/Household-plc-really-filthy-bathrooms.html |url-status=dead |archive-date=23 February 2009 |title=Household plc: really filthy bathroom |date=13 September 2003 |work=[[The Daily Telegraph]] |location=London | vauthors = Simhon R |access-date=31 March 2010}}</ref> and building construction.<ref name="G&E" /> [[petroleum#Extraction|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.<ref name='ceh'/>

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:
:{{chem2|CaCO3 + 2 HCl -> CaCl2 + CO2 + H2O}}

Many chemical reactions involving hydrochloric acid are applied in the production of food, food ingredients, and [[food additive]]s. 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.<ref name='ceh'/><ref name="G&E" />

==Presence in living organisms==
[[File:Stomach mucosal layer labeled.svg|left|Diagram of alkaline [[mucous membrane|mucous layer]] in stomach with mucosal defense mechanisms|thumb]]
[[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.<ref name=maton>{{Cite book | vauthors = Maton A, Hopkins J, McLaughlin CW, Johnson S, Warner MQ, LaHart D, Wright JD |title=Human Biology and Health |publisher=Prentice Hall |year=1993 |location=Englewood Cliffs, New Jersey, USA |isbn=978-0-13-981176-0 |url-access=registration |url=https://archive.org/details/humanbiologyheal00scho }}</ref><ref>{{cite web |title=Digestive Aids: Hydrochloric acid |url=http://www.healthy.net/scr/article.aspx?id=1863 |website=healthy.net | vauthors = Haas E |date = 6 December 2000}}</ref> Chloride (Cl<sup>−</sup>) and hydrogen (H<sup>+</sup>) ions are secreted separately in the [[stomach fundus]] region at the top of the stomach by [[parietal cell]]s of the [[gastric mucosa]] into a secretory network called [[canaliculus (parietal cell)|canaliculi]] before it enters the stomach lumen.<ref name="arthur">{{cite book | vauthors = Arthur C, Guyton MD, Hall JE |title=Textbook of Medical Physiology |publisher=W.B. [[Saunders (imprint)|Saunders]] Company |edition=10th |year=2000 |isbn=978-0-7216-8677-6}}</ref>

Gastric acid acts as a barrier against [[microorganism]]s to prevent infections and is important for the digestion of food. Its low pH [[denaturation (biochemistry)|denatures]] [[protein]]s and thereby makes them susceptible to degradation by [[digestive enzyme]]s 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 [[sodium bicarbonate|bicarbonate]].<ref name='maton'/>

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 ulcer]]s 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 [[antacid]]s are used to neutralize excessive existing acid.<ref name='maton'/><ref>{{Cite web |url=http://www.vivo.colostate.edu/hbooks/pathphys/endocrine/gi/secretin.html |title=Control and Physiologic Effects of Secretin |publisher=Colorado State University |vauthors=Bowen R |date=18 March 2003 |access-date=16 March 2009 |archive-date=16 August 2018 |archive-url=https://web.archive.org/web/20180816212554/http://vivo.colostate.edu/hbooks/pathphys/endocrine/gi/secretin.html |url-status=dead }}</ref>

Hydrochloric acid is also used by [[osteoclasts]] alongside [[proteases]] for [[bone resorption]].<ref>{{cite journal |last1=Väänänen |first1=H K |last2=Zhao |first2=H |last3=Mulari |first3=M |last4=Halleen |first4=J M |title=The cell biology of osteoclast function |journal=Journal of Cell Science |date=February 2000 |volume=113 |issue=3 |pages=377–81 |doi=10.1242/jcs.113.3.377 |pmid=10639325 |url=https://journals.biologists.com/jcs/article/113/3/377/26281/The-cell-biology-of-osteoclast-function |access-date=13 April 2024|url-access=subscription }}</ref>

==Safety==
<div style="float: right; margin-left: 0.5em">[[File:UN transport pictogram - 8.svg|100px|alt=A rhombic-shaped label with letters 8 and "corrosive", indicating that drops of a liquid corrode materials and human hands.]] [[File:GHS-pictogram-acid.svg|100px|alt=Causes severe skin burns and eye damage.]] [[File:GHS-pictogram-exclam.svg|100px|alt=May cause respiratory irritation.]]
</div>

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.<ref name="Ullmann" />

Vapors or [[mist]]s are a respiratory hazard, which can be partially mitigated by use of a [[respirator]] equipped with cartridges specifically designed to capture hydrochloric acid. Airborne acid is an irritant to the eyes, and may require the use of protective goggles or a facemask.{{citation needed|date=November 2023}}

{| class="wikitable" style="float: center; clear: right;"
|-
! [[Mass fraction (chemistry)|Mass <br/>fraction]]
! Classification<ref>{{Cite web |title=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 |url=https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:32008R1272&qid=1568410668462&from=EN |publisher=EUR-lex |access-date = 16 December 2008}}</ref>
! [[GHS hazard statements|List of <br />H-phrases]]
|-
| 10% ≤ C < 25%
| Causes skin irritation, Causes serious eye irritation, 
| {{H-phrases|315|319}}
|-
| C ≥ 10%
| May cause respiratory irritation
| {{H-phrases|335}}
|-
| C ≥ 25%
| Causes severe skin burns and eye damage
| {{H-phrases|314}}
|}

== Legal status ==
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]].<ref name="incb">{{Cite book|publisher=[[International Narcotics Control Board]] |url=http://www.incb.org/pdf/e/list/red.pdf |title=List of precursors and chemicals frequently used in the illicit manufacture of narcotic drugs and psychotropic substances under international control |issue=Annex to Form D ("Red List") |edition=Eleventh |date=January 2007 |url-status=dead |archive-url=https://web.archive.org/web/20080227224025/http://www.incb.org/pdf/e/list/red.pdf |archive-date=2008-02-27 }}</ref><ref>{{Cite report |url=https://www.unodc.org/documents/india/ccch2.pdf |title=Chemicals Required for the Illicit Manufacture of Drugs |date=1998 |publisher=UNDOC |access-date=July 20, 2022}}</ref>

== See also ==
* [[Chloride]], inorganic salts of hydrochloric acid
* [[Hydrochloride]], organic salts of hydrochloric acid from sodium hydroxide
* [[Aqua regia]]

== References ==
{{reflist}}

== External links ==
{{commons category|Hydrochloric acid}}
{{Spoken Wikipedia|Hydrochloric acid.ogg|date=April 23, 2005}}

* [http://webbook.nist.gov/ NIST WebBook, general link]
* [http://www.periodicvideos.com/videos/mv_HCl1.htm Hydrochloric Acid – Part One] and [http://www.periodicvideos.com/videos/mv_HCl2.htm Hydrochloric Acid – Part Two] at ''[[The Periodic Table of Videos]]'' (University of Nottingham)
* Calculators: [http://www.aim.env.uea.ac.uk/aim/surftens/surftens.php surface tensions], and [http://www.aim.env.uea.ac.uk/aim/density/density_electrolyte.php densities, molarities and molalities] of aqueous HCl

=== General safety information ===
* [https://web.archive.org/web/20040824093117/http://www.epa.gov/ttn/atw/hlthef/hydrochl.html EPA Hazard Summary]
* [https://web.archive.org/web/20030818121338/http://grover.mirc.gatech.edu/data/msds/50.html Hydrochloric acid MSDS by Georgia Institute of Technology]
* [https://www.cdc.gov/niosh/npg/npgd0332.html NIOSH Pocket Guide to Chemical Hazards]

=== Pollution information ===
* [http://www.npi.gov.au/substances/hydrochloric-acid/index.html National Pollutant Inventory – Hydrochloric Acid Fact Sheet]

{{Digestives}}
{{Chlorides}}
{{Blood substitutes and perfusion solutions}}
{{Authority control}}

{{DEFAULTSORT:Hydrochloric Acid}}
[[Category:Mineral acids]]
[[Category:Alchemical substances]]
[[Category:Chlorides]]
[[Category:Nonmetal halides]]
[[Category:Photographic chemicals]]
[[Category:Stomach]]
[[Category:Solutions]]
[[Category:E-number additives]]
[[Category:Chlorine-containing natural products]]