View source for Hydrogen chloride

{{Use dmy dates|date=December 2023}}{{Use American English|date=December 2023}}
{{Redirect|HCl|the aqueous solution|Hydrochloric acid|5=HCL (disambiguation)}}
{{Chembox
| Verifiedfields = changed
| Watchedfields  = changed
| verifiedrevid  = 477163297
| ImageFileL1    = Hydrogen-chloride-2D-dimensions.svg
| ImageFileL1_Ref = {{chemboximage|correct|??}}
| ImageNameL1    = Skeletal formula of hydrogen chloride with a dimension
| ImageFileR1    = Hydrogen-chloride-3D-vdW.svg
| ImageFileR1_Ref = {{chemboximage|correct|??}}
| ImageNameR1    = Space-filling model of hydrogen chloride with atom symbols
| IUPACName      = Hydrogen chloride<ref>{{Cite web|url = https://www.ebi.ac.uk/chebi/searchId.do?chebiId=17883|title = hydrogen chloride (CHEBI:17883)|work = Chemical Entities of Biological Interest (ChEBI)|location = UK|publisher = European Bioinformatics Institute}}</ref>
| OtherNames     = Hydrochloric acid gas<br>Hydrochloric gas<br>Hydrochloride
| SystematicName = Chlorane<ref>{{cite book|title=Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013|editor-last=Favre |editor-first=Henri A. |editor2-first=Warren H.|editor2-last=Powell|date=2014|publisher=The [[Royal Society of Chemistry]]|place=Cambridge|isbn=9781849733069 | page=131}}</ref>
| Section1       = {{Chembox Identifiers
| CASNo = 7647-01-0
| PubChem = 313
| CASNo_Ref = {{cascite|correct|CAS}}
| ChemSpiderID = 307
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| UNII = QTT17582CB
| UNII_Ref = {{fdacite|correct|FDA}}
| EINECS = 231-595-7
| UNNumber = 1050
| KEGG = D02057
| KEGG_Ref = {{keggcite|correct|kegg}}
| MeSHName = Hydrochloric+acid
| ChEBI_Ref = {{ebicite|correct|EBI}}
| ChEBI = 17883
| ChEMBL = 1231821
| ChEMBL_Ref = {{ebicite|changed|EBI}}
| RTECS = MW4025000
| Beilstein = 1098214
| Gmelin = 322
| SMILES = Cl
| StdInChI = 1S/HCl/h1H
| StdInChI_Ref = {{stdinchicite|changed|chemspider}}
| InChI = 1/HCl/h1H
| StdInChIKey = VEXZGXHMUGYJMC-UHFFFAOYSA-N
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
| InChIKey = VEXZGXHMUGYJMC-UHFFFAOYAT
}}
| Section2       = {{Chembox Properties
| Formula = HCl
| Appearance = Colorless gas
| Odor = pungent; sharp and burning
| MolarMass = 36.46 g/mol
| Density = 1.49 g/L<ref>{{CRC91|page=4–67}}</ref>
| MeltingPtC = −114.22
| BoilingPtC = −85.05
| ConjugateAcid = [[Chloronium]]
| ConjugateBase = [[Chloride]]
| Solubility = 823 g/L (0&nbsp;°C) <br> 720 g/L (20&nbsp;°C) <br> 561 g/L (60&nbsp;°C)
| SolubleOther = soluble in [[methanol]], [[ethanol]], [[diethyl ether|ether]] and [[water]]
| VaporPressure = 4352 kPa (at 21.1&nbsp;°C)<ref>[http://encyclopedia.airliquide.com/Encyclopedia.asp?GasID=17 Hydrogen Chloride]. Gas Encyclopaedia. Air Liquide</ref>
| pKa = −3.0;<ref>Tipping, E.(2002) ''[https://books.google.com/books?id=Qq25R0E5KIsC]''. Cambridge University Press, 2004.</ref> −5.9 (±0.4)<ref>Trummal, A.; Lipping, L.; Kaljurand, I.; Koppel, I. A.; Leito, I. "Acidity of Strong Acids in Water and Dimethyl Sulfoxide"  ''J. Phys. Chem. A''. '''2016''', ''120'', 3663-3669. {{doi|10.1021/acs.jpca.6b02253}}</ref>
| pKb = 17.0
| RefractIndex = 1.0004456 (gas) <br> 1.254 (liquid)
| Viscosity = 0.311 cP (−100&nbsp;°C)
 }}
| Section3       = {{Chembox Structure
| MolShape = linear
| Dipole = 1.05 D
}}
| Section4       = {{Chembox Thermochemistry
| DeltaHf = −92.31 kJ/mol
| DeltaHc = −95.31 kJ/mol
| Entropy = 186.902 J/(K·mol)
| HeatCapacity = 0.7981 J/(K·g)
}}
| Section5       = {{Chembox Pharmacology
| ATCCode_prefix = A09
| ATCCode_suffix = AB03
| ATC_Supplemental = {{ATC|B05|XA13}}
}}
| Section6       = {{Chembox Hazards
| MainHazards= Toxic, corrosive
| ExternalSDS = [http://hazard.com/msds/mf/baker/baker/files/h3880.htm JT Baker MSDS]
| GHSPictograms = {{GHS corrosion}} {{GHS skull and crossbones}}
| GHSSignalWord = Danger
| HPhrases = {{H-phrases|314|331}}
| PPhrases = {{P-phrases|261|280|303+361+353|304+340+310|305+351+338|410+403}}
| NFPA-H = 3
| NFPA-F = 0
| NFPA-R = 1
| NFPA-S = COR
| LD50 = 238 mg/kg (rat, oral)
| IDLH = 50 ppm<ref name=PGCH>{{PGCH|0332}}</ref>
| REL = C 5&nbsp;ppm (7&nbsp;mg/m<sup>3</sup>)<ref name=PGCH/>
| PEL = C 5&nbsp;ppm (7&nbsp;mg/m<sup>3</sup>)<ref name=PGCH/>
| LC50 = 3124 ppm (rat, 1&nbsp;[[hour|h]])<br />1108 ppm (mouse, 1&nbsp;h)<ref name=IDLH>{{IDLH|7647010|Hydrogen chloride}}</ref>
| LCLo = 1300 ppm (human, 30&nbsp;[[minute|min]])<br />4416 ppm (rabbit, 30&nbsp;min)<br />4416 ppm (guinea pig, 30&nbsp;min)<br />3000 ppm (human, 5&nbsp;min)<ref name=IDLH/>
}}
| Section7       = {{Chembox Related
| OtherCompounds = [[Hydrogen fluoride]]<br />[[Hydrogen bromide]]<br />[[Hydrogen iodide]]<br />[[Hydrogen astatide]]
}}
}}

The [[Chemical compound|compound]] '''hydrogen chloride''' has the [[chemical formula]] {{chem2|HCl}} and as such is a [[hydrogen halide]]. At [[room temperature]], it is a colorless [[gas]], which forms white fumes of [[hydrochloric acid]] upon contact with atmospheric [[water vapor]]. Hydrogen chloride gas and hydrochloric acid are important in technology and industry. Hydrochloric acid, the [[aqueous solution]] of hydrogen chloride, is also commonly given the formula HCl.

== Reactions ==
[[File:Hydrochloric acid 01.jpg|thumb|150px|left|Hydrochloric acid fumes turning pH paper<!-- The orange section of the strip means it's pH paper, not litmus paper. Litmus paper is red or blue--> red showing that the fumes are acidic]]
<!-- use this to start full-width paragraph:<br style="clear:both;"> -->
Hydrogen chloride is a [[diatomic molecule]], consisting of a [[hydrogen]] [[atom]] H and a [[chlorine]] atom Cl connected by a [[Polar-covalent bond|polar covalent bond]]. The chlorine atom is much more [[Electronegativity|electronegative]] than the hydrogen atom, which makes this bond polar. Consequently, the molecule has a large [[Molecular dipole moment|dipole moment]] with a negative [[partial charge]] (δ−) at the chlorine atom and a positive partial charge (δ+) at the hydrogen atom.<ref>{{cite book|author1=Ouellette, Robert J. |author2=Rawn, J. David |title=Principles of Organic Chemistry|url=https://books.google.com/books?id=U-CoBAAAQBAJ&pg=PA6|date=2015|publisher=Elsevier Science|isbn=978-0-12-802634-2|pages=6–}}</ref> In part because of its high polarity, HCl is very [[soluble]] in [[water]] (and in other polar [[solvent]]s).

Upon contact, {{chem2|H2O}} and HCl combine to form [[hydronium]] [[cation]]s {{chem2|[H3O]+}} and [[chloride]] [[anion]]s {{chem2|Cl−}} through a reversible [[chemical reaction]]:
:{{chem2|HCl + H2O → [H3O]+ + Cl−}}

The resulting solution is called [[hydrochloric acid]] and is a [[strong acid]]. The [[acid dissociation constant|acid dissociation]] or ionization constant, ''K''<sub>a</sub>, is large, which means HCl dissociates or ionizes practically completely in water. Even in the absence of water, hydrogen chloride can still act as an acid. For example, hydrogen chloride can dissolve in certain other solvents such as [[methanol]]:
:{{chem2|HCl + CH3OH → [CH3OH2]+ + Cl−}}

Hydrogen chloride can [[protonation|protonate]] molecules or ions and can also serve as an acid-[[catalyst]] for chemical reactions where [[anhydrous]] (water-free) conditions are desired.

Because of its acidic nature, hydrogen chloride is a [[corrosive substance]], particularly in the presence of moisture.

=== Structure and properties ===
{{multiple image
| align             = left
| image1            = DCl Neutron powder.png
| width1            = 200
| alt1              = 
| caption1          = 
| image2            = DCl Neutron powder, side.png
| width2            = 160
| alt2              = 
| caption2          = 
| footer            = The structure of solid DCl, as determined by [[neutron diffraction]] of DCl powder at 77&nbsp;K. DCl was used instead of HCl because the deuterium nucleus is easier to detect than the hydrogen nucleus. The extended linear structure is indicated by the dashed lines.
| direction         = 
| total_width       = 
}}

Frozen HCl undergoes a phase transition at {{cvt|98.4|K}}. X-ray powder diffraction of the frozen material shows that the material changes from an [[Orthorhombic crystal system|orthorhombic]] structure to a [[Cubic crystal system|cubic]] one during this transition. In both structures the chlorine atoms are in a [[Cubic crystal system#Bravais lattices|face-centered array]]. However, the hydrogen atoms could not be located.<ref>{{Cite journal
| last = Natta
| first = G.
| title = Struttura e polimorfismo degli acidi alogenidrici
| journal = Gazzetta Chimica Italiana
| volume = 63
| pages = 425–439
| year = 1933
| language = it }}
</ref> Analysis of spectroscopic and dielectric data, and determination of the structure of DCl (deuterium chloride) indicates that HCl forms zigzag chains in the solid, as does [[Hydrogen fluoride|HF]] (see figure on right).<ref>{{Cite journal
| last1 = Sándor
| first1 = E.
| author2-link=Robin F. C. Farrow
| last2 = Farrow
| first2 = R. F. C.
| title = Crystal Structure of Solid Hydrogen Chloride and Deuterium Chloride
| journal = Nature
| volume = 213
| pages = 171–172
| year = 1967
| doi = 10.1038/213171a0
| issue=5072|bibcode = 1967Natur.213..171S | s2cid = 4161132
}}
</ref>

{|class="wikitable" style="text-align:center"
|+Solubility of HCl (g/L) in common solvents<ref>[https://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=313#x27 Hydrochloric Acid – Compound Summary]. Pubchem</ref>
|-
! Temperature (°C) || 0 || 20 || 30 || 50
|-
| Water|| 823 || 720 || 673 || 596
|-
| Methanol || 513 || 470 || 430 ||
|-
| Ethanol|| 454 || 410 || 381 ||
|-
| Ether || 356 || 249 || 195 ||
|}

[[File:HCl2.jpg|thumb|left|320px|Infrared (IR) absorption spectrum]]
[[File:HCl37.JPG|thumb|left|320px|One doublet in the IR spectrum resulting from the isotopic composition of chlorine]]
The [[infrared spectrum]] of gaseous hydrogen chloride, shown on the left, consists of a number of sharp absorption lines grouped around 2886&nbsp;cm<sup>−1</sup> (wavelength ~3.47&nbsp;μm). At room temperature, almost all molecules are in the ground vibrational state ''v''&nbsp;=&nbsp;0. Including anharmonicity the vibrational energy can be written as:
::<math>E_\mathrm{v}= h\nu_e\left(v + \tfrac{1}{2}\right)+ hx_e\nu_e\left(v+\tfrac{1}{2}\right)^2</math>

To promote an HCl molecule from the ''v''&nbsp;=&nbsp;0 to the ''v''&nbsp;=&nbsp;1 state, we would expect to see an infrared absorption about ''ν''<sub>o</sub>&nbsp;=&nbsp;''ν''<sub>e</sub>&nbsp;+&nbsp;2''x''<sub>e</sub>''ν''<sub>e</sub> =&nbsp;2880&nbsp;cm<sup>−1</sup>. However, this absorption corresponding to the Q-branch is not observed due to it being forbidden by symmetry. Instead, two sets of signals (P- and R-branches) are seen owing to a simultaneous change in the rotational state of the molecules. Because of quantum mechanical selection rules, only certain rotational transitions are permitted. The states are characterized by the rotational quantum number ''J''&nbsp;= 0, 1, 2, 3, ... selection rules state that Δ''J'' is only able to take values of ±1.
::<math>E(J)_\mathrm{rot} = h \cdot B \cdot J(J+1)</math>
The value of the rotational constant ''B'' is much smaller than the vibrational one ''ν''<sub>o</sub>, such that a much smaller amount of energy is required to rotate the molecule; for a typical molecule, this lies within the microwave region. However, the vibrational energy of HCl molecule places its absorptions within the infrared region, allowing a spectrum showing the rovibrational transitions of this molecule to be easily collected using an [[infrared spectrometer]] with a gas cell. The latter can even be made of quartz as the HCl absorption lies in a window of transparency for this material.

Naturally abundant chlorine consists of two isotopes, <sup>35</sup>Cl and <sup>37</sup>Cl, in a ratio of approximately 3:1. While the spring constants are nearly identical, the disparate [[reduced mass]]es of H<sup>35</sup>Cl and H<sup>37</sup>Cl cause measurable differences in the rotational energy, thus doublets are observed on close inspection of each absorption line, weighted in the same ratio of 3:1.
{{clear|left}}

== Production ==
Most hydrogen chloride produced on an industrial scale is used for [[hydrochloric acid]] production.<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>

===Historical routes===
In the 17th century, [[Johann Rudolf Glauber]] from [[Karlstadt am Main|Karlstadt am Main, Germany]] used [[sodium chloride|sodium chloride salt]] and [[sulfuric acid]] for the preparation of [[sodium sulfate]] in the [[Mannheim process]], releasing hydrogen chloride. [[Joseph Priestley]] of [[Leeds, England]] prepared pure hydrogen chloride in 1772,<ref>{{cite journal | vauthors = Priestley J |year=1772 |doi=10.1098/rstl.1772.0021 |title=Observations on different kinds of air [i.e., gases] |journal=Philosophical Transactions of the Royal Society of London |volume=62 |pages=147–264 (234–244) |s2cid=186210131 |url=https://books.google.com/books?id=aBxWAAAAYAAJ&pg=PA234}}</ref> and by 1808 [[Humphry Davy]] of [[Penzance]], England had proved that the chemical composition included [[hydrogen]] and [[chlorine]].<ref>{{cite journal | vauthors = Davy H |year=1808 |title=Electro-chemical researches, on the decomposition of the earths; with observations on the metals obtained from the alkaline earths, and on the amalgam procured from ammonia |journal=Philosophical Transactions of the Royal Society of London |volume=98 |pages=333–370 |url=https://books.google.com/books?id=Kg9GAAAAMAAJ&pg=PA343 |quote= p. 343: When potassium was heated in muriatic acid gas [i.e., gaseous hydrogen chloride], as dry as it could be obtained by common chemical means, there was a violent chemical action with ignition; and when the potassium was in sufficient quantity, the muriatic acid gas wholly disappeared, and from one-third to one-fourth of its volume of hydrogene was evolved, and muriate of potash [i.e., potassium chloride] was formed. (The reaction was: 2HCl + 2K → 2KCl + H<sub>2</sub>) |doi=10.1098/rstl.1808.0023|doi-access= |bibcode=1808RSPT...98..333D |s2cid=96364168 }}</ref>

=== Direct synthesis ===
[[File:Hydrochloric Acid Burner Flame.ogg|thumb|Flame inside HCl oven]]
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.

The reaction can also be triggered by blue light.<ref>{{Cite AV media |first= Chris |last= Cramer |author-link= Christopher J. Cramer |title= Hydrogen Chloride Cannon |url= https://www.youtube.com/watch?v=BoC8LrNdnOc }}</ref>

=== Organic synthesis ===
The industrial production of hydrogen chloride is often integrated with the formation of [[halogenation|chlorinated]] and [[fluorine|fluorinated]] [[organic chemistry|organic]] compounds, e.g., [[Teflon]], [[Freon]], and other [[Chlorofluorocarbon|CFC]]s, as well as [[chloroacetic acid]] and [[polyvinyl chloride|PVC]]. Often this production of hydrochloric acid is integrated with captive use of it on-site. In the [[chemical reaction]]s, hydrogen [[atom]]s on the hydrocarbon are replaced by chlorine atoms, whereupon the released hydrogen atom recombines with the spare atom from the chlorine molecule, forming hydrogen chloride. Fluorination is a subsequent chlorine-replacement reaction, producing again hydrogen chloride:
:{{chem2|RH + Cl2 → RCl + HCl}}
:RCl + HF → RF + HCl
The resulting hydrogen chloride is either reused directly or absorbed in water, resulting in hydrochloric acid of technical or industrial grade.

=== Laboratory methods ===
Small amounts of hydrogen chloride for laboratory use can be generated in an ''HCl [[Gas generator|generator]]'' by dehydrating hydrochloric acid with either [[sulfuric acid]] or anhydrous [[calcium chloride]]. Alternatively, HCl can be generated by the reaction of sulfuric acid with sodium chloride:<ref>{{cite journal|title = A Convenient Way To Generate Hydrogen Chloride in the Freshman Lab|author = Francisco J. Arnsliz|journal = [[J. Chem. Educ.]]|year = 1995|pages = 1139|issue = 12|volume = 72|url = http://jchemed.chem.wisc.edu/journal/Issues/1995/Dec/abs1139.html|doi = 10.1021/ed072p1139|bibcode = 1995JChEd..72.1139A|access-date = 6 May 2009|archive-date = 24 September 2009|archive-url = https://web.archive.org/web/20090924082100/http://jchemed.chem.wisc.edu/Journal/issues/1995/Dec/abs1139.html|url-status = dead}}</ref>
:{{chem2|NaCl + H2SO4 → [[Sodium bisulfate|NaHSO4]] + HCl[[Gas evolution reaction|↑]]}}

This reaction occurs at room temperature. Provided there is NaCl remaining in the generator and it is heated above 200&nbsp;°C, the reaction proceeds further:
:{{chem2|NaCl + NaHSO4 → [[Sodium sulfate|Na2SO4]] + HCl↑}}
For such generators to function, the reagents should be dry.

Hydrogen chloride can also be prepared by the [[hydrolysis]] of certain reactive chloride compounds such as [[phosphorus chlorides]], [[thionyl chloride]] ({{chem2|SOCl2}}), and [[acyl chloride]]s. For example, cold water can be gradually dripped onto [[phosphorus pentachloride]] ({{chem2|PCl5}}) to give HCl:
:{{chem2|PCl5 + H2O → [[Phosphoryl chloride|POCl3]] + 2 HCl↑}}

== Applications ==
Most hydrogen chloride is consumed in the production of hydrochloric acid. It is also used in the production of [[vinyl chloride]] and many [[alkyl chloride]]s.<ref name=Ullmann/> [[Trichlorosilane]], a precursor to ultrapure silicon, is produced by the reaction of hydrogen chloride and silicon at around 300 °C.<ref>{{cite book |doi=10.1002/14356007.a24_001 | chapter=Silicon Compounds, Inorganic | title=Ullmann's Encyclopedia of Industrial Chemistry | date=2000 | last1=Simmler | first1=Walter | isbn=978-3-527-30385-4 }}</ref>
:{{chem2|Si + 3 HCl → HSiCl3 + H2}}

== History ==
Around 900, the authors of the Arabic writings attributed to [[Jabir ibn Hayyan]] (Latin: Geber) and the Persian physician and alchemist [[Abu Bakr al-Razi]] (c. 865–925, Latin: Rhazes) were experimenting with [[sal ammoniac]] ([[ammonium chloride]]), which when it was distilled together with [[vitriol]] (hydrated [[sulfates]] of various metals) produced hydrogen chloride.<ref>{{Cite book|last=Kraus|first=Paul|author-link=Paul Kraus (Arabist)|year=1942–1943|title=Jâbir ibn Hayyân: Contribution à l'histoire des idées scientifiques dans l'Islam. I. Le corpus des écrits jâbiriens. II. Jâbir et la science grecque|publisher=[[Institut Français d'Archéologie Orientale]]|location=Cairo|oclc=468740510|isbn=9783487091150}} vol. II, pp. 41–42; {{cite book|last=Multhauf|first=Robert P.|author-link=Robert P. Multhauf|year=1966|title=The Origins of Chemistry|location=London|publisher=Oldbourne|isbn=}} pp. 141-142.</ref> It is possible that in one of his experiments, al-Razi stumbled upon a primitive method to produce [[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. The relevant recipe reads as follows: "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." (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 these early experiments with [[chloride]] [[Salt (chemistry)|salts]], the gaseous products were discarded, and hydrogen chloride may have been produced many times before it was discovered that it can be put to chemical use.<ref>{{harvnb|Multhauf|1966|loc=p. 142, note 79}}.</ref>

One of the first such uses was the synthesis of [[mercury(II) chloride]] (corrosive sublimate), whose production from the heating of [[mercury (element)|mercury]] either with [[alum]] and ammonium chloride or with vitriol and sodium chloride was first described in the ''De aluminibus et salibus'' ("On Alums and Salts"), an eleventh- or twelfth century Arabic text falsely attributed to Abu Bakr al-Razi and [[Latin translations of the 12th century|translated into Latin]] by [[Gerard of Cremona]] (1144–1187).<ref>{{harvnb|Multhauf|1966|pp=160–163}}.</ref>

Another important development was the discovery by [[pseudo-Geber]] (in the ''De inventione veritatis'', "On the Discovery of Truth", after c. 1300) that by adding ammonium chloride to [[nitric acid]], a strong solvent capable of dissolving gold (i.e., ''[[aqua regia]]'') could be produced.<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>

After the discovery in the late sixteenth century of the process by which unmixed [[hydrochloric acid]] can be prepared,<ref>{{harvnb|Multhauf|1966|loc=p. 208, note 29; cf. p. 142, note 79}}.</ref> it was recognized that this new acid (then known as ''spirit of salt'' or ''acidum salis'') released vaporous hydrogen chloride, which was called ''marine acid air''. In the 17th century, [[Johann Rudolf Glauber]] used salt ([[sodium chloride]]) and [[sulfuric acid]] for the preparation of [[sodium sulfate]], releasing hydrogen chloride gas (see production, above). In 1772, [[Carl Wilhelm Scheele]] also reported this reaction and is sometimes credited with its discovery. [[Joseph Priestley]] prepared hydrogen chloride in 1772, and in 1810 [[Humphry Davy]] established that it is composed of [[hydrogen]] and [[chlorine]].<ref>{{cite journal|author = Hartley, Harold|title = The Wilkins Lecture. Sir Humphry Davy, Bt., P.R.S. 1778–1829|journal = [[Proceedings of the Royal Society A]] | year = 1960|volume = 255|issue = 1281|pages = 153–180|bibcode = 1960RSPSA.255..153H|doi = 10.1098/rspa.1960.0060|s2cid = 176370921|author-link = Harold B. Hartley}}</ref>

During the [[Industrial Revolution]], demand for [[alkaline]] substances such as [[soda ash]] increased, and [[Nicolas Leblanc]] developed a new industrial-scale process for producing the soda ash. In the [[Leblanc process]], salt was converted to soda ash, using sulfuric acid, limestone, and coal, giving hydrogen chloride as by-product. Initially, this gas was vented to air, but the [[Alkali Act]] of 1863 prohibited such release, so then soda ash producers absorbed the HCl waste gas in water, producing hydrochloric acid on an industrial scale. Later, the [[Hargreaves process]] was developed, which is similar to the Leblanc process except [[sulfur dioxide]], water, and air are used instead of sulfuric acid in a reaction which is exothermic overall. In the early 20th century the Leblanc process was effectively replaced by the [[Solvay process]], which did not produce HCl. However, hydrogen chloride production continued as a step in hydrochloric acid production.

Historical uses of hydrogen chloride in the 20th century include hydrochlorinations of [[alkynes]] in producing the chlorinated [[monomer]]s [[chloroprene]] and [[vinyl chloride]], which are subsequently [[polymer]]ized to make polychloroprene ([[Neoprene]]) and [[polyvinyl chloride]] (PVC), respectively. In the production of vinyl chloride, [[acetylene]] ({{chem2|C2H2}}) is hydrochlorinated by adding the HCl across the [[triple bond]] of the {{chem2|C2H2}} molecule, turning the triple into a [[double bond]], yielding vinyl chloride.

The "acetylene process", used until the 1960s for making [[chloroprene]], starts out by joining two [[acetylene]] molecules, and then adds HCl to the joined intermediate across the [[triple bond]] to convert it to chloroprene as shown here:

:[[File:Chloroprene synthesis.svg|500px]]

This "acetylene process" has been replaced by a process which adds [[chlorine|{{chem2|Cl2}}]] to the double bond of ethylene instead, and subsequent elimination produces HCl instead, as well as chloroprene.

== Safety ==
Hydrogen chloride forms corrosive [[hydrochloric acid]] on contact with water found in body tissue. [[Inhalation]] of the fumes can cause [[cough]]ing, [[choking]], [[inflammation]] of the nose, throat, and upper [[respiratory tract]], and in severe cases, [[pulmonary edema]], [[circulatory system]] failure, and death.<ref>{{Citation |last=Contaminants |first=National Research Council (US) Committee on Emergency and Continuous Exposure Guidance Levels for Selected Submarine |title=Hydrogen Chloride |date=2009 |work=Emergency and Continuous Exposure Guidance Levels for Selected Submarine Contaminants: Volume 3 |url=https://www.ncbi.nlm.nih.gov/books/NBK219917/ |access-date=2024-04-23 |publisher=National Academies Press (US) |language=en}}</ref> Skin contact can cause redness, [[pain]], and severe [[chemical burn]]s. Hydrogen chloride may cause severe burns to the eye and permanent eye damage.

The U.S. [[Occupational Safety and Health Administration]] and the [[National Institute for Occupational Safety and Health]] have established occupational exposure limits for hydrogen chloride at a ceiling of 5&nbsp;ppm (7&nbsp;mg/m<sup>3</sup>),<ref>[https://www.cdc.gov/niosh/npg/npgd0332.html CDC – NIOSH Pocket Guide to Chemical Hazards]</ref> and compiled extensive information on hydrogen chloride workplace safety concerns.<ref>{{Cite web|url=https://www.cdc.gov/niosh/topics/hydrogen-chloride/|title=Hydrogen Chloride|date=5 March 2012|website=CDC - NIOSH Workplace Safety and Health Topic|access-date=2016-07-15}}</ref>

== See also ==
* [[Gastric acid]], hydrochloric acid secreted into the [[stomach]] to aid [[digestion]] of [[protein]]s
* [[Chloride]], salts of hydrogen chloride
* [[Hydrogen bromide]] 
* [[Hydrochloride]], organic salts of hydrochloric acid
* [[Hydrochlorination]], addition reaction with alkenes

== References ==
{{Reflist|30em}}

== External links ==
{{Commons category|Hydrogen chloride}}
* {{ICSC|0163|01}}
* [[Thames & Kosmos]] Chem C2000 Experiment Manual

{{Hydrogen compounds}}
{{Molecules detected in outer space}}
{{chlorine compounds}}
{{Chlorides}}
{{Hydrides by group}}
{{Authority control}}

[[Category:Chlorides]]
[[Category:Halogen-containing natural products]]
[[Category:Hydrogen compounds]]
[[Category:Industrial gases]]
[[Category:Nonmetal halides]]
[[Category:Diatomic molecules]]
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