# hydrochloric acid

General
Surname hydrochloric acid
other names
• Hydrochloric acid
• Acidum hydrochloricum
Molecular formula HCl x (H 2 O) ${\ displaystyle \ cdot}$
Brief description

colorless, often yellowish, pungent smelling liquid due to impurities

External identifiers / databases
 CAS number 7647-01-0 ( hydrogen chloride ) EC number 933-977-5 ECHA InfoCard 100.210.665 PubChem 313 Wikidata Q2409
Drug information
ATC code

A09 AB03 , B05 XA13

properties
Molar mass 36.46 g mol −1
Physical state

liquid

density

1.19 g cm −3 (37 percent solution)

Melting point

−30 ° C (37 percent solution)

Vapor pressure

190 h Pa (20 ° C, 37 percent solution)

safety instructions
Please note the exemption from the labeling requirement for drugs, medical devices, cosmetics, food and animal feed
GHS hazard labeling from  Regulation (EC) No. 1272/2008 (CLP) , expanded if necessary

danger

H and P phrases H: 290-314-335
P: 260-280-303 + 361 + 353-304 + 340 + 310-305 + 351 + 338
MAK

2 ml m −3 or 3.0 mg m −3

As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Hydrochloric acid , and hydrochloric acid, called, is a aqueous solution of gaseous hydrogen chloride , in the oxonium - and chloride ions protolyzed is. It is a strong, inorganic acid and is one of the mineral acids . Their salts are called chlorides , the best known is sodium chloride (NaCl, table salt).

## history

Their use is already mentioned indirectly by Pliny , in the separation of gold and silver in mining, when table salt and vitriol form hydrochloric acid at high temperatures, which combine with the silver. It is possible that Georgius Agricola mentions a similar process for separating silver in his De Re Metallica of 1556 (the recipe given would give hydrochloric acid if salt meant table salt). Pseudo-Geber (13th century) described a reaction of mercury after heating with table salt and alum or iron sulfate, whereby fine white needles of mercury chloride were formed by reaction with hydrochloric acid. He and medieval alchemists were also familiar with aqua regia , which was produced by adding salmia (ammonium chloride) or table salt to nitric acid. In the 15th century, hydrochloric acid and its use to soften bones and ivory for carving was mentioned, first in an anonymous mid-15th century Italian manuscript in the University of Bologna, then in a recipe by Caterina Sforza (1490) . It was made by both authors by heating table salt and vitriol and distillation.

In the first half of the 15th century won Basil Valentine hydrochloric acid by reaction of halite (rock salt) with ferrous sulphate . In 1597, Libavius mentions hydrochloric acid in his book Alchemia , but it was also mentioned by Giambattista della Porta (Magiae Naturalis 1558, 1589) as the best means for whitening teeth. Johann Rudolph Glauber succeeded in producing it on a large scale from table salt and sulfuric acid in the 17th century. Lavoisier called hydrochloric acid acid muriatique ( lat. Muria , brine '). Sources containing table salt are still known today as Muriatic springs . In North America, hydrochloric acid is also called muriatic acid .

## Occurrence

In nature, hydrochloric acid is found in volcanic gases and strongly diluted in crater lakes. It occurs in free form in the gastric juice of vertebrates (0.1 to 0.5 percent by mass). The deposits of salts of hydrochloric acid, as rock salt and dissolved in sea ​​water, are almost inexhaustible .

## Presentation and extraction

Hydrochloric acid is produced in the laboratory from concentrated sulfuric acid and table salt (hence the name):

${\ displaystyle \ mathrm {NaCl _ {(s)} + {H_ {2} SO_ {4}} _ {(l)} \ longrightarrow {NaHSO_ {4}} _ {(s)} + HCl _ {(g)} }}$

The sulfuric acid displaces the hydrogen chloride from its salt. Since hydrogen chloride is gaseous, it is constantly withdrawn from the equilibrium, which is almost entirely on the side of the products. The resulting sodium hydrogen sulfate is an acidic sulfuric acid salt. The resulting hydrogen chloride gas is then introduced into water:

${\ displaystyle \ mathrm {HCl _ {(g)} + H_ {2} O _ {(l)} \ longrightarrow H_ {3} O _ {(aq)} ^ {+} + Cl _ {(aq)} ^ {-} }}$

Hydrochloric acid with higher mass proportions of hydrogen chloride is also known as fuming hydrochloric acid , as hydrogen chloride gas escapes and hydrochloric acid is formed again with the water from the humidity, so that a white mist forms over open vessels.

In the chemical industry, high-purity hydrogen chloride is obtained through the reaction of hydrogen with chlorine :

${\ displaystyle \ mathrm {H_ {2} + Cl_ {2} \ longrightarrow 2 \, HCl}}$

Here too, the hydrogen chloride is allowed to react with water.

Technically pure hydrochloric acid is mainly a by-product of the chlorination of organic compounds.

## properties

Mass
fraction w in%
Mass
concentration
β in g / l
Mole
concentration
c in mol / l
Density ϱ
in g / cm³
10.5 110 3.06 1.050
15.5 166 4.61 1.075
20.4 224 6.22 1,100
22.3 248 6.89 1,110
24.3 272 7.56 1.120
26.2 296 8.22 1.130
28.2 321 8.92 1,140
30.1 347 9.64 1.150
32.1 372 10.23 1.160
34.2 400 11.11 1.170
36.2 428 11.89 1.180
38.3 456 12.67 1.190
40.4 485 13.47 1,200

Hydrogen chloride gas dissolves very well in water : At 0 ° C, 1 liter of water, if it is still in the liquid phase , dissolves 815 g or 507 liters of gas with the development of heat. At 20 ° C, one liter of saturated hydrochloric acid contains 720 g of HCl. The concentration dependency of the density is shown in the adjacent table, whereby there happens to be a simple mathematical relationship between it and the percentage content of hydrogen chloride: The doubled decimal places roughly correspond to the concentration, e.g. B. a hydrochloric acid with a density of 1.10 g · cm −3 and an HCl content of 20 percent. ${\ displaystyle \ rho}$

${\ displaystyle \% = 200 \ cdot (\ rho -1)}$

The melting and boiling behavior of hydrochloric acid depends heavily on its composition. In the solid phase, four stoichiometric hydrates with defined melting points are formed. These are a monohydrate HCl · H 2 O with a melting point of −15 ° C, a dihydrate HCl · 2H 2 O with a melting point of −18 ° C, a trihydrate HCl · 3H 2 O with a melting point of −25 ° C and a hexahydrate HCl 6H 2 O with a melting point of −70 ° C. The phase diagram shows corresponding eutectic melts for compositions between the stoichiometric hydrates . These are for a mixture of mono- and dihydrate with a mass fraction of hydrogen chloride of 57.3% at −23 ° C, for di- and trihydrate with a mass fraction of 44.0% at -28 ° C, for tri- and hexahydrate with a mass fraction of 26.6% at −73 ° C and of hexahydrate and ice with a mass fraction of 23.0% at −75 ° C. In addition, a metastable eutectic is formed between the trihydrate and ice with a mass fraction of 24.8% at −87 ° C. In the concentration range from 0 to 25%, a sharp drop in the melting point is observed. The vapor-liquid phase diagram between hydrogen chloride and water shows a negative azeotrope . The resulting azeotropic boiling point maximum is at normal pressure with a mass fraction of 20.2% at 109 ° C. When evaporating hydrochloric acid solutions with a concentration deviating from the azeotrope composition, preference is given to initially evaporating the excess component, i. In other words, in the case of hydrochloric acid with a mass fraction of <20.2%, concentration takes place, and in the case of hydrochloric acid with> 20.2%, concentration is reduced until the constant-boiling azeotrope composition is reached. The boiling curve in the phase diagram above the azeotrope composition correlates with the solubility curve of hydrogen chloride in water. At 25 ° C the mass fraction is 42%, which corresponds to the "smoking" hydrochloric acid.

Hydrogen chloride dissociates completely in water , hydrochloric acid with 32% has a pH value of −1. In moist air, hydrogen chloride gas forms a mist of fine hydrochloric acid droplets. Dilute hydrochloric acid is a good conductor of electricity .

## Reactions

In the reaction of hydrochloric acid with aqueous ammonia solution to form white fumes of ammonium chloride .

Hydrochloric acid dissolves most metals with the exception of precious metals and a few others ( e.g. tantalum and germanium ) with the formation of chlorides and hydrogen , unless these are protected by passivation .

${\ displaystyle \ mathrm {Mg \ + \ 2 \ HCl \ \ longrightarrow \ MgCl_ {2} \ + \ H_ {2} \ uparrow}}$

It is very suitable for removing oxide layers from metals, as metal oxides react with hydrochloric acid to form chlorides and water:

${\ displaystyle \ mathrm {CuO \ + \ 2 \ HCl \ \ longrightarrow \ CuCl_ {2} \ + \ H_ {2} O}}$

Ammonium chloride can be obtained by neutralizing hydrochloric acid with aqueous ammonia solution :

${\ displaystyle \ mathrm {NH_ {3} + HCl \ longrightarrow NH_ {4} Cl}}$

A mixture of hydrochloric acid and nitric acid is called aqua regia because it can also dissolve gold , the "king of metals". In addition to the oxidizing effect of nitrosyl chloride and nascent chlorine, the reduction in the effective gold ion concentration through complex formation also contributes to this:

${\ displaystyle \ mathrm {Au ^ {3 +} \ + \ 4 \ Cl ^ {-} \ \ longrightarrow \ AuCl_ {4} ^ {-}}}$

## use

Hydrochloric acid is an important basic chemical with great importance in the chemical industry as an inorganic acid. It is used, for example, in the processing of ores and raw phosphate . It is used to stimulate petroleum and natural gas probes, especially in carbonate deposits, but also in sandstone deposits. With their help there are also z. B. Calcium carbonate deposits on equipment are removed and cleaning is carried out after drilling with filter gravel (" gravel pack ") and drilling holes themselves. In metal processing, it is used for pickling , etching and soldering . In addition, diluted hydrochloric acid is used in construction to remove mortar residues from masonry - so-called acidification .

Last but not least, hydrochloric acid is an important reagent in chemical analysis . It is able to separate a group of metals, which form poorly soluble chlorides, from other metals by precipitation . These can then be further analyzed separately (see hydrochloric acid group ). The Alkalimetry is another area of use of hydrochloric acid.

As a food additive , hydrochloric acid has the designation E 507.

In the pharmaceutical industry , hydrochloric acid is used to convert basic drugs that are poorly soluble or insoluble in water (examples: ciprofloxacin , citalopram , clenbuterol , clindamycin , dibenzepine ) into more soluble hydrochlorides .

One of the important uses of hydrochloric acid is the pickling of steel to rust or iron oxide scale from iron or steel prior to subsequent processing to remove such. B. by extrusion , rolling , electroplating and other techniques.

${\ displaystyle \ mathrm {Fe_ {2} O_ {3} + Fe + 6 \ HCl \ longrightarrow 3 \ FeCl_ {2} +3 \ H_ {2} O}}$

In the steel pickling industry, hydrochloric acid regeneration processes such as the spray roaster or the fluidized bed HCl regeneration process have been developed, which enable the recovery of HCl from used pickling liquid. The most common regeneration process is the pyrohydrolysis process:

${\ displaystyle \ mathrm {4 \ FeCl_ {2} +4 \ H_ {2} O + O_ {2} \ longrightarrow 8 \ HCl + 2 \ Fe_ {2} O_ {3}}}$

A closed acid cycle is created by recovering the used acid . The iron (III) oxide by- product of the regeneration process is valuable and used in a wide variety of secondary industries.

In normal acid-base reactions , hydrochloric acid can be used to produce numerous products that lead to inorganic compounds . These include chemicals for water treatment , such as iron (III) chloride and polyaluminum chloride .

${\ displaystyle \ mathrm {Fe_ {2} O_ {3} +6 \ HCl \ longrightarrow 2 \ FeCl_ {3} +3 \ H_ {2} O}}$

Both iron (III) chloride and polyaluminum chloride are used as flocculants in wastewater treatment , drinking water production and paper production.

Other inorganic compounds that are produced with hydrochloric acid are calcium chloride as road salt , nickel (II) chloride for electroplating and zinc chloride for electroplating and battery production .

${\ displaystyle \ mathrm {CaCO_ {3} +2 \ HCl \ longrightarrow CaCl_ {2} + CO_ {2} + H_ {2} O}}$

## Biological importance

In humans and animals, hydrochloric acid is a component of gastric juice , where it causes, among other things, the denaturation of proteins , but also serves to kill microorganisms before they enter the further digestive system. It also creates the acidic environment in which the digestive enzyme pepsin is most effective.

## proof

Hydrochloric acid is demonstrated on the one hand by its acidic character. Additionally, one identifies the chloride - anion in very dilute solution (with an excess of chloride ions is a soluble complex forms Dichloroargentat) by addition of silver nitrate solution , wherein the sparingly soluble silver chloride precipitate:

${\ displaystyle \ mathrm {HCl \ + \ AgNO_ {3} \ \ longrightarrow \ HNO_ {3} \ + \ AgCl}}$

If the resulting white precipitate dissolves in dilute ammonia water to form complexes, then evidence has been provided that it was chloride ions:

${\ displaystyle \ mathrm {AgCl \ +2 \ NH_ {3} \ \ longrightarrow \ [Ag (NH_ {3}) _ {2}] ^ {+} \ + \ Cl ^ {-}}}$
${\ displaystyle \ mathrm {AgCl \ + \ HCl \ \ longrightarrow \ [AgCl_ {2}] ^ {-} \ + \ H ^ {+}}}$

Is hydrochloric acid with manganese dioxide ( manganese heated), the result is chlorine :

${\ displaystyle \ mathrm {4 \ HCl \ + \ MnO_ {2} \ \ longrightarrow \ Cl_ {2} \ + \ MnCl_ {2} \ + \ 2 \ H_ {2} O}}$

The hydrochloric acid content of a solution is determined by titration with sodium hydroxide solution ( acidimitry , dimensional analysis ). Photometry can this provision and of chlorides by means of mercury salt of chloranilic perform. The content of hydrochloric acid in gastric juice is determined with Günzburg's reagent .

## Individual evidence

1. Entry on hydrochloric acid in the GESTIS substance database of the IFA , accessed on January 10, 2017(JavaScript required) .
2. Entry on Hydrochloric acid in the Classification and Labeling Inventory of the European Chemicals Agency (ECHA), accessed on February 1, 2016. Manufacturers or distributors can expand the harmonized classification and labeling .
3. On the early history of hydrochloric acid: Ladislao Reti, How old is hydrochloric acid?, Chymia, Volume 10, 1965, pp. 11-23
4. ^ Il libro dei colori, edited by O. Guerrini, C. Ricci, Bologna 1887
5. Printed in Pier Desiderio Pasolini, Caterina Sforza, 3 volumes, Rome 1893
6. ^ Entry on hydrogen chloride in the GESTIS substance database of the IFA , accessed on June 26, 2011 (JavaScript required)
7. all data from Gmelin's Handbook of Inorganic Chemistry, System Number 6 Chlorine, Verlag Chemie Berlin 1927 and Gmelin's Handbook of Inorganic Chemistry, System Number 6 Chlorine, Supplementary Volume Part B - Delivery 1, Verlag Chemie Weinheim 1968.
8. ^ Axel Kleemann , Jürgen Engel, Bernd Kutscher and Dietmar Reichert: Pharmaceutical Substances , 4th edition (2000), 2 volumes published by Thieme-Verlag Stuttgart, ISBN 978-1-58890-031-9 ; online since 2003 with biannual additions and updates.
9. ^ Hydrochloric Acid . In: Chemicals Economics Handbook . SRI International , 2001, pp. 733.4000A-733.3003F.
10. a b Greenwood and Earnshaw, pp. 946-48
11. Entry on Günzburg's reagent. In: Römpp Online . Georg Thieme Verlag, accessed on September 24, 2014.

## Web links

Wiktionary: Hydrochloric acid  - explanations of meanings, word origins, synonyms, translations
Commons : Hydrochloric acid  - Collection of pictures, videos, and audio files