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Hydrochloric acid

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                          Hydrochloric acid
                          Hydrochloric acid
                               General
    Systematic name                                  Hydrochloric acid
        Other names                                      Muriatic acid
  Molecular formula                               HCl in water (H[2]O)
         Molar mass                                  36.46 g/mol (HCl)
         Appearance                                 Clear colorless to
                                                   light-yellow liquid
         CAS number                                        [7647-01-0]
                              Properties
     Density, phase                                       1.18 g/cm³,
                                                         37% solution.
Solubility in water                                    Fully miscible.
      Melting point                                  −26 °C (247 K)
                                                         38% solution.
      Boiling point                     110 °C (383 K),
                                        20.2% solution;
                                        48 °C (321 K),
                                                         38% solution.
  Acid dissociation
     constant pK[a]                                               −8.0
          Viscosity                              1.9 mPa·s at 25 °C,
                                                        31.5% solution
                               Hazards
               MSDS                                      External MSDS
           NFPA 704

                    0
                    3
                    1

                                                                            32-38% solution
       Main Hazards                                  Highly corrosive.
        Flash point                                     Non-flammable.
      R/S statement                                          R34, R37,
                                                         S26, S36, S45
       RTECS number                                          MW4025000
                       Supplementary data page
      Structure and
         properties                                      n, ε[r], etc.
      Thermodynamic
               data                                    Phase behaviour
                                                    Solid, liquid, gas
      Spectral data                                    UV, IR, NMR, MS
                          Related compounds
       Other anions                                        HF, HBr, HI
      Other cations                                                N/a
      Related acids                     Hydrobromic acid
                                        Hydrofluoric acid
                                        Hydroiodic acid
                                                         Sulfuric acid
           Except where noted otherwise, data are given for
                materials in their standard state (at 25 °C, 100 kPa)
                                     Infobox disclaimer and references

   The chemical compound hydrochloric acid is the aqueous ( water-based)
   solution of hydrogen chloride gas (HCl). It is a strong acid, the major
   component of gastric acid and of wide industrial use. Hydrochloric acid
   must be handled with appropriate safety precautions because it is a
   highly corrosive liquid.

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

   From the Industrial Revolution, it became an important industrial
   chemical for many applications, including the large-scale production of
   organic compounds, such as vinyl chloride for PVC plastic, and MDI/ TDI
   for polyurethane, and smaller-scale applications, such as production of
   gelatin and other ingredients in food, and leather processing. About 20
   million metric tonnes of HCl gas are produced annually.

History

   Hydrochloric acid was first discovered around 800 AD by the alchemist
   Jabir ibn Hayyan (Geber), by mixing common salt with vitriol (sulfuric
   acid). Jabir discovered many important chemicals, and recorded his
   findings in over twenty books, which carried his chemical knowledge of
   hydrochloric acid and other basic chemicals for hundreds of years.
   Jabir's invention of the gold-dissolving aqua regia, consisting of
   hydrochloric acid and nitric acid, was of great interest to alchemists
   searching for the philosopher's stone.
   Jabir ibn Hayyan, medieval manuscript drawing
   Jabir ibn Hayyan, medieval manuscript drawing

   In the Middle Ages, hydrochloric acid was known to European alchemists
   as spirit of salt or acidum salis. Gaseous HCl was called marine acid
   air. The old (pre- systematic) name muriatic acid has the same origin
   (muriatic means "pertaining to brine or salt"), and this name is still
   sometimes used. Notable production was recorded by Basilius Valentinus,
   the alchemist- canon of the Benedictine priory Sankt Peter in Erfurt,
   Germany in the fifteenth century. In the seventeenth century, Johann
   Rudolf Glauber from Karlstadt am Main, Germany used sodium chloride
   salt and sulfuric acid for the preparation of sodium sulfate in the
   Mannheim process, releasing hydrogen chloride gas. Joseph Priestley of
   Leeds, England prepared pure hydrogen chloride in 1772, and in 1818
   Humphry Davy of Penzance, England proved that the chemical composition
   included hydrogen and chlorine.

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

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

   Hydrochloric acid is listed as a Table II precursor under the 1988
   Convention Against Illicit Traffic in Narcotic Drugs and Psychotropic
   Substances because of its use in the production of heroin, cocaine, and
   methamphetamine.

Chemistry

   Acid titration
   Acid titration

   Hydrogen chloride (HCl) is a monoprotic acid, which means it can
   dissociate (i.e., ionize) only once to give up one H^+ ion (a single
   proton). In aqueous hydrochloric acid, the H^+ joins a water molecule
   to form a hydronium ion, H[3]O^+:

                HCl + H[2]O → H[3]O^+ + Cl^−

   Molecular model of hydrogen chloride.
   Molecular model of hydrogen chloride.

   The other ion formed is Cl^−, the chloride ion. Hydrochloric acid can
   therefore be used to prepare salts called chlorides, such as sodium
   chloride. Hydrochloric acid is a strong acid, since it is fully
   dissociated in water.

   Monoprotic acids have one acid dissociation constant, K[a], which
   indicates the level of dissociation in water. For a strong acid like
   HCl, the K[a] is large. Theoretical attempts to assign a K[a] to HCl
   have been made. When chloride salts such as NaCl are added to aqueous
   HCl they have practically no effect on pH, indicating that Cl^− is an
   exceedingly weak conjugate base and that HCl is fully dissociated in
   aqueous solution. For intermediate to strong solutions of hydrochloric
   acid, the assumption that H^+ molarity (a unit of concentration) equals
   HCl molarity is excellent, agreeing to four significant digits.

   Of the seven common strong acids in chemistry, all of them inorganic,
   hydrochloric acid is the monoprotic acid least likely to undergo an
   interfering oxidation-reduction reaction. It is one of the least
   hazardous strong acids to handle; despite its acidity, it produces the
   less reactive and non-toxic chloride ion. Intermediate strength
   hydrochloric acid solutions are quite stable, maintaining their
   concentrations over time. These attributes, plus the fact that it is
   available as a pure reagent, mean that hydrochloric acid makes an
   excellent acidifying reagent and acid titrant (for determining the
   amount of an unknown quantity of base in titration). Strong acid
   titrants are useful because they give more distinct endpoints in a
   titration, making the titration more precise. Hydrochloric acid is
   frequently used in chemical analysis and to digest samples for
   analysis. Concentrated hydrochloric acid will dissolve some metals to
   form oxidized metal chlorides and hydrogen gas. It will produce metal
   chlorides from basic compounds such as calcium carbonate or copper(II)
   oxide. It is also used as a simple acid catalyst for some chemical
   reactions.

Physical properties

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

                                 Conc. (w/w)
                         c : kg HCl/kg  Conc. (w/v)
                           c : kg HCl/m^3 Density
                             ρ : kg/l Molarity
                                   M   pH
                                  Viscosity
                            η : mPa·s Specific
                                    heat
                            s : kJ/(kg·K) Vapor
                                  pressure
                             P[HCl] : Pa Boiling
                                    point
                                b.p. Melting
                                    point
                                    m.p.
        10% 104.80 1.048 2.87 M -0.5 1.16 3.47 0.527 103 °C -18 °C
         20% 219.60 1.098 6.02 M -0.8 1.37 2.99 27.3 108 °C -59 °C
         30% 344.70 1.149 9.45 M -1.0 1.70 2.60 1,410 90 °C -52 °C
        32% 370.88 1.159 10.17 M -1.0 1.80 2.55 3,130 84 °C -43 °C
        34% 397.46 1.169 10.90 M -1.0 1.90 2.50 6,733 71 °C -36 °C
        36% 424.44 1.179 11.64 M -1.1 1.99 2.46 14,100 61 °C -30 °C
        38% 451.82 1.189 12.39 M -1.1 2.10 2.43 28,000 48 °C -26 °C

    The reference temperature and pressure for the above table are 20 °C
                         and 1 atmosphere (101 kPa).

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

Production

Direct synthesis

   The large scale production of hydrochloric acid is almost always
   integrated with other industrial scale chemicals production. In the
   chlor-alkali industry, salt solution is electrolyzed producing chlorine
   (Cl[2]), sodium hydroxide, and hydrogen (H[2]). The pure chlorine gas
   can be re-combined with the hydrogen gas, forming chemically pure
   hydrogen chloride gas. As the reaction is exothermic, the installation
   is called an HCl oven.

                Cl[2] + H[2] → 2 HCl

   The resulting pure hydrogen chloride gas is absorbed in deionized
   water, resulting in chemically pure hydrochloric acid.

Organic synthesis

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

                R-H + Cl[2] → R-Cl + HCl
                R-Cl + HF → R-F + HCl

   The resulting hydrogen chloride gas is either reused directly, or
   absorbed in water, resulting in hydrochloric acid of technical or
   industrial grade.

Clandestine synthesis

   Clandestine labs often produce HCl gas by reacting sulfuric acid and
   sodium chloride (table salt). They commonly use HCl gas to convert a
   freebase compound into its hydrochloride salt, as this form is much
   more stable and convenient for distribution.

Industrial market

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

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

Applications

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

Regeneration of ion exchangers

   An important application of high-quality hydrochloric acid is the
   regeneration of ion exchange resins. Cation exchange is widely used to
   remove ions such as Na^+ and Ca^2+ from aqueous solutions, producing
   demineralized water.

                Na^+ is replaced by H[3]O^+
                Ca^2+ is replaced by 2 H[3]O^+

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

pH Control and neutralization

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

                OH^− + HCl → H[2]O + 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 neutralizing waste streams and swimming
   pool treatment.

Pickling of steel

   Pickling is an essential step in metal surface treatment, to remove
   rust or iron oxide scale from iron or steel before subsequent
   processing, such as extrusion, rolling, galvanizing, and other
   techniques. Technical-quality HCl at typically 18% concentration is the
   most commonly-used pickling agent for the pickling of carbon steel
   grades.

                Fe[2]O[3] + Fe + 6 HCl → 3 FeCl[2] + 3 H[2]O

   The spent acid has long been re-used as ferrous chloride solutions, but
   high heavy-metal levels in the pickling liquor has decreased this
   practice.

   In recent years, the steel pickling industry has however 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:

                4 FeCl[2] + 4 H[2]O + O[2] → 8 HCl+ 2 Fe[2]O[3]

   By recuperation of the spent acid, a closed acid loop is established.
   The ferric oxide by product of the regeneration process is a valuable
   by-product, used in a variety of secondary industries.

   HCl is not a common pickling agent for stainless steel grades.

Production of inorganic compounds

   Numerous products can be produced with hydrochloric acid in normal
   acid-base reactions, resulting in inorganic compounds. These include
   water treatment chemicals such as iron(III) chloride and polyaluminium
   chloride (PAC).

                Fe[2]O[3] + 6 HCl → 2 FeCl[3] + 3 H[2]O

   Both iron(III) chloride and PAC are used as flocculation and
   coagulation agents in wastewater treatment, drinking water production,
   and paper production.

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

Production of organic compounds

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

Other applications

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

   Many chemical reactions involving hydrochloric acid are applied in the
   production of food, food ingredients, and food additives. Typical
   products include aspartame, fructose, citric acid, lysine, hydrolyzed
   (vegetable) protein as food enhancer, and in gelatin production.
   Food-grade (extra-pure) hydrochloric acid can be applied when needed
   for the final product. Mixing simple aluminium foil with hydrochloric
   acid produces hydrogen. The hydrogen can then be stored in a balloon.
   Upon application of a flame to the balloon it will produce a fiery
   explosion.

Presence in living organisms

Physiology and pathology

   Hydrochloric acid constitutes the majority of gastric acid, the human
   digestive fluid. In a complex process and at a large energetic burden,
   it is secreted by parietal cells (also known as oxyntic cells). These
   cells contain an extensive secretory network (called canaliculi) from
   which the HCl is secreted into the lumen of the stomach. They are part
   of the fundic glands (also known as oxyntic glands) in the stomach.

   Safety mechanisms that prevent the damage of the epithelium of
   digestive tract by hydrochloric acid are the following:
     * Negative regulators of its release
     * A thick mucus layer covering the epithelium
     * Sodium bicarbonate secreted by gastric epithelial cells and
       pancreas
     * The structure of epithelium ( tight junctions)
     * Adequate blood supply
     * Prostaglandins (many different effects: they stimulate mucus and
       bicarbonate secretion, maintain epithelial barrier integrity,
       enable adequate blood supply, stimulate the healing of the damaged
       mucous membrane)

   When, due to different reasons, these mechanisms fail, heartburn or
   peptic ulcers can develop. Drugs called proton pump inhibitors prevent
   the body from making excess acid in the stomach, while antacids
   neutralize existing acid.

   In some instances, not enough of hydrochloric acid gets produced in the
   stomach. These pathologic states are denoted by the terms
   hypochlorhydria and achlorhydria. Potentially they can lead to
   gastroenteritis.

Chemical weapons

   Phosgene (COCl[2]) was a common chemical warfare agent used in World
   War I. The main effect of phosgene results from the dissolution of the
   gas in the mucous membranes deep in the lung, where it is converted by
   hydrolysis into carbonic acid and the corrosive hydrochloric acid. The
   latter disrupts the alveolar- capillary membranes so that the lung
   becomes filled with fluid ( pulmonary edema).

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

Safety

                            Dangerous goods labels
    Dangerous goods label for hydrochloric acid: corrosive Dangerous goods
      label for hydrochloric acid: corrosive Caution: Hydrochloric acid is
                                                                 corrosive

   Hydrochloric acid in high concentrations forms acidic mists. Both the
   mist and the solution have a corrosive effect on human tissue,
   potentially damaging respiratory organs, eyes, skin and intestines.
   Upon mixing hydrochloric acid with common oxidizing chemicals, such as
   bleach (NaClO) or permanganate (KMnO[4]), the toxic gas chlorine is
   produced. To minimize the risks while working with hydrochloric acid,
   appropriate precautions should be taken. For example, water should
   never be added to the concentrated acid, as this releases enough energy
   to make the water boil and splatter acid; rather, acid should be added
   to water. See references for details.

   The hazards of solutions of hydrochloric acid depend on the
   concentration. The following table lists the EU classification of
   hydrochloric acid solutions:

                    Concentration
                      by weight  Classification R-Phrases
                       10%–25%   Irritant (Xi)  R36/37/38
                        >25%     Corrosive (C)   R34 R37

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