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  Hydrochloric Acid
 

The chemical compound hydrochloric acid is the aqueous (water-based) solution of hydrogen chloride (HCl) gas. It is a strong acid, the major component of gastric acid and of wide industrial use. Hydrochloric acid should only 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. During 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. At present, production is approximately 20 million metric tones annually (20 Mt/a) of HCl gas.
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, H3O+:
HCl + H2O → H3O+ + Cl−

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 practically fully dissociated in water.
Monoprotic acids have one acid dissociation constant, Ka, which indicates the level of dissociation in water. For a strong acid like HCl, the Ka is large.
Theoretical attempts to assign a Ka to HCl have been made.[2] 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+ morality (a unit of concentration) equals HCl morality 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 morality 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/m3
Density
ρ : kg/l
Morality
M
pH
Viscosity
η : mPa·s
Specific
heat

s : kJ/(kg·K)
Vapor
pressure

PHCl : Pa
Boiling
point

b.p.
Melting
point

m.p.
10%
104.80
1.048
2.87M
-0.5
1.16
3.47
0.527
103 °C
-18 °C
20%
219.60
1.098
6.02M
-0.8
1.37
2.99
27.3
108 °C
-59 °C
30%
344.70
1.149
9.45M
-1.0
1.70
2.60
1410
90 °C
-52 °C
32%
370.88
1.159
10.17M
-1.0
1.80
2.55
3130
84 °C
-43 °C
34%
397.46
1.169
10.90M
-1.0
1.90
2.50
6733
71 °C
-36 °C
36%
424.44
1.179
11.64M
-1.1
1.99
2.46
14100
61 °C
-30 °C
38%
451.82
1.189
12.39M
-1.1
2.10
2.43
28000
98 °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 H2O has a constant-boiling azeotrope at 20.2% HCl and 108.6 °C (227 °F). There are four constant-crystallization eutectic points for hydrochloric acid, between the crystal form of HCl·H2O (68% HCl), HCl·2H2O (51% HCl), HCl·3H2O (41% HCl), HCl·6H2O (25% HCl), and of course ice (0% HCl). There is also a metastable eutectic at 24.8% between ice and the HCl·3H2O crystallization.

Production process
Direct synthesis
The large scale production of hydrochloric acid is almost always integrated with other industrial scale chemical production. In the chlor-alkali industry, salt solution is electrolyzed producing chlorine, sodium hydroxide, and hydrogen. The pure chlorine gas can be re-combined with the hydrogen gas, forming chemically pure HCl gas. As the reaction is exothermic, the installation is called an HCl oven.
Cl2 + H2 → 2 HCl
The resulting pure hydrogen chloride gas is absorbed in demineralized 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, chloro-acetic 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 re-combines with the spare atom from the chlorine molecule, forming hydrogen chloride. Fluorination is a subsequent chlorine-replacement reaction, producing again hydrogen chloride
R-H + Cl2 → R-Cl + HCl
R-Cl + HF → R-F + HCl
The resulting hydrogen chloride gas is either re-used directly, or absorbed in water, resulting in hydrochloric acid of technical or industrial grade.

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 tones 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 Ca2+ from aqueous solutions, producing demineralized water.
Na+ is replaced by H3O+
Ca2+ is replaced by 2 H3O+
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 → 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 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.
Fe2O3 + Fe + 6 HCl → 3 FeCl2 + 3 H2O
The spent acid has long been re-used as ferrous chloride solutions, but a high heavy-metal level 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 FeCl2 + 4 H2O + O2 → 8 HCl+ 2 Fe2O3
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).
Fe2O3 + 6 HCl → 2 FeCl3 + 3 H2O
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 aluminum 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 epithelial 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 (COCl2) 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.
 

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