Last updated
Chloride anion
Chloride ion.svg
Systematic IUPAC name
Chloride [1]
3D model (JSmol)
PubChem CID
  • InChI=1S/ClH/h1H/p-1 Yes check.svgY
  • [Cl-]
Molar mass 35.45 g·mol−1
Conjugate acid Hydrogen chloride
Std molar
153.36 J·K−1·mol−1 [2]
−167 kJ·mol−1 [2]
Related compounds
Other anions
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

The term chloride refers either to a chloride ion (Cl), which is negatively charged chlorine atom, or a non-charged chlorine atom covalently bonded to the rest of the molecule by a single bond (−Cl). Many inorganic chlorides are salts. Many organic compounds are chlorides. The pronunciation of the word "chloride" is /ˈklɔːrd/ . [3]


The chloride ion is an anion (negatively charged ion) with the charge Cl-. Chloride salts such as sodium chloride are often soluble in water. [4] It is an essential electrolyte located in all body fluids responsible for maintaining acid/base balance, transmitting nerve impulses and regulating liquid flow in and out of cells. Other examples of ionic chlorides are calcium chloride CaCl2 and ammonium chloride [NH4]Cl.

The chloride is also a neutral chlorine atom covalently bonded by a single bond to the rest of the molecule. For example, methyl chloride CH3Cl is an organic compound with a covalent C−Cl bond in which the chlorine is not an anion. Other examples of covalent chlorides are carbon tetrachloride CCl4, sulfuryl chloride SO2Cl2 and monochloramine NH2Cl.

Electronic properties

A chloride ion (diameter 167  pm) is much larger than a chlorine atom (diameter 99 pm). The chlorine atom's hold on the valence shell is weaker because the chloride anion has one more electron than it does. [5] The ion is colorless and diamagnetic. In aqueous solution, it is highly soluble in most cases; however, for some chloride salts, such as silver chloride, lead(II) chloride, and mercury(I) chloride, they are only slightly soluble in water. [6] In aqueous solution, chloride is bonded by the protic end of the water molecules.

Reactions of chloride

Chloride can be oxidized but not reduced. The first oxidation, as employed in the chlor-alkali process, is conversion to chlorine gas. Chlorine can be further oxidized to other oxides and oxyanions including hypochlorite (ClO, the active ingredient in chlorine bleach), chlorine dioxide (ClO2), chlorate (ClO
), and perchlorate (ClO

In terms of its acid–base properties, chloride is a weak base as indicated by the negative value of the pKa of hydrochloric acid. Chloride can be protonated by strong acids, such as sulfuric acid:

NaCl + H2SO4 → NaHSO4 + HCl

Ionic chloride salts reaction with other salts to exchange anions. The presence of halide ions like chloride can be detected using silver nitrate. A solution containing chloride ions will produce a white silver chloride precipitate: [7]

Cl + Ag+ → AgCl

The concentration of chloride in an assay can be determined using a chloridometer, which detects silver ions once all chloride in the assay has precipitated via this reaction.

Chlorided silver electrodes are commonly used in ex vivo electrophysiology. [8]

Other oxyanions

Chlorine can assume oxidation states of −1, +1, +3, +5, or +7. Several neutral chlorine oxides are also known.

Chlorine oxidation state−1+1+3+5+7
Namechloride hypochlorite chlorite chlorate perchlorate
Structure Chloride-ion-3D-vdW.png Hypochlorite-3D-vdW.png Chlorite-3D-vdW.png Chlorate-3D-vdW.png Perchlorate-3D-vdW.png

Occurrence in nature

In nature, chloride is found primarily in seawater, which has a chloride ion concentration of 19400 mg/liter. [9] Smaller quantities, though at higher concentrations, occur in certain inland seas and in subterranean brine wells, such as the Great Salt Lake in Utah and the Dead Sea in Israel. [10] Most chloride salts are soluble in water, thus, chloride-containing minerals are usually only found in abundance in dry climates or deep underground. Some chloride-containing minerals include halite (sodium chloride NaCl), sylvite (potassium chloride KCl), bischofite (MgCl2∙6H2O), carnallite (KCl∙MgCl2∙6H2O), and kainite (KCl∙MgSO4∙3H2O). It is also found in evaporite minerals such as chlorapatite and sodalite.

Role in biology

Chloride has a major physiological significance, which includes regulation of osmotic pressure, electrolyte balance and acid-base homeostasis. Chloride is present in all body fluids, [11] and is the most abundant extracellular anion which accounts for around one third of extracellular fluid's tonicity. [12] [13]

Chloride is an essential electrolyte, playing a key role in maintaining cell homeostasis and transmitting action potentials in neurons. [14] It can flow through chloride channels (including the GABAA receptor) and is transported by KCC2 and NKCC2 transporters.

Chloride is usually (though not always) at a higher extracellular concentration, causing it to have a negative reversal potential (around −61 mV at 37 °C in a mammalian cell). [15] Characteristic concentrations of chloride in model organisms are: in both E. coli and budding yeast are 10–200  mM (dependent on medium), in mammalian cells 5–100 mM and in blood plasma 100 mM. [16]

The concentration of chloride in the blood is called serum chloride, and this concentration is regulated by the kidneys. A chloride ion is a structural component of some proteins; for example, it is present in the amylase enzyme. For these roles, chloride is one of the essential dietary mineral (listed by its element name chlorine). Serum chloride levels are mainly regulated by the kidneys through a variety of transporters that are present along the nephron. [17] Most of the chloride, which is filtered by the glomerulus, is reabsorbed by both proximal and distal tubules (majorly by proximal tubule) by both active and passive transport. [18]


The structure of sodium chloride, revealing the tendency of chloride ions (green spheres) to link to several cations. NaCl polyhedra.png
The structure of sodium chloride, revealing the tendency of chloride ions (green spheres) to link to several cations.

The presence of chlorides, such as in seawater, significantly worsens the conditions for pitting corrosion of most metals (including stainless steels, aluminum and high-alloyed materials). Chloride-induced corrosion of steel in concrete lead to a local breakdown of the protective oxide form in alkaline concrete, so that a subsequent localized corrosion attack takes place. [19]

Environmental threats

Increased concentrations of chloride can cause a number of ecological effects in both aquatic and terrestrial environments. It may contribute to the acidification of streams, mobilize radioactive soil metals by ion exchange, affect the mortality and reproduction of aquatic plants and animals, promote the invasion of saltwater organisms into previously freshwater environments, and interfere with the natural mixing of lakes. Sodium chloride has also been shown to change the composition of microbial species at relatively low concentrations. It can also hinder the denitrification process, a microbial process essential to nitrate removal and the conservation of water quality, and inhibit the nitrification and respiration of organic matter. [20]


The chlor-alkali industry is a major consumer of the world's energy budget. This process converts sodium chloride into chlorine and sodium hydroxide, which are used to make many other materials and chemicals. The process involves two parallel reactions:

2 ClCl
+ 2  e
2 H
+ 2 e → H2 + 2 OH
Basic membrane cell used in the electrolysis of brine. At the anode (A), chloride (Cl ) is oxidized to chlorine. The ion-selective membrane (B) allows the counterion Na to freely flow across, but prevents anions such as hydroxide (OH ) and chloride from diffusing across. At the cathode (C), water is reduced to hydroxide and hydrogen gas. Chloralkali membrane.svg
Basic membrane cell used in the electrolysis of brine. At the anode (A), chloride (Cl ) is oxidized to chlorine. The ion-selective membrane (B) allows the counterion Na to freely flow across, but prevents anions such as hydroxide (OH ) and chloride from diffusing across. At the cathode (C), water is reduced to hydroxide and hydrogen gas.

Examples and uses

An example is table salt, which is sodium chloride with the chemical formula NaCl. In water, it dissociates into Na+ and Cl ions. Salts such as calcium chloride, magnesium chloride, potassium chloride have varied uses ranging from medical treatments to cement formation. [4]

Calcium chloride (CaCl2) is a salt that is marketed in pellet form for removing dampness from rooms. Calcium chloride is also used for maintaining unpaved roads and for fortifying roadbases for new construction. In addition, calcium chloride is widely used as a de-icer, since it is effective in lowering the melting point when applied to ice. [21]

Examples of covalently-bonded chlorides are phosphorus trichloride, phosphorus pentachloride, and thionyl chloride, all three of which are reactive chlorinating reagents that have been used in a laboratory.

Water quality and processing

A major application involving chloride is desalination, which involves the energy intensive removal of chloride salts to give potable water. In the petroleum industry, the chlorides are a closely monitored constituent of the mud system. An increase of the chlorides in the mud system may be an indication of drilling into a high-pressure saltwater formation. Its increase can also indicate the poor quality of a target sand.[ citation needed ]

Chloride is also a useful and reliable chemical indicator of river and groundwater fecal contamination, as chloride is a non-reactive solute and ubiquitous to sewage and potable water. Many water regulating companies around the world utilize chloride to check the contamination levels of the rivers and potable water sources. [22]


Chloride salts such as sodium chloride are used to preserve food and as nutrients or condiments.

See also

Related Research Articles

<span class="mw-page-title-main">Alkali metal</span> Group of highly reactive chemical elements

The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). Together with hydrogen they constitute group 1, which lies in the s-block of the periodic table. All alkali metals have their outermost electron in an s-orbital: this shared electron configuration results in their having very similar characteristic properties. Indeed, the alkali metals provide the best example of group trends in properties in the periodic table, with elements exhibiting well-characterised homologous behaviour. This family of elements is also known as the lithium family after its leading element.

<span class="mw-page-title-main">Chlorine</span> Chemical element, symbol Cl and atomic number 17

Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine is a yellow-green gas at room temperature. It is an extremely reactive element and a strong oxidising agent: among the elements, it has the highest electron affinity and the third-highest electronegativity on the revised Pauling scale, behind only oxygen and fluorine.

<span class="mw-page-title-main">Electrochemistry</span> Branch of chemistry

Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outcome of a particular chemical change, or vice versa. These reactions involve electrons moving via an electronically-conducting phase between electrodes separated by an ionically conducting and electronically insulating electrolyte.

<span class="mw-page-title-main">Halogen</span> Group of chemical elements

The halogens are a group in the periodic table consisting of six chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), astatine (At), and tennessine (Ts), though some authors would exclude tennessine as its chemistry is unknown and is theoretically expected to be more like that of gallium. In the modern IUPAC nomenclature, this group is known as group 17.

<span class="mw-page-title-main">Hydroxide</span> Chemical compound

Hydroxide is a diatomic anion with chemical formula OH. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, a ligand, a nucleophile, and a catalyst. The hydroxide ion forms salts, some of which dissociate in aqueous solution, liberating solvated hydroxide ions. Sodium hydroxide is a multi-million-ton per annum commodity chemical. The corresponding electrically neutral compound HO is the hydroxyl radical. The corresponding covalently bound group –OH of atoms is the hydroxy group. Both the hydroxide ion and hydroxy group are nucleophiles and can act as catalysts in organic chemistry.

<span class="mw-page-title-main">Ionic bonding</span> Chemical bonding involving attraction between ions

Ionic bonding is a type of chemical bonding that involves the electrostatic attraction between oppositely charged ions, or between two atoms with sharply different electronegativities, and is the primary interaction occurring in ionic compounds. It is one of the main types of bonding, along with covalent bonding and metallic bonding. Ions are atoms with an electrostatic charge. Atoms that gain electrons make negatively charged ions. Atoms that lose electrons make positively charged ions. This transfer of electrons is known as electrovalence in contrast to covalence. In the simplest case, the cation is a metal atom and the anion is a nonmetal atom, but these ions can be of a more complex nature, e.g. molecular ions like NH+
or SO2−
. In simpler words, an ionic bond results from the transfer of electrons from a metal to a non-metal in order to obtain a full valence shell for both atoms.

In chemistry, a salt is a chemical compound consisting of an ionic assembly of positively charged cations and negatively charged anions, which results in a compound with no net electric charge. A common example is table salt, with positively charged sodium ions and negatively charged chloride ions.

<span class="mw-page-title-main">Electrolysis</span> Technique in chemistry and manufacturing

In chemistry and manufacturing, electrolysis is a technique that uses direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of elements from naturally occurring sources such as ores using an electrolytic cell. The voltage that is needed for electrolysis to occur is called the decomposition potential. The word "lysis" means to separate or break, so in terms, electrolysis would mean "breakdown via electricity".

An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon dissolving, the substance separates into cations and anions, which disperse uniformly throughout the solvent. Solid-state electrolytes also exist. In medicine and sometimes in chemistry, the term electrolyte refers to the substance that is dissolved.

<span class="mw-page-title-main">Ammonium</span> Polyatomic ion (NH₄, charge +1)

The ammonium cation is a positively-charged polyatomic ion with the chemical formula NH+4 or [NH4]+. It is formed by the protonation of ammonia. Ammonium is also a general name for positively charged or protonated substituted amines and quaternary ammonium cations, where one or more hydrogen atoms are replaced by organic groups.

<span class="mw-page-title-main">Sodium chloride</span> Chemical compound with formula NaCl

Sodium chloride, commonly known as salt, is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35.45 g/mol respectively, 100 g of NaCl contains 39.34 g Na and 60.66 g Cl. Sodium chloride is the salt most responsible for the salinity of seawater and of the extracellular fluid of many multicellular organisms. In its edible form, salt is commonly used as a condiment and food preservative. Large quantities of sodium chloride are used in many industrial processes, and it is a major source of sodium and chlorine compounds used as feedstocks for further chemical syntheses. Another major application of sodium chloride is de-icing of roadways in sub-freezing weather.

The chlorite ion, or chlorine dioxide anion, is the halite with the chemical formula of ClO
. A chlorite (compound) is a compound that contains this group, with chlorine in the oxidation state of +3. Chlorites are also known as salts of chlorous acid.

<span class="mw-page-title-main">Chlorate</span> Anion and term for chemical compounds containing it

The chlorate anion has the formula ClO3-. In this case, the chlorine atom is in the +5 oxidation state. "Chlorate" can also refer to chemical compounds containing this anion; chlorates are the salts of chloric acid. "Chlorate", when followed by a Roman numeral in parentheses, e.g. chlorate (VII), refers to a particular oxyanion of chlorine.

<span class="mw-page-title-main">Hypochlorite</span> Ion

In chemistry, hypochlorite is an anion with the chemical formula ClO. It combines with a number of cations to form hypochlorite salts. Common examples include sodium hypochlorite and calcium hypochlorite. The Cl-O distance in ClO is 1.69 Å.

The chloralkali process is an industrial process for the electrolysis of sodium chloride (NaCl) solutions. It is the technology used to produce chlorine and sodium hydroxide, which are commodity chemicals required by industry. Thirty five million tons of chlorine were prepared by this process in 1987. The chlorine and sodium hydroxide produced in this process are widely used in the chemical industry.

<span class="mw-page-title-main">Counterion</span> Ion which negates another oppositely-charged ion in an ionic molecule

In chemistry, a counterion is the ion that accompanies an ionic species in order to maintain electric neutrality. In table salt the sodium ion is the counterion for the chloride ion and vice versa.

<span class="mw-page-title-main">Oxygen compounds</span>

The oxidation state of oxygen is −2 in almost all known compounds of oxygen. The oxidation state −1 is found in a few compounds such as peroxides. Compounds containing oxygen in other oxidation states are very uncommon: −12 (superoxides), −13 (ozonides), 0, +12 (dioxygenyl), +1, and +2.

Chlorine gas can be produced by extracting from natural materials, including the electrolysis of a sodium chloride solution (brine) and other ways.

<span class="mw-page-title-main">Lead compounds</span> Type of compound

Compounds of lead exist with lead in two main oxidation states: +2 and +4. The former is more common. Inorganic lead(IV) compounds are typically strong oxidants or exist only in highly acidic solutions.

Mixed oxidant solution is a type of disinfectant which is used for disinfecting, sterilization and eliminating pathogenic microorganisms in water and in many other applications. Using a mixed oxidant solution for water disinfection, compared to other methods, may have various benefits such as higher disinfecting power, stable residual chlorine in water, improved taste and smell, elimination of biofilm, and safety. A mixed-oxidant solution is produced by electrolysis of sodium chloride and is a mixture of disinfecting compounds. The main component of this product is chlorine and its derivatives (ClO, HClO and Cl2 solution). It may also contain high amounts of chlorine dioxide solution, dissolved ozone, hydrogen peroxide, and oxygen, from which the name "mixed oxidant" is derived.


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