Chloride

The chloride ion is the anion (negatively charged ion) Cl&minus; . It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride salts such as sodium chloride are often very soluble in water. It is an essential electrolyte located in all body fluids responsible for maintaining acid/base balance, transmitting nerve impulses and regulating fluid in and out of cells. The word chloride can also form part of the name of chemical compounds in which one or more chlorine atoms are covalently bonded. For example, methyl chloride, more commonly called chloromethane, (CH3Cl) is an organic compound with a covalent C-Cl bond. It is not a source of chloride ion.

Electronic properties
Chloride ion is much larger than a chlorine atom, 167 and 99 pm, respectively. The ion is colorless and diamagnetic. In aqueous solution, it is highly soluble in most cases, however some chloride salts, such as silver chloride, lead(II) chloride, and mercury(I) chloride are slightly soluble in water. When in aqueous solution, Chloride is bound by the protic end of the water molecules.

Occurrence in nature
Sea water contains 1.94% chloride. Some chloride-containing minerals include the chlorides of sodium (halite or NaCl), potassium (sylvite or KCl), and magnesium (bischofite), hydrated MgCl2. Called serum chloride, the concentration of chloride in the blood is regulated by the kidneys. A chloride ion is a structural component of some proteins, e.g., it is present in the amylase enzyme.

Role in commerce
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 Cl−  →     +  2 e−
 * 2  +  2 e− →   H2  +  2 OH−



Water quality and processing
Another 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.

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

Domestic uses
Chloride salts such as sodium chloride are used to preserve food.

Corrosion
The presence of chlorides, e.g. in seawater, significantly aggravates the conditions for pitting corrosion of most metals (including stainless steels, aluminum, aluminum alloys, and high-alloyed materials) by enhancing the formation and growth of the pits through an autocatalytic process.
 * [[Image:Halit-Kristalle.jpg|thumb|right|220px|Crystals of sodium chloride, which, like most chloride salts is colorless and water-soluble.]]
 * [[Image:NaCl polyhedra.png|thumb|right|220px|The structure of sodium chloride, revealing the tendency of chloride ions (green spheres) to link to several cations.]]

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, chlorate (ClO3−), and perchlorate (ClO4−).

In terms of its acid-base properties, chloride is a very 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 chloride is often detected by its formation of an insoluble silver chloride upon treatment with silver ion:
 * Cl− +  Ag+   →   AgCl

Examples
An example is table salt, which is sodium chloride with the chemical formula NaCl. In water, it dissociates into Na+ and Cl&minus; ions. Salts such as calcium chloride, magnesium chloride, potassium chloride have varied uses ranging from medical treatments to cement formation. Another example is calcium chloride with the chemical formula CaCl2. Calcium chloride 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.

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

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