chloride

 

(klōr'īd, klôr'–) , chemical compound containing chlorine. Most chlorides are salts that are formed either by direct union of chlorine with a metal or by reaction of hydrochloric acid (a water solution of hydrogen chloride) with a metal, a metal oxide, or an inorganic base. Chloride salts include sodium chloride (common salt), potassium chloride, calcium chloride, and ammonium chloride. Most chloride salts are readily soluble in water, but mercurous chloride (calomel) and silver chloride are insoluble, and lead chloride is only slightly soluble. Some chlorides, e.g., antimony chloride and bismuth chloride, decompose in water, forming oxychlorides. Many metal chlorides can be melted without decomposition; two exceptions are the chlorides of gold and platinum. Most metal chlorides conduct electricity when fused or dissolved in water and can be decomposed by electrolysis to chlorine gas and the metal. Chlorine forms compounds with the other halogens and with oxygen; when chlorine is the more electronegative element in the compound, the compound is called a chloride. Thus, compounds with bromine and iodine are bromine chloride, BrCl, and iodine chloride, ICI, but compounds with oxygen or fluorine (which are more electronegative than chlorine) are oxides (e.g., chlorine dioxide, ClO₂) or fluorides (e.g., chlorine fluoride, ClF) respectively. Many organic compounds contain chlorine, as is indicated by common names such as carbon tetrachloride, methylene chloride, and methyl chloride. However, in the nomenclature system for organic chemistry adopted by the International Union of Pure and Applied Chemistry (IUPAC), the presence in a compound of chlorine bonded to a carbon atom is indicated by the prefix or infix chloro; thus, carbon tetrachloride is tetrachloromethane, methylene chloride is dichloromethane, and methyl chloride is chloromethane.

The chloride ion is formed when the element chlorine picks up one electron to form an anion (negatively-charged ion) Cl⁻. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. An example is table salt, which is sodium chloride with the chemical formula NaCl. In water, it dissolves into Na⁺ and Cl⁻ ions.

The word chloride can also refer to a chemical compound in which one or more chlorine atoms are covalently bonded in the molecule. This means that chlorides can be either inorganic or organic compounds. The simplest example of an inorganic covalently-bonded chloride is hydrogen chloride, HCl. A simple example of an organic covalently-bonded (an organochloride) chloride is chloromethane (CH₃Cl), often called methyl chloride.

Other examples of inorganic covalently-bonded chlorides that are used as reactants are:

• phosphorus trichloride, phosphorus pentachloride, and thionyl chloride, all three of which reactive chlorinating reagents that have been used in a laboratory
• disulfur dichloride (S₂Cl₂), used for vulcanization of rubber.

Human health

Chloride ions have important physiological roles. For instance, in the central nervous system, the inhibitory action of glycine and some of the action of GABA relies on the entry of Cl⁻ into specific neurons. Also, the chloride-bicarbonate exchanger biological transport protein relies on the chloride ion to increase the blood's capacity of carbon dioxide, in the form of the bicarbonate ion.

The North American Dietary Reference Intake recommends a daily intake of between 2300 and 3600 mg/day for 25-year-old males.

Other applications

Chloride is also a useful and reliable chemical indicator of river / groundwater faecal contamination, as chloride is a non-reactive solute and ubiquitous to sewage.

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