Bromide

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A bromide is a chemical compound containing bromide ion, that is bromine atom with effective charge of −1. The class name can include ionic compounds such as caesium bromide or covalent compounds such as sulfur dibromide.

Natural occurrence

Bromide is present in typical seawater (35 PSU) with a concentration of around 65 mg/L, which is around 0.2% of all dissolved salts. Seafoods generally have high levels of bromide, while foods derived from land have variable amounts.

Chemistry

One can test for a bromide ion by adding dilute nitric acid (HNO₃), then silver nitrate (AgNO₃). A cream precipitate forms.

Medical uses

Bromide compounds, especially potassium bromide, were frequently used as sedatives in the 19th and early 20th century. This gave the word "bromide" its colloquial connotation of a boring cliché, a bit of conventional wisdom overused as a sedative.

The bromide ion is antiepileptic, and bromide salts are still used as such, particularly in veterinary medicine. The half-life of bromide in human kidneys (12 days) is long compared with many pharmaceuticals, making dosing difficult to adjust (a new dose may require several months to reach equilibrium). Bromide ion concentrations in the cerebrospinal fluid are about 30% of those in blood, and are strongly influenced by the body's chloride intake and metabolism.

Chronic toxicity from bromide can result in bromism, a syndrome with multiple neurological symptoms. Bromide toxicity can also cause a type of skin eruption. See potassium bromide.

Lithium bromide was used as a sedative beginning in the early 1900s, but it fell into disfavor in the 1940s when some heart patients died after using it as a salt substitute. Like lithium carbonate and lithium chloride it was used as treatment for bipolar disorder.

In biology

Bromide is needed by eosinophils (white blood cells of the granulocyte class, specialised for dealing with multi-cellular parasites), which use it to generate antiparasitic brominating compounds by the action of eosinophil peroxidase, a haloperoxidase enzyme which is able to use chloride, but preferentially uses bromide when available. Despite this use by the body, bromide is not known to be strictly necessary for life, as its functions may generally be replaced (though in some cases not as well) by chloride.

Bromide salts are also sometimes used in hot tubs and spas as mild germicidal agents, using the action of an added oxidizing agent to generate in situ hypobromite, in a similar fashion to the peroxidase in eosinophils.

The average concentration of bromide in human blood is 5.3±1.4 mg/L and varies with age and gender. Much higher levels may indicate exposure to brominated chemicals (e.g. methyl bromide). However, bromide occurs in relatively high concentration in seawater and many types of seafood, and bromide concentrations in the blood are heavily influenced by seafood contributions to the diet.

History

In some countries, bromide salts remain available in a liquid form at pharmacies, although since the 1950s they have been removed as over-the-counter sedatives in most countries in the West.

References

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