A bromide is a chemical compound containing a bromide ion or ligand. This is a bromine atom with an ionic charge of −1 (Br−); for example, in caesium bromide, caesium cations (Cs+) are electrically attracted to bromide anions (Br−) to form the electrically neutral ionic compound CsBr. The term "bromide" can also refer to a bromine atom with an oxidation number of −1 in covalent compounds such as sulfur dibromide (SBr2).
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 and deep sea plants generally have high levels of bromide, while foods derived from land have variable amounts. Bromargyryte - natural, crystalline silver bromide - is the most common bromide mineral currently known. It is still very rare. Beside silver, bromine is sometimes found in minerals combined with mercury and copper.
Bromide compounds, especially potassium bromide, were frequently used as sedatives in the 19th and early 20th century. Their use in over-the-counter sedatives and headache remedies (such as Bromo-Seltzer) in the United States extended to 1975, when bromides were withdrawn as ingredients, due to chronic toxicity.
The bromide ion is antiepileptic, and bromide salts are still used as such, particularly in veterinary medicine. Bromide ion is excreted by the kidneys. The half-life of bromide in the human body (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.
Since bromide is still used in veterinary medicine (particularly to treat seizures in dogs) in the United States, veterinary diagnostic labs can routinely measure blood bromide levels. However, this is not a conventional test in human medicine in the U.S., since there are no FDA-approved uses for bromide, and (as noted) it is no longer available in over-the-counter sedatives. Therapeutic bromide levels are measured in European countries like Germany, where bromide is still used therapeutically in human epilepsy.
Lithium bromide was used as a sedative beginning in the early 1900s, but it fell into disfavor in the 1940s, possibly due to the rising popularity of safer and more efficient sedatives (specifically, barbiturates) and when some heart patients died after using a salt substitute (see lithium chloride). Like lithium carbonate and lithium chloride it was used as treatment for bipolar disorder.
It has been said that during World War I, British soldiers were given bromide to curb their sexual urges, although this is not well supported by documentation, and has been disputed as an urban myth, as the sedative effects of bromide would have hampered military performance. Lord Dunsany mentions a soldier being given bromide as a sedative for nervous exhaustion and overwork in his play Fame and the Poet (1919).
There are more substantiated reports that bromide was used in the food served at some concentration camps during the Holocaust. This was apparently done in an effort to both chemically restrain the interned and prevent menstruation in females.
According to one study, bromine (as bromide) is an essential cofactor in the peroxidasin catalysis of sulfilimine crosslinks in collagen IV. This post-translational modification occurs in all animals, and bromine is an essential trace element for humans.
Bromide is needed by eosinophils (white blood cells of the granulocyte class, specialized for dealing with multi-cellular parasites), which use it to generate antiparasitic brominating compounds such as hypobromite, by the action of eosinophil peroxidase, a haloperoxidase enzyme which is able to use chloride, but preferentially uses bromide when available. Other than its role in collagen IV production and its facultative use in eosinophils by the body, bromide is not known in other cases necessary for animal life, as its functions may generally be replaced (though in some cases not as well) by chloride. Land plants do not use bromide.
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.
Bromide is perhaps a minor necessary nutrient for collagen IV-producing animals in the sea. However, a few sea animals, such as Murex snails, use bromide to make organic compounds. Bromide ion is also heavily concentrated by some species of ocean algae, which construct methyl bromide and a great number of bromoorganic compounds with it, using the unusual enzymes called vanadium bromoperoxidases to do these reactions.
The average concentration of bromide in human blood in Queensland, Australia 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, since bromide occurs in relatively high concentration in seawater and many types of seafood, bromide concentrations in the blood are heavily influenced by seafood contributions to the diet.