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Concept
Bromide
Summary
A bromide ion is the negatively charged form (Br−) of the element bromine, a member of the halogens group on the periodic table. Most bromides are colorless. Bromides have many practical roles, being found in anticonvulsants, flame-retardant materials, and cell stains. Although uncommon, 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. The bromide ion has an ionic radius of 196 pm.
Bromide is present in typical seawater (35 PSU) with a concentration of around 65 mg/L, which is about 0.2% of all dissolved salts. Seafood and deep sea plants generally have higher levels than land-derived foods. Bromargyrite—natural, crystalline silver bromide—is the most common bromide mineral known but is still very rare. In addition to silver, bromine is also in minerals combined with mercury and copper.
Bromide salts of alkali metal, alkaline earth metals, and many other metals dissolve in water (and even some alcohols and a few ethers) to give bromide ions. The classic case is sodium bromide, which fully dissociates in water:
NaBr → Na+ + Br−
Hydrogen bromide, which is a diatomic molecule, takes on salt-like properties upon contact with water to give an ionic solution called hydrobromic acid. The process is often described simplistically as involving formation of the hydronium salt of bromide:
HBr + H2O → H3O+ + Br−
Bromine readily reacts with water, i.e. it undergoes hydrolysis:
Br2 + H2O → HOBr + HBr
This forms hypobromous acid (HOBr), and hydrobromic acid (HBr in water). The solution is called "bromine water". The hydrolysis of bromine is more favorable in the presence of base, for example sodium hydroxide:
Br2 + NaOH → NaOBr + NaBr
This reaction is analogous to the production of bleach, where chlorine is dissolved in the presence of sodium hydroxide.
One can test for a bromide ion by adding an oxidizer. One method uses dilute HNO3.
Balard and Löwig's method can be used to extract bromine from seawater and certain brines.
Official source
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