What is a bromide

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Detection of bromide

Pre-trial

A preliminary test is not actually proof, but often provides helpful information about the presence or absence of an element.

For the preliminary test, the solid substance to be analyzed is mixed with concentrated sulfuric acid and, if necessary, heated. If there is bromide, it escapes as brown vapor:

warning
Beware of the possible presence of chlorate,: This creates chlorine dioxide, which decomposes explosively. This preliminary test may only be carried out in the fume hood.

Precipitation of silver bromide

The clear solution of the soda extract is acidified with dilute nitric acid (check with paper!) And silver nitrate solution is added drop by drop. If bromide is present, a light yellow, light-sensitive precipitate is formed which, after centrifugation and washing with water, dissolves in concentrated ammonia solution, but not in dilute ammonia solution or in ammonium carbonate solution. When the ammonia solution is diluted, silver bromide precipitates again.

The light yellow precipitate (solubility product) is stable in dilute nitric acid. At an ammonia concentration, as it is present in concentrated ammonia solution, the silver diammine complex is formed,. If the ammonia concentration is reduced by dilution with water, it precipitates again, any existing remains in solution.

Note
Under the specified conditions, chloride also precipitate as white silver chloride,, and iodide as lemon-yellow silver iodide,,. Silver chloride dissolves again in ammonium carbonate solution with the formation of the silver diammine complex, silver iodide does not dissolve even in concentrated ammonia solution.

Oxidation to elemental bromine

The colorless solution of the soda extract is acidified with dilute sulfuric acid and underlaid with chloroform or dichloromethane (approx. 1). After adding a few drops of chlorine-water, the organic phase turns brown. Another addition of chlorine-water causes the brown color to lighten.

Bromide is oxidized by chlorine to bromine, which dissolves in the organic phase with a brown color. Adding excess chlorine results in the formation of pale yellow bromine chloride, an interhalogen compound.

Note
If the analysis contains iodide, this is primarily oxidized to iodine under the specified conditions, which dissolves in the organic phase with a purple color.
  1. If bromide and iodide are present next to each other, the slow, dropwise addition of chlorine-water causes the following to precipitate first because of the easier oxidizability (stronger reducing agent):: violet
  2. The iodine is then further oxidized to: /: colorless
  3. Then the weaker reducing agent bromide is oxidized to bromine:: brown
  4. If more chlorine-water is added, the bromine is further oxidized:: yellowish

If you work carefully with an excess of chlorine-water, bromide can be detected alongside iodide. To do this, you have to add chlorine-water until the organic phase, which is initially purple in color from the iodine, has turned brown (positive bromide detection) or has completely discolored (negative bromide detection).