The aim here is to produce insoluble precipitates of silver halides.
The colour of the solid precipitate formed varies depending
on the halide ion present.
The test is very simple to carry out. Simply add dilute nitric acid to a solution of various halide ions in a boiling tube. Addition of silver nitrate will produce an insoluble precipiate of the silver halide. Note this test does not work for fluoride ions (F-) since silver fluoride is soluble and so no precipitate is produced. Each of the silver halides produced is a different colour, so it is possible to identify the halide ion present in the initial solution from the colour of the precipitate produced.
The nitric acid is neeeded in case the solution under test contains ions as such as carbonates or hydroxides as an impurity, these ions will also react with the silver nitrate and produce precipitates, however these impurities will react with the nitric acid and so will not be able to form any precipitates which would interfere with the test. As an example consider the addition of a silver nitrate solution to a sodium chloride solution containing a few drops of nitric acid in a boiling tube, an equation for the reaction taking place is given below:
Similar reactions occur if we swap the sodium chloride solution for either the bromide or iodide solutions. Here precipiates of silver bromide and silver iodide are produced. Silver bromide is produced as a cream coloured precipitate while silver iodide is a yellow coloured precipitate.
One of the problems with using the above test to identify chlorides, bromides and iodides is that it can be difficult to tell the
difference between white, cream and yellow coloured precipitates, the differences can be subtle at times and difficult to distinguish, particularly
between white and cream coloured precipitates. However we can use differences in the solubilities of silver chlorides, bromides and
iodides in ammonia solutions to back up the silver nitrate test.
|Test||fluoride ion (F -)||chloride ion (Cl -)||bromide ion (Br -)||iodide ion (I -)|
|addition of silver nitrate solution||no precipitate produced||white precipitate of silver chloride||cream precipitate of silver bromide||yellow precipitat of silver iodide|
|solubility in ammonia solution||no reaction||soluble in dilute ammonia solution||insoluble in dilute ammonia solution but soluble in concentrated ammonia solution||insoluble in dilute and concentrated ammonia solutions|