Solutions which contain halide ions will react with silver ions (Ag+) to form insoluble precipitates of silver halides; this is shown in the equation below:
We can test for the presence of halide ions (Cl-, Br- and I-) using two simple techniques:
As shown in the equation above when a few drops of a silver nitrate solution is added to a solution containing either a chloride, bromide or iodide ion an insoluble precipitate of the silver halides is formed.
The colour of the solid precipitate formed varies depending
on the halide ion present, the colours of these solid silver halide precipitates give a good hint at which type of halide ion is present, the colours of these solid silver halide precipitates are shown in the image opposite.
The test is very simple to carry out. Simply add dilute nitric acid to an aqueous solution which contains a suspected halide ion
in a boiling tube. Addition of silver nitrate to this acidified halide ion solution
will produce an insoluble precipitate 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 needed 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 if any of these ions are present as an impurity they will react with the nitric acid and so will not be able to form any precipitates which would
obviously interfere with the test for the halide ion and give a potentially false result.
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 sodium bromide or iodide solutions. Here precipitates 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, equations to show the formation of these two coloured precipitates is shown below:
Silver bromide like silver chloride is light sensitive and breaks down in a similar fashion to silver chloride however silver iodide is more stable when exposed to light. However the yellow colour of solid silver iodide generally has a greyish hint to it due to the presence of metallic silver, produced by the photochemical decomposition of silver iodide when it is exposed to sunlight.
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, for example:
| 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 precipitate 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 |