Testing for halide ions using silver nitrate

Testing for halide ions

We can test for the presence of halide ions using two simple techniques:

Precipitation reactions using silver nitrate solution

colours of silver halide precipitates after addition of silver nitrate 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:

sodium chloride(aq) + silver nitrate(aq) silver chloride(s) + sodium nitrate(aq)
NaCl(s) + AgNO3(aq)AgCl(s) + NaNO3(aq)
The solid precipitate of silver chloride produced can be seen as a WHITE solid on the bottom of the boiling tube (see image above). The silver chloride precipitate produced is a light sensitive chemical and if left exposed to sunlight for more than a few mintes darkens to form a grey-violet solid. This is due to the fact that silver chloride decomposes to form metallic silver: Silver nitrate is a photosensitive chemical and is stored in dark bottles to keep out sunlight
silver chloride(s) → silver(s) + chlorine(g)
2AgCl(s) → Ag(s) + Cl2(g)

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.

NaBr(s) + AgNO3(aq)AgBr(s) + NaNO3(aq)
and
NaI(s) + AgNO3(aq) AgI(s) + NaNO3(aq)
Silver bromide like silver chloride is light senstive 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.

Solubility of the silver halides in ammonia solution

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.

Key Points

The table below summarises the results of halide testing:
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

Practice questions

Check your understanding - Questions on testing for halides.

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