3.2.5 Group 7(17), the Halogens - Identification of halide ions using silver nitrate

Specification

The reactions of Halide ions with Silver ions

Silver ions form insoluble products with halide ions. The colour and behaviour of these silver halides can be used to identify the halide ion used:

• Silver chloride is a white insoluble solid
• Silver bromide is a creamy white insoluble solid.
• Silver iodide is a yellow insoluble solid

All three precipitates darken on exposure to light. The light stimulates the reduction of the silver ions to silver atoms. This is the original basis behind black and white photography, which used silver bromide as the light sensitive compound.

The general equation for their formation is as follows:

Ag⁺(aq) + X^-(aq) --> AgX(s)

NOTE: The silver ions are added in the form of acidified (using nitric acid) silver nitrate solution, as this is one of the few soluble silver salts. The nitric acid is there to decompose and carbonate ions present, which would interfere with the test and give a false positive white precipitate of silver carbonate.

2Ag⁺(aq) + CO₃^-(aq) --> Ag₂CO₃(s)

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Using ammonia

The silver halide precipitate identity can be further confirmed by addition of ammonia solution. The ammonia molecules present in the ammonia solution form a complex ion with silver(I) ions which then allows the precipitate to re-dissolve.

However, the ease with which this happens depends on the nature of the halide. Silver chloride forms the complex ion easily and dilute ammonia solution suffices:

NH₃ + AgCl --> [Ag(NH₃)₂]⁺ + Cl^-

The silver complex ion, diammine silver(I), is a linear ion with the ammonia ligands arranged at 180 degrees to one another.

Silver bromide also dissolves in ammonia, but only if the ammonia is in concentrated solution.

NH₃ + AgBr --> [Ag(NH₃)₂]⁺ + Br^-

Silver iodide does not dissolve in even concentrated ammonia.

This then affords a means of confirming the identity of the halide ion present. It's not much use for fluoride ions, however, as silver fluoride is soluble in water and so fluoride ions do not form a precipitate with silver ions.

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Summary

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