Because the colours of the silver halide precipitates can be hard to tell apart, ammonia solution is added to the precipitate to confirm which halide is present. The silver halides dissolve in ammonia to different extents:AgCl (white): dissolves in dilute ammonia, giving a colourless solution.
AgBr (cream): does not dissolve in dilute ammonia, but dissolves in concentrated ammonia.
AgI (yellow): insoluble even in concentrated ammonia.A precipitate dissolves because ammonia molecules bond to the silver ions, forming a soluble complex ion, the diamminesilver(I) ion:AgCl(s) + 2NH3(aq) → [Ag(NH3)2]+(aq) + Cl−(aq) The solubility of the silver halides in ammonia decreases from AgCl to AgBr to AgI. Silver iodide is the least soluble, so even concentrated ammonia cannot dissolve it.Concentrated ammonia gives off a pungent, toxic and irritating vapour, so it should be used in a fume cupboard.Worked example: two precipitates from the silver nitrate test both look pale yellow. Dilute ammonia is added to each and neither dissolves. Concentrated ammonia is then added: precipitate 1 dissolves, but precipitate 2 does not.
Precipitate 1 dissolves only in concentrated ammonia, so it is silver bromide, and the solution contained bromide ions:
AgBr(s) + 2NH3(aq) → [Ag(NH3)2]+(aq) + Br−(aq)
Precipitate 2 is insoluble in concentrated ammonia, so it is silver iodide, and the solution contained iodide ions.
AgBr (cream): does not dissolve in dilute ammonia, but dissolves in concentrated ammonia.
AgI (yellow): insoluble even in concentrated ammonia.A precipitate dissolves because ammonia molecules bond to the silver ions, forming a soluble complex ion, the diamminesilver(I) ion:
Precipitate 1 dissolves only in concentrated ammonia, so it is silver bromide, and the solution contained bromide ions:
Precipitate 2 is insoluble in concentrated ammonia, so it is silver iodide, and the solution contained iodide ions.