Halide Ions
- Halide ions can be identified in an unknown solution by dissolving the solution in nitric acid and then adding silver nitrate solution dropwise
- The nitric acid is to prevent any false positive results from carbonate ions precipitating out with silver ions
- The halide ions will react with the silver nitrate solution as follows:
Ag+ (aq) + X- (aq) → AgX (s)
(ionic equation)
-
- Where X- is the halide ion
- The state symbols are key in this equation
- If the unknown solution contains halide ions, a precipitate of the silver halide will be formed (AgX)
A silver halide precipitate is formed upon addition of silver nitrate solution to halide ion solution
- Silver chloride (AgCl) is a white precipitate
- Silver bromide (AgBr) is a cream precipitate
- Silver iodide (AgI) is a yellow precipitate
The silver halide precipitates are dense and characteristically coloured
Adding ammonia
- Because the white, cream and yellow precipitates could look very similar in colour, ammonia is often used as a follow up test to determine which halide ion is present
- Dilute followed by concentrated ammonia is added to the silver halide solution to identify the halide ion
- If the precipitate dissolves in dilute ammonia the unknown halide is chloride
- If the precipitate does not dissolve in dilute, but does dissolve in concentrated ammonia the unknown halide is bromide
- If the precipitate does not dissolve in dilute or concentrated ammonia, then the unknown halide is iodide
Silver chloride and silver bromide precipitates dissolve on addition of ammonia solution whereas silver iodide is insoluble in ammonia
Reaction of Halide Ions with Silver Nitrate & Ammonia Solutions
Reaction of Halide Ions with Silver Nitrate & Ammonia Solutions