Tests for Halide Ions (Chloride, Bromide, Iodide)

Purpose of Halide Ion Tests

Halide ion tests are used to identify the presence of chloride (Cl9), bromide (Br9), and iodide (I9) ions in a compound. These tests help distinguish between these ions by producing different coloured precipitates when reacted with silver nitrate solution. The colour and solubility of the precipitate confirm which halide ion is present.

Procedure for Halide Ion Tests

The standard procedure involves:

• Adding dilute nitric acid (HNO 3) to the sample solution. This removes any carbonate or hydroxide impurities that might interfere with the test, preventing the formation of unwanted precipitates such as silver carbonate or silver hydroxide.
• Adding silver nitrate (AgNO 3) solution to the acidified sample.
• Observing the formation of a precipitate and noting its colour.

For instance, if a solution contains chloride ions, adding dilute nitric acid followed by silver nitrate will produce a white precipitate of silver chloride.

Example:

If you add dilute nitric acid and then silver nitrate to a solution containing chloride ions, a white precipitate forms. This indicates the presence of chloride ions.

Precipitate Colours and Solubility

Each halide ion produces a characteristic silver halide precipitate with silver nitrate:

• Chloride (Cl9): White precipitate of silver chloride (AgCl)
• Bromide (Br9): Cream precipitate of silver bromide (AgBr)
• Iodide (I9): Yellow precipitate of silver iodide (AgI)

To further confirm the identity of the halide ion, the precipitates solubility in ammonia solution is tested:

• Silver chloride (AgCl): Dissolves in dilute ammonia solution.
• Silver bromide (AgBr): Does not dissolve in dilute ammonia but dissolves in concentrated ammonia solution.
• Silver iodide (AgI): Insoluble in both dilute and concentrated ammonia solutions.

Example:

A cream precipitate forms when silver nitrate is added to a solution. When this precipitate is added to dilute ammonia, it does not dissolve. However, it dissolves in concentrated ammonia. This confirms the halide ion is bromide.