Testing for halide ions

Here’s how to test for halide ions in a sample:

1. Add a few drops of dilute nitric acid (HNO3) to the sample.

  • The dilute nitric acid is added to remove any carbonate ions. Carbonate ions can also react with silver nitrate to form a white precipitate, which could be confused with the precipitate of halide ions, leading to a false positive result.

2. Add a few drops of dilute silver nitrate (AgNO3) solution.

3. Observe the colour of the precipitate that forms. The colour of the precipitate identifies the halide ion present in the sample.

The table below shows the precipitate colour for each halide ion.

Halide ionPrecipitate colourEquation
Chloride (Cl⁻)WhiteAg⁺ (aq) + Cl⁻ (aq) → AgCl (s)
Bromide (Br⁻)CreamAg⁺ (aq) + Br⁻ (aq) → AgBr (s)
Iodide (I⁻)YellowAg⁺ (aq) + I⁻ (aq) → AgI (s)

AgCl producing a white precipitate in a test tube, AgBr with a cream precipitate in another test tube, and AgI showing a yellow precipitate in a third test tube.