Testing for Halide Ions (AQA A-Level Chemistry): Revision Notes

2.3.3 Testing for Halide Ions

How to Test for Halide Ions

Halide ions (Cl^-, Br^-, I^-) can be identified in aqueous solutions using silver nitrate (AgNO_3) solution, and the resulting precipitates can be further distinguished using ammonia solution. The procedure for testing halide ions is widely used in qualitative analysis and is essential for identifying different halides.

Method

1. Addition of Nitric Acid (HNO_3)

• Before adding silver nitrate, dilute nitric acid is added to the test solution.
• Purpose: Nitric acid reacts with and removes any impurities such as carbonates (CO_3^{2-}) or hydroxides (OH^-) that could interfere with the test by forming precipitates with silver ions.

2. Addition of Silver Nitrate (AgNO_3)

• After the nitric acid, silver nitrate is added to the solution. The halide ions (if present) will react with the silver ions (Ag^+) to form a precipitate.
• Reactions:
  • Ag^+(aq) + Cl^-(aq) → AgCl(s) (white precipitate)
  • Ag^+(aq) + Br^-(aq) → AgBr(s) (cream precipitate)
  • Ag^+(aq) + I^-(aq) → AgI(s) (yellow precipitate)
• Note: Fluoride ions (F^-) do not form a precipitate with silver nitrate because silver fluoride (AgF) is soluble in water. Therefore, this test cannot be used to detect fluoride ions.

Use of Ammonia Solution

To further distinguish between the precipitates formed by chloride, bromide, and iodide ions, an ammonia solution is used:

Silver Chloride (AgCl)

• Solubility: Dissolves in both dilute and concentrated ammonia to form a colourless solution.

Silver Bromide (AgBr)

• Solubility: Does not dissolve in dilute ammonia, but dissolves in concentrated ammonia to form a colourless solution.

Silver Iodide (AgI)

• Solubility: Insoluble in both dilute and concentrated ammonia. This test is especially useful in distinguishing between bromide and iodide ions, as their precipitates are similar in appearance but differ in their solubility in ammonia.

Displacement Reactions

Halogens can displace less reactive halide ions from solutions. This principle can also be used to test for the presence of halide ions by adding halogen solutions (e.g., chlorine water) to halide salt solutions (e.g., KCl, KBr, KI):

Example reactions

• Chlorine (Cl_2) will displace bromide and iodide ions, forming bromine (Br_2) or iodine (I_2) in solution.
• Bromine (Br_2) can displace iodide ions, forming iodine. Displacement reactions confirm the oxidising power of halogens and provide an additional method of identifying halides in solution.