Testing halide ions involves forming distinctive silver–halide precipitates and confirming their identity using ammonia. These reactions allow reliable differentiation between chloride, bromide and iodide ions in qualitative analysis.

Testing for Halide Ions: Core Principles

Halide ions (Cl⁻, Br⁻, I⁻) are identified using silver nitrate, followed by confirmatory dissolution tests with aqueous ammonia. These steps are central to OCR A-Level qualitative analysis.

Halide identification relies on the low solubility of many silver halides, which produce characteristic colours that differ sufficiently to allow confident visual distinction. The follow-up ammonia test strengthens reliability by ensuring that similar-looking precipitates can still be differentiated.

Preparing the Test Sample

Before carrying out halide testing, the sample must be acidified using dilute nitric acid. This prevents false positives by removing interfering anions.

Acidifying with nitric acid avoids unwanted formation of insoluble carbonates or sulfites, both of which could otherwise precipitate silver salts and compromise identification.

Why Nitric Acid Is Required

Nitric acid is used because:

• It removes carbonate impurities by converting them into carbon dioxide.

• It does not introduce new ions that could form precipitates with silver nitrate.

• Other acids (e.g., hydrochloric acid) would introduce halide ions and invalidate the test.

Formation of Silver Halide Precipitates

Once the sample is acidified, aqueous silver nitrate (AgNO₃) is added. Each halide ion forms a distinctively coloured silver halide precipitate, allowing visual identification.

Characteristic Precipitates

• Chloride ions (Cl⁻): form a white precipitate of silver chloride (AgCl).

• Bromide ions (Br⁻): form a cream precipitate of silver bromide (AgBr).

• Iodide ions (I⁻): form a yellow precipitate of silver iodide (AgI).

Chloride, bromide and iodide ions give white, cream and yellow precipitates of silver halides when AgNO₃(aq) is added to the acidified solution.

Three test tubes showing the silver halide precipitates formed with chloride (white), bromide (cream) and iodide (yellow) ions after addition of acidified silver nitrate solution. The photo illustrates the trend of increasingly deeper colour down Group 17. This image includes only the observations required by the syllabus and does not add extra content beyond the precipitate colours. Source

One difficulty students face is accurately distinguishing cream from pale yellow. For this reason, the specification requires a second confirmatory test using ammonia to eliminate ambiguity.

Ionic Equations for Precipitate Formation

These equations, while simple, emphasise the direct 1:1 reaction typical of halide precipitation processes.

Confirming Halides Using Aqueous Ammonia

The second stage involves adding dilute aqueous ammonia, then concentrated aqueous ammonia if required. Differences in silver halide solubility produce a predictable pattern used to confirm halide identity.

The dissolution behaviour depends on the formation of complex ions, which increases silver ion solubility for some halides.

Behaviour of Silver Halides with Ammonia

• Silver chloride (AgCl):

  • Dissolves in dilute ammonia, producing a colourless solution.

• Silver bromide (AgBr):

  • Insoluble in dilute ammonia, but dissolves in concentrated ammonia.

• Silver iodide (AgI):

  • Insoluble in both dilute and concentrated ammonia, remaining yellow.

Adding dilute then concentrated ammonia distinguishes the halides further because the silver halide precipitates have different solubilities.

Equilibrium diagram showing solid silver chloride reacting with aqueous ammonia to form the soluble complex ion [Ag(NH₃)₂]⁺ and chloride ions. This illustrates why the white AgCl precipitate dissolves in dilute ammonia, whereas other silver halides are less soluble. The diagram focuses on AgCl only and does not show the behaviour of AgBr or AgI, which must be inferred separately from the study notes. Source

Underlying Chemistry: Formation of Complex Ions

When soluble, silver halides interact with ammonia to form the diamminesilver(I) complex, which is highly soluble.

This equation demonstrates why AgCl and AgBr behave differently: the strength of Ag–X bonding decreases AgX solubility down the group.

Practical Procedure for Testing Halide Ions

To ensure clarity and consistency, the OCR specification expects students to apply the correct test sequence and reagent additions.

Step-by-Step Method

• Step 1: Add dilute nitric acid to the sample to remove interfering ions.

• Step 2: Add aqueous silver nitrate and observe precipitate colour.

• Step 3: Add dilute ammonia and record any dissolution.

• Step 4: If still undissolved, add concentrated ammonia and observe further changes.

This sequence reduces risk of misidentification and ensures compliance with standard qualitative analysis protocols.

Key Observations to Record

Students should pay close attention to:

• Initial precipitate colour (white, cream, or yellow).

• Effect of dilute ammonia (dissolves AgCl only).

• Effect of concentrated ammonia (dissolves AgBr but never AgI).

• Whether any precipitate remains, confirming iodide.

Analytical Considerations and Good Laboratory Practice

Careful observation is critical because lighting conditions can alter perceived colours. Recording results immediately prevents confusion, especially when comparing cream and yellow precipitates. Additionally, clean glassware must be used, as residual ions can distort results.

Acidification should always precede silver nitrate addition; otherwise, unwanted precipitates such as Ag₂CO₃ may form, giving a false suggestion of halides. Awareness of these potential errors increases student reliability when interpreting test results.

Finally, the halide test plays a central role in multi-stage qualitative analysis, and its consistent structure—precipitation followed by selective dissolution—makes it robust, predictable and highly examinable within OCR A-Level Chemistry.