Qualitative Analysis: Tests for Anions and Cations

Learning Objectives (OCR Spec 3.1.4)

By the end of this lesson you should be able to:

• Describe the chemical test and expected observation for each of:
  • Carbonate ion (CO3^2-)
  • Sulfate ion (SO4^2-)
  • Halide ions (Cl-, Br-, I-) using AgNO3 and ammonia
  • Ammonium ion (NH4+)
• Carry out tests in the correct sequence to avoid false positives
• Write ionic equations for each test
• Appreciate that OCR does not require recall of flame test colours (though you should know what a flame test is)

The Importance of Sequence

When identifying unknown anions, the sequence matters because some tests can interfere with others. The OCR-recommended order is:

graph TD
  A[Unknown solution] --> B[1. Test for carbonate dilute HCl]
  B --> B1[Gas evolved? -> CO3 2- present]
  B --> C[2. Test for sulfate BaCl2 + dilute HCl]
  C --> C1[White precipitate -> SO4 2- present]
  C --> D[3. Test for halide AgNO3 + dilute HNO3]
  D --> D1[Coloured precipitate -> halide present]
  D --> E[Confirm with ammonia NH3]

Why this order?

• Testing for sulfate before carbonate fails because BaCO3 is also an insoluble white precipitate → false positive for sulfate
• Testing for halide before sulfate fails because Ag2SO4 is sparingly soluble and precipitates → false positive for halide
• Testing for halide before carbonate fails because Ag2CO3 is also insoluble → false positive

Always acidify with the correct acid: dilute HCl for sulfate (removes carbonate as CO2), dilute HNO3 for halide (removes any residual carbonate or sulfate).

1. Test for Carbonate Ion (CO3^2-)

Test: Add dilute hydrochloric acid (or any strong acid) to the solid or solution.

Observation: Effervescence (bubbles of colourless gas). Collect the gas and bubble through limewater. If limewater turns cloudy/milky, the gas is CO2 and the anion is carbonate.

Ionic equation:

CO3^2-(s/aq) + 2H+(aq) → CO2(g) + H2O(l)

Limewater test for CO2:

Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l) (calcium carbonate is the white insoluble precipitate that makes limewater cloudy)

Example: Na2CO3(s) + 2HCl(aq) → 2NaCl(aq) + CO2(g) + H2O(l)

2. Test for Sulfate Ion (SO4^2-)

Test: Add dilute hydrochloric acid first (to eliminate carbonate), then add aqueous barium chloride (or barium nitrate).

Observation: White precipitate of BaSO4.

Ionic equation:

Ba2+(aq) + SO4^2-(aq) → BaSO4(s)

The HCl acidifies and removes any carbonate as CO2. Without this step, BaCO3 (also a white precipitate) would give a false positive. BaSO4 is extremely insoluble - the Ksp is about 10^-10 - and the white precipitate is a clear, unambiguous confirmation of sulfate.

3. Test for Halide Ions (Cl-, Br-, I-)

Test: Add dilute nitric acid first (to eliminate carbonate and sulfate), then add aqueous silver nitrate.

Observation (colour of precipitate):

Halide	Ionic equation	Precipitate colour
Cl-	Ag+(aq) + Cl-(aq) → AgCl(s)	White
Br-	Ag+(aq) + Br-(aq) → AgBr(s)	Cream
I-	Ag+(aq) + I-(aq) → AgI(s)	Yellow

The colours are subtle - white/cream/yellow can be hard to distinguish, especially in dim light. To confirm, perform a second test with aqueous ammonia.

Confirming with Ammonia

Precipitate	Dilute NH3	Concentrated NH3
AgCl (white)	Dissolves	Dissolves
AgBr (cream)	Insoluble	Dissolves
AgI (yellow)	Insoluble	Insoluble

The silver halide forms a soluble complex ion [Ag(NH3)2]+ when dissolved:

AgCl(s) + 2NH3(aq) → [Ag(NH3)2]+(aq) + Cl-(aq)

The solubility in ammonia mirrors the trend in silver halide solubility: AgI is the most insoluble (because Ag-I is more covalent, larger polarisable ion), so it resists even concentrated ammonia.

Full Halide Flowchart

graph TD
  A[Unknown with AgNO3 + dil HNO3] --> B{Precipitate colour?}
  B -->|White| C[AgCl - Cl- present]
  B -->|Cream| D[AgBr - Br- present]
  B -->|Yellow| E[AgI - I- present]
  C --> C1[Add dil NH3]
  C1 --> C2[Dissolves: confirms Cl-]
  D --> D1[Add dil NH3]
  D1 --> D2[No change]
  D2 --> D3[Add conc NH3]
  D3 --> D4[Dissolves: confirms Br-]
  E --> E1[Add dil NH3]
  E1 --> E2[No change]
  E2 --> E3[Add conc NH3]
  E3 --> E4[No change: confirms I-]

4. Test for Ammonium Ion (NH4+)