Qualitative analysis (o-level chemistry)

This is a branch of chemistry that requires the use of known chemicals and reagents called bench reagents to identify unknown chemicals or reagents based on changes in color and other observable reactions that take place when one chemical is added to another.  As part of the school curriculum, students are expected to understand the use of the bench reagents to identify the simple cations and anions.

The syllabi

For the purpose of the examinations, Ordinary level students should be familiar with the use reagent such as:

Dilute sodium hydroxide solution,

Dilute ammonia solution

Dilute hydrochloric acid solution

Dilute Nitric acid solution

Dilute sulphuric acid

Potassium iodide solution

Sodium carbonate solution

Potassium hexacyanoferrate (II) solution

Potassium hexacyanoferrate (III) solution

Lime water

Barium chloride/nitrate solution

Silver nitrate solution

Lead nitrate/ethanoate solution

Copper turning

Litmus paper

Ammonium thiocyanate

Cations and anions to be identified

Cations: Cu²⁺, Fe²⁺, Pb²⁺, Zn²⁺ Al³⁺, NH₄⁺ and Fe³⁺

Anions: SO₄^2-, Cl^–, NO₃^–, and CO₃^2-

Identification of cations

1. The color of a compound gives a clue about a cation present in a compound.

The table common colors of cations are shown in the table below

Color of compounds	Suspected cations
White	Pb2+, Zn2+ Al3+, NH4+
Blue	Cu2+
Green	Fe2+, Cu2+
Brown	Fe3+

2. Observation when cations are reacted with dilute sodium hydroxide dropwise until in excess.

Cations in solution	Observation	Comment
NH4+	No observable change	On boiling the resultant mixture, a gas that turns red litmus paper blue is given off. This is the only way of showing that a compound contains ammonium ion.
Zn2+	White precipitate soluble in excess Equation Zn2+(aq) + 2OH–(aq) → Zn(OH)2(s) Then Zn(OH)2 (s) + 2OH–(aq) →Zn(OH)42-(aq)	Note that Al3+, Zn2+, and Pb2+ react with sodium hydroxide to give the same observation.
Pb2+	White precipitate soluble in excess Equation Pb2+(aq) + 2OH–(aq) → Pb(OH)2(s) Then Pb(OH)2 (s) + 2OH–(aq) →Pb(OH)42-(aq)	Note that Al3+, Zn2+, and Pb2+ react with sodium hydroxide to give the same observation.
Al3+	White precipitate soluble in excess Equation Al3+(aq) + 3OH–(aq) → Al(OH)3(s) Then Al(OH)3 (s) + 2OH–(aq) → Al(OH)4–(aq)	Note that Al3+, Zn2+, and Pb2+ react with sodium hydroxide to give the same observation.
Cu2+	Blue precipitate insoluble in excess Cu2+(aq) + 2OH–(aq)   → Cu(OH)2(s)	Formation of the blue precipitate is an indication of the presence of Cu2+ ions but this is not a confirmatory test for Cu2+.
Fe2+	Dirty green precipitate insoluble in excess. Fe2+(aq) + 2OH–(aq) → Fe(OH)2(s)	Formation of the dirty green precipitate is an indication of the presence of Fe2+ ions at O-level but this is not a confirmatory test for Fe2+.
Fe3+	Brown precipitate insoluble in excess	Formation of the brown precipitate is an indication of the presence of Fe3+ ions but this is not a confirmatory test for Fe3+.

3. Observation when cations are reacted with dilute ammonia dropwise until in excess.

Cations in solution	Observation	Comment
NH4+	No observable change	This is not a significant test for NH4+
Zn2+	White precipitate soluble in excess Equation Zn2+(aq) + 2OH–(aq) → Zn(OH)2(s) Then Zn(OH)2 (s) + 4NH3(aq) →Zn(NH3)42+(aq)	This test distinguishes Zn2+ions from Al3+ and Pb2+ ions that form white precipitates with ammonia solution insoluble in excess. The test is thus used to confirm the presence of Zn2+ ions
Pb2+	White precipitate soluble in excess Equation Pb2+(aq) + 2OH–(aq) → Pb(OH)2(s)	Note that Al3+ and Pb2+ show the same observation with the ammonia solution. The insolubility of the precipitate in excess distinguishes Al3+ and Pb2+ ions from Zn2+ ion whose precipitate is soluble in excess ions.
Al3+	White precipitate soluble in excess Equation Al3+(aq) + 3OH–(aq) → Al(OH)3(s)	Note that Al3+ and Pb2+ show the same observation with the ammonia solution. The insolubility of the precipitate in excess distinguishes Al3+ and Pb2+ ions from Zn2+ ion whose precipitate is soluble in excess ions.
Cu2+	Blue precipitate soluble in excess to give deep blue solution Cu2+(aq) + 2OH–(aq)   → Cu(OH)2(s) Then Cu(OH)2(aq) + 4NH3(aq) → Cu(NH3)42+(aq)	The formation of blue precipitate soluble in excess ammonia is an indication of the presence of Cu2+ ions. At O-level this test is used to confirm the presence of Cu2+ ions I solution
Fe2+	Dirty green precipitate insoluble in excess. Fe2+(aq) + 2OH–(aq) → Fe(OH)2(s)	Formation of a dirty green precipitate is an indication of the presence of Fe2+ ions at O-level but this is not a confirmatory test for Fe2+.
Fe3+	Brown precipitate insoluble in excess	Formation of a brown precipitate is an indication of the presence of Fe3+ ions but this is not a confirmatory test for Fe3+.

4. Confirmatory test: After testing the solution with sodium hydroxide and then ammonia solution, a confirmatory test is carried out to tell whether the suspected ion is the exact one.

Suspected ion  in solution	Confirmatory test	Observation/comment
NH4+	When sodium hydroxide is added and solution boiled	A gas that turns damp red litmus paper blue is given of
Zn2+	Addition of ammonia solution	A white precipitate soluble in excess This distinguishes Zn2+ from Pb2+ Al3+ ion whose precipitate with ammonia are insoluble in excess
Pb2+	Potassium iodide solution	A yellow precipitate of PbI2.
Al3+	Potassium iodide solution	No observable change. Al3+ is confirmed by a negative test distinguishing it from Pb2+ that form a yellow ppt
Cu2+	Ammonia solution	Blue precipitate soluble in excess to form deep solution
Cu2+	Potassium hexacyanoferrate II	Brown precipitate
Cu2+	Potassium iodide	Brown suspension: note that a solution of Pb2+ is colorless and gives yellow precipitate while that of Cu2+ is blue.
Fe2+	Potassium hexacyanoferrate III	Deep blue solution Note that potassium hexacyanoferrate III confirms Fe2+ while potassium hexacyanoferrate II confirms Fe3+ but the observations are the same: deep blue solution
Fe3+	Potassium hexacyanoferrate II	Deep blue solution
Fe3+	Ammonium thiocyanate	A red solution is formed

Identification of anions

(a) Texture

(i) Salts containing SO₄^2-, Cl^– or NO₃^– or usually crystalline

(ii) Carbonates are usually in powder form

(b) Solubility in water

(i) Salts containing SO₄^2-, Cl^– or NO₃^– or usually soluble

(ii) Carbonates are usually insoluble

(c) Effect on heat

(i) Sulfate, SO₄^2-, decompose to release acid gas that turns damp blue litmus red and turn damp potassium dichromate paper from orange to green. The gas is SO₂

(ii) Nitrates, NO₃^–, decompose to release brown fumes of nitrogen dioxide, NO₂. This test is used to confirm the presence of nitrate ions.

(iii) Carbonates, CO₃^2-, decompose to give a gas (CO₂) that change damp blue litmus paper pink and lime water milky.

Note that, every time you are required to heat a compound in a test tube, test the gas given using both blue and red litmus papers. Compounds containing ammonium ions, NH₄+ give off a gas that turns damp red litmus paper blue.

(d) Testing the solution with lead (ii) nitrate solution

(i) Sulphate ions, SO₄^2-, give white precipitates insoluble on boiling the final mixture

(ii) The chloride ion, Cl^–, form white precipitate soluble on boiling

(iii) Nitrates, NO₃^–, give no observable change

To distinguish solutions containing sulphate ions, SO₄^2-, from those containing Chloride ions, Cl^–.

Sulphate ions, SO₄^2-, form white precipitate with barium chloride insoluble in dilute nitric acid.

Chloride ions, Cl^–, form a white precipitate with silver nitrate solution insoluble in dilute nitric acid soluble in excess ammonia

Testing for carbonates

Most carbonates used in the qualitative analysis are insoluble in water and are usually remain as residues on the filter paper

Addition of acid to a carbonate, effervescence is observed, a gas that turns limewater milky is produced.

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