Identification of metallic and non-metallic ions
Francine Taylor-Campbell, Contributor

You should be able to:

• Identify the following cations; Pb,²⁺, Al³⁺, Ca²⁺, Zn²⁺, Fe²⁺, Fe²⁺, NH₄⁺ and Cu²⁺
• Identify the following gases; H₂, O₂, NH₃, SO₂, HCl, Cl₂, NO₂, H₂O
• Identify the following anions; CO₃^2-,, SO₄^2-, SO₃^2-, NO₃^-, Br^-, I^-, Cl^-

Points to note

Metallic ions or cations are identified based on the colour and solubility of the hydroxides formed from their reaction with aqueous ammonia and sodium hydroxide.

Non-metallic ions or anions are generally identified based on the evolution and identification of gases produced when the compounds are heated or reacted with acids. The colour and solubility of precipitates formed on reaction with barium ions and ammonia (for the halide ions) are also used.

Comments

1. Whenever the reagent silver nitrate is used, this implies that we are searching for halide ions. Cl-, Br- and I- form white, cream and yellow precipitates with silver nitrate solution, respectively. If this solution is added and no precipitate is seen, this suggests that no halide ion is present.

2. Whenever barium ions are added to a solution, one can assume that we are searching for either CO₃²,SO₄^2- or SO₃^2- ions. The addition of acid allows us to determine which of these ions is present. CO₃^2- and SO₃^2- will react with the acid and give off CO₂ and SO₂ gases, respectively. If CO₂ is present, it will turn lime water (calcium hydroxide) milky, while SO₂ will cause potassium dichromate paper to change from orange to green. Please note that both gases are acidic and will turn damp blue litmus to red. SO₄^2-ion does not react and is in fact insoluble in the acid.

3. There are three possible ions (Zn²⁺, Al³⁺ and Pb²⁺) that produce a white precipitate, which is soluble when excess sodium hydroxide is added. These three ions must be listed and further tests and observations done to determine which ion is present. Alkaline substances turn red litmus paper to blue. Ammonia is therefore the gas evolved, as it is alkaline in nature and would be formed from an ammo-nium ion (NH₄⁺).

4. To differentiate between Al³⁺and Pb²⁺, solutions of iodide, chloride or sulphate can be added. In all cases, lead ions will form a precipitate, while aluminium ions will not. Remember that Pb2+ forms precipitates with Cl-, I- and SO42- to form PbCl2 (white), PbI2 (yellow) and PbSO4 (white), respectively.

5. On heating the substance being tested, the gases produced should always be tested. If the gas that turns damp red litmus blue, this suggests that the gas is ammonia as it is alkaline in nature. Hence, the ammonium ion (NH₄⁺) would be present.

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Francine Taylor-Campbell is an independent contributor.