Chemistry – Experimental techniques and chemical analysis - Identification of ions and gases | e-Consult

Test 1: Reduction with Aluminium and Sodium Hydroxide

• A small amount of the unknown solution is mixed with a piece of aluminium foil in dilute aqueous sodium hydroxide (NaOH).
• The mixture is allowed to react.
• The aluminium reduces the nitrate ions (NO₃^-) to ammonia (NH₃).
• The reaction is as follows: 4NO₃^-(aq) + 3Al(s) + 6H₂O(l) → 3NH₃(g) + 4Al(OH)₃(aq)

Test 2: Testing for Ammonia Gas

• The gas evolved in the first test is passed into damp litmus paper.
• If ammonia gas is present, the damp litmus paper will turn blue.
• Alternatively, the gas can be passed into lime water (calcium hydroxide solution, Ca(OH)₂). If ammonia is present, the lime water will turn milky due to the formation of calcium ammonium hydroxide.
• The balanced chemical equation for the reaction with lime water is: NH₃(g) + Ca(OH)₂(aq) → Ca(OH)₂(aq) + NH₃(aq)

Conclusion: If the aluminium foil reacts vigorously with sodium hydroxide and ammonia gas is produced, it confirms the presence of the nitrate ion.