The method available for the detection of anions is not as systematic as for the cations, so we have to depend much on the preliminary tests. A group of certain radicals is just indicated by a common test and then individual radicals are identified by performing their specific tests. For example, some of the anions on reaction with dil. H₂SO₄ produce specific gases which are helpful to detect the anions. For example,

     ZnCO₃ + H₂SO₄     →       ZnSO₄ + H₂O + CO₂
2NaHCO₃ + H₂SO₄    →       Na₂SO₄ + 2H₂O + 2CO₂
  Na₂S₂O₃ + H₂SO₄    →      Na₂SO₄ + H₂O + SO₂ + S
       Na₂S + H₂SO₄      →      Na₂SO₄ + H₂S
    2KNO₂ + H₂SO₄      →      K₂SO₄ + H₂O + NO + NO₂

On the other hand, some other anions on reaction with conc. H₂SO₄ and heating produce specific gases, e.g., chlorides produce hydrochloric acid; bromides produce bromine; iodides give iodine; nitrates produce nitrogen peroxide; acetates give acetic acid vapours, oxalates produce a mixture of Co and CO₂ as:

          NaCI + H₂SO₄        →      NaHSO₄ + HCI
            2KBr + 3H₂SO₄     →      2KHSO₄ + 2H₂O + Br₂ + SO₂
              2KI + 3H₂SO₄       →       2KHSO₄ + 2H₂O + I₂ + SO₂
        4KNO₃ + 4H₂SO₄       →       4KHSO₄ + 4NO₂ +O₂ + 2H₂O
(CH₃COO)₂ Ca + H₂SO₄    →        CaSO₄ + 2CH₃COOH
 COONa
 |                   +  H₂SO₄     →     Na₂SO₄ + CO₂ + CO + H₂O
 COONa

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