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Confirmatory Tests for Anions

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 + H2SO     →      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 + H2SO       →      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
 |                   +  H2SO4     →     Na2SO4 + CO2 + CO + H2O

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