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Qualitative Analysis

Qualitative Analysis:

 

The process of identification of cations and anions is called qualitative analysis or salt analysis. We identify which cations (positive ions) are present in a salt by using aqueous sodium hydroxide, and aqueous ammonia. For cation identification, learn the following table.

 

Sodium Hydroxide solution, NaOH (aq) Aqueous Ammonia, NH3 (aq)
Observations on adding

  1. a few drops of NaOH
  2. excess NaOH
 Observations on adding

  1. a few drops of NH3
  2. excess NH3
Al3+
  1. a white precipitate is formed
  2. the precipitate dissolves in excess to give a colourless solution
  1. a white precipitate is formed
  2. precipitate insoluble in excess
Ca2+
  1. white ppt formed
  2. ppt insoluble in excess
  1. no ppt
  2. no ppt
Zn2+
  1. white ppt formed
  2. ppt dissolves in excess to give a colorless solution
  1. white ppt formed
  2. ppt dissolves in excess to give a colorless solution
Cu2+
  1. light blue ppt
  2. ppt insoluble
  1. light blue ppt
  2. ppt dissolves in excess to give a deep blue solution
Fe2+
  1. green ppt
  2. ppt insoluble
  1. green ppt
  2. ppt insoluble
Fe3+
  1. reddish-brown ppt
  2. ppt insoluble
  1. reddish-brown ppt
  2. ppt insoluble
NH4+
  1. no ppt is formed
  2. On heating, ammonia gas is given off. Ammonia turns moist red litmus paper blue.
 No reaction

 

 

To identify anions, learn these reactions:

 

CO32- (carbonate):

Test: add dilute hydrochloric acid. Pass the gas given off into limewater.

Observation: effervescence is observed. Gas given off forms a white ppt with lime water. Carbon dioxide gas is given off.

 

Cl– (chloride):

Test: add dilute nitric acid, then add silver nitrate solution.

Observation: a white ppt of silver chloride is formed.

 

I– (iodide):

Test: add dilute nitric acid, then add silver nitrate solution

Observation: a yellow ppt of silver iodide is formed.

 

NO3– (nitrate):

Test: add dilute sodium hydroxide. Then add a piece of aluminium foil. Warm the mixture. Test the gas given off with a piece of moist red litmus paper.

Observation: the moist red litmus paper turns blue. Ammonia gas is given off.

 

SO42- (sulphate):

Test: add dilute nitric acid, then add barium nitrate solution.

Observation: a white ppt of barium sulphate is formed.