Our objective is to determine the anion present in the given salt

Our Objective

Our objective is to determine the anion present in the given salt.

The Theory

What is Qualitative Inorganic Analysis?

The qualitative inorganic analysis is a method of analytical chemistry that seeks to find out the elemental composition of inorganic compounds through various reagents. It is mainly focused on detection of ions in an aqueous solution, so the substances in other states need to be converted into an aqueous solution before starting the experiment.

The procedure for detecting ions (cations and anions) in aqueous solutions are called Cation Analysis and Anion Analysis.

Let's discuss the Qualitative Analysis of Anions.

Qualitative Analysis of Anions

Preliminary Tests

Some preliminary tests are done before going to the anion analysis.

(A) Physical Examination: Colour and Smell

The physical examination of the unknown salt involves the study of colour, smell and density. Characteristic smell helps to identify some anions such as acetate, sulphide etc.

(B) Dry Heating Test

This test is performed by heating a small amount of salt in a dry test tube. Quite valuable information can be gathered by carefully performing and noting the observations. On heating, some salts undergo decomposition, thus evolving the gases.

(C) Identification of Anions (Acid Radicals)

The identification of the radicals is first done on the basis of the preliminary tests. The Dry heating test is one of the preliminary tests performed earlier which may give some important information about the acid radical present. The other preliminary tests are based on the fact that:

• 
  CO₃^2-, S^2-, NO₂^- and SO₃^2- react with dil. H₂SO₄ to give out CO₂, H₂S, NO₂ and SO₂ gases respectively. These gases on identification indicate the nature of the anion present in the salt.
  
• 
  Cl^-,Br^-,I^-,NO_3- and C₂O₄^2- and CH₃COO^- react with conc. H₂SO₄ but not with dil. H₂SO₄ to produce characteristic gases.
  
• 
  SO₄^2- and PO₄^3- react neither with dil H₂SO₄ nor with conc. H₂SO₄. These are, therefore, identified by individual tests.
  

Thus, these anions may be identified by performing the following tests below:

1) Dil. H₂SO₄ Tests

Treat a pinch of the salt with dil. H₂SO₄ and identify the gas evolved.

Chemical Reactions Involved in Dil.H₂SO₄ Test:

Dilute H₂SO₄ decomposes carbonates, sulphides, sulphites and nitrites in cold to give gases. These gases on identification indicate the nature of the anion present in the salt.