How to carry out Qualitative Analysis in Chemistry

 

    Qualitative analysis is the type of chemical analysis which its objective is to identify the ions, atoms or functional groups in an unknown substance.

   To identify the ions, the chemical properties of the unknown substance are determined by carefully reacting the unknown substance with different reagents. By defining what the particular reaction will be if a particular ion is present, the ions that are presence in the solution of the unknown substance can be identified. For example, adding barium chloride in a solution of a compound containing tetraoxosulphate (VI), trioxosulphate (IV) or trioxocarbonate (IV) gives a white precipitate. The precipitate from the last two will dissolve in dilute acid to produce sulphur (IV) oxide and carbon (IV) oxide respectively. These reactions help to differentiate between trioxocarbonate (IV) and trioxosulphate (IV) radicals.

 Another method employed in qualitative analysis is checking the solubility of the various compounds in water, this is because solubility of different compounds vary in water. The solubility of some substances in water is listed below:

·         All trioxonitrate (V) salts, NO₃^- are soluble

·        All hydrogen trioxocarbonate (IV), HCO₃^-  are soluble

·        All chlorides Cl^-   except those of silver, copper and lead

·        All tetraoxosulphate(VI) salts, SO₄^2- except those of barium, calcium,silver, lead and mercury.   

·        All salts of sodium and potassium are soluble in water

·        All ammonium salts

Examples of substances which are not soluble in water is listed below:

·        All trioxocarbonate (IV), CO₃^2- except those of potassium, sodium and ammonium

·        All hydroxide, OH^-, except those of sodium, potassium and ammonia

  It is important to note that coloured ions are characteristics of transition elements or ions. Most transition elements form coloured ions in solution due to d-block electrons. This is a very important hint for identifying transition ions in solution.

Example Question

Specimen X is an inorganic compound carry out the following tests and identify and any gas evolved.

S\N	Test	Observation	inference
a	X plus heating	Water condenses at the cooler part of the test tube, its colour changes from dark brownish yellow to dark green	X has water of crystallization, it is coloured, transition ions likely present
b	X + water and divide into four portions	Soluble in water,and gives a light brownish yellow solution	Transition ions Fe2+, Fe3+ etc. likely present
c	To the first portion, add concentrated H2SO4  + MnO2 + heating, and place damp starch- iodide paper at tip of the test-tube	A greenish yellow gas evolved, with pungent smell and darken starch-iodide paper	Chloride ions, chlorine likely present
d(I)	To the second portion, add dilute HNO3 and silver trioxonitrate (V)solution	White precipitate formed which is insoluble in excess acid and the precipitate decolourized on exposure to sunlight	Chloride ions, chlorine likely present
d(II)	To the mixture above d(i) add ammonia solution in excess	The white precipitate dissolved and gives a colourless solution	Chloride confirmed and chlorine present
e	To the third portion, add dilute NaOH solution	Reddish-brown gelatinous precipitate formed	Fe3+ present
f	To four portion, add ammonia solution	Reddish-brown gelatinous precipitate formed	Fe3+ confirmed

 

 

Other tests for Iron (III) ions include:

1.    Test with potassium hexacyanoferrate (II) solution: iron (III)ions produce a deep blue precipitate with the reagent

 

2.    Test with potassium thiocyanate solution or ammonium thiocyanate solution: if the unknown solution contains iron(III)ions, it will form deep blood red colouration