Question

Nitrogen detection in an organic compound is carried out
by Lassaigne’s test. The blue colour formed corresponds to
which of the following formulae? [NEET Kar. 2013]
(a) Fe₃[Fe(CN)₆]₃ (b) Fe₃[Fe(CN₆]₂
(c) Fe₄[Fe(CN)₆]₃ (d) Fe₄[Fe(CN)₆]₂

Answer 1

In detection of the organic compound

Nitrogen by lassaigne test the prussian blue colour formed is of

Fe₄[Fe(CN)₆]₃

1) The carbon and nitrogen present in the organic compound on fusion with sodium metal gives sodium cyanide then this is converted in to sodium ferrocyanide by addition of ferrous sulphate. Ferric ions generated during this process react with ferrocyanide to form prussian blue precipitate

2) following is the reaction involved

Na4[Fe(CN)6] + Fe3+ → Fe4[Fe(CN)6]3

Answer 2

Fe₄[Fe(CN)₆]₃  is the complex.

Explanation:

• Lassaigne’s test is a very popular test for organic compounds.

• This is a qualitative test for the presence of nitrogen and carbon in an organic compound.

• The organic compound is taken in a closed container with sodium metal and the metal is fused.

• This gives the sodium cyanide if carbon and nitrogen is present.

• This solution is then mixed with freshly prepared ferrous sulphate solution.

• This gives sodium sulphate and ferrosoferric cyanide complex.

• This complex has a Prussian blue colour.

• Na4[Fe(CN)6] + Fe3+ → Fe4[Fe(CN)6]3

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