Question

Why is a fresh solution of FeSO4 used in the test for nitrogen?

Answer 1

Answer:

Lassaigne’s Test for detection of Nitrogen in Organic compound involves  

first the formation of NaCN (IONIC) in the Sodium fusion extract, then its treatment to fresh FeSO4+H2SO4.

Initial product is Na4[Fe(CN)6] (Iron in II oxidation state), which after adding H2SO4 forms Ferri ferrocyanide (Prussian Blue Colour).{ Fe4[Fe(CN)6]3}

So, we see the initial product formation is detrimental for the coloured compound but it needs Fe2+.

Fe2+ has great tendency to form Fe3+ (as Fe3+ has d5 config while Fe2+ has d6, former being half filled and hence more stable). [Facts are agreeable to Reduction Potential Values too.]

So, in fresh sol of FeSO4 isnt used, sol will have predominant Fe3+ which wont give the test satisfactorily. Thus is the need to use Fresh Sol of FeSO4

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