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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.

Why is fresh solution of FeSO4 used in the detection of nitrogen in Lassaigne’s test?

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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.