Three students, Manish, Ramesh and Rajini were determining the extra elements present in an organic compound given by their teacher. They prepared the Lassaigne's extract (L.E.) independently by the fusion of the compound with sodium metal. Then they added solide `FeSO_(4)` and dilute sulphuric acid to a part of Lassaigne's extract. Manish and Rajni obtained prussian blue colour but Ramesh got red colour.
Ramesh repeated the test with the same Lassaigne's extract, but again got red colour only. They were surprised and went to their teacher and told him about their observation. Teacher asked them to think over the reason for this. Can you help them by giving the reason for this observation. Also, write the chemical equations to explain the formation of compounds of different colour.

Explanation

If the organic compound contains both N and S, then while fusion it may for form either a mixture of sodium cyanide `(NaCN)` and sodium sulphide `(Na_(2)S)` or sodium thiocyanate `(NaSCN)` depending on the amount of `Na` metal used.
If less sodium metal is used, only `NaSCN` is obtained.
This then gives red colour on reacting with `Fe^(3+)` ions (produced by oxidation of `Fe^(2+)` ions while preparing Lassaigne's extract) due to the formation of ferric thiocyanate.
`F^(2+) overset("Aerial oxidation")rarr Fe^(3+)`
`Fe^(3+) + 3NaSCN rarr underset(underset(("red"))("Ferric thicyanate"))(Fe(SCN)_(3)) + 3Na^(+)`
In case, excess of sodium metal is used, the intially formed sodium thicoyanate decomposes as follows.
`underset(underset("thiocyanate")("Sodium"))(NaSCN) + 2Na overset(Delta)rarr underset(underset("cyanide")("Sodium"))(NaCN ) + underset(underset("sulphide")("sodium")) (Na_(2)S)`
This `NaCN` then reacts with `FeSO_(4), Fe^(3+)` ions and `NaCN`, it given prussian blue colour due to the formation of ferric ferrocyanide or iron (III) hexacyanoferrate (II).
`2NaCN + FeSO_(4) rarr Na_(2) SO_(4) + Fe(CN)_(2)`
`Fe(CN)_(2) + 4NaCN rarr underset(underset("ferrate (II)")("sodium hexacyano -"))(Na_(4) [Fe(CN)_(6)])`
On the basis of above results, it is clear that Ramesh used less sodium and hence `NaSCN` formed in the Lassaigne's extract which gave red colouration due to `Fe(SCN)_(3)` formation while Manish and Rajni used excess sodium and hence `NaCN` formed in the Lassaigne's extract which gave prussian blue colour of `Fe_(4) [Fe(CN)_(6)]`