Question

The violet colour obtained during the test of sulphur by Lassaigne’s test is due to formation of
(A)$N{a_2}[Fe{(CN)_5}S]$
(B) $N{a_2}[Fe{(CN)_5}NO]$
(C) $N{a_2}[Fe{(CN)_6}]$
(D)$N{a_4}[Fe{(CN)_5}NOS]$

Answer 1

Hint: Lassaigne’s test is also known as the fusion test. It is used for detecting elements such as halogens, nitrogen and sulphur other than carbon, hydrogen and oxygen. It is used for the elemental analysis for the qualitative determination of the presence of foreign elements.

Complete step-by-step answer:We know that Lassaigne’s test is also known as the sodium fusion test which is used for the determination of elements such as nitrogen, sulphur and hydrogen. In this process, sodium metal is heated in a fusion tube till it melts. Now a small amount of substance is added and a tube is heated strongly at a higher temperature. The red hot tube is kept into distilled water in a china dish. The contents are boiled for some time and then they are cooled and filtered. The filtrate is called sodium extract or Lassaigne’s extract.
 Now the extract contains sodium sulphide formed by the reaction of sulphur present in the compound and sodium. The reaction taking place is as follows
 $2Na + S \to N{a_2}S$
Then the lassaigne’s extract undergoes sodium nitroprusside test to determine the presence of sulphur in the extract. A few drops of sodium nitroprusside solution are added to the other part of the Lasseigne's extract. The appearance of purple colour confirms the presence of sulphur. The reaction taking place is as follows
$N{a_2}S + N{a_2}[Fe{(CN)_5}NO] \to N{a_4}[Fe{(CN)_5}NOS]$.
The compound with violet colour is sodium thio nitroprusside , $N{a_4}[Fe{(CN)_5}NOS]$

Hence, the correct answer is option (D).

Note:The Lassaigne’s extract is also used in lead acetate test to determine the presence of sulphur in the given compound. The qualitative analysis test for nitrogen and sulphur are difficult to perform due to the covalent bonding with the organic compound . They are converted into ionic forms using the metal by sodium fusion test.