Question

The blood red colour in the combination test of nitrogen and sulphur in organic compound is due to the formation of:
A. ferric sulphocyanide
B. ferric acetate
C. ferrous sulphocyanide
D. ferricyanide

Answer 1

Nitrogen, sulphur, halogens present in any organic compound can be identified using Lassaigne’s test which is a type of combination test. In this reaction there are mainly two steps first is preparation of sodium fusion extract and detection of element using SFE.

Complete answer:
Ferric chloride that is $FeC{l_3}$ reacts with sodium thiocyanate $(NaCNS)$ which results in ferric thiocyanate $[Fe{\left( {CNS} \right)_3}]$ . This test is used to identify the presence of nitrogen and sulphur in organic compound
The reaction involved is as follows:
$F{e^{3 + }} + 3NaCNS \to Fe{(CNS)_3}$
The obtained product $[Fe{\left( {CNS} \right)_3}]$ is blood red in colour.
The correct answer is A.

Note:
Ferric sulphocyanide is also called ferric thiocyanate. In lassaigne’s test, a fresh solution of ferric chloride should be used or else it can easily get oxidized through aerial oxidation. If sodium fusion is carried with excess of sodium the thiocyanate can decompose into $C{N^ - }$ and ${S^{2 - }}$ .