Formation of $$Na_4[Fe(CN)_5NOS]$$, a purple coloured complex formed by addition of sodium nitroprusside in sodium carbonate extract of salt indicates the presence of :

In qualitative inorganic analysis, sulphide ion $$S^{2-}$$ is identified by the sodium nitroprusside test.

First, the salt is fused with solid sodium carbonate and the fused mass is extracted with water to obtain the carbonate extract that contains all anions in their ionic forms.

When a few drops of freshly prepared aqueous sodium nitroprusside $$Na_2[Fe(CN)_5NO]$$ are added to this extract, the following reaction takes place if $$S^{2-}$$ is present:

$$Na_2[Fe(CN)_5NO]^{2-} + S^{2-} \;\longrightarrow\; Na_4[Fe(CN)_5NOS]^{4-}$$

The product $$Na_4[Fe(CN)_5NOS]$$ is a purple-coloured complex. The appearance of the purple colour is therefore a confirmatory test for the sulphide ion.

No such colour is produced with sulphate $$SO_4^{2-}$$, sulphite $$SO_3^{2-}$$, or simple metal cations such as $$Na^+$$, so these ions are ruled out.

Hence, the formation of the purple complex $$Na_4[Fe(CN)_5NOS]$$ indicates the presence of the sulphide ion.

Correct option: Option C (Sulphide ion).