Question

Schiff reagent loses its colour by passing
1) ${\rm{S}}{{\rm{O}}_{\rm{2}}}$
2) ${{\rm{N}}_{\rm{2}}}$
3) ${{\rm{O}}_{\rm{2}}}$
4) ${\rm{S}}{{\rm{O}}_{\rm{3}}}$

Answer 1

Hint: The product of the reaction of a dye formulation such as sodium bisulfite and fuchsin, pararosaniline (aromatic methyl group is absent), and a new fuchsin is Schiff's reagent. This reagent is used in the detection of aldehyde functional groups in an organic compound.

Complete step by step answer:
Let’s discuss the preparation of Schiff reagent. The preparation of Schiff reagent is done by passing sulphur dioxide solution into the solution of fuchsin dye. This reaction turns the solution colourless due to the formation of an additional product. The colour of the solution is restored because of the abstraction of sulphurous acid from the Schiff’s reagent by an aldehyde group. So, we can say that on passing the sulphur dioxide gas through the Schiff reagent, colour of the reagent loses.

Hence, correct answer is option (1), that is, ${\rm{S}}{{\rm{O}}_{\rm{2}}}$.

Additional Information:

An aldehyde reacts with Schiff’s reagent to produce a product of bright red colour. So, the identification of the aldehyde group in a compound can be detected by reacting the compound with Schiff’s reagent.
Generally, ketones do not respond to the reaction of Schiff’s reagent. If the reaction is subjected to heat, then the reaction gives a light pink colour.
Alcohol and ester do not react with Schiff’s reagent.

Note: Schiff’s reagent distinguishes between primary and secondary alcohols. On the reaction of an aldehyde with Schiff’s reagent, if its colour changed to red quickly, then, the aldehyde was formed from a primary alcohol. If no colour changes of Schiff’s reagent, then, the alcohol is secondary.