Question

Which one of the following chemicals is used as reducing agent in printing?

• A. Sodium chlorite
• B. Sodium hydrosulphite
• C. Sodium nitrate
• D. Sodium phosphate

Hint:

Sodium hydrosulphite (also called sodium dithionite) is a common reducing agent used in vat dyeing and discharge printing.

Answer 1

The Correct Option is B

Step 1: Understanding Reducing Agents in Printing: Reducing agents are used in textile printing, particularly with vat dyes and discharge printing. Vat Dye Printing: Vat dyes are insoluble in water. They must be reduced to a soluble "leuco" form to be applied to the fiber. After dyeing, the leuco form is oxidized back to the insoluble form, trapping the dye within the fiber. 
Discharge Printing: A reducing agent is used to destroy the color of a previously dyed fabric, creating a white or light-colored design on a darker background. 

Step 2: Identifying the Reducing Agent: Sodium Hydrosulphite (Sodium Dithionite, \(Na_2S_2O_4\)): This is a powerful reducing agent commonly used in vat dyeing and discharge printing. 
Sodium Chlorite (\(NaClO_2\)): This is an oxidizing agent, used for bleaching, not reducing. 
Sodium Nitrate (\(NaNO_3\)): This is an oxidizing agent. 
Sodium Phosphate (\(Na_3PO_4\)): This is a buffering agent, not a reducing agent.