Question

The dye exhaustion of direct dyeing with cotton depends on :
A. electrolytic concentration
B. temperature of dye bath
C. pH of dye bath
D. oxidizing agents Choose the correct answer from the options given below:

• A. B, C, D only
• B. A, C, D only
• C. A, B, D only
• D. A, B, C only

Hint:

For direct dyes, "Salt is the Driver." Without salt (electrolyte), the dye and fiber would simply repel each other like similar poles of a magnet.

Answer 1

The Correct Option is D

Concept: Direct dyes are water-soluble anionic dyes. "Exhaustion" refers to the movement of dye molecules from the dye liquor onto the fiber surface.

Step 1: Analyzing Dyeing Parameters.

• A (Electrolytic concentration): Cotton develops a negative surface charge in water, which repels negatively charged direct dyes. Adding an electrolyte (like salt) neutralizes this "zeta potential," allowing the dye to reach the fiber.
• B (Temperature): Dyeing is a kinetic process. Temperature controls the rate at which dye molecules diffuse into the fiber's interior.
• C (pH): The acidity or alkalinity of the bath affects both the solubility of the dye and the swelling behavior of the cotton fiber.

Step 2: Exclusion.
D (Oxidizing agents) are used in Vat or Sulfur dyeing to convert the dye into its final insoluble form. Direct dyes do not require oxidation to develop their color or fix to the fiber.