Question

The maximum damage to cotton in sodium hypochlorite is found at \(\underline{\hspace{2cm}}\) pH.

• A. 3 to 4
• B. 5 to 6
• C. 7 to 8
• D. 9 to 10

Hint:

For hypochlorite bleaching of cotton: - Safe Zone (Controlled Bleaching): Alkaline pH (9-11). Active species: OCl\(^-\) (mild). - Danger Zone (Maximum Damage): Neutral pH (7-8). Active species: HOCl (aggressive).

Answer 1

The Correct Option is C

Step 1: Understand the bleaching action of sodium hypochlorite (NaOCl). NaOCl is an oxidizing agent. Its bleaching and degrading effects on cotton (cellulose) are due to oxidation. 
 

Step 2: Analyze the chemistry of NaOCl at different pH values. The active species in a hypochlorite solution changes with pH. 

- In alkaline conditions (pH 9-11), the dominant species is the hypochlorite ion (OCl\(^-\)). This is a relatively slow and controlled oxidizing agent, which is why commercial bleaching is done in this range. 

- In acidic conditions (pH < 5), chlorine gas (Cl\(_2\)) can be released, and the primary species is hypochlorous acid (HOCl). 

- At neutral or near-neutral pH (pH 7-8), there is a maximum concentration of the undissociated hypochlorous acid (HOCl). Hypochlorous acid is a much more powerful and aggressive oxidizing agent than the hypochlorite ion.

Step 3: Relate the active species to cellulose damage. Because hypochlorous acid (HOCl) is the most potent oxidizing agent in the system, the maximum rate of cellulose degradation (damage or tendering) occurs at the pH where its concentration is highest. This is the neutral pH range of 7 to 8. For this reason, bleaching is always avoided in this pH range.