Water treatment chemicals: The percentage of available chlorine in fresh bleaching powder (calcium hypochlorite mixture) is approximately in which range?

Introduction / Context:
Bleaching powder is a common disinfectant in water treatment and emergency chlorination. Its effectiveness depends on the available chlorine content, which decreases with age, heat, and exposure to moisture and carbon dioxide.

Given Data / Assumptions:

• Fresh, good-quality bleaching powder as stored correctly.
• Available chlorine refers to the oxidizing capacity expressed as an equivalent percentage of chlorine.

Concept / Approach:
Textbook values indicate that fresh bleaching powder contains roughly 30–35% available chlorine. This value is used for dose calculations, jar testing, and stock management. Higher figures (e.g., 50–60%) align with high-grade calcium hypochlorite products, not typical bleaching powder used historically in municipal works.

Step-by-Step Solution:

Verification / Alternative check:
Confirm available chlorine by iodometric titration for precise plant dosing, especially for aged stocks.

Why Other Options Are Wrong:
50–60 and 40–50 are typical for stronger hypochlorite formulations; 20–25 or 10–15 are too low for fresh material and imply degraded stock.

Common Pitfalls:
Not accounting for storage degradation; using nominal percentages without periodic assay leads to under- or over-chlorination.

Final Answer:
30 to 35