Water treatment chemicals: The percentage of available chlorine in fresh bleaching powder (calcium hypochlorite mixture) is approximately in which range?
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A50 to 60
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B30 to 35
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C40 to 50
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D20 to 25
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E10 to 15
Answer
Correct Answer: 30 to 35
Explanation
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:
Identify material: bleaching powder (not high-test hypochlorite).Recall standard available chlorine range: ~30–35%.Select the matching option for dosing calculations.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