Confined-space gas check: A lead acetate paper kept for about 5 minutes at a manhole turns black. Which gas is indicated?
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AHydrogen sulphide
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BCarbon dioxide
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CMethane
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DOxygen
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ENone of these
Answer
Correct Answer: Hydrogen sulphide
Explanation
Introduction / Context:Before manhole entry, simple qualitative field tests are sometimes used alongside instrumented monitoring. Lead acetate paper is a classic indicator of hydrogen sulphide (H₂S), a highly toxic sewer gas produced by anaerobic decomposition of sulphates and organic matter.
Given Data / Assumptions:
- Paper is moistened with lead acetate solution.
- Exposure time ≈ 5 minutes.
- Blackening of the paper is observed.
Concept / Approach:
H₂S reacts with lead acetate to form black lead sulphide (PbS). This visual reaction is specific enough for a quick qualitative alert but must be complemented by calibrated multi-gas meters for safe entry decisions (O₂, H₂S, LEL, CO, etc.).
Step-by-Step Solution:
Expose lead acetate strip at manhole opening.Observe for darkening (PbS formation) due to H₂S.If darkening occurs, ventilate and re-test with an electronic detector.Proceed only if gas readings are within permitted exposure limits.Verification / Alternative check:
Electronic sensors for H₂S provide quantitative ppm readings and alarms; cross-check oxygen percentage and LEL for methane/volatile hazards.
Why Other Options Are Wrong:
CO₂ does not blacken lead acetate; methane is flammable but non-reactive in this test; oxygen is not a contaminant gas and does not cause blackening.
Common Pitfalls:
Relying solely on paper tests; not accounting for oxygen deficiency; inadequate ventilation before entry.
Final Answer:
Hydrogen sulphide