Confined-space gas check: A lead acetate paper kept for about 5 minutes at a manhole turns black. Which gas is indicated?

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:

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