Safety testing in sewer manholes: The lead acetate paper test is conducted to detect the presence of which hazardous gas before entry and maintenance operations?

Introduction / Context:
Confined-space entry into sewer manholes requires pre-entry atmospheric testing for toxic and asphyxiating gases. A classic field indicator is the lead acetate paper test, commonly referenced in public health engineering and maintenance manuals as a quick screen for certain sewer gases.

Given Data / Assumptions:

• Sewer manhole atmosphere may contain a mixture of gases (H₂S, CH₄, CO₂, low O₂).
• Lead acetate paper darkens in the presence of hydrogen sulphide.
• The test is qualitative, not a replacement for calibrated gas detection meters.

Concept / Approach:

Hydrogen sulphide (H₂S) produced by anaerobic decomposition reacts with lead acetate to form lead sulphide, causing a characteristic brown/black discoloration. This quick check alerts workers to a toxic environment requiring ventilation and continuous monitoring before entry.

Step-by-Step Solution:

Verification / Alternative check:

Confirm using an electronic multi-gas detector with specific H₂S sensor and alarms; cross-check oxygen levels and explosive limits to ensure comprehensive safety.

Why Other Options Are Wrong:

Methane (CH₄) is flammable but does not blacken lead acetate; carbon dioxide (CO₂) is asphyxiant without this reaction; diesel vapours are not targeted by the lead acetate test.

Common Pitfalls:

Relying solely on the paper test; ignoring oxygen deficiency; skipping ventilation; entering without a confined-space permit and attendant.

Final Answer:

Hydrogen sulphide (H₂S)