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Declining water pH level

Q: A lime-treated water sample had a neutral pH. When preserved for few days, its pH declines. What is the probable reason?

A: My first guess is acid gas. The most probable culprit is CO₂ from human activity, including breathing and boiler emissions. My second guess would be excessive SO₂ in the region, but that's less likely than CO₂ or other acid gas that may be in the plant or the lab. Lime water is often used to scrub CO₂.