Several redox dyes may replace methylene blue. When the environment is very rich, algae blooms and excessive weeds lead to oxidation of organic matter and subsequent oxygen depletion. Go to Top Makeup Ans. Each time the flask is filled the solutions turns colorless. Each time the flask is half-full and shaken vigorously the solution turns blue.
Air is mixed with the other reactants during shaking.
The air is oxidizing the methylene blue.