What is the purpose of using methylene blue solution in an experiment to identify which sample of water is more polluted? In the experiment, the methylene blue solution will decolourise after it was put in the samples of water. And the level of water pollution is based on the time it took to decolourise. So the longer it took to decolourise, the more polluted the water is, or what?

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Methylene blue is an indicator used in colorimetric testing of water. Methylene blue is an indirect indicator of presence of dissolved oxygen. In presence of oxygen, the indicator exhibits a blue color. However, in a reducing environment, it converts to a colorless leuco-compound. Thus, it can be used to figure...

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Methylene blue is an indicator used in colorimetric testing of water. Methylene blue is an indirect indicator of presence of dissolved oxygen. In presence of oxygen, the indicator exhibits a blue color. However, in a reducing environment, it converts to a colorless leuco-compound. Thus, it can be used to figure out if there are oxygen-consuming compounds in the water sample. Generally, organic wastes consume dissolved oxygen from water and make it unfit for consumption or for aquatic life (which generally need at least 3-4 mg/l dissolved oxygen for survival). When introduced in a water sample, methylene blue will cause the sample to turn blue and the coloration will be lost depending on the concentration of oxygen-consuming substances. The more time it takes for discoloration to take place, lesser is the quantity of oxygen consuming contaminants and hence more pure the water is (that is, less polluted).

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