The reduced form of methylene blue is often used to indicate the presence of oxy

Question

The reduced form of methylene blue is often used to indicate the presence of oxygen contaminants in anaerobic systems. Explain how this species can be used as an indicator in these systems. Be sure to include any 1/2 reactions involved.

(Background info: This question is in terms of the reaction of ethylene blue being reduced by ascorbic acid, being converted into the colorless H2MB+ compound. The dissolved oxygen will affect the results of the experiment because the reduction potential of oxygen is greater than the reduction potential of the methylene blue, meaning that the oxygen will react rather than the methylene blue, as intended.

Two half reactions:

MB+ +2e- + 2H+ --> H2MB+1

O2+2H2O + 4e- --> 4OH-. )

Explanation / Answer

MB+ +2e- + 2H+ --> H2MB+1

O2+2H2O + 4e- --> 4OH-.

if both reactions are combined

MB+ +6e- + 2H+ + O2+2H2O ==> H2MB+1 + 4OH-.

so, H2MB+1 is produced

and hence will act as indicator to the reaction

so,  this species can be used as an indicator

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