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Dissolved oxygen related to redox potential

  1. May 11, 2010 #1
    I have read a little bit about some industries deaerating water used in boilers made of iron to prevent them from becoming damaged.

    I think this is because removing dissolved oxygen from the water lowers its redox potential. I think that this works because once the redox potential of the surrounding environment is lowered past a certain point it is no longer possible for Fe+2 to oxidize into Fe+3.
    Am I right about any of this?

    I was also wondering about the reverse reaction, I have read a little bit about using electrolysis for this. If I had some other method of lowering the redox potential of the system far enough would the Fe+3 go back to Fe+2?

    I'm sure I shouldn't be I'm sort of thinking of this as a parallel to melting point/boiling point.
     
  2. jcsd
  3. May 11, 2010 #2

    Borek

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    Staff: Mentor

    I don't think potential plays important role here. Oxygen is an oxidizer strong enough to work on iron even in very low concentrations. It can be easily calculated from Nernst equation, at the moment I am only referring to my intuition. Call it an educated guess.

    However, if you take a look at the reaction stoichiometry, it becomes obvious that amount of iron oxidized is limited by the amount of oxygen present. Less oxygen present, less iron oxidized.

    --
    methods
     
  4. May 13, 2010 #3
    Okay I see the oxygen as a limiting reactant idea now and that makes sense, it's simpler than I was making it out to be.

    I'll try to restate the second part of my question.

    If I had some Fe203 in an anaerobic solution could it change to Fe304?
    Could it take an electron from H20 somehow?
    Would it help if the redox potential of the solution was very low?

    Sorry if this is nonsense, I have not taken a chemistry course in a long time and do not really know how these things work.
     
  5. Jul 19, 2012 #4
    The answer is not straightforward, but simple in that it would attempt to reach equilibrium:
    Yes, it could steal things from water (O, H or electrons and break up an H2O)
    A solution of Fe2O3 would change some to Fe3O4 as needed to reach equilibrium

    Your boiler situation is a little different, you are oxidizing zero-valent iron (Fe) to other states (both Fe2+ and Fe3+) depending on redox potential.
     
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