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Oxidation of iron and galvanised iron in bleach

  1. May 7, 2016 #1
    In a laboratory experiment I have tested plain carbon steel in a beaker of bleach (sodium hypochlorite), immersed for a week.

    The result is a magenta-coloured solution with a large amount of ferrous hydroxide deposits.

    Q1) I haven't been able to find anywhere anything that might be causing this magenta colour. Fe(II) is meant to be green, so is Fe(II) Chloride, Fe(OH)2 is brown and insoluble.... so why is is deep magenta?

    Q2) There is also a large amount of trapped gas at the surface of the solution. Why is this? The most likely reduction reaction is the ClO- + H2O+2e- => Cl- +2OH-, followed by O2+H2O+4e- => 4OH-. In both of these cases, no gas is involved, or gas is actually used up.
    I have found that sodium hypochlorite decomposes into O2 gas (2NaClO=>2NaCl+O2), which is catalysed by the presence of metals, but WHY is this the case? In this equation the oxygen is oxidised, but in my situation it should be the iron being oxidised and therefore I need a reduction reaction. So where is the gas coming from?

    Thank you!
    (2nd year engineering student)
     
  2. jcsd
  3. May 8, 2016 #2

    Borek

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    Staff: Mentor

    No idea about the magenta - it is definitely not an iron compound. Color makes me think about Mn, and it is not an uncommon element in steel - but even then I am not convinced it would be oxidized to permanganate.

    Decomposition can occur as a side process, occurring completely separately from the iron oxidation.
     
  4. May 8, 2016 #3

    Nidum

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    Science Advisor
    Gold Member

    Have you used laboratory grade pure chemical or commercial bleach ?
     
  5. May 8, 2016 #4
    Thanks Borek. Do you know why the presence of iron catalyses the decomposition?

    Re: the magenta- my lecturer apparently mumbled something to someone else in my lab group about the fact that it was due to the presence of some kind of iron compound. No idea what....Maybe he was making it up?? But it was definitely extremely pink/magenta, not just a small hint due to trace elements of Mn. Strange
     
  6. May 8, 2016 #5
    Commercial bleach
     
  7. May 8, 2016 #6
    Also there were a lot more bubbles present in the beaker with plain carbon steel, compared to the beaker with galvanised steel, although theoretically the reduction reaction would be the same right?
     
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