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ORP Element in Water

  1. Jan 7, 2016 #1
    Dear PF Forum,
    I would like to thank this forum for providing me invaluable helps lately. I'd like to understand ORP in water.
    I'm aware that PF discourages member for not doing research first. I've (been) reading redox, antioxidant, pH, covalent, including: https://en.wikipedia.org/wiki/Reduction_potential and http://chemwiki.ucdavis.edu/Analyti.../Redox_Chemistry/Standard_Reduction_Potential
    Frankly, I have no idea how ORP works.

    So, perhaps I should give a condition first.
    There are 1 liter pure H2O, 55.50622 moles.
    In it I disolve 1 gr NaCl, 0.017112 moles. So that the water can be electrolyzed.
    Now, as you know there are many water ionizer machines out there. Supposed I pump this solution to the machine, so that the water is divided by 2.
    500 ml of it has PH = 9 and ORP = -300.
    A: PH = 9 means that in this 500 ml of water there are 0.5 * 10-9 moles of H+, about 0.5 nanogram and 0.5 * 10-5 OH-, about 0.45 miligram. Is this true?
    B: What about ORP. What element can make this water has ORP -300 milivolt?
    Since I only use H2O and NaCl, I think there are some possible answers here:
    H+, OH-, Na+, Na (neutral), HCl, anything?

    Thank you very much.
  2. jcsd
  3. Jan 7, 2016 #2


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    Staff: Mentor

    When you dissolve NaCl in water the solution is more or less just neutral, it doesn't become automagically pH 9 for no apparent reason.

    As far as I am aware in normal conditions redox potential you can measure after putting an electrode in water would be that of O2/OH- system (where O2 comes from the atmosphere). The exact potential will depend on the pH and partial pressure of oxygen and it will be given by the Nernst equation.
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