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H2O, HDO, D2O equilibrium concentrations

  1. Oct 25, 2015 #1
    I have seen something to this effect stated dozens of times on the internet : "..because the deuterons and the protons are exchanging so quickly, you will end up with a statistical distribution: 25% H2O, 25% D2O, and 50% HDO." They are referring to a sample of water which is 50 atom % deuterium (relative to hydrogen) . In other words, it is not possible to isolate a sample of pure HDO.

    But how can this be true if the deuterium-oxygen bond is significantly stronger than the hydrogen-oxygen bond? Won't the deuterium show higher affinity to the oxygen and displace the hydrogen there statistically? What would be the pH of such a solution? Is the hydronium/ deuteron ion concentration the same as it would be in 100% HDO
    (were that possible). Would pH = pD ?
     
  2. jcsd
  3. Oct 25, 2015 #2

    jbriggs444

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    For a neutral solution, you are talking about 10-7 molar concentration of hydronium ions along with a 10-7 concentration of hydroxyl ions. That is a negligible adjustment to a 25%/50%/25% distribution of water molecules. (Those being roughly 13 molar, 26 molar and 13 molar concentrations).
     
    Last edited: Oct 25, 2015
  4. Oct 26, 2015 #3
    But what is the concentration of "deuteroxyl" (OD-) and deuteron ions (D+)? Perhaps we should include D3O+ ?
     
  5. Oct 26, 2015 #4

    jbriggs444

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    Beats me. I just know both are small enough to be negligible compared to the concentrations of D2O, DOH and H2O. It is those concentrations that your original post started out to ask about.
     
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