H2O, HDO, D2O equilibrium concentrations

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Discussion Overview

The discussion revolves around the equilibrium concentrations of H2O, HDO, and D2O in a sample of water containing 50 atom % deuterium. Participants explore the implications of bond strengths, statistical distributions, and the effects on pH and ion concentrations in such a solution.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the validity of the statistical distribution of 25% H2O, 25% D2O, and 50% HDO, suggesting that the stronger deuterium-oxygen bond should lead to a higher affinity of deuterium for oxygen.
  • Another participant notes that for a neutral solution, the concentration of hydronium and hydroxyl ions is negligible compared to the overall distribution of water molecules.
  • A participant raises questions about the concentrations of deuteroxyl (OD-) and deuteron ions (D+) and suggests the possible inclusion of D3O+ in the discussion.
  • Another reply reiterates uncertainty regarding the concentrations of deuteroxyl and deuteron ions, stating they are likely negligible compared to the concentrations of D2O, HDO, and H2O.

Areas of Agreement / Disagreement

Participants express differing views on the statistical distribution of the water molecules and the implications of bond strengths, indicating that multiple competing views remain without a consensus.

Contextual Notes

There are unresolved questions regarding the exact concentrations of various ions in the solution, as well as the assumptions underlying the statistical distribution of the water molecules.

ScepticAmatuer
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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 ?
 
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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:
But what is the concentration of "deuteroxyl" (OD-) and deuteron ions (D+)? Perhaps we should include D3O+ ?
 
ScepticAmatuer said:
But what is the concentration of "deuteroxyl" (OD-) and deuteron ions (D+)? Perhaps we should include D3O+ ?
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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