Light Heavy and Semiheavy Water Equilibrium

[Vanadium 50]

If I start with a mix of half H2O and half D2O, when it equilibrates it will be half HDO, a quarter H2O and a quarter D2O. My question is "how long does this take?". Ballpark is fine - microsecodnds? Days? Centuries?

 

[Bystander]

If I start with a mix of half H2O and half D2O, when it equilibrates it will be half HDO, a quarter H2O and a quarter D2O. My question is "how long does this take?". Ballpark is fine - microsecodnds? Days? Centuries?

What temperature/phase(solid, liquid, vapor)? Liquid kinetics are O(m) different from solid/vapor.

 

[Vanadium 50]

Liquid water, room temperature. Maybe a little cooler.

 

[Bystander]

https://en.wikipedia.org/wiki/Norwegian_heavy_water_sabotage ;
Totally forgot about this. Liquid phase? Fast as you can mechanically mix/stir it together.

 

[Frabjous]

https://en.wikipedia.org/wiki/Norwegian_heavy_water_sabotage ;
Totally forgot about this. Liquid phase? Fast as you can mechanically mix/stir it together.

I believe he is talking about hydrogen-deuterium exchange, not just mixing.

 

[Borek]

Mixing limited, technically this is not much different from acid/base reactions and these are quite fast, in nanosecond range if memory serves me well.

 

[Vanadium 50]

Thanks. I guess I could think of it as an acid-base reaction, between some very week acids and bases. I don't know the pH of D2O, but imagine it's around 7.3.

Calculating from 1st principles looks like a nightmare, since you have a 6-way equilibrium between H2O, HDO, D2O, H+, D+, OH-, and OD-. (Plus the complication of whether H+ is really H3O+) But "a tint fraction oif a second" is a good enough answer for me.

 

[Bystander]

https://www.google.com/search?q=aut...512l4.18337j0j7&sourceid=chrome&ie=UTF-8#ip=1

 

[Vanadium 50]

I am not sure what that link is meant to say. It's just there.

It dies not mention a time, which is my original question. It does say, indirectly, that water (H2O) has a pH of 7. which is not news. (D2O I looked up and it is 7.4, I estimated 7.3)

I think @Borek answered my question. "As fast as they physically can mix"

 

[Borek]

Handwavy, but more detailed answer. If you put several water molecules side by side, they will get linked by hydrogen bonds. Resulting four membered ring is highly symmetrical (even if not flat) and the bonds will start oscillating, resulting in a very quick exchange of H and D between water molecules. In the presence of H⁺ from water autodissociation this is made even easier, as H⁺ will attach itself to one of the lone electron pairs of a molecule (red outlined part). Charge will delocalize to all hydrogens three hydrogens (as in H₃O⁺), which makes them even more eager to bond to neighbor water molecules, which further speeds up bond oscillations, to the point where charge easily jumps between water molecules, rearranging which hydrogen is attached to each oxygen on the way. That's actually why limiting ion conductivity of H⁺ is anomalously high, several times higher than that of any other ion - H⁺ doesn't have to travel by itself, it is charge that jumps (not the case of, say, Na⁺, which has to meticulously navigate between water molecules).

This explanation is far from being strict, but gives good intuition, and shows why individual water molecules in liquid water are not as "separate" as molecules in other liquids.