Is it possible to separate heavy water by refrigeration?

It may be difficult to separate "theory" from "application," but let''s see if this helps facilitate the discussion.
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Chris Bradley
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Is it possible to separate heavy water by refrigeration?

Post by Chris Bradley » Mon May 07, 2012 11:24 am

Just an idling thought - as heavy water ice is heavier that water and melts at 3.8C, if you have a cold plate at the bottom of a tank of water held accurately to 1C, and have a slow flow of replenishing water in, then would heavy-water ice form at the bottom on the cold plate over time?

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Rich Feldman
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Re: Is it possible to separate heavy water by refrigeration?

Post by Rich Feldman » Tue May 08, 2012 12:08 am

I doubt it, for a couple of reasons.

First, you are proposing a sort of "freeze distillation", which is never a black-and-white proposition. Consider the analogy of chilling a mixture of water and ethanol. When it starts to freeze, you don't get pure water ice. You get a solid phase somewhat more water-rich, and a liquid phase somewhat more water-poor. One ref:
http://en.wikipedia.org/wiki/Fractional ... stillation

Second, let's suppose regular water on Earth has a H:D ratio of 6000. That doesn't mean 1/6000 of the molecules are D2O. It means 1/3000 of them are HDO, and at any instant perhaps 1/9000000 of them are D2O. That reduces the efficiency of any enrichment using molecular water.

And those H's and D's are promiscuous. Remember that pH=7 means 1e-7 is the equilibrium concentration of dissociated ions H+ and OH-. They're all busy recombining and re-ionizing, so O atoms swap partners many times each second.
Chemistry experts, please correct me if I have this wrong:
If you mixed equal parts of H2O and D2O,
within the blink of an eye 50% of the molecules would be HDO.
Mike echo oscar whisky! I repeat! Mike echo oscar whisky, how do you copy? Over.

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Frank Sanns
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Re: Is it possible to separate heavy water by refrigeration?

Post by Frank Sanns » Tue May 08, 2012 2:33 am

There are a few things working against you here. For one, temperature is average molecular motion so when freezing points are close, there is a chance for some frozen materials to become unfrozen. This is complicated in very dilute solutions like with water. Add to that a large affinity of water molecules to each other because of hydrogen bonding, be they H or D, and it is an even more difficult task. Pure water also can super cool significantly so it would have to be seeded with frozen D2O to have a chance.

It would be a waste of time for even a fool to try such a thing......well, actually, I have to confess, I tried it. I tried it with a twist. I spun water up at 800,000 g in a centrifuge for a couple of hours at 1 degree C to see if I could first enrich the density of D2O at the bottom of the tubes. It did not freeze so I started to drop the temperature. At -10 for an hour there was still not crystallization starting. I very slowly brought the centrifuge to a stop over the period of nearly 15 minutes so as to not disturb any gradients that I might have. I seeded it with a mg or so of some pure frozen D2O that I had and the entire tube froze with long crystals. No stratification or liquid was left.

The next try was to cool things down to 1C over night so everything was in equilibrium and then I put a few mg of frozen D2O into the tube and started the centrifuge. After 2 hours I slowly brought it to a stop but there was only liquid again. The only thing I could think was the temperature in the tube may have warmed a degree or so as I had things open and the seed D2O melted.

I wanted to try one more time by going down to 0 C and seeding with a larger quantity of D2O but it starts getting to the point of not being worth it as there is not much pure D2O in water to begin with so I just shelved the idea. Like I said, a foolish thing to try but sometimes you just have to give it a go!

Frank Sanns

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