On 19 June, David L. Dettman wrote:

...   Does anyone have thoughts on how long the mixed waters should sit
aroundbefore use in order to allow H2O + D2O to go to 2HDO? What is the rate of
diffusion / exchange? Is there a catalyst we could use to speed this up (if
it is slow)?

Proton mobilities in water are very high and can be represented by
reactions such as:

  H20 + H3O+ = H3O+ + H2O  (k1 = 10.6 +/- 4 x 10**9 litre mol(-1) sec(-1))

thus the half life of the proton transfer reaction for this second order
exchange is about 2 x 10-12 sec.  There will be an isotope effect for
deuterium (deuton?) exchange that would decrease the exchange rate
constant, but in practical terms the deuterium exchange reaction would
still be instantaneous.

So the short answer is that the exchange is practically instantaneous. In
artificially prepared tracer water, say up to +1000 per mille, the
contribution of D2O to mass 4 is vanishingly small.

Jeffrey Turner

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   Dr Jeffrey Turner
   Centre for Groundwater Studies
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