Re: - C14 Decay rates

[Larry Cartwright <exit60@CABLESPEED.COM>]

Jim Green wrote:
> This is a migration from another list:
> > It is well known that the rates of chemical reactions are different for
> > different isotopes.

Yes.

The most dramatic physical application of that phenomenon might be the
separation of "heavy water" from "light water" by electrolysis.  H-1 H20
molecules are broken 1000-2000% (10-20 times) faster than H-2 H20
molecules, so protium oxide molecules are preferentially decomposed by
electrolyis, leaving the remaining water enriched with deuterium oxide.
If you perform electrolysis on a liter of tap water, by time you get
down to 35 mL remaining you theoretically should have nearly pure H-2
H20 (or D2O).  It should have a mass substantially greater than 35
grams; if you freeze it, the resulting ice cube should sink in ordinary
water.  But I digress.

Yes, any chemical reaction is going to preferentially yield
light-isotope products to some degree. However, the effect is only
dramatically substantial in the case of hydrogen.

> This means that a chemical reaction which continues over
> > thousands of years, for instance the autoxidation of glucose, could result
> > in the preferential loss of C12O2 to the atmosphere relative to the loss of
> > C14O2.
>
> Does this list agree?

I read or heard somewhere that the ratio of reaction rates is, roughly,
inversely proportional to the square root of the ratio of masses.  That
exists only as a penciled margin note in my ancient copy of Sienko &
Plane, so I can't claim a credentialed source for that relationship.

If that is true, then C-12 CO2 is going to react about 2% (1.02 times)
faster than C-14 CO2.  So, yes, I don't have any doubt that this is
going to cause a slight preference for the production C-12 CO2 and is
going to accumulate a slightly increased C-14 concentration in the
source of the glucose.

However, it seems to me that the higher proportion of C-12 CO2 given off
by the oxidation of glucose is going to be offset by the equally higher
proportion of C-12 CO2 in the photosynthetic uptake which forms glucose
in the first place.  This is a fact that will have to be taken into
consideration in any *interpretation* of the preferential loss statement
you quoted.

Best wishes,

Larry

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Larry Cartwright   <exit60@cablespeed.com>
Retired (June 2001) Physics Teacher
Charlotte MI 48813 USA
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~