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Re: - C14 Decay rates



Jim Green wrote:
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;
This is an interesting statement: Does this mean that tap water is 3.5%
D2O by volume??? It must be late at night and I am missing something.

That should have been *1000 liters* (not 1000 mL) of distilled tap water
to yield 35 mL of D2O. Oh well, I was right except for the unit, do I
get partial credit? It was late at night and I was definitely missing
something.

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.

I am mystified: "any chemical reaction" ??? Why should "any chemical
reaction" be preferential to lower atm wt? The rate of reaction of D2 + O2
is slower than H2 + O2? How about Fe? Does Fe56 rust faster than
Fe58? And leaves a coat of what on the surface?

According to what I was taught many moons ago, every chemical reaction
is subject to isotope effect. I'm assuming this is still in intro chem
texts. _Chemistry_, Sienko and Plane, 1961, P. 311: "... a bond to a
protium atom can be broken as much as 18 times faster than a bond to a
deuterium atom. As an example, H2 reacts with Chlorine2 13.4 times as
fast as D2 does. ... For elements heavier than hydrogen the isotope
effect is much smaller."

I'm just performing journalist duty here, and I don't understand *why*
this occurs. Perhaps we have a physical chemistry guru in the Phys-L
membership who can elaborate.

U238 _diffuses_ differently from U235 -- Initially that is how it was
separated. But oxidize differently?

If that is true, then C-12 CO2 is going to react about 2% (1.02 times)
faster than C-14 CO2.

I don't understand the calculation here.

I thought that it was said that "the effect is only dramatically
substantial in the case of hydrogen."

*If* the relationship I quoted (for which I have no credentialed source
at this time) is true, the reaction rate (r) relates to mass (m)
approximately by R = r1/r2 = sqrt(m2/m1). I was assuming that m1 and m2
are molecular mass, so then for C-12 CO2 m1=44amu and for C-14 CO2
m2=46amu and R = sqrt(46/44) = 1.02

If the reaction rate is determined only by the carbon, then R =
sqrt(14/12) = 1.08 and the rate is faster than I surmised, though
nowhere close to the ratio for H-1 and H-2. In any event C-12 reaction
rate is just a few percent faster than C-14, the only question being
exactly what that percentage is.

I call 1000-2000% (10-20 times) faster "dramatically substantial"
compared to 2%-8% (1.02-1.08 times) faster.

For Uranium isotopes apparently the reaction rate ratio is so small that
separation by reaction rate is not feasible, so the diffusion technique
is used.

Best wishes,

Larry

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