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Non-natural-isotopic-mix for lithium compounds.



John Denker said that some lithium
compounds may not have the "natural"
mix of lithium isotopes.

I knew separated lithium isotopes were
being manufactured, but I was unaware
that some of the processed lithium was
finding its way into chemical bottles
on our stockroom shelves.

Does anyone have any idea what chemical
companies are doing this? Do you have
any idea if they are listing the true
molecular weight on the bottle as opposed
to the "natural" molecular weight for
the particular compound?

If they are listing the natural molecular
weight (i.e. not divulging the lithium
is not the natural mix of isotopes) then
analytical chemists are going to get
goofy results. With an element this small,
the difference in weights would indeed make
a noticeable difference when making
analytical reagent solutions.

Also, during reactions, isotope effects
would make some difference since this is
a small atom.



Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton College
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu




-----Original Message-----
From: Forum for Physics Educators [mailto:PHYS-L@lists.nau.edu] On
Behalf Of John Denker
Sent: Thursday, May 13, 2004 1:17 PM
To: PHYS-L@lists.nau.edu
Subject: Re: molecular weight of dry air


Michael Edmiston wrote:

.... multiplying these percentages by the masses
and summing,
we get the average atomic mass of natural oxygen as:

15.99941

and this is the number I find on my periodic table.

If we figure the molecular mass of O2 by
simply doubling this number, we get:

31.99881

.... If we do the statistics [for the various flavors
of O2 molecule] we actually get:

31.99881

which is the same number as obtained by simply
doubling the average atomic mass found on
the periodic table.

Think about this for a minute.

I don't think I need to think about it for very long.

The result has got nothing to do with statistics; the statistical
calculation could be not quite right (and indeed *is* not quite right)
and the same result still holds. There's a sum rule in action here.
The physical principle here is that in chemical reactions, _mass is
conserved_ to a very high degree of precision.

Starting from that principle, the rest is just arithmetic. Averaging is
a linear operation. Finding the mass of a molecule by summing the mass
of the constituents is a linear operation. As my friend Yann once said
to me:

"They're both linear. I suspect they commute."

(As to why I needed to be told that, readers can use
their imagination.)


Chemists [have] been
doing it right. The atomic masses on the periodic table
are the weighted averages (weighted by natural abundance)
of the atomic masses (not mass numbers) of the naturally
occurring isotopes of the elements. It is specifically done > this
way to allow us to find average natural molecular > masses by simply
adding the atomic masses listed > in the periodic table.

Well, some chemists sometimes do it right. But meanwhile it
is also true that there are some tremendous misconceptions running
around.

1) Perhaps the mother of all conceptions in this area is
the notion that there is any such thing as "THE" natural abundance.

In particular, for years there were two different scales
for measuring atomic mass, a "chemical" scale based on the so-called
natural mixture of oxygen isotopes, and a "physical" scale based on 16O.
Some people kept on using the former for years past the point where it
was clear that place-to-place variations in "THE" natural abundance made
the scale irreproducible and self-inconsistent. An complicated
political process led to replacing both scales with the current 12C
scale in 1961.

Reference: Encyclopedia B.


2a) Also, we ought not assume that the reagent-grade stuff
in the stock bottle conforms to the "NATURAL" abundance.

For one example, it turns out that for very scary reasons, certain
parties have on hand a lot of lithium enriched in 7Li (or, more to the
point, significantly depleted in 6Li). They got the bright idea that
they would sell it in the reagent market. Your typical periodic table
quotes the natural atomic mass of lithium with a precision of 0.002,
which is already markedly worse precision than for other common elements
... but even so you can get "reagent" lithium that is outside those
error bars by a factor of 10 or 20, resulting in relative errors on the
order of half a percent or so!

2b) Even leaving aside cases where the abundance has been willfully
doctored, there are industrial processes that unintentionally
fractionate the isotopes ... for example the nitrogen in synthetic
fertilizers is waaay outside the error bars for the "natural" abundance.

2c) Even leaving aside all human intervention, there are natural
processes that fractionate chromium and thallium waaay outside the
"official" error bars.

Reference:
http://www.iupac.org/publications/pac/2002/pdf/7410x1987.pdf