Stuart Altmann on thu 24 jun 99
Paul Lewing wrote, "A molecular analysis is actually counting molecules,
not weighing them. Do they do a weight analysis first and then convert it
using atomic weights....How do they do the analysis...What is the tool or
machine...And how accurate are they?
The chemical analysis is done in terms of mass (so-called "weight," though
not really that). But thanks to Avogadro's number, we can readily convert
from mass to the number of atoms or molecules. The number of molecules in
one gram-mole of a substance, defined as the molecular weight in grams, is
6.022 times 10 to the 23rd power--which is Avogadro's number. For example,
the molecular weight of oxygen is 32, so one gram-mole of oxygen has a mass
of 32 g and contains 6.022 x 10^23^ molecules. Isn't that conversion amazing?
Many techniques are used for analysis of chemical elements, depending on
the level of accuracy required, the equipment on hand, and so forth. For a
good description of available techniques, see the Encyclopedia Brittanica's
Macropedia, under Analysis and Measurement. An ordinary, relatively
inexpensive analysis provides values good to two (or three) significant
figures, and that's probably all the accuracy you can expect --or need--
for most pottery work.
The most accurate apparatus now being used for routine chemical analysis is
the high-resolution mass spectroscope, which can resolve a difference as
small as one part in 10,000,000. With that kind of precision, you can tell
the masses not just of chemical elements but of the various isotopes of an
element, e.g. carbon-12 vs. carbon-14, or hydrogen vs. deuterium vs.
tritium. (In our work on baboons, we use this apparatus to analyze
quantitatively for the rare isotopes of hydrogen and oxygen, in order to
estimate the animals' rate of energy expenditure -- but that's another story.)
("In the unity part of a formula there are parts of a molecule listed for
each oxide...") No, a unity formula does not indicate the number of
molecules or parts of molecules. It indicates the relative amounts
(proportions) of the listed oxides. This common confusion would be avoided
if unity formulas were expressed as percentages instead of proportions--but
that would probably lead to a different confusion, of thinking that these
are the percentages of oxides in your recipe.
Paul asks, "Do they take many samples from different parts of the mine...Is
it different for more consistent materials?"
The more heterogeneous the material, the larger the sample size needed to
achieve any given degree of accuracy in estimating mean composition.
Getting representative samples is fraught with potential biases; much
attention has been directed to methods for minimizing them. Do mining
companies make use of modern sampling methods? Ah, thereby hangs a tale!
Stuart Altmann
email: salt@princeton.edu office 'phone: 609/258-4520
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