Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: chiralSPO on 14/02/2016 02:33:25
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As I understand it, Avogadro's number is currently defined as the number of atoms of 12C that together have a mass of exactly 12 grams (and 1 gram is currently defined as 1/1000 the mass of the official kg bar in France https://en.wikipedia.org/wiki/Kilogram). The kg is currently the only SI unit not defined by a natural (universal) standard.
By current definitions, Avogadro's number is about 6.022140857x1023. It cannot be defined with any more precision due to the precision of the standard kg, which changes with time...
As it happens, 279 = 604462909807314587353088; a mere 0.37% larger than the currently defined value. The half lives of many radioactive nuclei can be determined with high precision (http://www.nist.gov/pml/data/halflife-html.cfm), for instance 18F, 67Ga, and 111In are all currently known to within 1ppm. So a mole of 111In (t½= 2.80477 days) could be defined as the mass required to observe a decay for 221.577 days. (or could define it by the instantaneous decay rate...)
Thoughts?
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The half lives of many radioactive nuclei can be determined with high precision
The problem with radioactive materials is that they are continually shooting out subatomic particles (that changes their mass slightly), and the fact that they are radioactive makes them a bit hazardous to handle, and could affect the operation of nearby electronics making the measurement.
You can't easily average measurements made on 10 successive days, because the decay rate will differ on every day.
While the original element may be quite stable, fission decay products are sometimes a bit unpredictable, and some of them may react with oxygen in the air.
There has been debate for some time about redefining the SI kilogram.
One of the proposals was to base the kilogram on a certain number of atoms of isotopically pure silicon. So various teams produced highly-polished silicon spheres, and then attempted to count the atoms, by various methods. This would have defined Avogadro's number in a new way.
For some of the saga, see: http://spectrum.ieee.org/consumer-electronics/standards/the-kilogram-reinvented
However, the current recommendation is to define the kilogram in terms of Plank's constant. This in turn will in turn produce a more traceable value for Avogadro's number.
See: http://en.wikipedia.org/wiki/Proposed_redefinition_of_SI_base_units#Kilogram
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By current definitions, Avogadro's number is about 6.022140857x1023. It cannot be defined with any more precision
The half lives ..... are all currently known to within 1ppm.
So you want to replace a number known to 1 part in 1010 with one known to 1 part in 106. In what way does this constitute progress?
There is a special place in Hell for Disciples of the Box - beware of guilt by association!
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You can't easily average measurements made on 10 successive days, because the decay rate will differ on every day.
While the original element may be quite stable, fission decay products are sometimes a bit unpredictable, and some of them may react with oxygen in the air.
So you want to replace a number known to 1 part in 210 (sic) with one known to 1 part in 26 (sic). In what way does this constitute progress?
I only posed the question. I am happy to have answers explaining why it is a bad idea. :-) I did notice as I was looking at the numbers that the half lives were known with less precision than I had initially thought. Still, I would say it is progress if we could define our unit of mass in such a way that it could be transmitted anywhere in the universe and still be valid.
There is a special place in Hell for Disciples of the Box - beware of guilt by association!
Now THAT was uncalled for!
There has been debate for some time about redefining the SI kilogram.
One of the proposals was to base the kilogram on a certain number of atoms of isotopically pure silicon. So various teams produced highly-polished silicon spheres, and then attempted to count the atoms, by various methods. This would have defined Avogadro's number in a new way.
For some of the saga, see: http://spectrum.ieee.org/consumer-electronics/standards/the-kilogram-reinvented
However, the current recommendation is to define the kilogram in terms of Plank's constant. This in turn will in turn produce a more traceable value for Avogadro's number.
See: http://en.wikipedia.org/wiki/Proposed_redefinition_of_SI_base_units#Kilogram
Interesting. I do like the idea of using Plank's constant--really anything to get away from a hunk of metal that has no universal meaning.
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Interesting. I do like the idea of using Plank's constant--really anything to get away from a hunk of metal that has no universal meaning.
This method has my vote as well, referenced to a constant precisely defined.
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Nice idea, but the official value of h has changed by about 3 ppm in the last 100 years!
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Nice idea, but the official value of h has changed by about 3 ppm in the last 100 years!
I think Plank's constant can be allowed some leeway to settle down in its first decades after discovery. People have been weighing lumps of metal on balances for millennia, so you would think they could get that right by now...
All that metrologists can say is that the master kilogram seems to have lost as much as 50 µg over the course of a century relative to its siblings.
Mmmm... 50 µg out of 1 kg is 50 ppb (parts per US billion) for the platinum/iridium kg.
The Planck constant is (with one or two exceptions) the fundamental physical constant which is known to the lowest level of precision, with a 1σ relative uncertainty ur of 1.2×10−8.
1.2×10−8 is 12 ppb. I think Plank is still ahead.
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1.2×10−8 is 12 ppb. I think Plank is still ahead.
But still way behind Avogadro at 1 in 1010