A Standard Mass You Can Count On: Refining Avogadro Constant to Boost Kilogram's Precision

By Cevallos, Marissa | Science News, November 20, 2010 | Go to article overview

A Standard Mass You Can Count On: Refining Avogadro Constant to Boost Kilogram's Precision


Cevallos, Marissa, Science News


The kilogram may finally get a break from its yo-yo diet. An international team of scientists is closer to redefining the unit of mass based on fundamental constants, instead of a piece of metal in France that loses weight only to put it back on again.

Since 1889, the international standard for the kilogram has been a cylinder of platinum stored in a vault outside Paris. But despite exceedingly stringent storage conditions, the cylinder (and six exact copies kept with it) gains weight from dust in the atmosphere, requiring regular steam baths to remove the crud. On top of that, the cylinders change mass relative to each other by micrograms per century, for reasons no one can fully explain.

Scientists want to redefine the basic metric unit of mass based on something truly constant, just as the meter is defined as the distance light travels in 1/299,792,458th of a second. Several teams have been trying to define the kilogram in terms of the Avogadro constant, well-known to chemistry students as the number of atoms or molecules in one "mole" (about 6.022 times [10.sup.23]).

[ILLUSTRATION OMITTED]

German scientists determined the constant by counting the atoms in painstakingly crafted one-kilogram spheres of silicon-28. Because, by definition, one mole of silicon-28 atoms weighs 28 grams, a kilogram of the material should contain 35.7142857 moles of atoms.

In principle, counting the atoms is like estimating how many Coca-Cola cans are in a giant mound of 12-pack cartons, says Arnold Nicolaus of the Physikalisch-Technische Bundesanstalt in Braunschweig, Germany. …

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