Avogadro constant
The Avogadro constant (symbol: L or NA) is a physical constant that relates measurements made at the macroscopic scale to those made on an atomic scale. It is usually defined as the number of entities (such as atoms or molecules) in one mole, and the 2006 CODATA recommended value is 6.022 141 79(30) × 1023 mol−1. It was orginally called Avogadro's number, and is named in honour of the Italian physicist Amadeo Avogadro.
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International Avogadro Coordination
The International Avogadro Coordination (IAC), often simply called the "Avogadro project", is a collaboration begun in the early 1990s between various national metrology institutes to measure the Avogadro constant by the X-ray crystal density method to a relative uncertainty of 2 × 10−8 or less.[1][2]
Results using one kilogram spheres of silicon (natural isotopic composition) gave an uncertainty of 3.1 × 10−7.[3][4] In 2007, a 4.8-kg single crystal of isotopically-enriched silicon (99.94% 28Si) was grown, and two one-kilogram spheres cut from it. Diameter measurements on the spheres are repeatable to within 0.3 nm, and the uncertainty in the mass is 3 µg. Full results from these deteminations are expected in late 2010.[5]
Proposed redefinition
In the context of the "New SI", based around the redefinition of the kilogram in terms of a universal physical constant, there have been several proposals that would give the Avogadro constant a defined value in SI units.
Notes and references
Notes
References
- ↑ Avogadro Project; National Physical Laboratory, <http://www.npl.co.uk/engineering-measurements/mass-force-pressure/mass/research/avogadro-project>. (accessed 19 August 2010).
- ↑ Jabbour, Zeina J. Getting Closer to Redefining The Kilogram. Weighing & Measurement Magazine 2009 (October), 24–26, <http://www.nist.gov/customcf/get_pdf.cfm?pub_id=903635>.
- ↑ Becker, Peter Tracing the definition of the kilogram to the Avogadro constant using a silicon single crystal. Metrologia 2003, 40 (6), 366–75. DOI: 10.1088/0026-1394/40/6/008.
- ↑ Williams, E. R. Toward the SI System Based on Fundamental Constants: Weighing the Electron. IEEE Trans. Instrum. Meas. 2007, 56 (2), 646–50. DOI: 10.1109/TIM.2007.890591.
- ↑ Report of the 11th meeting of the Consultative Committee for Mass and Related Quantities (CCM); International Bureau of Weights and Measures, 2008; p 17, <http://www.bipm.org/utils/common/pdf/CCM11.pdf>.
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