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Projekt Avogadro (IAC) i redefinicja jednostki liczności materii

Chyla W. T.

Wiadomości Chemiczne

2012

[Z] 66, 7-8

767-787

The paper presents the International Avogadro Coordination (IAC ) and the problem of redefinition of the unit of amount of substance (the mole) in the context of a comprehensive reform of the international system of units (SI ) that is expected to conclude in a few years. The redefinition program, known as the New SI or the Quantum SI , draws on Maxwell’s concept of replacing artifact standards with atomic standards, which are considered stable and available – at least in principle – to everybody, everywhere and at any time; the idea has been generalized and the present tendency is to define base units in terms of physical constants. Redefinition of the mole is a spin-off project associated with redefinition of the kilogram; two high-purity, 28Si-enriched silicon spheres were manufactured to make possible very accurate measurements of their parameters in order to determine (“count”) the number of silicon atoms in each of the two spheres. Initially, the project has been designed to determine the Avogadro constant, with the intent to redefine the kilogram as the mass of an exactly specified number of atoms. Once the consensus had been reached that the kilogram should be defined by fixing the numerical value of the Planck constant and the unit of mass should be realized with the use of the watt balance, the Avogadro project of silicon spheres was reinterpreted and became the basis for the redefinition and realization of the mole. In this paper, I discuss the origins of the Avogadro project (IAC ), the physical principle of a very accurate measurement of the Avogadro constant and technical details of the realization of that project. The problem of resolving the discrepancy between results of the IAC and the watt balance project is reviewed. The current status of the IAC is discussed and difficulties with the proposed wording of the New SI redefinition of the mole are indicated. It is expected that all the technical problems can be resolved before the next meeting of the CGPM, where voting on implementation of the New SI is expected.