Ten reasons NOT to fix the numerical value of the Avogadro constant

Nigel Wheatley

doi:10.1038/npre.2011.5828.1

Abstract

AbstractThis discussion paper, circulated in advance of the 17th meeting of the Consultative Committee for Amount of Substance, addresses the proposed redefinition of the mole in terms of a fixed numerical value for the Avogadro constant. It gives ten reasons why the Avogadro constant should not be given a fixed numerical value in the International System of Units, noting that there would be no metrological benefit from such a change, and that the proposed redefinition would be more conceptually complex that the current one and divorced from practical measurement and historical background. As the conditions for redefinition of the kilogram have not been met, and no mise en pratique has been produced for the proposed new definition, the paper concludes that the proposal should be rejected.

Highlights

The Comité consultatif pour la quantité de matière – metrologie en chimie (CCQM) restated its “preference” for a redefinition of the mole based on a fixed [numerical] value of the Avogadro constant at its 15th meeting in 2009 [4]
It is widely accepted that fixing the numerical values of the Planck constant h and the elementary charge e would allow the results of many measurements to be expressed more precisely in SI units, so-called “quantum metrology”
There are two nontrivial physical constants whose uncertainties would depend on the uncertainty in NA: the Faraday constant F and the molar gas constant R.1

Summary

Introduction

The Comité consultatif pour la quantité de matière – metrologie en chimie (CCQM) restated its “preference” for a redefinition of the mole based on a fixed [numerical] value of the Avogadro constant at its 15th meeting in 2009 [4]. Relative measurement uncertainties (109ur) in various physical constants under the current definitions of the SI base units and under three possible redefined systems: redefinition of the kilogram and the ampere, with additional redefinition of the kelvin or of the kelvin and the mole.

Results

Conclusion