The Avogadro Constant: the Unit and the Scaling Factor

Some considerations are made on the importance of the Avogadro constant. It serves as the unit for amount-of-substance measurements and its numerical value serves as a scaling factor between quantities measured on the macroscopic scale and the same quantities on the atomic scale. The question is raised whether it would not be useful to define the Avogadro constant to 6 significant digits exactly, pending a further improvement of its determination to an uncertainty of <1·10 -7 N A. Any improvement is highly unlikely to suggest a significant change and, if so, such a change will be well within the implied uncertainty of the number of quoted digits. The question is based on our perception of the “particulate” nature of matter and the ensuing concept of “numerosity”. Such a definition of the Avogadro constant as basis for the unit for amount-of-substance measurement, would be very useful to the SI. It would not only simplify the establishment of traceability of chemical amount measurement results to the SI (i.e. to the definition of the SI unit for amount-of-substance), but also enable to establish traceability to the SI of microbiological and possibly other biochemical measurement results.