Letters April 2019

Peter Nelson (Chemistry World, February 2019, p4) raises the same concerns about the revision of the International System of Units (SI) as he has done previously (Chemistry World, October 2017, p4 and Chemistry World, October 2011, p43).

There is nothing ‘new’ about the SI units – instead it is the definitions of four of the SI base units that are changing in order to future-proof the global measurement system. The definition of units with respect to fixed numerical values of fundamental constants allows their realisation across a range of magnitudes of the unit’s scale, and not just with an uncertainty optimised for a specific material artefact. It also allows for realisation according to any workable experiment. A definition of the kilogram (or, for that matter, the mole) based on a fixed number of silicon-28 atoms, as he proposes, is inferior in both respects.

He also suggests that fixing the numerical value of the Avogadro constant will affect the molar mass constant. This is not correct. The fixed numerical value of the Avogadro constant has been carefully chosen to ensure that the molar mass constant is consistent with its historic value to within its newly acquired relative uncertainty of one part in 2 billion (a figure entirely insignificant for practical chemistry).

As for the variation of fundamental constants over time and space, there remains no compelling evidence of this, let alone over the time and length scales that would have any relevance for practical measurements.

Finally, in reply to Franz Wimmer (also Chemistry World, February 2019, p4), the mole is neither just a number, nor is it an arbitrary choice – recent publications on the mole’s redefinition explain why.