Future requirements for non-decimal unit prefixes in chemical measurement

  • Richard J. C. BrownEmail author
Discussion Forum


The 2019 revision of the International System of Units (SI), including the redefinition of the mole, and the progress of analytical chemistry technologies allowing single-molecule resolution has brought into sharp focus the dilemma between expressing measurements of ultra-low quantities of elementary entities in terms of amount of substance or in terms of counting quantities. We should prefer the use of amount of substance for these measurements for a number of reasons including to allow intensive and extensive quantities in chemistry to be distinguished and to separate these measurements from those involving the counting of non-elementary entities. The discussion article highlights how a new, non-decimal approach to unit prefixes might in future enable amount of substance to be used usefully down to its natural limit—a single elementary entity. A new prefix symbol ‘σ’ is proposed to represent the multiplier 1/{NA}.


Chemical metrology Prefixes Counting quantities Revised SI Amount of substance 



The funding of the National Measurement System by the Department for Business, Energy and Industrial Strategy is gratefully acknowledged.


  1. 1.
    The International System of Units (SI) 8th Edition (2006) International bureau of weights and measures. BIPM, Sèvres FranceGoogle Scholar
  2. 2.
    26th Meeting of the CGPM (13–16 November 2018) Draft resolution a — on the revision of the international system of units (SI). BIPM, Sèvres FranceGoogle Scholar
  3. 3.
    Brown RJC (2018) The evolution of chemical metrology: distinguishing between amount of substance and counting quantities, now and in the future. Metrologia 55:L25–L33CrossRefGoogle Scholar
  4. 4.
    Cooper G, Humphry SM (2012) The ontological distinction between units and entities. Synthese 187:393–401CrossRefGoogle Scholar
  5. 5.
    De Bièvre P (2013) Second opportunity for chemists to re-think the mole. Accred Qual Assur 18:537–540CrossRefGoogle Scholar
  6. 6.
    Leonard BP (2011) Why the invariant atomic-scale unit, entity, is essential for understanding stoichiometry without ‘Avogadro anxiety’. Accred Qual Assur 16:133–141CrossRefGoogle Scholar

Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2019

Authors and Affiliations

  1. 1.Chemical, Medical and Environmental Science DepartmentNational Physical LaboratoryTeddingtonUK

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