Future requirements for non-decimal unit prefixes in chemical measurement

Abstract

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}.