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The evolution of chemical metrology: distinguishing between amount of substance and counting quantities, now and in the future

This discussion article begins by highlighting the benefits of the mole’s incorporation within the international system of units (SI), in particular by bringing chemical measurement within formal metrology structures. The origins of the confusion that has consistently existed between amount of substance (the base quantity of which the mole is the SI base unit) and counting quantities are examined in detail and their differentiating characteristics fully elaborated on. The importance and benefits of distinguishing between these different quantities and the role that the Avogadro constant plays in doing this are highlighted. It is proposed that these issues are becoming increasingly important for two reasons. First, as chemistry and biology consider increasingly small size domains, measurements are being made of significantly reduced collections of entities. Second, the proposed re-definition of the mole makes the link between amount of substance and the number of elementary entities more transparent. Finally, proposals for new ways of expressing very low amounts of substance in terms of new prefixes based on the numerical value of the Avogadro constant are presented as a way to encourage the use of the mole, when appropriate, even for ultra-low level chemical measurement.