CCQM owes chemists a description of the concept 'amount of substance'

CCQM, the ‘Consultative Committee on amount of substance—metrology in chemistry’ to the International Committee for Weights and Measures (CIPM), is being renamed ‘CC on amount of substance—metrology in chemistry and biology’. This seems to be an excellent occasion to review the status of the understanding of the concept ‘amount of substance’ in the chemical community at large. It is broadly admitted that the concept (and term) ‘amount of substance’ has been a source of confusion ever since it was used in the International System of Units (SI) in 1971 by the General Conference on Weights and Measures (CGPM) as the base quantity for the definition of the unit mole [1]. Thus, the International Organization for Standardization (ISO) through its Technical Committee 12, responsible for quantities, added it as such to the International System of Quantities, ISQ [2]. It was also included in the 2nd and 3rd edition of the International Vocabulary of Metrology [3, 4]. The term mole had been in existence for a very long time (almost a 100 years), albeit subject to various interpretations. However, there is a long-standing problem. A unit—of necessity—requires a quantity of which it is the unit: the kilogram is the unit of the quantity ‘mass’, the second is the unit of the quantity ‘time’, the metre is the unit of the quantity ‘length’, etc. The absence of a clearly understood and accepted description in the chemical community of the quantity ‘amount of substance’ having the unit mole, especially in the analytical community, was—and still is—a problem. That is not good. Sometimes, ‘amount of substance’ is simply described as ‘the quantity of which the mole is the unit’ [5, 6], a description from which we do not really learn very much. Neither is the text in a well-known textbook helpful wherein it is stated that ‘The mole is an SI base unit ... The physical quantity to which it refers is called the ‘amount of substance’, n. However, practicing chemists commonly prefer to talk about the number of moles. Take the advice of your instructor on whether to use or not the official term’ [7]. A very worthwhile attempt was already made in 1990s to replace the term ‘amount of substance’ by ‘numerosity’ [8], a term for indeed a property of matter (see definition of ‘quantity’ in [3, 4]) that stresses the fact that matter is discontinuous in structure and consists of discrete entities. It was not disapproved in the literature (on the contrary) nor considered and replied to in the ‘New SI’ [9]. Rather ‘amount of substance’ was kept, stressing a continuous character as property of matter, a property that was disqualified by modern science since more than a hundred years. Also, the ‘Reproposition’ [10] of the suggestion underwent the same fate rather than be examined seriously. Hence, let us attempt to understand—or at least attempt to clarify some problems—with the SI quantity ‘amount of substance’. Around 1990s, the need for ‘traceability of chemical measurement results to the SI’ started to take off in the literature because there was a rising need to compare measurement results across borders in trade, in commerce, in food and feed measurements, in environmental and clinical measurements, in international and intercontinental interlaboratory comparisons, to name just a few. Also, The author is a member of the Joint Committee on Guides for Metrology (JCGM), Working Group 2 (VIM). The opinions expressed in this column do not necessarily represent the view of the Working Group or of ACQUAL.