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Definition of the Mole

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Definition of the Mole IUPAC Definition of the mol e For Peer Review Only Journal: Pure and Applied Chemistry Manuscript ID PAC-REC-17-01-06 Manuscript Type: Recommendation Date Submitted by the Author: 11-Jan-2017 Complete List of Authors: Marquardt, Roberto; Université Louis Pasteur, Laboratoire de Chimie Quantique Theorique Meija, Juris; National Research Council Canada, Measurement Science and Standards Mester, Zoltan; National Research Council of Canada, Institute for National Measurement Standards Towns, Marcy; Purdue University System, Department of Chemistry Weir, Ron; Royal Military College of Canada, Dep. of Chemistry & Chemical Engineering Davis, Richard Stohner, Jürgen; ZHAW Zürich Univ. for Applied Sciences, ICBC Institute for Chemistry and Biological Chemistry; ETH-Zürich, Physical Chemistry Keywords: the mole, definition, Avogadro constant, Avogadro number Author-Supplied Keywords: P.O. 13757, Research Triangle Park, NC (919) 485-8700 Page 1 of 5 IUPAC Pure Appl. Chem. 201X; aop 1 2 3 Sponsoring bodies: IUPAC Physical and Biophysical Chemistry Division, IUPAC Inorganic Chemistry Division, IUPAC Analyt- 4 ical Chemistry Division, IUPAC Committee on Chemistry Education, and IUPAC Interdivisional Committee on Terminology, 5 Nomenclature and Symbols: see more details on page XXX. 6 IUPAC Provisional Recommendation 7 8 9 Roberto Marquardt, Juris Meija, Zoltán Mester, Marcy Towns, Ron Weir, Richard Davis, 10 and Jürgen Stohner* 11 12 Definition of the mole (IUPAC Provisional 13 14 For Peer Review Only 15 Recommendation 201X) 16 17 18 DOI: 10.1515/pac-201X-XXXX, Received ...; accepted ... 19 Abstract: In 2011 the General Conference on Weights and Measures (CGPM) noted the intention of the 20 21 International Committee of Weights and Measures (CIPM) to revise the entire International System of 22 Units (SI) by linking all seven base units to seven fundamental physical constants. Of particular interest 23 to chemists, new definitions for the kilogram and the mole have been proposed. A recent IUPAC Technical 24 Report discussed these new definitions in relation to immediate consequences for the chemical community. 25 This IUPAC Recommendation on the preferred definition of the mole follows from this Technical Report. 26 It supports a definition of the mole based on a specified number of elementary entities in contrast to the 27 present 1971 definition. 28 29 Keywords: SI; the mole; definition; Avogadro number; Avogadro constant 30 31 32 1 Introduction 33 34 The 9th General Conference on Weights and Measures (CGPM) instructed the International Committee 35 for Weights and Measures (CIPM) in 1948 “to make recommendations for a single practical system of units 36 of measurement, suitable for adoption by all countries adhering to the Metre Convention” [1]. In 1954 the 37 10th CGPM adopted a practical system of units of measurements for international use. It contained six 38 base units: the metre, kilogram, second, ampere, degree Kelvin (later renamed kelvin), and candela [2]. This 39 international system was named “Système International d’Unités” (engl. International System of units) and 40 41 abbreviated as SI by the 11th CGPM [3]. The seventh base unit, the mole, was added to the SI in 1971 by 42 the 14th CGPM [4]. 43 The International Bureau of Weights and Measures (BIPM) publishes the SI Brochure with the purpose 44 “to define and promote the SI, which has been used around the world as the preferred language of science 45 46 Roberto Marquardt: Laboratoire de Chimie Quantique, Institut de Chimie, Université Louis Pasteur, 1 Rue Blaise Pascal, 47 F-67008 Strasbourg France 48 Juris Meija: National Research Council Canada, Measurement Science and Standards, 1200 Montreal Road M-12, Ottawa, 49 ON K1A 0R6 Canada 50 Zoltán Mester: National Research Council Canada, Measurement Science and Standards, 1200 Montreal Road M-12, Ot- 51 tawa, ON K1A 0R6 Canada 52 Marcy Towns: Purdue University, Department of Chemistry, 560 Oval Drive, West Lafayette, Indiana USA 53 Ron Weir: Royal Military College of Canada, Department of Chemistry & Chemical Engineering, P.O. Box 17000, Stn. 54 Forces, Kingston, ON K7K 7B4 Canada 55 Richard Davis: Bureau International des Poids et Mesures, Pavillon de Breteuil F-92312 Sèvres Cedex France 56 *Corresponding Author: Jürgen Stohner: ZHAW Zürich University of Applied Sciences, ICBT Institute for Chemistry & 57 Biotechnology, Einsiedlerstrasse 31, CH 8820 Wädenswil Switzerland , email: [email protected] and Guest scientist at: ETHZ 58 Swiss Federal Institute of Technology, Laboratory for Physical Chemistry, ETH Hönggerberg, Wolfgang-Pauli-Strasse 10, 59 CH-8092 Zürich, Switzerland, email: [email protected] 60 P.O. 13757, Research Triangle Park, NC (919) 485-8700 IUPAC Page 2 of 5 2 Marquardt et al, Definition of the mole 1 2 3 and technology since its adoption in 1948” [5]. The SI is an evolving system which reflects current best 4 measurement practices. Thus, changing the definition of base units of measurement is not an unprecedented 5 nor unusual action. The evolving definition of the metre is an example of this. Early adoption of the metre as 6 the unit of length by the French government resulted in its definition as the 1:10 000 000 part of the meridian 7 through Paris between the North Pole and the equator. A century later, the first meeting of the CGPM 8 in 1889 sanctioned a new platinum-iridium prototype metre bar which served as the official international 9 metre until 1960. The desire to adopt an indestructible standard and advances in interferometry led the 10 11th CGPM to further redefine the metre as “the length equal to 1650 763.73 wavelengths in vacuum of 11 12 the radiation corresponding to the transition between the levels 2p10 and 5d5 of the krypton 86 atom” 13 in 1960 [6]. With fixing the velocity of light to a constant value, the metre was last redefined by the 14 17th CGPMFor in 1983 Peer as “the length ofReview the path travelled by lightOnly in vacuum during a time interval of 15 1/299 792 458 of a second” [7]. 16 The present SI has several important shortcomings (see [8] and references therein). Notably, the base 17 unit kilogram is based on an artifact, a platinum-iridium cylinder (the international prototype kilogram, 18 IPK) manufactured in 1879 and stored at the BIPM. There is an intrinsic uncertainty in the long-term 19 stability of the IPK. In principle, this instability also impacts other SI units that depend on the kilogram, 20 21 among which is the mole. 22 In 1999 the 21st CGPM recommended that “national laboratories continue their efforts to refine exper- 23 iments that link the unit of mass to fundamental or atomic constants with a view to a future redefinition 24 of the kilogram” [9]. Furthermore, in 2011 the CGPM noted the intention of the International Commit- 25 tee of Weights and Measures (CIPM) to revise the entire SI by linking all seven base units to seven 26 constants [10, 11]. 27 In a recent IUPAC Technical Report [8], a thorough analysis of the discussion on the impact of the 28 proposed new definitions of two key units for chemists was undertaken: the kilogram and the mole. The 29 present recommendation focuses on the unit mole. In Section 2 a brief summary of the discussion is recalled, 30 31 in Section 3 the recommended definition of the mole is formulated, and in Section 4 some additional remarks 32 and explanations are presented. 33 34 35 36 2 Discussion 37 38 Since the proposals for a new definition of the mole were put forward, a major division of opinions among 39 chemists was whether to retain the current definition based on the mass of carbon-12 or to adopt a definition 40 that is based on a fixed value of the Avogadro constant. 41 The amount of chemical substances is traditionally measured by mass or volume. Since the introduction 42 of relative atomic masses (also called “atomic weights”) by Dalton (see for example [12]), chemists are able 43 to express their observations in a quantity that is proportional to the number of elementary entities. The 44 unit of the SI quantity “amount of substance” in its present definition still relies on the mass of the 45 46 international prototype kilogram [4]: 47 “1. The mole is the amount of substance of a system which contains as many elementary entities as 48 there are atoms in 0.012 kg of carbon 12; its symbol is “mol”. 49 2. When the mole is used, the elementary entities must be specified and may be atoms, molecules, 50 ions, electrons, other particles, or specified groups of such particles.” 51 52 Thus, the 1971 definition refers to well-defined number of atoms in a certain fixed mass of carbon-12. 53 With the recent advances in science and measurement practice, our ability to determine the value of the 54 55 Avogadro constant has now reached a level of relative uncertainty that allows a redefinition of the mole 56 in terms of the explicit number of elementary entities. While this conceptual change does not bring any 57 immediate practical benefits to our ability to better realize the mole, it realigns the definition of the mole 58 with the way most chemists understand it [8]. 59 60 P.O. 13757, Research Triangle Park, NC (919) 485-8700 Page 3 of 5 IUPAC Marquardt et al, Definition of the mole 3 1 2 3 3 The definition 4 5 The amount of substance, symbol n, is a measure of the number of specified elementary entities. An 6 7 elementary entity may be an atom, a molecule, an ion, an electron, any other particle or specified group of 8 particles. 9 10 The mole, symbol mol, is the SI unit of amount of substance. One mole contains exactly 23 11 6.022 140 8Y × 10 elementary entities.
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