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Vol. 32 No. 1
January-February 2010

What is a Mole?: Old Concepts and New

The following subarticles follow this article:

A Fixed Avogadro Constant or a Fixed Carbon-12 Molar Mass: Which One to Choose?, by Yves Jeannin

Closing Comments from Ian M. Mills

In the March-April 2009 issue of Chemistry International, Ian Mills and Martin Milton reviewed concepts familiar to chemists: the quantity “amount of substance” and its unit, the “mole.” They also presented a possible new definition for the mole. The reasoning behind the possible new definition is currently being debated in the community. The IUPAC Interdivisional Committee on Terminology, Nomenclature and Symbols was invited to review the question during its recent meeting in Glasgow in August 2009. CI asked the ICTNS Chair Jack Lorimer to recap the issue.

 

by Jack Lorimer

The current definition of the base unit for the SI base quantity “amount of substance,” the “mole,” was adopted in 1971 by the CGPM (Conférence Générale des Poids et Mesures). The CGPM is the body in charge of maintaining the International System of Units (SI), in accordance with the requirements of the Metre Convention, which is the legal basis for use of the SI in the many countries that subscribe to the convention. The definition reads1:

  1. The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12; its symbol is “mol”.
  2. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of particles.
  3. (Addendum in 1980) In this definition it is understood that unbound atoms of carbon 12, at rest and in their ground state, are referred to.

Since that time, two associated problems have been recognized. First, the name of the base quantity is awkward, with the word “substance” being a source of confusion. Other names, such as “chemical amount” and the problematic “enplethy” have been suggested,2 among others. The second problem is more fundamental for metrology, because the current definition makes use of a second SI base unit, the kilogram.

In 2005, at the ICTNS meeting in Beijing, a proposal from the CCU (Consultative Committee on Units) of the BIPM (Bureau International des Poids et Mesures) to redefine not only the mole but all other SI base units (kg, m, s, A, K, cd) in terms of fundamental physical constants led to a resolution to the IUPAC Bureau in support of this general goal, but specific recommendations were only in a preliminary stage.

Some of the scientists whose breakthroughs contributed to the modern definition of the mole (from left): Lord Kelvin, Johann Josef Loschmidt, Amedeo Avogadro, and Stanislao Cannizzaro.

In 2009, Mills and Milton3 published an article in CI that brought the attention of the ICTNS to a specific proposal for redefinition of the mole in terms of fundamental physical constants. ICTNS also received information from CCU giving specific recommendations for revision of the base units, and in particular, asking for support of the redefinition of the mole by IUPAC, given that the mole has special interest for chemists. At its Glasgow meeting at the IUPAC General Assembly, ICTNS devoted a half-day session to this request, which started with a presentation by Ian Mills, IUPAC’s representative on the CCU and currently president of that body. Vigorous discussion followed, leading to conclusions that the redefinition should be supported, and that redefining the unit should provide an excellent opportunity to redefine the name of the base quantity at the same time. The outcome of the session was a resolution to the Bureau:

“Given that: (a) definition of the mole in a way that is independent of mass is desirable; (b) the mole is often thought of by chemists as an Avogadro number of entities; and (c) the name of the ISQ (International System of Quantities) base quantity “amount of substance” has been a source of much confusion, ICTNS recommends to the Bureau that:

The recommendation of the CCU (Consultative Committee on Units) of the BIPM, that the mole be defined as follows:

“The mole, unit of amount of substance of a specified elementary entity, which may be an atom, molecule, ion, electron, any other particle or a specified group of such particles, is such that the Avogadro constant is equal to exactly 6.022 141 79 x 1023 per mole.

Thus, we have the exact relation NA = 6.022 141 79 x 1023 mol-1. The effect of this definition is that the mole is the amount of substance of a system that contains 6.022 141 79 x 1023 specified elementary entities.”


be supported by the IUPAC, with the following suggestions:

  1. The greatest effort should be made to change the name of the ISQ base quantity “amount of substance” at the same time that a new definition of the mole is approved.
  2. A note should accompany the new definition to explain that the molar mass of 12C will be an experimental quantity, with a relative measurement uncertainty of about 1.4 x 10-9.”

The ICTNS had at its disposal, prior to the meeting, a number of relevant documents. These included a dissenting view to the recommendation by former IUPAC President Yves Jeannin on behalf of the Chemistry Section, French Academy of Sciences, and a supporting view from the U.S. National Institute of Standards and Technology (NIST). One of the co-authors of the NIST paper is Peter Mohr, who is also the current chair of SUNAMCO (Symbols, Units, Nomenclature and Atomic Masses Committee), the counterpart of ICTNS in the International Union of Pure and Applied Physics. The definition in the ICTNS resolution was taken directly from this latter document. The document by Jeannin is reproduced below, and followed by closing comments by Ian Mills, in which the arguments are summarized and the relation between the old and new definitions is discussed.

It may be of interest to readers to know the sequence of resolutions that must accompany any approved change in the SI, and also to be aware of the alphabet soup of acronyms that describes the various committees involved. The BIPM was set up in 1875 by the Metre Convention to ensure worldwide unification of measurements,1 and has it headquarters and laboratories in Sèvres, just outside Paris, on international territory ceded by the French government. It operates under supervision of the CIPM (International Committee on Weights and Measures), which in turn is under the authority of the CGPM (General Conference on Weights and Measures). Delegates from Member States of the Metre Convention attend the General Conference every four years, and ratify recommendations that arise, in this case, through (in succession) the CCU, CIPM, and CGPM, with the CGPM having responsibility for final decisions. Any changes in the SI are thus subjected to extensive scrutiny over a number of years. The process of redefining the SI base units is currently at the CIPM stage. Input from IUPAC is possible at either the CCU or CIPM stages through ICTNS, which has responsibility for interactions with international organizations outside IUPAC, but in important cases, ICTNS makes recommendations to the Bureau. As noted, IUPAC has a representative on the CCU, and the director of BIPM is a member of ICTNS.

The ICTNS hoped that presentation of these articles would provide IUPAC members with a broad picture of the problems associated with definition of the mole and with the cogent arguments that led to the support of ICTNS for redefinition. Those interested in the redefinitions of the other SI base units will also find relevant information.

References

  1. Le système international d’unités/The International System of Units, SI. (the SI Brochure) 8th ed., BIPM (Bureau international des poids et mesures), Sèvres (2006); pp. 95, 115.
  2. Quantities, Units and Symbols in Physical Chemistry. 3rd ed. (the IUPAC Green Book). RSC Publishing, Cambridge, UK (2007); p. 4.
  3. I.M. Mills and M. Milton, Chemistry International 31 (March-April), 3–7 (2009).

J.W. Lorimer <lorimer@uwo.ca> is an emeritus professor of chemistry at the University of Western Ontario, in London, ON, Canada. He was chair of ICTNS from January 2004 to December 2009.

Post Scriptum: On behalf of the Bureau, the IUPAC Executive Committee at its 2 October 2009 meeting reviewed and endorsed the ICTNS recommendations to support the redefinition of the mole as proposed by the CCU.


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