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Vol. 33 No. 5
September-October 2011

The Kilogram in the “New SI”

The kilogram, kept by the Bureau International des Poids et Mesures.

For good reason, chemists regularly follow progress made by the Bureau International des Poids et Mesures (BIPM) toward the so-called “New SI.” The International Committee for Weights and Measures (CIPM) of the BIPM continues to ponder over the redefinition of the kilogram, which is the only base unit of the SI still tied to an artifact. CIPM drafted a Resolution for consideration this coming October at the General Conference on Weights and Measures (CGPM) meeting, noting its intention to redefine four of the SI base units, namely the kilogram, the ampere, the kelvin, and the mole, in terms of invariants of nature. The new definitions would be based on fixed numerical values of the Planck constant (h), the elementary charge (e), the Boltzmann constant (k), and the Avogadro constant (NA), respectively. In order to encourage communication, awareness, and debate on the possible revision of the SI, BIPM has provided key draft documents on its website at www.bipm.org/en/si/new_si

Readers of CI have been informed of these developments and in several instances Ian Mills, IUPAC representative on CIPM Consultative Committee on Units (CCU) and president of CCU, has reviewed the issues; see “An Update on the Kilogram” (CI Sep-Oct 2005, pp. 12–15); “Amount and Substance and the Mole,” I. Mills and M. Milton (CI Mar-Apr 2009, pp. 3–7); “What is a Mole? Old Concepts and New” by Jack Lorimer, “A Fixed Avogadro Constant or a Fixed Carbon-12 Molar Mass: Which One to Choose? by Y. Jeannin, and “Closing Comment” by Ian Mills (CI Jan–Feb 2010, pp. 6–11)

In this two-part feature, Albert C. Censullo et al. start with a review of the status of the kilogram and their understanding of it and end by offering an alternative proposal. In part II, Ian Mills responds by reviewing the rationale behind the proposed new SI, the concept of explicit-constant definitions, and the implications of the definitions of both the kilogram and mole.

Part I—From the Current “Kilogram Problem” to a Proposed Definition

The metric system of measurements has served the international scientific and technical communities well since its inception over 200 years ago. By the mid 1800s, three so-called “base units” were in place, for measuring distance, mass, and time (centimetre, gram, second, or CGS system). These base units evolved into the metre, kilogram, and second (MKS system). The ampere became the fourth base unit in 1946. In 1954, the kelvin and candela became new base units. Finally, in 1971, the mole became the seventh base unit, for amount of substance. The definitions of each base unit have undergone continuous evolution, corresponding to improvements in measuring capabilities, and recognition of shortcomings of prior definitions. As stated by BIPM, “The SI is not static but evolves to match the world’s increasingly demanding requirements for measurement.” (read more)

Part II—Explicit-Constant Definitions for the Kilogram and for the Mole

Censullo et al. suggest the following changes to the present proposals for the new SI. They prefer explicit-unit definitions over the proposed explicit-constant definitions of all the units. They would prefer the kilogram to be redefined to fix the mass of an atom, such as the carbon 12, rather than to fix the value of the Planck constant. They would prefer the mole, and hence the Avogadro constant, to be defined to fix the molar mass of carbon 12 (which is the current definition) rather than by defining the mole explicitly by specifying the number of entities in a mole, and thus fixing the numerical value of the Avogadro constant. Each of these issues is discussed in turn below. A more detailed discussion can be found in reference 1. The copy of Resolution A for the 24th CGPM to be held in October 2011,2 and the FAQs,3 are also relevant; both are available on the BIPM website. (read more)


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