Why the dalton should be redefined exactly in terms of the kilogram

Abstract

With a redefined kilogram fixing the Planck constant, h , and a redefined mole fixing the Avogadro constant, N A , as recently proposed, retaining the carbon-12-based dalton creates a fundamental incompatibility in the stoichiometric equations. To avoid this incompatibility, the dalton should be redefined, as well: exactly in terms of the fixed- h kilogram. Specifically, with the mole defined as mol = N * / N A , where N * is an exact dimensionless constant, the dalton should be redefined as Da = (1/1000 N * ) kg, exactly, so that N A /mol −1 remains exactly equal to the gram-to-dalton mass-unit ratio—as required by the fundamental compatibility condition relating the kilogram, mole and dalton. This would necessarily decouple the dalton from the carbon-12 reference mass, m a ( 12 C)/12, used for cataloguing nuclidic mass ratios to extremely high precision. Relative atomic masses, defined as A r (X) = m a (X)/Da, would then have uncertainties dominated by that of m a ( 12 C)/kg; but since this would be of order 10 −9 or less, the values of relative atomic masses, to the precision used in stoichiometry, would be totally unaffected. This is a much more straightforward and easily comprehended strategy than those using confusing inexact correction factors or the equivalent, as recommended in recent publications and implicitly endorsed by the CGPM.

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