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

ISSN： 0026-1394

Source： Metrologia, Vol.49, Iss.4, 2012-08, pp. : 487-491

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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 = (1/1000N^*) 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(¹²C)/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(¹²C)/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.