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 carbon12based 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 fixedh 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 gramtodalton massunit ratio—as required by the fundamental compatibility condition relating the kilogram, mole and dalton. This would necessarily decouple the dalton from the carbon12 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.
 Publication:

Metrologia
 Pub Date:
 August 2012
 DOI:
 10.1088/00261394/49/4/487
 Bibcode:
 2012Metro..49..487L