Masses of moles

From the JCBN/NC-IUB Newsletter 1981 [1]

The molar mass (symbol: M) of glucose is 180 g mol-1; the relative molecular mass (symbol: Mr) of glucose is 180; the molecular mass of glucose is 180 atomic mass units (symbol: u).

These are all correct ways of saying the same thing by present IUPAC recommendations: Manual of Symbols and Terminology for Physicochemical Quantities and Units, 1979, Pergamon, Oxford; also in Pure Appl. Chem. 51, 1-41 (1979). Although Mr [once called 'molecular weight', but changed in these recommendations because the word 'weight' is dimensionally incorrect and hence confusing] is the most used, M is sometimes needed, as in the Svedberg equation M = R . T . s/D (1 - r). The third form of expression may sometimes be useful, or would be if the name of the unit and its symbol were more convenient. NC-IUB and JCBN are therefore asking IUB and IUPAC to apply to the Comité international des poids et mesures (International Committee of Weights and Measures) to "approve the name 'dalton' (symbol: Da) as an alternative to the name 'atomic' mass unit' (symbol: u)".

In asking other IUPAC bodies their views on this proposal, the Interdivisional Committee for Nomenclature and Symbols states that it finds merit in the proposal and adds the following arguments and notes.


1. The word 'dalton' is already used in the biochemical literature [see LT.Edsall, Nature (Lond.) 228, 888-889 (1970) and M. A. Paul, Nature (Lond.) 229, 142-143 (1971)].

2. The name 'atomic mass unit' is unwieldy. Its symbol (u) is strange in that this one unit should be symbolized with a 'u' for unit, and some of its multiples and submultiples, e.g. 'mu', may be confusing.

3. The name 'dalton' and symbol 'Da' would allow adjectival use, e.g. a 144-Da fragment, a 15-kilodalton molecule, a 2.6-MDa ribosome.


1. The numerical value of the molecular mass in daltons is equal to the numerical value of the molar mass (Mm or M) in g mol-1 and to the relative molecular mass (Mr).

2. The dalton is one twelfth of the mass of the nuclide 12C. Its relation to the kilogram therefore depends on experimental determination and cannot be stated exactly. It is 1.6605655 x 10-27 kg within about 6 parts per million.

3. Since molecular mass is a mass, no special symbol for the quantity other than m is needed.


Units. Names of units do not begin with capital letters. Hence. dalton, not Dalton, as with volt, joule, etc. Symbols for units based on proper names do begin with capital letters, e.g. J for joule but m for metre.

Quantities. Symbols for quantities, unlike those for units, are italicized when derived from the Latin alphabet (some quantities are symbolized by Greek letters), e.g. l for length, A for absorbance, v for rate of reaction. Some are upper case, some lower.

The Comité international des poids et mesures did not accept the request. However, NC-IUB and JCBN believe that the dalton is valuable; they know that it is widely used in biochemistry and support its continued use.

1. IUPAC-IUB Joint Commission on Biochemical Nomenclature (JCBN), and Nomenclature Commission of IUB (NC-IUB),.Newsletter 1981, Arch. Biochem. Biophys., 1981, 206, 458-462; Eur. J. Biochem., 1981, 114, 1-4; Hoppe-Seyler's Z. Physiol. Chem., 1981, 362, I-IV; J. Biol. Chem., 1981, 256, 12-14.

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