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1I was stirred by two recent articles in the January 2003 issue of this Journal (1, 2). The persistent perceived problem with the base units kilogram and mole addressed in those articles is resolvable once it is finally recognized that we have been using a double standard: the international platinum–iridium kilogram prototype and 12C. I believe this practice is unnecessary. It has led arguably to bewilderment and a plethora of commentary in this Journal and elsewhere (see citations in 1, 2), some of which has just compounded the confusion. Among the seven SI base units, the second has been defined since 1967 as (3): the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.
Also the meter has been defined since 1983 as (3):
the length of path traveled by light in vacuum during a time interval of 1/299,792,458 of a second (the reciprocal of the integral speed of light, defined exactly). Both of these base unit definitions involve integers and are not subject to experimental uncertainty. I propose, following the lead of what the physicists did for time and distance, that we consider the SI base unit, the mole, to be an integer, which logically it should be. Avogadro’s number is exactly the integer 602,214,180,000,000,000,000,000 and the mole is equal to Avogadro’s number.2 The US National Institute of Standards and Technology (NIST) for several years has been addressing the unsatisfactory situation of the standard kilogram ingot by designing a highly sophisticated watt balance from which the “electronic kilogram” could be standardized (4). This yeoman’s achievement would lead to a more precise and stable standard, but one still a step shy of reaching the fundamental status shared by other base units. Avogadro’s number is also the number of atoms of 12C in 12 grams of 12C. It leads to an exact definition of the kilogram, the mass of (1000/12) moles 12C atoms, or the mass of 50,184,515,000,000,000,000,000,000 atoms of 12C. The unified atomic mass unit is then defined exactly (in grams) as the reciprocal of Avogadro’s number (or as the reciprocal of 1000 times Avogadro’s number for kg). Only integers are involved in the definitions. The (platinum–iridium) international prototype of the kilogram, established in 1901 (which should tell you something) is currently the only SI base unit entrenched in an artifact rather than in a fundamental natural property. With the proposed re-definition, that relationship is relegated to being of secondary importance. The mole is a number. I regard “amount of substance”, used in the mole’s existing controversial definition, as an unacceptable alternative. Contributing to my bias in this regard is the fact that spectroscopists have a unit, the einstein. which is defined as a mole of radiation quanta, the latter definitely not being a substance under accepted terminology. Notes- The proposal herein received unanimous endorsement by the ACS Committee on Nomenclature, Terminology, and Symbols by those attending its March 24, 2003, meeting in New Orleans, LA.
- This is in the same sense as a dozen is equal to the number 12.
Literature Cited- Freeman, R. D. J. Chem. Educ. 2003, 80, 16–21.
- Gorin, G. J. Chem. Educ. 2003, 80, 103–104.
- Mills, I.; Cvitas, T.; Homann, K.; Kallay N.; Kuchitsu, K. Quantities, Units and Symbols in Physical Chemistry; Blackwell Scientific: Oxford, 1988.
- Schwarz, J. P.; Liu, R. M.; Newell, D. B.; Steiner, R. L.; Williams, E. R.; Smith, D.; Erdemir, A.; Woodford, J. J. Res. NIST 2001, 106, 627–640.
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