Revising ‘Revised Atomic Weights’

The News of the Week article titled “Revised Atomic Weights” has some errors that should concern us as chemists (C&EN, Oct. 7, 2013, page 10). The particularly erroneous part is the statement, “Both constants [the Planck constant and the Avogadro constant] are being used to redefine the kilogram.” This is not entirely true.

Each of these constants has been proposed as the basis for redefining the kilogram, but the selection has not been made. The method used is quite different depending on which constant would be used. The silicon sphere in the photograph is the means by which the kilogram would be redefined by the Avogadro constant. The watt balance is the method by which the kilogram would be redefined by the Planck constant. It is my understanding that neither has been selected to date.

The more recent proposal and more likely result is that the Planck constant would be used to redefine the kilogram and the Avogadro constant would be used to redefine the mole.

Furthermore, the General Conference on Weights & Measures, a treaty organization established in 1875, will be the body deciding on the redefinitions, possibly during its next scheduled meeting. The Committee on Data for Science & Technology is working on consensus values of the constants.

Peter F. Rusch
Mountain View, Calif.