What Is a Kilogram in the Revised International System of Units (SI)?

Abstract The definition of the kilogram, the unit of mass in the International System of Units (SI), has not changed in more than 125 years. The kilogram is still defined by the mass of a Pt–Ir cylinder conserved at the International Bureau of Weights and Measures. Science and technology have progressed to the point where it is likely the kilogram will be redefined in 2018 in terms of a constant of physics─the Planck constant, which is closely linked to the Avogadro and atomic mass constants. In this article, we illustrate by means of a simple experiment on how analytical chemistry is contributing to this project, how the new definition of the kilogram will be put into practice and what it may mean for chemists. Surprisingly, perhaps, this simple experiment allows us to determine the mass of an aluminum-27 atom, the mass of a carbon-12 atom (and the atomic mass constant), the Avogadro constant, and the Planck constant─all with uncertainty less than 1%─in close analogy to the way the most accurate experiment of this type achieves an uncertainty of parts in 108. Finally, we point out that a very different area of modern technology, namely voltage and resistance measurements using quantum electrical devices, provides a second high-accuracy way to implement the new definition.