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Analytical Chemistry

New definitions for the kilogram and mole

Vote passes to redefine all SI units in terms of physical constants

by Laura Howes
November 16, 2018


A silicon sphere in an interferometer.
Credit: Physikalisch-Technische Bundesanstalt
This spherical interferometer measured the diameter of silicon spheres down to a few nanometers, enabling the definition of Avogadro's constant.

It doesn’t happen too often, but after a vote that took place earlier today near Paris, science textbooks really will have to be rewritten.

At the Congress Chamber in the Palace of Versailles, assembled metrologists voted to redefine four fundamental units of measure in the International System of Units (SI): the ampere, kelvin, kilogram, and mole. These units will join the meter, candela, and second in being defined not in reference to physical artifacts, but in reference to fundamental physical constants. Scientists say redefining these units to be based on a physical constant will make measurements more accurate and stable. The unanimous passing of the vote was greeted with a standing ovation among the participants from over 60 countries.

The international prototype kilogram under three glass cloches.
Credit: PTB/BIPM
The IPK, a cylinder of platinum-iridium alloy that currently defines the kilogram and will soon be retired, sits in Paris.

The redefined units, which will take effect on May 20, 2019, World Metrology Day, are the result of years of work, discussion, and competition to measure the fundamental constants of nature to an incredible degree of certainty. Although most people will not notice the change, the increased precision will make the SI system more robust, says Frank Härtig of PTB, the national metrology institute of Germany. “We have completely new possibilities,” he explains, adding that, as analytical techniques become more advanced and can measure ever smaller amounts of material, the new definitions ensure those measurements will be precise.

Since 1889, the SI unit of mass, the kilogram, has been defined as being equal to the mass of the international prototype kilogram (IPK). The IPK is a cylinder of platinum-iridium alloy that sits in the International Bureau of Weights & Measures, near Paris. On stage in Versailles, Bill Phillips of the U.S. National Institute of Standards & Technology (NIST) described that situation as scandalous.

When the IPK was created in the 1880s, so were other identical prototype cylinders, which were distributed to various countries. Over the years, the IPK has lost mass when compared with those prototypes.

The uncertainty that this mass change has created also impacts the mole. This SI unit, used by chemists to define an amount of atoms or molecules, has been defined since 1971 in relation to the kilogram, as “the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kg of carbon-12.”

Beginning in May 2019, the kilogram will be defined with respect to Planck’s constant, and the mole will be defined as being an amount of entities equal to Avogadro’s number. Similarly, the ampere will be defined with respect to electric charge carried by a single proton, dubbed the “elementary electric charge,” and the kelvin will be defined with respect to the Boltzmann constant. To allow for this switch, these constants have had to be measured precisely and with a high degree of certainty. That work has taken over 10 years and given rise to several technological breakthroughs. As Härtig describes it, the research has been “competition amongst friends”—different metrological institutes have vied to measure the most accurate values for these constants yet.

To define the value of Planck’s constant, two independent methods competed. The Kibble balance, which won, offsets the weight of a test mass against the force produced when an electrical current runs through a coil of wire suspended in a magnetic field. Two different Kibble balances, one at NIST and a second at the National Research Council Canada, made the measurements that define Planck’s constant to be 6.62607015 × 10−34 J s.

The method that didn’t win the competition for Planck’s constant, called the counting method, actually made it possible to define Avogadro’s number instead. Härtig’s team at PTB created incredibly precise spheres enriched in the isotope silicon-28 and measured their volumes with interferometry. Robert Vocke and Savelas Raab at NIST then worked to determine the precise proportions of silicon isotopes in the crystal lattice with mass spectrometry. With the precise volume of the sphere and the make-up of the crystal lattice known, the scientists could determine the value of Avogadro’s constant as 6.02214076 × 1023 mol−1.


Back in the classroom, those rewritten textbooks might actually make things easier for students tackling the concept of the mole. Pedagogically, says Marcy Towns of Purdue University, the new definition is not a big shift. “If you read the educational literature,” she explains, “students understand the definition of the mole as 6.022 x 1023 particles, and teachers very much use that definition,” rather than defining the unit in relation to the kilogram. Towns should know, having undertaken a huge review of the subject as part of the International Union of Pure & Applied Chemistry’s Interdivisional Committee on Terminology, Nomenclature & Symbols.

Multiple sources who spoke to C&EN for this story described the task of redefining these SI units as being a highlight of their scientific career, a once-in-a-lifetime opportunity to be part of an era-defining event. They underlined the philosophical change in moving to definitions that will retain their significance beyond Earth. Rather than officials on this planet sending out the IPK or other artifacts to explain these units, Härtig explains, “other intelligent cultures will be able to understand what we understand when we say ‘kilogram.’ ”


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