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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


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.

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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ANUJ MONDAL (November 16, 2018 8:34 PM)
The change was required . And it may again change in future .
John Chassin (November 21, 2018 12:18 PM)
Why didn't they just drop the ampere as an SI unit and replace it with charge (coulomb), This looks like they are trying to prop up the ampere to avoid real changes in the literature.
Charles Pathirana (December 1, 2018 1:21 PM)
IPK losing mass does not make sense. This needs further clarification.
john tamine (November 21, 2018 4:33 PM)
the article says that the IPK has "lost mass" relative to the other clone standards without explaining exactly how that conclusion was reached, or if true, what possible mechanism(s) could account for the loss. how are we to have confidence that the error is not in the current measurments, or that the difference existed all along but was only revealed thru more precise measurements than were possible in the past, rather than an actual loss of mass?
Nicolas Large (November 21, 2018 4:53 PM)
Interesting article. And Interesting initiative. I understand the need to define units in the most stable way. However, the concept of defining the SI units (m, s, kg, ...) from the values of the constants that were obtained based on a previous definition of the units is really puzzling. For example the speed of light, c is equal to 299,792,458 m/s precisely because the meter and the second have been previously defined and set in stone from a prototype of some sort. To that respect, it's a self definition. We can wonder if it's logical to define the units from the constants which values have been determined precisely from this units. Wouldn't it mean that 1m =1m because we defined the meter to be 1m?
Jean-Claude Bünzli (November 21, 2018 9:13 PM)
I certainly concur with John, the article is imprecise. What is the weight difference found between the IPK and the clones? Was this difference detectable at the time the clones were manufactured? And besides, one does not figure out how the IPK could lose weight.
Fred Egbamuno (November 22, 2018 5:31 AM)
Great work! I congratulate the team that made this happen and the whole science world. This is interesting, more so, that it's happening in my lifetime. Most of the scientific constants and notations have been there before me. As a chemist I'm elated that I'm witnessing this.

I just must agree that the IPK may have changed overtime either due to handling as atoms may be knocked off or even added by the manipulation of the object when handling.

This new approach I think, would challenge students of science and technology to be more intuitive and insightful in order to comprehend the measurements in the absence of a physical reference object.
Michael Webb (December 12, 2018 12:03 PM)
The key phrase is 'when compared with those prototypes'. Looking at some other sources, there is apparently a protocol for handling the standards, and surface spectroscopy suggests some have picked up Hg, (possibly from outgassing manometers used to verify the vacuums the samples are evidently stored under?), and the protocols won't clean these deposits. Supposing the IPK may have been taken out less over the years, that makes sense. But perhaps it is more clear to say most/all of the secondary prototypes have picked up mass relative to the IPK? Still, since first learning of this in Jr. High, a single artifact defining something so fundamental has held a certain philosophical romance. We change with the times.
Umar Abubakar Birnin-Yauri (November 23, 2018 4:49 AM)
I am happy that the change is infinitesimally negligible.
Bilal Ahmad (December 7, 2018 11:14 AM)
it was a quite new thing to me but it also give us information about what's happening in the world.
Zada O'Connor (December 7, 2018 1:49 PM)
This article was very interesting.
Dr. Paul C. Li (May 23, 2019 4:23 AM)
Dear Honorable and Respected Achievers in Science & Technology: The search of accuracy and correctness is an endless endeavor when M.C.Escher’s art work is viewed in one’s mind in shocks and echos at the moment we visit Huang San (黄山)on a trolley flying in the mist of circumferential valley of greens.The article inspired my belief the beauties of art, science and technology in one. Submitted to your attention by a life long chemist.
Stanton R de Riel (May 23, 2019 1:53 PM)
Ah, yes -- I can foresee the interaction when (humans) are describing the history of the mole to a considerably more advanced extraterrestrial species : "Carbon-12? Oh, you mean, you regarded matter as composed of atoms? Oh, that's funny!"

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