The Very Standard Kilogram
Gravimetry, the science of measuring weight, plays a critical role in analytical sciences. All analytical chemistry labs are equipped with one or more calibrated scientific balances to measure weight. Specifically, in quantitative gravimetric analyses, incorrectly calibrated balances or erroneous mass measurements (often the first step of much larger experimental workflows) may have catastrophic consequences to the final results. At Emery Pharma, we take great care in maintaining our balances with strict Standard Operating Procedures guiding their use and regular qualification and calibration.
The central unit in gravimetry is the Kilogram (kg) set by the International System of Units (SI). All weight, mass measurements, balances, and calibrations are ultimately traced to the standard kilogram. However consequential this standard may be, the definition of 1 Kilogram has evolved through the years: originally defined in 1795 as the mass of 1 litre of water, later established as the mass of a cylinder of platinum-iridium (Pt–Ir), the International Prototype of the Kilogram (IPK) in 1889. And then, in 2019, the definition—and hence the actual mass that corresponds to a kilogram—was updated yet again as part of a major kilogram redefinition in metrology.
The issue with the then‑established identity of the kilogram was addressed in our previous post in 2017. We questioned the ability of the IPK—a golf-ball‑sized Pt–Ir cylinder—to serve as a primary standard that could endure the weathering of time. Indeed, the biggest issue with the official mass standard was that the standard kilogram was an artifact, meaning it is realized by a designated physical object and thus susceptible to damage, erosion, chemical reactions, and wear.
Kilogram No. 20, in the U.S., is one of several “working standards”. Credit: Science Source. https://www.scientificamerican.com/article/redefining-the-kilogram/
Simply put, as an artifact, no matter how carefully it is handled or stored, it is prone to subtle mass drift. It has been shown that exact copies of the IPK have lost, on average, about 50 μg over 100 years. And if exact copies experience mass drift, the IPK itself must experience drift as well. While negligible to the average person, in a world where increasingly precise measurements are required, this fluctuation in mass becomes unacceptable as the rigors of science demand greater accuracy.
One Standard, One Constant
In light of this finding, the International Committee for Weights and Measures (CIPM) recommended redefining the kilogram. Several approaches to redefine the kilogram were examined by the scientific community, all focused on one goal: to define the kilogram based on a fundamental, universal constant. Unlike a physical object—where even the simple act of weighing can leave behind trace material, altering the object—the constant-based definition remains invariant.
For example, the meter was redefined from a marked platinum/iridium bar (created alongside the IPK) to the constant of the speed of light—specifically, the distance light travels in a vacuum in 1/299,792,458 of a second. A standard defined by a universal constant can be independently reproduced in different laboratories by following a written specification, not preserved artifacts.
Measuring the Measurement – How to redefine the Kilogram?
One approach is to count the number of atoms that compose one kilogram. Elements proposed for this method include silicon, carbon‑12, and gold or bismuth ions. The gold‑ion method defined the mass of a gold atom and calculated the number of atoms required (3,057,443,620,887,933,963,384,315 gold atoms); however, due to technical issues, ion‑accumulation prototypes proved impractical.
Silicon and carbon‑12 were other candidates, with ultrapure monocrystalline silicon being more feasible. Yet impurities and oxides in silicon resulted in reproducibility challenges and insufficient precision for metrological standards.
Another approach used ampere‑based force, defining kilogram mass with applied magnetic force, linking directly to mass and acceleration. However, because of reproducibility limitations, this method was abandoned.
A magnet levitating above a superconductor cooled by liquid nitrogen. While not the contraption investigated to measure the kilogram, the concept is the same: utilizing a magnetic field to generate the force required to derive the kilogram. Credit: Mai-Linh Doan/Wikimedia Commons, CC BY-SA 3.0. https://www.scientificamerican.com/article/how-do-they-do-that-a-closer-look-at-quantum-magnetic-levitation/
In 2019, the approach accepted by the CIPM was defining the kilogram in terms of the Planck constant. The Planck constant correlates fundamental natural entities such as mass and energy and was fixed at 6.62607015 × 10⁻³⁴ kg·m²·s⁻¹. The mass is realized using a Kibble balance, essentially a high-precision weighing scale that measures the electrical power required to counteract a kilogram’s weight under Earth’s gravity. The kilogram is then defined in terms of the second and the meter, each defined by fixed physical constants (the frequency of cesium‑133 and the speed of light, respectively). By tying SI units to fixed natural constants, the kilogram is now derived rather than tied to a variable physical artifact.
Impact
With the 2019 redefinition, the kilogram is no longer dependent on a physical object like the IPK. This redefinition was part of a broader shift—SI redefinitions—that also modernized the ampere, kelvin, and mole—but the kilogram was the final unit to make this transition.
All seven SI units are now defined by universal physical constants. Diagram by Emilio Pisanty – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=50713156
Of the four redefined units, the kilogram was the only—and the final—unit to be redefined away from a physical object.
Will this shift from a metal artifact to a fixed number drastically change everyday weighing? Practically speaking, no. Yet for scientists and metrologists requiring extremely accurate measurements, the kilogram now has a reproducible definition traceable to universal physical constants—a revolution in metrological accuracy.
At Emery Pharma we use state-of-the-art, fully calibrated, and qualified instruments and devices to ensure constant and consistent results. Contact us online or call +1 (510) 899-8814 to inquire about our services and qualifications!
About the Author
Authored by Hubert Lin, Director of Quality Assurance.
References
- Bureau International des Poids et Mesure (BIPM). July 7, 2021. “Mise en pratique for the definition of the kilogram in the SI”. SI Brochure – 9th edition (2019) – Appendix 2.
- Clara Moskowitz. 1 November 2018. “Redefining the Kilogram”. Scientific American. 319, 5, 19 (November 2018). doi:10.1038/scientificamerican1118-19.
- Robinson, Ian A.; Schlamminger, Stephan (2016). “The watt or Kibble balance: A technique for implementing the new SI definition of the unit of mass”. Metrologia. 53 (5): A46–A74. doi:10.1088/0026-1394/53/5/A46.
- National Institute of Standards and Technology U.S. Department of Commerce online. “Kilogram: Silicon Spheres and the International Avogadro Project.” Created 14 May 2018, updated 2 November 2018.
- Bowers, Mary, The Caravan, September 1–15, 2009: “Why the World is Losing Weight”.
