The kilogram, the actual kilogram, sits in a vault in Sèvres, France under numerous bell jars. It is the last SI unit to be defined based on a physical quantity—in this case one kilogram of platinum and iridium—as opposed to fundamental constants. There is, however, a push underway to change this, to phase out the hunk of metal and define a kilogram in terms of a fundamental constants of the universe.

Researchers collaborating as part of an international effort known as Project Avogadro are working to define the kilogram in terms of Avogadro's constant (N A ). The chemically inclined among the readers will know what this is without even thinking: it is the number of atoms that make up a mole of a substance—about 6.022 x 1023. As it stands today, our knowledge of this number is not known to a high enough degree of precision to redefine the kilogram and rid ourselves of the artifact in France.

The committee that oversees the definition of a kilogram has required that, to redefine the kilogram in terms of Avogadro's constant, they must know this number to within a relative uncertainty of 2x10-8. To achieve this goal, members working on Project Avogadro have created two single-crystal spheres of silicon-28, each weighing a kilogram. To calculate avogardo's constant using these spheres, they need to accurately measure the molar volume of 28Si and the volume of a unit cell of its crystal.

The two spheres of 28Si are approximately 93.6 mm in diameter and are already considered the most perfectly round objects on Earth (if they were the size of the Earth, the total difference between the highest peak and sea level would be a mere 2.4 meters). Tests on these spheres, meant to determine the lattice parameter (size of a unit cell) of the crystals, their masses and volumes, and the chemistry of their surfaces, were carried out all over the world.

Higher than expected levels of metallic impurities from the polishing process were found on the surfaces, and that had an effect on the entire crystal structure that had to be taken into account. All these tests and corrections resulted in a new measurement for Avogadro's constant: 6.02214078(18) x 1023 mol-1.

With a relative uncertainty of 3 x 10-8N A , this is the most precise measurement ever made of Avogadro's number. Unfortunately, it is still higher then the desired 2 x 10-8N A error level required before the physical kilogram can be discarded. The researchers feel that, with a greater understanding of the impurities and measurements performed with the latest and greatest interferometers, this level of error can be met and surpassed in the next couple of years.

All mole fans can rest assured though: even with these more precise measurements, International Mole Day will not need to change. We can all wake up at 6:02 in the morning on October 23 and drink our 18 grams of water to kick it off each year.

Physical Review Letters, 2011. DOI: 10.1103/PhysRevLett.106.030801

Listing image by Physikalisch-Technische Bundesanstalt (PTB)