WEIGHTY TASK
RETHINKING THE MOLE AND KILOGRAM
A new watt balance at NIST will be operational in 2015.
ACS MEETING NEWS: Chemists debate a new approach to the International System of Units JYLLIAN KEMSLEY, C&EN WEST COAST NEWS BUREAU
Some chemists dislike the fact that the and weights go back thousands of years, proposed SI labels the Avogadro constant suggesting that people have long cared as the official way to measure “amount of about standardized measurements. Today, substance.” The Avogadro constant and the the International System of Units (SI) mole are fine for talking about molecular defines seven base units of measure: the reactions, but they don’t work across the meter, kilogram, second, ampere, kelvin, board. “Auto exhaust emission is in grams mole, and candela. But the system might be per mile, and sulfur in gasoline is in parts getting a makeover. per million per gallon,” noted Willie E. If international consensus can be May, acting director of the National Instireached, by 2018, the SI will instead set tute of Standards & Technology (NIST). would be set at exactly 6.626X × 10–34 kg m2 seven fundamental constants of nature. The mole was the last of the base units s–1. To get from the Planck constant to the The base units will then be derived from added to the SI, and some scientists now kilogram, scientists could use a watt balance, those constants. propose eliminating it from the list of SI which balances the weight of a test object by For example, the period taken up by a constants. Analytical chemists “think in passing a current through a coil suspended second will be calculated from a set freterms of Avogadro’s number, a scaling facin a magnetic field. “The math is simple, but quency, 9,192,631,770 Hz, that describes tor to bridge the atomic to the macroscopic don’t ask me to explain the physics,” said the ground-state hyperfine splitting of the level,” said Paul De Bièvre, an independent Carmen J. Giunta, a professor of chemistry cesium-133 atom. And the length of a meter metrology consultant. He and others sugat Le Moyne College. Scientists already use will come from defining the speed of light gested that it would be better to treat Avowatt balances to determine the Planck conin vacuum to be exactly 299,792,458 m s–1. gadro’s number as a counting number, akin stant from the kilogram standard. Other base units would be derived from to a dozen or googol. Paul J. Karol, a professor emeritus of the Planck (kilogram), Boltzmann (kelvin), The kilogram, meanwhile, is the only chemistry at Carnegie Mellon University, and Avogadro (mole) constants; elemenremaining SI base unit defined in terms of insists that using the watt balance to go in tary charge (ampere); and the so-called lua physical artifact: It is the mass of a plumthe reverse direction to define the kilogram minous efficacy of specific monochromatic sized cylinder of platinum-iridium alloy from the Planck constant, is “an absolute radiation (candela). kept in a vault at the International Bureau mistake.” Besides the conceptual difficulty Among chemists, redefining the mole of Weights & Measures, in France. Estabof connecting electrical current to the kiloand kilogram with this new approach is lished as the official kilogram standard in gram, Karol pointed out, the mass balance causing much debate. The subject was the 1889, the artifact’s mass has diverged over approach depends on the local gravitational center of discussion during a symposium time from the masses of copies of the cylinfield, which can vary depending on geoat the American Chemical Society nader by about 50 µg. graphical location, tides, and other factors. tional meeting in San Francisco earlier this Metrologists would now like to peg the Karol and others would like to see the month. The session was sponsored by the kilogram to something fundamental and kilogram defined in terms of the mass of one ACS Committee on Nomenclature, Termiinvariant in nature. The favored approach atom of 12C, another unvarying property of nology & Symbols. so far is to use the Planck constant, which nature. But with this approach, the Planck The mole is currently deconstant would continue to be fined as the amount of subderived from the kilogram, someUPDATING THE SI stance that contains as many thing physicists are against. Proposed defining specified entities as there For most chemists, the proCurrent base units constants are atoms in 0.012 kg of 12C. posed SI revisions will go unMole: The amount of substance of a system Avogadro: NA is exactly In the new SI, the Avogadro noticed: Only the most extreme which contains as many elementary entities 6.022X × 1023 mol–1 as there are atoms in 0.012 kg of 12C constant would be defined as analytical measurements would exactly 6.022X × 1023 mol–1, be affected. Said NIST’s May, Kilogram: The mass of the international Planck: h is exactly where X represents addition“Our aims are to make sure chem–34 2 –1 prototype of the kilogram 6.626X × 10 kg m s al digits to be set according to ical and biological measurements the best value available when made around the world are comX = Additional digits to be set when definition is adopted. the definition is adopted. parable over space and time.” ◾ CEN.ACS.ORG
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AUGUST 25, 2014
CURT SUPLEE/N IST
RELICS IN THE FORM OF rods, rulers,
