NEWS OF THE WEEK
REVISED ATOMIC WEIGHTS
Precise measurements of this single-crystal silicon sphere will help finetune the values of Planck’s constant and Avogadro’s number.
STANDARDS: Updates of 19 elements
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foreshadow change to the kilogram
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ASS SPECTROMETRY—a technology that
is constantly being made more accurate and reliable—is used every two years to revise the standard atomic weights of select elements. This year, the International Union of Pure & Applied Chemistry (IUPAC), which oversees these revisions, has released revised atomic weights for 19 elements. But a planned redefinition of the kilogram, the standard unit of mass, could take the reassessments of atomic weights to a new level of accuracy. This year’s element revisions include changes in atomic weights for cadmium, molybdenum, selenium, and thorium, on the basis of recent determinations of terrestrial abundances of each element’s multiple stable isotopes. Revisions of the other 15 elements reflect updated assessments of their single stable isotopes. “High-precision measurements of atomic weights play an important role in science,” comments Juris Meija, secretary of IUPAC’s Commission on Isotopic
DESTROYING SYRIA’S CHEMICAL WEAPONS ARMS CONTROL: Goal is to eliminate
Syrian arsenal by the middle of 2014
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NTERNATIONAL INSPECTORS arrived in Syria last
week to begin the daunting task of overseeing the destruction of President Bashar al-Assad’s chemical weapons program. The country, which has been wracked by civil war, agreed to the action after the threat of a U.S. military strike and international condemnation. The team of chemists and disarmament specialists from the Organisation for the Prohibition of Chemical Weapons (OPCW) began setting up a base of operations in Damascus. From there they will launch their complex mission of finding, dismantling, and ultimately destroying an estimated 1,000-ton arsenal of deadly chemical agents. Mustard gas and the nerve agent sarin are believed to be among them. OUAI BESHARA/AFP/GETTY IMAGES/NEWSCOM
Chemical weapons experts leave a Damascus hotel to investigate the alleged use of chemical weapons.
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Abundances & Atomic Weights. Although the weight changes don’t affect the daily work of most scientists, they can have wide-ranging effects. In the 1980s, for example, measurements of the atomic weight of silver helped settle the value of the Faraday constant, Meija explains. “Nowadays, the atomic weight of silicon is an important variable in setting the high-precision values for the Planck constant and for the Avogadro constant.” Both constants are being used to redefine the kilogram, Meija says, which is the only standard unit of measure in the metric system still based on a physical artifact—a platinum-iridium cylinder in Severn, France. Redefining a kilogram in terms of universal constants would allow the standard to be reproduced accurately without having to physically measure an artifact. A fixed value for Avogadro’s number and a kilogram defined by constants would, in effect, shift any uncertainty onto the mass of 12C, which is taken to be an exact value. Subsequently, any revisions of the mass of 12C would require a slight recalibration of mass spectrometers. The Committee on Data for Science & Technology, a sister organization to IUPAC, will make the final determination on redefining the kilogram. IUPAC’s new atomic weight revisions are, in some ways, “creating the foundation for this redefinition,” says Carl J. Williams of NIST’s Physical Measurement Laboratory.— CRAIG BETTENHAUSEN
Inspectors must first inventory and secure the Syrian munitions and facilities. But it is still unclear where and how the stockpile will be destroyed, and officials are not releasing more details. Traditionally, chemical weapons are destroyed incountry at or near weapons storage depots, says James Lewis, communications director of the Center for Arms Control & Non-Proliferation, a think tank in Washington, D.C. “OPCW will likely make a recommendation, but it is Syria’s ‘responsibility’ to destroy their stockpile,” Lewis tells C&EN. Paul F. Walker, an expert on chemical weapons at the environmental organization Green Cross International, says the demilitarization effort will likely have to be undertaken within Syria. And to be successful, he adds, OPCW will need the help of other countries with experience in destroying chemical munitions, namely the U.S. and Russia. OPCW’s plan was authorized by the United Nations Security Council on Sept. 27. The plan calls for the destruction of Syria’s chemical weapons production capability by Nov. 1 and the elimination of all chemical weapons by mid-2014. Many experts, however, think this timeline is highly ambitious, at best. The Hague-based OPCW is a UN-affiliated body that administers the Chemical Weapons Convention, an arms control agreement that bans the production, stockpiling, and use of chemical weapons and their precursors.—GLENN HESS
OCTOBER 7, 2013
