The Impact of Depleted 6Li on the Standard Atomic Weight of Lithium
by Norman E. Holden
Li (lithium) is one of a handful of elements whose stable isotopic ratio varies in natural terrestrial samples to the extent that the resulting atomic weight variation exceeds the measurement uncertainty on the value. As a result, the standard atomic weight of lithium is more accurately characterized as a range of atomic weight values from 6.9387 to 6.9959. Lithium has become the least accurately known atomic weight because of the existence and the distribution in the distant past of some chemical reagents, which were depleted in the 6Li isotope of natural lithium. This background story brings to light an interesting page of history.
Lithium is an element with only two stable isotopes, 6Li and 7Li, and so there is only one stable isotope ratio involved (see Figure 1). The standard isotopic reference material for lithium,1 IRMM-016, has a measured stable isotope ratio that leads to a mole fraction for 6Li of 0.0759 (which corresponds to an isotopic abundance value of 7.59%) and a mole fraction for 7Li of 0.9241 (which corresponds to the isotopic abundance value of 92.41%). The product of each isotope’s atomic mass and its isotopic abundance, summed over both isotopes leads to a calculated value of 6.94 for the atomic weight of lithium. For the isotopically fractionated lithium samples with depleted 6Li in our story, the mole fractions in the extreme case2 would be 6Li is 0.02007 (or isotopic abundance of 2.007%) and 7Li is 0.97993 (or isotopic abundance of 97.993%). These mole fractions lead to a value of about 7.00 for the atomic weight of the lithium sample that is depleted in 6Li.
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| Figure 1: Lithium cellproposed for IUPAC’s Periodic Table of the Isotopes. |
At this point, let it be noted that the isotopic abundance values are also weighting factors that relate the thermal neutron absorption cross section (or probability that a neutron reaction will occur) of each stable isotope to the thermal neutron absorption cross section of the natural chemical element. In the case of lithium, the thermal neutron cross section reaction for one of its isotopes, 6Li, had an interesting impact on the atomic weight of lithium in reagents found on the shelves of chemists.
The majority of the thermal neutron absorption in the various target chemical elements usually involves the neutron capture reaction. In this reaction, the neutron projectile is absorbed by the target nucleus and any excess energy created in this process is released by the emission of a gamma-ray photon. This energy release allows the product nucleus to decay from the excited state to the normal ground state. However, in the case of a 6Li target nucleus, a much larger contribution to the absorption cross section results from the neutron reaction: 6Li (n, 3H) 4He. The neutron cross section for this reaction has a very large value. The value is approximately 940 barns† (or 940 x 10-28 m2),
compared to values of a milli-barn (or 1 x 10-31 m2) for typical neutron capture cross sections in light elements targets.
From the late 1940s to the early 1950s, a number of nations, which had previously developed and tested nuclear fission weapons, were attempting to construct thermonuclear weapons of mass destruction (or in the vernacular, hydrogen bombs). The approach involved the use of the 2H3H reaction (or DT reaction), which released a large amount of energy. The successful method that was suggested for producing this reaction was to irradiate lithium deuteride with neutrons. To improve the efficiency for generation of the tritium component, the lithium sample was enriched‡ in 6Li.
Rather than waste all of the leftover by-product of these isotopically fractionated lithium samples, this by-product, which would be enriched in 7Li, was commercially distributed in laboratory reagents. Because of the fact that the enrichment of 6Li was part of a classified military weapons program, the general scientific community and the public were never provided information that the lithium being distributed in the chemical reagents was depleted in 6Li. This distribution resulted in labels on containers of reagents, which had incorrect atomic weight values listed on them.
The isotopic fractionation of lithium was first noted when measurements of the neutron cross section of various materials, that were normalized to the natural lithium standard cross-section value, provided results that were much lower than those same cross sections when measured against all other neutron cross-section standards.§
The large discrepancy in the isotopic abundance of 6Li in reagents was later measured via neutron activation analysis and by mass spectrometric measurements. The detection of this problem was published in the open scientific literature at various times in 1958,3 1964,4 1966,5 1968,6 1973,7 and 1997,8 with ever increasing depletion of 6Li in the commercial samples noted. Figure 2 shows the variation in isotopic composition and atomic weight of selected lithium-bearing materials. Note that lithium enriched in 7Li has made its way into ground waters (see Figure 2), and the lithium isotopic composition has been used as an environmental tracer to identify lithium compounds in waste waters down gradient of a mental institution using pharmaceuticals containing lithium (T. Bullen, U.S. Geological Survey, written communication).
Although many of lithium’s elemental properties would not affected by the use of depleted lithium, the incorrect atomic weight would lead to errors in the concentration of the lithium being used. It has a major effect when isotopically fractionated lithium is used as a reference in mass spectrometric measurements. In the neutron cross-section field, natural lithium was eliminated as a measurement standard more than half a century ago because of the problem of depleted 6Li.
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| Figure 2. Variation in atomic weight with isotopic composition of selected lithium-bearing materials (modified from reference 2). Isotopic reference materials are designated by solid black circles. The previous (2007) standard atomic weight of lithium was 6.941 ± 0.002. |
The atomic weight of terrestrial and commercial lithium sources varies between 6.9387 and 6.9959.2 If the standard isotopic reference material’s atomic weight is recommended, the value would be 6.94 (6), where the number in parentheses indicates the uncertainty needed to cover the isotopically fractionated lithium sources, which is an uncertainty of about 0.9% (see Figure 2). If a value were recommended that is accurate to one in the last quoted digit, the atomic weight becomes 6.9 (1), and an uncertainty of about 14%. In either case, lithium is the element with the least accurate atomic weight, and all because of the unacknowledged distribution of depleted 6Li in chemical reagents in the distant past.
It has been noted on many occasions by the Commission on Isotopic Abundances and Atomic Weights that the published standard atomic weight is chosen to apply to samples for all potential users, no matter which terrestrial or commercial sample they may be using. If the published value of the standard atomic weight in the Commission’s report is not of adequate accuracy for a particular application when the uncertainty budget is determined, one needs to measure the atomic weight value for the specific sample.
References
1. H.P. Qi, P.D.P. Taylor, M. Berglund and P. De Bievre, Int. J. Mass Spectrom. Ion Phys. 171, 263–268 (1997).
2. T.B. Coplen et.al., Pure Appl. Chem. 74, 1987–2017 (2002).
3. A. Klemm, Angew. Chem.70, 21–24 (1958).
4. D.C. Aumann and H.J. Born, Radiochim. Acta3, 62–73 (1964).
5. J.J.M. De Goeij, J.P.W. Houtman and J.B.W. Kanij, Radiochim. Acta 5, 117–118 (1966).
6. J. Pauwels, K.F. Lauer, Y. Le Duigou, P. De Bievre and G.H. Debus, Anal. Chim. Acta43, 211–220 (1968).
7. P. De Bievre, Z. Anal. Chem. 264, 365–371 (1973).
8. H.P. Qi, T.B. Coplen, Q.Zh. Wang and Y.H. Wang, Anal. Chem. 69, 4076–4078 (1997).
9. Bureau International des Poids et Mesures, Le Système International d’Unités (SI). 8th French and English Editions, BIPM, Sevres, France, (2006).
Norman Holden <holden@bnl.gov> works at the National Nuclear Data Center of the Brookhaven National Laboratory, in Upton, New York. He is a member of the IUPAC Inorganic Chemistry Division and is actively involved in multiple projects. He is chair of the project to develop an isotopic periodic table for the educational community, and of another on the assessment of fundamental understanding of isotopic abundances and atomic weights of the chemical elements.
† The International System of Units9 (SI) has a unit of area of meters2 (m2). The barn can be expressed as 10-28 m2. (The history of the origin of the name of the unit “barn” would also make an interesting story). The large value of 940 barns for the neutron isotopic cross section of 6Li would correspond to a natural element cross section of about 71 barns (which is also a relatively large value) for “normal” lithium. This large value led to the use of natural lithium as a neutron cross section standard. For isotopically fractionated lithium depleted in 6Li, the natural elemental cross section would be about 19 barns. Neutron cross-section measurements that were made relative to the lithium standard that was depleted in 6Li would be too low by almost a factor of 4.
‡ It is interesting to note that the 7Li component of the lithium deuteride also provided a source of additional tritium. It was not initially realized that the cross section at high neutron energies for the reaction 7Li (n, 2n) was so significant. Since there was not a very large source of 6Li available at the start, the initial lithium was not very highly enriched and this lithium had a significant amount of 7Li in it. The total yield (energy release) from the explosion of the first dry lithium deuteride weapon’s test was two and one half times greater than originally anticipated and this had unexpected consequences.
§ A similar (although a much less dramatic) result occurred from the use of natural boron as a neutron cross-section standard. This was due to the large value (about 3838 barns) of the cross section for the reaction 10B (n, 4He) 7Li. There are two major boron sources in the world, which have different ratios of 10B and 11B in their samples. (However, that would also be a story for another day). The direct result of these problems with lithium and boron resulted in natural lithium and natural boron being eliminated as neutron cross-section standards by the late 1950s.
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The Impact of Depleted 6Li on the Standard Atomic Weight of Lithium
IUPAC in Glasgow, Scotland: Division Roundups
Committee on Chemistry Education
by Christiane Reiners, national representative from Germany, and with contributions from Chris Brouwer*
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| Mei-Hung Chiu, chair of the CCE Subcommittee on Chemistry Education for Development, discusses the Flying Chemists Program. |
Among the many committee meetings at IUPAC’s General Assembly in Glasgow in August 2009, the passion and enthusiasm for the International Year of Chemistry in 2011 was perhaps most evident in the deliberations of the Committee on Chemistry Education (CCE). After all, this committee was instrumental in building support for the UN Declaration of IYC2011 and it will play a lead role in planning and organizing IYC events. However, the committee’s meeting on 2–3 August encompassed much more than IYC. The “normal” committee business was simply condensed into about half the allotted time.
Shortly into the meeting, CCE Chair Peter Mahaffy framed the magnitude of what lies ahead, calling IYC an “opportunity of a lifetime for the professional chemistry community.” Against this backdrop, much of the meeting was devoted to discussing “How best can we contribute to the IYC?” Mahaffy encouraged committee members to “focus on the importance of chemistry in our lives” as they devised strategies and developed ideas for activities.
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| “Isn’t it delightful to have friends visiting from afar!” (Mei-Hung Chiu). |
“It is impressive to see what has happened all ready in national chemical societies,” said Mahaffy about IYC progress so far. “My hope is that there be something of a scientific legacy that we leave behind.” As he explained, the year of geophyics in 1957 resulted in extensive atmospheric monitoring, which then led to our understanding of climate change.
In order to contemplate such grand ideas, and smaller ones too, CCE members broke into working groups for a portion of the meeting to identify, formulate, and plan projects that CCE could coordinate. Before the working groups met, Tony Wright (Australia) and Mustafa Sözbilir (Turkey) presented the results of a task group that had considered the best types of IYC activities for CCE to pursue.
Since IUPAC has limited financial and human resources, Wright said, there should be an emphasis on activities that support developing countries and a focus on helping teachers. In addition, the task group suggested that IYC activities should be evaluated to see if they meet the following criteria:
reinforce curiosity among elementary school
students
encourage cooperative learning rather than
didactic
teach responsible stewardship, which includes sustainable development and ethical issues
facilitate appropriate curriculum development and learning
Four main proposals emerged from CCE’s meeting at the GA: (1) global experiments; (2) celebrations of national stories of chemistry; (3) coordination of an international chemistry day or week; and (4) efforts to directly engage the general public. For a more detailed description of these proposals, see the November 2009 CI (p. 10, “WCLM Generates Ideas for IYC2011”).
As noted previously, CCE plans to emphasize IYC2011 through some of its existing activities, including the Young Ambassadors for Chemistry and the Flying Chemists Program. Mei-Hung Chu, chair of the Subcommittee on Chemistry Education for Development, provided an overview of the Flying Chemists Program, which, since 2005, has provided resources to developing countries that want to promote chemistry. Chu reported that in 2011, the program will focus on Ethiopia, which is fitting since the Ethiopian Chemical Society was the lead petitioner to UNESCO and then the United Nations in the successful designation of 2011 as the International Year of Chemistry.
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| Thomas Tritton, president of the Chemical Heritage Foundation, addresses the CCE meeting. |
The meeting included presentations by Lida Schoen, who discussed the Young Ambassadors for Chemistry (YAC) project, and by Natalia Tarasova, who discussed the UN-Decade for Education for Sustainable Development. Furthermore, presentations were made about several important groups: the Network for Inter-Asian Chemistry Educators (NICE), the Australian Collaborative Education Network, FACS, the National Association of Research in Science Teaching, Chemical Heritage Foundation, and OPCW. All these activities aim at bringing partners and stakeholders together and underline the versatility of chemical education, which is a focusing and radiating enterprise at the same time.
Apart from the activities within CCE, it was interesting and encouraging to listen to the contributions from divisional representatives and from the standing committee representatives of COCI (Chemistry and Industry) and CHEMRAWN (Chemical Research for Applied World Needs). On the one hand, those interactions support the idea that chemistry education needs strong partners in other disciplines as chemistry education without chemistry is knitting without wool. On the other hand, chemistry teachers turn out to be important multipliers for spreading innovations in chemistry. Consequently, the interactions with other divisions helped to build up a close communication network, smoothing the way to an International Year of chemistry.
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High on the meeting agenda was CCE’s flagship activity: the International Conference on Chemical Education. Morton Hoffman, the CCE member responsible for the series, reviewed the successful ICCE held in Mauritius in 2008. He was followed by Mei-Hung Chiu, who made a compelling case for attending the 21st ICCE in Taiwan 2010, to be held 8 to 13 August. Meeting participants were then asked to consider competing bids from Poland and Italy to host the 22nd ICCE in 2012. The presentations by the Polish and Italian representatives were impressive and convincing at the same time, which made the final vote rather difficult. After the final tally, Italy was declared the winner, which means that the 22nd ICCE will take place in Rome. But, before then, see you in Taiwan!
Chris Brouwer, production editor of CI and principal of pubsimple, contributed to this report.
Division IV: Polymer
by Michael Hess, division secretary
The Polymer Division gathered in sunny Glasgow on 31 July to 1 August 2009, with 36 participants from more than 20 countries. The division, which has Christopher Ober (Cornell University, USA) as president and Michael Buback (University Göttingen, Germany) as vice president, comprises six subcommittees:
Polymer Terminology
Developing Polymer Materials
Polymer Education
Molecular Characterization of Polymers
Structure and Properties of Commercial Polymers
Modeling of Polymerization Kinetics
The chairs of these subcommittees reported the results of their work since the last division meeting at the IUPAC World Polymer Congress 2008 in Taipei. At this meeting and in Glasgow, minisummits were held between the Polymer Division and representatives of many international polymer societies (e.g., The European Polymer Federation, The Japanese Society of Polymer Science, The Korean Polymer Society, the American Chemical Society) in order to contemplate tangible cooperation in certain areas. Ideas that emerged from these meetings include the following:
Organize joint symposia and conferences with large international organizations (e.g, during the meetings of the European Polymer Federation or the Asian Pacific Federation as well as at the 2011 GA in Puerto Rico).
Improve the division’s presence on the Internet. The Polymer Division has established a polymer education website <www.iupac.org/polyedu> that has generated strong interest.
Improve public awareness of the importance and the value of polymer science and technology to our societies. Contacts with industry are being cultivated for a fruitful implementation of the division’s ideas.
In order to arouse public interest and to improve visibility of IUPAC activities, the Polymer Division administers the IUPAC-SAMSUNG Polymer Scientists’ Award, the DSM Performance Materials Award in cooperation with the Polymer Division, and the IUPAC-Polymer International Award. In particular, these awards acknowledge the activities of young scientists in the field. For 2008, the DSM Award went to Craig Hawker (USA), the IUPAC Polymer International Award to Zhenan Bao (USA), and the Samsung Award to Eric Cloutet (France) in 2008.
An International Research Funding (Pilot) Project was launched by the Polymer Division with the cooperation of the IUPAC task group on International Research Funding in Chemical Sciences. Discussions in Washington, D.C., in 2008 resulted in a detailed plan to call for proposals involving (at least) three scientists and students from a minimum of three countries as a part of the division’s educational efforts. The call was launched in October 2009 (for more details see www.iupac.org/polyedu/DivIVCall/). A symposium assembling all participants is planned during the IYC 2011.
Part of the Glasgow meeting involved updates on activities of individual subcommittees. Following is a sampling of some of these updates.
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| Group photo of Division IV at the General Assembly in Glasgow. |
Polymer Terminology
The Subcommittee on Polymer Terminology consists of 38 members from 15 countries. In the past two years, the subcommittee has worked on 24 projects, 7 of which are concerned with polymer nomenclature or are nomenclature related and which involve interdivisional cooperation, specifically with Division VIII. The most important publication is the new edition of the Purple Book, or Compendium of Polymer Terminology and Nomenclature, which was finally completed by a group of editors, headed by Richard G. Jones. The compendium comprises 13 chapters of terminology and 9 chapters related to nomenclature, all of which are based on documents previously published in PAC. Another five glossaries containing recommendations have been published in PAC.
Developing Polymer Materials
The subcommittee consists of 25 members and has currently two projects in progress. One of its goals is to identify promising developments in the forefront of polymer science.
Polymer Education
A major focus of the Polymer Education Subcommittee is preparing for IYC, but it also is working on providing new teaching materials for free online and improving international research funding. In addition, the subcommittee tries to encourage the hiring of students and post-docs from developing regions to improve their training and broaden their scientific networks. Ongoing projects are the UNESCO/IUPAC Postgraduate Course organized by Pavel Kratochvìl at the Institute of Macromolecular Chemistry in the Czech Republic and the tutorial (Short Course in Polymer Characterization) offered before the annual IUPAC-sponsored POLYCHAR Conference (Delhi/Lucknow, India, in 2008; Rouen, France, in 2009).
Molecular Characterization of Polymers
This subcommittee is currently working on five projects with a high number of participants from industry. Many of the projects tackle statistical problems in chromatographic characterization of polymers, such as reproducibility and reliability of results, but also basic problems involving the description of the separation process that are important when the validity of results has to be considered.
Structure and Properties of Commercial Polymers
This subcommittee has the most members from industry, with 33 out of 65 members in total from 12 countries. The subcommittee is divided into an Asian-Pacific and a European-American group, which each have two co-chairs. Since the GA in Torino, four projects were completed dealing with topics such as scratch resistance, structure and properties of cyclic polyolefins, and guidelines for rheological characterization.
Modeling of Polymerization Kinetics
The subcommittee consists of 34 members from 11 countries. Modeling and mechanistic studies into free-radical polymerizations are important for science and industry, but often completely different model assumptions and parameter values are reported for ostensibly the same systems. Projects of the subcommittee aim to rectify this situation by producing critically evaluated kinetic parameters, whose values are reliable and which can be used by the international polymer community.
Conferences
There were 16 IUPAC-sponsored international conferences in almost all continents since the last GA, from which six volumes of Macromolecular Symposia (Wiley & Sons) were produced, totaling nearly 1000 pages.
The next meeting of the Polymer Division will be at the IUPAC-World Polymer Congress 2010 in Glasgow, Scotland.
Division II: Inorganic Chemistry
by Leonard Interrante, division secretary
The Inorganic Chemistry Division meeting in Glasgow was attended by 25 division members and guests, including 4 Young Observers. It was preceded by a meeting (23–30 July 2009 in Vienna) of the Commission on Isotopic Abundances and Atomic Weights and its major working subcommittees at which isotopic data and atomic weights for the 2007–2009 period were evaluated.
A particularly interesting aspect of the Glasgow meeting was the enthusiasm and involvement of the Young Observers present, with several YO’s presenting excellent ideas for the upcoming International Year of Chemistry. In addition to reports from the division president, commission representative Tiping Ding, the subcommittee chairs, and from our project coordinator, Ty Coplen, on the various active and completed projects, along with the proposals submitted and in preparation, we had presentations from CCE, COCI, and the Analytical Division regarding the activities of these groups and from Fabienne Meyers on Chemistry International.
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| Group photo of Division II at the General Assembly in Glasgow. |
Among the topics discussed at this meeting was the name and symbol of the new element with atomic number 112: A provisional recommendation for the name “copernicium” and the symbol “Cn” was made by the division and is now available for public comment. (See <www.iupac.org/reports/provisional/abstract09/corish_310110.html> or page 23 in print). The provisional recommendation is co-authored by Kasuyuki Tatsumi and John Corish, and is open for public comment until 31 January 2010. At the end of the review period, the division will consider the comments received and make the final recommendation.
Another important outcome was approval of the recommendation of the Subcommittee on Materials Chemistry to transform itself into a truly Interdivisional Subcommittee on Materials Chemistry by developing a new structure that would recognize the interdisciplinary and interdivisional character of this subject. Following the meeting in Glasgow, two division members, Leonard Interrante and Tony West, attended a meeting with members of Divisions I (Physical Chemistry) and IV (Polymer) at Cornell University in Ithaca, New York, USA, on 17 October 2009 to set up this new ISMC structure and plan its activities for the coming biennium.
Division II will begin 2010 with a new division president and vice president. Since Division President Kazuyuki Tatsumi was elected vice president of IUPAC in Glasgow, Bob Loss, current division vice president, will become president in January 2010. In a special election held just after the Glasgow meeting, titular member Jan Reedijk was elected vice president of the division, also effective January 2010.
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The Impact of Depleted 6Li on the Standard Atomic Weight of Lithium
R is for Rutherford
He may well be the best-known scientist born and raised in New Zealand and the most famous physicist to receive the Nobel Prize in Chemistry (1908). Ernest Rutherford was born in 1871 in a rural community near Nelson, on the South Island of New Zealand. He received his early education at local schools and then attended Canterbury College (1890–1895), where he obtained B.A., M.A., and M.Sc. degrees in math and physics and did research on the magnetic properties or iron exposed to high-frequency oscillations. After a three-year stint at Trinity College in Cambridge, England, he accepted a position as a professor of physics at McGill University in Montreal, where he conducted most of the work that led to the Nobel Prize “for his investigations into the disintegration of the elements and the chemistry of radioactive substances.” He subsequently investigated the nature of alpha rays and established the nuclear structure of the atom while at the University of Manchester (1907–1919). From there, he succeeded J.J. Thomson as head of the famous Cavendish Laboratory at Cambridge, where he remained until his death in 1937. A talented experimentalist and gifted mentor, he is regarded as one of the most important scientists of the 20th century. Element 104 (rutherfordium, Rf) is named after him.
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The stamp illustrated herein is part of an eclectic set of 26 stamps (A through Z . . ) issued by New Zealand Post on 6 August 2008 to celebrate the achievements and cultural heritage of New Zealanders. Thus, G is for Goodnight Kiwi, a beloved cartoon character that used to signal the end of nightly broadcasts on New Zealand television, and K is for Kia Ora, a traditional Maori greeting that literally means “be well” but is indistinctly used to say hello or goodbye. Rutherford, coincidentally bestowed with the honor of representing the letter R stamp on the centennial of his Nobel Prize, is the only scientist portrayed on the set. He may have spent most of his professional career in Canada and the UK, but there’s little doubt that most people in New Zealand consider him a source of national pride, a cultural icon, and a symbol of kiwi ingenuity.
Written by Daniel Rabinovich <drabinov@uncc.edu>.
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