The Background and Implications of the “New SI” for Analytical Chemists

The International System of Units (SI) is not in itself a complete philosophical system; it has been developed over time to deal with practical needs. It started during the French revolution with units of unified length and mass. By the mid-1800’s three base units were in place, for measuring distance, mass and time (centimetre, gram and second, the CGS system). These base units evolved into the metre, kilogram and second (the MKS system). The ampere became the fourth base unit in 1946. In 1954 the kelvin and the candela were added as new base units. Finally, in 1971, the mole became the seventh base unit, for the amount of substance1,2. The definitions of each base unit have undergone continuous evolution to deal with improvements in measuring capabilities and following the realisation of any shortcomings in the prior definitions. The revision presently on the way will lead to the “New SI” that again comprises of seven base units, namely the second, s; the metre, m; the kilogram, kg; the ampere, A; the kelvin, K; the mole, mol and the candela, cd 3. From the Present SI to the New SI The “weak” element in the present SI system is the kilogram, which is defined by an artefact, the international prototype of the kilogram (IPK). This is a cylinder made of an alloy in which the mass fraction of platinum is 90 % and the mass fraction of iridium is 10 %, cast in 1879 by Johnson Matthey, and kept in air under three bell jars at the International Bureau for Weights and Measures (BIPM) in a vault in Sèvres (Paris, France). Three comparisons have been carried out between the IPK and the official copies stored under different conditions at BIPM, the most recent being in 1989. These comparisons show a trend towards larger average mass of the copies with respect to the IPK of approximately 50 μg over 100 years4,5. These small changes are thought to be due to contamination from atmospheric mercury and the growth of a carbonaceous layer, the latter can be removed by UV/ozone treatment6. Such drifts go unnoticed among the typical uncertainties of mass measurements in analytical chemistry laboratories. The same holds for the ampere and mole (as well as for the candela) whose definitions depend on the kilogram; however these small drifts do not go unnoticed in other areas, such as quantum and nuclear physics and astronomy. In addition to the kilogram problem, the definition of ampere became considered to be old- fashioned, based on the force between two wires, rather than a charge flow as used in everyday practice and in electrical metrology. Although the candela could be dispensed with it will continue as it is so enmeshed in its special applications of photometry and radiometry -28- Journal of the Association of Public Analysts (Online) 2013 41 28-44 Thorburn Burns et al for industrial use and in the environmental field7,8. Thus, for the reasons given above it was considered by BIPM desirable to update the whole system9. A proposal for such an update has been prepared by the Consultative Committee on Units (CCU), which reports to the International Committee for Weights and Measures (CIPM) who produce the necessary resolutions for consideration and eventual decision by the General Conference on Weights and Measures (CGPM) of the BIPM acting under the Metre Convention1. The case for the proposed new system has been debated, mainly positively, in a series of papers in Meterologia9-14 and in papers presented at the Royal Society Discussion Meeting – The New SI Based on Fundamental Constants, 24-25 January 201115.This discussion was designed to follow on from the earlier Royal Society meeting – The Fundamental Constants of Physics, Precision Measurements and the Base Units of the SI”, held 14-15 February 200516. The International Union of Pure and Applied Chemistry (IUPAC) position, the custodian of the International Table of Atomic weights, is as stated by J. Lorimer in the post scriptum to his article “Old Concepts and New” as follows: “On behalf of the Bureau, the IUPAC Executive Committee at its meeting 2 October 2009 reviewed and accepted the ICTNS recommendations to support the mole as proposed by the CCU”17 ICTNS is the IUPAC Interdivisional Committee on Terminology, Nomenclature and Symbols. The IUPAC has informed of the impending change to the SI in a series of articles in Chemistry International, in the main they promote the New SI18-23. Discussions, particularly in the journal Accreditation and Quality Assurance, indicate that not all chemists and engineers are happy with the proposed changes to the SI24-33. Given that the decision to adopt the New SI is in effect a fait accompli, due to the authority vested in the BIPM set up by the Metre Convention of 1875 and amended in 19211, those affected will have to adapt to live with the new system. In this article are outlined the basic aspects of the New SI system that affects chemistry, its advantages and the problems with which chemists will have to deal, specifically with regard to the kilogram and the mole, in day to day practice and also when teaching new entrants to the profession. What is New about the “New SI”? The revision of the SI system has given the opportunity to reinforce the basis of the system. The idea was to go from measurable artefacts to defined, “universal” or “fundamental” constants of nature, those which according to current knowledge seem not to change in time and space. If in reality they are strictly constant they would be the ideal anchor points for a Unit System. A set of seven constants was chosen for reference. -29-