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Home , Mole (unit), Realisation (metrology)

A general introduction on and traceability

Paul Brewer LNG metrology workshop 15th June 2016 National Physical Laboratory

• Develop and disseminate UK’s standards, ensure international acceptance • Knowledge transfer and advice between industry, government and academia • Support Industry, trade, regulation, legislation, quality of life, science and innovation

industrial environment

energy and Particle particles Metrology The Fundamentals of Metrology

• What is metrology and what is it for?

• What is an NMI and what is it for?

• What is the mole and what is it for? What is ‘Metrology’?

. Metrology is “the science of measurement, embracing both experimental and theoretical determinations at any level of uncertainty in any field of science and technology.”

. Almost all of science and industry involves making and interpreting measurement – why is metrology special? The Proclamation Regarding Weights and Measures, 1556 by Ford Madox Brown (1889) The electrochemical characteristics of platinum phthalocyanine

. Quantitative conclusions inferred; but what was the accuracy, repeatability, reproducibility and uncertainty of these ? . Would this have affected the conclusions? Metrology’s main activities

. The definition of internationally accepted units of measurement, e.g. the . The realisation of units of measurement by scientific methods . The establishment of (metrological) traceability chains by disseminating and documenting the value and accuracy of a measurement . Traceability implies the calculation of an associated measurement uncertainty . These activities may be fundamental (scientific) or applied (practical, industrial, legal) International vocabulary of metrology The Results of Metrology . Generates and frameworks for quantification and through these underpins consistency and assurance in all measurement . Gives a quantified level of confidence in the measurement through an uncertainty statement . Provides a measurement infrastructure which is stable over , comparable between locations, and coherent, allowing measurements of different properties using different methods to be combined (without scaling factors) . Removes barriers to trade, improves efficiency and competitiveness, enables technological development, encourages global agreement and collaboration confidence in trends confidence in spatial data . Provides a measurement infrastructure which is stable over time, comparable between locations, and coherent, allowing measurements of different properties using different methods to be combined (without scaling factors) confidence that data can be used directly in the equations of chemical physics – unique to the SI How did we get here?

. After the French Revolution (1789) old units of measurement associated with the old regime were replaced by new units . The meridional definition of the was soon replaced by a metre bar, ‘Mètre des Archives’, in 1799 . As more countries adopted this new ‘metric’ there was a danger of lack of comparability, or rival systems emerging . Prompted by the need to unify geodesic measurement, 17 governments signed “the ” in 1875 . This diplomatic treaty established a permanent organizational structure for member governments to act in common accord on all matters relating to units of measurement. . Initially covering just and standards, the coverage grew to encompass the current ‘International System of Units (SI)’ The Metre Convention

. The UK was initially reluctant to join. Not to do with non-acceptance of the but more the fear that the endeavour would grow in size and end up costing additional money . In April 1884 H. J. Chaney, Warden of Standards in London, contacted the BIPM to calibrate some UK metre standards. O.-J. Broch, director of the BIPM replied that he could not perform any calibrations for non-member states . On 17 September 1884, the British Government signed the convention on behalf of the United Kingdom

National Prototype Metre Bar No. 27, Creating the metre-alloy in 1874 at the made in 1889, given to the USA served as Conservatoire des Arts et Métiers the standard for length from 1893 to 1960 International Infrastructure

Originally responsible for curating the primary artefactual

standards NMI NMI interaction The SI system of units . The 11th CGPM (1960) adopted the name International System of Units, SI, for the recommended practical system of units of measurement . The base units are a choice of seven units which are regarded as dimensionally independent: the metre, the kilogram, the , the , the , the mole, the . Derived units are those formed by combining base units according to the algebraic relations linking the corresponding quantities . The SI is not static but evolves to match evolving requirements for measurement The seven dimensionally independent SI base units Examples of derived units National Metrology Institutes (NMIs)

. The move away from single artefactual realisations of units and the burgeoning need for calibrations and local support for trade and industry resulted in the establishment of NMIs . 1887: PTR, Germany (PTB from 1945), 1900: NPL, 1901: NBS, USA (NIST from 1988) . NMIs are institutes designated by nation states to develop, improve and maintain national measurement standards for one or more quantities . NMIs represent their country internationally in relation to the NMIs of other countries, Regional Metrology Organisations, and the BIPM . The NMI or its government may appoint other institutes to hold specific national standards: ‘Designated Institutes’ Expansion of the Metre Convention

. This has prompted: a) The requirement for regional organisation of NMIs b) Need to understand the comparability of NMIs Regional Metrology Organisations

. Performs regional coordination of metrology activities – often with wider scope than the Metre Convention (e.g. EMRP), development of smaller NMIs, regional comparisons, etc CIPM Mutual Recognition Arrangement (MRA)

. The MRA was a response to a growing need to give users reliable quantitative information on the comparability of national metrology services. Its objectives: a) To establish the of equivalence of national measurement standards at NMIs and DIs b) To provide for the mutual recognition of calibration and measurement certificates issued by NMIs and DIs c) Provide a secure technical foundation for wider international agreements relating to trade, commerce and regulation . The CIPM MRA has been signed by the representatives from 57 Member States, 41 Associates of the CGPM, and 4 international organizations – and covers a further 150 institutes designated by the signatory bodies. CIPM MRA in practice

. Participation in key comparisons . CMC claims . Coordinated by CCQM . Input from Euramet Chemical metrology and the mole

. Metrology has developed mainly from physical measurement where measurement equations and traceability chains are transparent . Chemical measurements are more complex because of the multi- parameter nature of the problem:

 Component  Amount ()  Matrix  Isotopic distribution

. Chemical measurements also more interested in qualitative measurement (presence or absence) than the physical measurement . In bio-metrology these problems have additional complexity which are not fully part of the SI: what is it (identity), is it alive (and other nominal properties), how many are there (counting quantities)? The mole

. 14th CGPM in 1971 adopted the mole as the 7th of the SI (3) . The mole is the of a system which contains as many elementary entities as there are in 0.012 kilogram of 12; its symbol is "mol" . When the mole is used, the elementary entities must be specified and may be atoms, , , , other particles, or specified groups of such particles . In this definition, it is understood that unbound atoms of carbon 12, at rest and in their ground state, are referred to Why do we need the mole? Lessons from 1971:

. expressed the need for a quantity which was defined as directly proportional to the number of entities in a sample of a substance . It was preferable to adopt a convention with amount of substance having its own dimension. This convention was in wide use by chemists and already recommended by IUPAC, IUPAP and ISO . The wish for chemists to adopt the SI – but the need to incorporate a base unit for amount of substance into the SI to make this happen . A basis to establish stoichiometric relationships (which are not describable in mass terms) Additional benefits . Introduced to . Brings the equations of chemical physics within the SI . Allows intensive and extensive quantities in chemistry to be distinguished (e.g. m3/mol rather than just m3) . Resolved the confusion arising from the use of both:  g-mol and kg-mol  12C and 16O basis . To express chemical quantities using user-friendly numbers, for instance: 0.1 mol/dm3 Realising the mole . Like other base units the mole may be ‘realised’ (mise en pratique) using ‘primary methods’ . Unlike the other base units the mole is rarely realised directly as an extensive quantity . Instead the mole is realised via related intensive quantities  The composition of mixtures  Matrix reference

materials RJC Brown, Intern. J. Environ. Anal. Chem. 2008, 88, 681–687 Uncertainty of realisation

. Unlike the IPK, the realisation of the mole has an associated uncertainty . The uncertainty associated with this realisation is not only the uncertainty estimated by individual NMIs . The best estimate of the uncertainty of realisation come from international comparisons of these realisations . Elucidates systematic biases invisible to any one laboratory Dissemination of units and traceability

. Our role is to realise the mole at the highest level (for gas and particle measurements) . A great privilege and responsibility . A huge number of measurements rely on our work . Establishing traceability and evaluating the uncertainties of our measurements are our key metrological outputs Re-definition of the base units

. The driving force for redefinition has been the replacement of IPK, the last artefact . In the "New SI" four of the SI base units, namely the kilogram, the ampere, the kelvin and the mole, will be redefined in terms of invariants of nature . Based on fixed numerical values of the (h), the (e), the (k), and the Avogadro

constant (NA), respectively . The metre and the second will not change as these are already based on invariants of nature; the candela will not change either Benefits/drawbacks for chemistry?

. The definition of the mole (as for s) becomes independent of other base units . The definition is in line with those of the other base units . The definition is now more generic, conceptually simpler, and not focussed on a specific element. Is it easier to teach though? . No practical change for realisation, still performed by weighing at uncertainties that are orders of magnitude above the definition