Amount of substance, measurement

P. De Bievre, S. Valkiers, and P. D. P. Taylor, The Importance of Avogadro s Constant for Amount-of-Substance Measurements, Fresenius J. Anal. Chem. 1998,361, 227. 

Abstract By the definition of the mole as a base unit for amount-of-substance measures within the International System of Units (SI), chemists can make chemical measurements in full compliance with established metrological principles. Since the mole requires exact knowledge of the chemical entity, which is often neither available nor of practical relevance to the purpose of the measurement, the SI units of mass or length (for volume) are unavoidable in the expression of results of many chemical measurements. Science, technology, and trade depend upon a huge and ever increasing number and variety of chemical determinations to quantify material composition and quality. Thus, international harmonization in the assessments of processes, procedures, and results is highly desirable and clearly cost effective. The authors, with relevant experience and re-... 

Most chemists will agree that the majority of chemical measurements are, or could be, expressed as amount-of-substance measurements. When appropriate, they will in this paper be so described. However, whereas mass or length (volume) measurements at the smallest attainable uncertainty do not generally require a detailed understanding of the material whose property is quantified, amount-of-substance measurements require reference to the exact composition of the measured entity, to interfering impurities, and to the material - by composition, mass, or volume - within which that entity is measured. 

For amount-of-substance measurements we include kilogram mass units, which are linked to the amount-of-substance unit in SI by the atomic-weight values. The latter differ greatly in uncertainty for different chemical entities, but are always available, with the best estimates by current knowledge of their uncertainties,... 

One may be inclined to suppose that for each type of chemical measurement there is a need to build a measurement system based on the pyramid concept [7, 32], For the practicing chemist, however, this would be seen only as an unhelpful imposition. Previously discussed limitations of such a pyramid system would apply equally to the use of RMs. In addition, there is a major difficulty due to the previously discussed differences between RM matrix and sample matrices. Whereas for extrinsic measurements the composition of an RM or other traveling standard is of little or no concern, intrinsic amount-of-substance measurements are generally affected by the internal composition, structure, and texture of the RM. 

The mole is associated with a specific chemical entity as defined by its chemical formula [15], Its structural formula, isotopic composition, isomeric form, crystal structure, or chirality may have to be given in order to completely specify the entity of interest. The achievable uncertainty of amount-of-substance measurement is limited by that of its apparent molar mass. This consideration affects not only measurements on entities with variable molar mass, but those on pure substances. It is related to the traditional and important concern about purity. 

Establishing Traceability to the SI of results of Amount-of-Substance measurements... 

A third and very important example is one where the unknown amount is not measured by a PMM directly in SI- or other units, but against commonly accepted references, usually values carried by reference materials (in whatever units). This is especially important in cases where the substance cannot (yet) be unequivocally identified as a unique substance (e.g. protein in beef or fibre in com flakes, both very important in trade). Since an amount-of-substance measurement is not (yet) possible because the substance to be measured is not (yet) uniquely defined, we go back to the traditional definition of calibration as illustrated in Fig. 5 [2], Several reference samples of different contents are prepared (or agreed) and their contents ex-... 

In amount of substance measurements, we almost never face the need to determine the sum of all components. We try to determine specific substances that form a part of the whole. Amount of substance, which is of course an extensive quantity, can be considered as having partial nature - this is supported by its definition, too (entities must be stated). Any prepared stand-... 

The Slovak Institute of Metrology (SMU) has recently rebuilt its high-precision coulometric equipment to be used as a national standard for amount of substance measurements. Its main purpose is certification of primary reference materials of composition with directly determined main component. These primary reference materials can be subsequently used for disseminating traceability into different chemical measurements. 

Another possibility to establish the traceability of amount of substance measurements is to use the following primary methods indicated by the Comite Consultatif pour la Quantite de Matiere (CCQM) isotope dilution with mass spectrometry, coulometry, gravimetry, titrimetry, determination of freezing-point depression, and methods which provide a direct traceability to SI units. 

In practice, many of the conditions of measurement in practical chemical measurement are specified in terms of physical measurements. So, too, are many of the inputs to a given calculation. Though the establishment of traceability in these fields has been far from trivial, it is now essentially a routine matter for laboratories to obtain suitable calibrated equipment for measuring quantities such as length, volume, mass, temperature and time. The problem for most laboratories is related to their chemical reference values for amount of substance measurements. 

In the next sections as well as in the next chapter, important fundamental terms and concepts will be discussed. Among these will be substance, amount of substance, measures of composition, and energy, all of which students are probably familiar with from high school. For this reason, it should be easy to start right in with Chap. 3 (Entropy) or even Chap. 4 (Chemical Potential). Chemical potential puts us right at the heart of matter dynamics. Using this as a starting point opens up a multitude of areas of application. Chapters 1 and 2 can then be considered reference work for fimdamental terms and concepts. 

The two base quantities that are most important for quantitative chemical analysis are amount of substance (measured in moles, mol) and mass (measured in kilograms, kg), although length (measured in meters, m) is also important via its derived quantity volume in view of the convenience introduced by our common use of volume concentrations for liquid solutions. 

The number of formula units, N, in a sample of any substance is proportional to the amount of substance measured in moles, denoted by n ... 

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