THE IMPORTANCE OF THE AVOGADRO CONSTANT FOR AMOUNT-OF-SUBSTANCE MEASUREMENTS

One of the routes to re-determine the Avogadro constant requires ultra -high accuracy isotopic measurements of amount ratios of Si isotopes t o 10(-5) reproducibilities on the ratio measurements and calibrating t he results by synthetic isotope mixtures prepared to 2 . 10(-5) relati ve combined uncertainty. The route to these results is described, foll owed by a description of the improvement of measurements. The same rou te has recently been opened for other gases and has opened the way to the establishment of ''Primary Standards of Measurements'' for gas iso topic measurements. It is described how these highly accurate measurem ents of ratios of amount of substance our knowledge of the Avogadro co nstant N-A through these measurements. The same route has recently bee n opened for other gases and has opened the way to the establishment o f ''Primary Standards of Measurements'' for gas isotopic measurements. It is described how these highly accurate measurements of ratios of a mount of substance have considerably contributed to clearer thinking a bout concepts for measurements of amount of substance. The acquired ex pertise in measurement instrumentation and measurement procedures can now be extended to a more general use in measurements of isotope amoun t ratio (i.e. in other elements). it can also be combined with isotope dilution. This latter combination opens up a new route towards a real ization of traceability of an amount-of-substance measurement to the S I and to the (measurement procedure and instrumentation leading to) Av ogadro constant.