Redefinition of the mole and uncertainty of analytical measurements

Redefinition of the basic units of the International System of Units (SI) — the kilogram, mole, ampere, and kelvin, — which are now expressed in terms of fundamental physical constants means a substantial revision of the system. In particular, the new definition of the mole fixing the value of the Avogadro constant sets a unit of the amount of substance, which is independent of the unit of mass. We consider some consequences of redefining (the mole and kilogram) and focus on the uncertainty of measuring the amount of substance and related quantities which are important for description of the mixture composition. The issue regarding the molar mass of the substance and associated uncertainty is considered in detail It is noted that calculation of the molar mass using relative atomic masses, involves the molar mass constant which is not equal exactly to 1 g/'mol in the new SI. This introduces an additional, though very small, uncertainty of less than 1 x 10 -9 in relative terms. The budget of uncertainty for the amount of substance determined through the mass measurements when the mass is measured with the highest accuracy is scrutinized. It is demonstrated that for substances of less than 99.98% purity, the uncertainty associated to the purity is comparable to that of relative atomic masses of the elements. For high-purity substances, the uncertainty in the relative atomic masses have the largest contribution to the budget. Anyhow, the uncertainty associated to the molar mass constant is three orders of magnitude less than the nearest contribution to the uncertainty attributed to weighing. In the case of derived quantities which are the ratio of two quantities of the same kind, the additional uncertainty does not arise at all. This is illustrated by the calculation of the mole fraction of a component in the gravimetrically prepared gas mixture.