What Chemistry Teachers Should Know about the Revised International System of Units (Système International)

Carmen J Giunta

doi:10.1021/acs.jchemed.8b00707

Abstract

In November 2018, a revision of the International System of Units (Systeme International, SI) was approved at the 26th meeting of the General Conference on Weights and Measures (CGPM). What are the implications of the revision of all seven definitions of SI base units for the teaching of chemistry? (1) No names of base units or their corresponding physical quantities have changed. For the precision typically employed in educational settings (say six or fewer significant figures), no values of base units have changed either. (2) The mole is officially defined as a definite number (6.02214076 × 1023) of elementary entities. (3) The kilogram is defined by setting a fixed value for the Planck constant. (4) The revision defines units indirectly by assigning fixed values to key physical constants (explicit-constant definitions rather than explicit-unit definitions). (5) Some quantities previously regarded as exact now have small uncertainties and vice versa.

Highlights

In November 2018, the 26th meeting of the General Conference on Weights and Measures (Confeŕ ence Geń eŕ ale des Poids et Mesures, CGPM) approved a revision to the International System of Units (Systeme International, SI) to go into effect May 20, 2019.1 This revision, sometimes referred to as the “New SI”,2 formulates explicit-constant definitions of all seven SI base units and newly fixes numerical values for four important physical constants
The kilogram was the only base unit still defined by an artifact, a platinum−iridium ingot known as the International Prototype Kilogram (IPK) maintained at the International Bureau of Weights and Measures
Third the revised definition of the mole is an exception in that it is clearer than the old definition, and it includes explicit definitions of both the unit and its corresponding constant, NA. (That prompted grumbling from some metrologists for spoiling the elegance of the otherwise exclusively explicit-constant system.)[14] ordinary laboratory procedures are unchanged: even though the kilogram, for example, is defined by the Planck constant, macroscopic mass measurements still involve balances calibrated to standards traceable to the defined unit; even though the kelvin is defined by the Boltzmann constant, laboratory temperature measurements continue to be made with tubes containing a liquid whose thermal expansion is well characterized or devices made from materials with wellcharacterized thermoelectric properties

Summary

■ INTRODUCTION

In November 2018, the 26th meeting of the General Conference on Weights and Measures (Confeŕ ence Geń eŕ ale des Poids et Mesures, CGPM) approved a revision to the International System of Units (Systeme International, SI) to go into effect May 20, 2019.1 This revision, sometimes referred to as the “New SI”,2 formulates explicit-constant definitions of all seven SI base units and newly fixes numerical values for four important physical constants. The value of the meter has not changed at all in the revision, but the definition has changed: “It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit m s−1.”4 Logically, this definition is exactly equivalent to the old one. (That prompted grumbling from some metrologists for spoiling the elegance of the otherwise exclusively explicit-constant system.)[14] ordinary laboratory procedures are unchanged: even though the kilogram, for example, is defined by the Planck constant, macroscopic mass measurements still involve balances calibrated to standards traceable to the defined unit; even though the kelvin is defined by the Boltzmann constant, laboratory temperature measurements continue to be made with tubes containing a liquid whose thermal expansion is well characterized or devices made from materials with wellcharacterized thermoelectric properties. The ratio[10] of the molar mass of 12C in grams per mole to the atomic mass of 12C in daltons is very close to 1 (to within about 5 parts in 1010) but not exactly 1. (This high level of precision is not determined using a balance but rather a mass spectrometer.)

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES