Determination of the Atomic Weight of ²⁸Si-Enriched Silicon for a Revised Estimate of the Avogadro Constant

The much anticipated overhaul of the International System of Units (SI) will result in new definitions of base units in terms of fundamental constants. However, redefinition of the kilogram in terms of the Planck constant (h) cannot proceed without consistency between the Avogadro and Planck constants, which are both related through the Rydberg constant. In this work, an independent assessment of the atomic weight of silicon in a highly enriched ²⁸Si crystal supplied by the International Avogadro Coordination (IAC) was performed. This recent analytical approach, based on dissolution with NaOH and its isotopic characterization by multicollector inductively coupled plasma mass spectrometry, is critically evaluated. The resultant atomic weight A_r(Si) = 27.976 968 39(24)_k=1 differs significantly from the most recent value of A_r(Si) = 27.976 970 27(23)_k=1. Using the results generated herein for A_r(Si) along with other IAC measurement results for mass, volume, and the lattice spacing, the estimate of the Avogadro constant becomes N_A = 6.022 140 40(19) × 10²³ mol^–1.