Regular Article

The molar mass of a new enriched silicon crystal: maintaining the realization and dissemination of the kilogram and mole in the new SI

Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116 Braunschweig, Germany

^* e-mail: axel.pramann@ptb.de

Received: 13 September 2019
Received in final form: 20 December 2019
Accepted: 24 December 2019
Published online: 5 February 2020

Abstract

The local distribution of the isotopic composition and molar mass M of a new silicon crystal (Si28-24Pr11) highly enriched in the ²⁸Si isotope is reported, with focus on the experimental methods as well as on the associated uncertainties. The crystal was used in 2018 for the production of two additional silicon spheres for the realization and verification of the Avogadro constant N_A using the “X-ray-crystal-density (XRCD) method” which is a primary method for the dissemination of the revised SI units mole and kilogram. 17 subsamples have been investigated (from five different axial and in several radial positions) by isotope ratio mass spectrometry using a multicollector-inductively coupled plasma mass spectrometer (MC-ICP-MS). The average molar mass of the crystal is M = 27.976 933 787(77) g/mol with a relative combined uncertainty u_c,rel(M) = 2.7 × 10⁻⁹. The mean amount-of-substance fraction of ²⁸Si is x(²⁸Si) = 0.999 993 104 (66) mol/mol indicating that this crystal has the highest enrichment in this isotope which has ever been used for the determination of N_A. No local variations in M and x(ⁱSi) (i = 28, 29, and 30) could be identified due to material properties. The results are compared with those from two previous enriched crystals.