Recent Developments in the Study of the Surface-Stability of Platinum and Platinum-Iridium Mass Standards

Abstract

We review developments in the study of the stability of platinum-iridium standard weights, in particular the kilogram prototypes manufactured from alloy supplied by Johnson Matthey in the 1880s that still stand at the heart of the International System of Units (abbreviated SI from the French: Systeme international d’unites). The SI has long since moved on from length standards based on physical artefacts fabricated from this alloy, but the SI unit of mass is still defi ned in this way, as the mass of a real physical object. The stability of these reference masses has been a concern since the 1930s, with mass loss or gain at the surface being the principal concern. In recent years X-ray photoelectron spectroscopy (XPS) has been particularly valuable in elucidating the types of contamination present and the mechanism by which contamination takes place. While direct studies on the International Prototype Kilogram are understandably diffi cult, at Newcastle University we have examined the surfaces of six Pt mass standards also manufactured in the mid-19th century, using XPS to identify contamination chemically. XPS shows a signifi cant quantity of mercury on the surfaces of all six. The most likely source of Hg vapour is the accidental breakage of thermometers and barometers, and the mechanism of contamination may be similar to the poisoning of platinum group metal (pgm) catalysts by Hg, an effect known for almost a century.