STABILITY OF REFERENCE MASSES .3. MECHANISM AND LONG-TERM EFFECTS OF MERCURY CONTAMINATION ON PLATINUM-IRIDIUM MASS STANDARDS

In a previous study (Part I) we showed that the material used for national prototype and reference masses, Pt-10% Ir, is susceptible to mercury contamination even at the very low levels of mercury present in air in weighing laboratories. In this study we measure the mass uptake per unit area of Pt-10% Ir exposed to mercury vapour, as a function of time, using the mass response of a quartz crystal microbalance with electrodes of Pt-10% Ir. This system can detect with accuracy mass increases equivalent to less than 0,1 mug on a prototype kilogram. It is shown that there is a rapid adsorption of the first monolayer of mercury, but this mercury then diffuses into the solid allowing further mercury to adsorb. The quantity of mercury absorbed does not tend towards any immediate limit, but instead continues to grow in proportion to the square root of time. A consideration of the morphology of polished surfaces suggests this growth will continue for between 10 and 600 years. We conclude that our earlier recommendation to clean all environments where reference masses are housed, particularly with regard to mercury, is essential for maintaining the stability of Pt-10% Ir masses. Significant mass instability due to mercury sorption will occur for atmospheric mercury levels well below current health and safety limits.