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 nati onal prototype and reference masses, Pt-10% Ir, is susceptible to merc ury contamination even at the very low levels of mercury present in ai r in weighing laboratories. In this study we measure the mass uptake p er 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 el ectrodes of Pt-10% Ir. This system can detect with accuracy mass incre ases equivalent to less than 0,1 mug on a prototype kilogram. It is sh own that there is a rapid adsorption of the first monolayer of mercury , but this mercury then diffuses into the solid allowing further mercu ry to adsorb. The quantity of mercury absorbed does not tend towards a ny immediate limit, but instead continues to grow in proportion to the square root of time. A consideration of the morphology of polished su rfaces suggests this growth will continue for between 10 and 600 years . We conclude that our earlier recommendation to clean all environment s where reference masses are housed, particularly with regard to mercu ry, is essential for maintaining the stability of Pt-10% Ir masses. Si gnificant mass instability due to mercury sorption will occur for atmo spheric mercury levels well below current health and safety limits.