Determination of the amount of physical adsorption of water vapour on platinum-iridium surfaces

This paper presents the measurement of the physical adsorption of water vapour on platinum-iridium surfaces using a vacuum mass comparator. This value is of importance for redefining the kilogram, which will be realized under vacuum in the near future. Mirror-polished artefacts, consisting of a reference artefact and a test artefact, were manufactured for this experiment. The surface area difference between the reference and test artefacts was 226.2 cm(2). This surface area difference was approximately 3.2 times the geometric surface area of the prototype of the kilogram made of platinum-iridium (71.7 cm(2)). The measurement results indicate that the amount of physical adsorption at a relative humidity of 50% is 0.0129 mu g cm(-2), with a standard uncertainty of 0.0016 mu g cm(-2). This value is 0.03 to 0.16 times that observed in other studies.