I was wondering about this. I have heard, and known for a while, that the famous and celebrated Prototype Kilogram - the lump of metal whose mass is used to define the standard mass scale worldwide - is slowly changing, degrading in some fashion, and this has prompted the search for a new and better definition of the kilogram mass unit on something more stable (the lead is now Planck's constant, but the change has not yet been adopted by the BIPM and looks like it won't be until at least next year.).

However what I'm curious about is the reason for the mass change. Apparently it's said it's not clear as to what exactly could be causing it, but I was wondering: is it chemically possible that this mass change could be due to oxidation - tarnishing - of the kilogram and removal of the oxide? I would imagine that the scientists who have studied this must have thought of this, so I'd be more interested in why it's dismissed as an explanation. In particular, it's said that the material it's made of - mostly platinum - has a very low oxidation rate, but "very low" need not equal "zero", and the mass change is reported as up to (unclear, because it's the measuring standard itself!) 50 micrograms over 100 years - which is nonetheless a very slow rate of loss - about $10^{-16}$ mol Pt/s, so that it would take $10^{16}$ s (10 Ps) to oxidize just one mole, or about 300 million years. Could this be within range of the very-slow-but-not-zero range of oxidization of platinum in air? In particular I find this:


talking about platinum oxidizing in air upon mild heating, and even losing mass. Now it seems that a rule of physics is "whatever isn't forbidden is compulsory", that is, whatever isn't absolutely impossible will eventually happen given a suitably long time period, and so if the oxidation is possible could it still be going on at very reduced rates at room temperature that nonetheless might still be sufficient to degrade the kilogram? Or is the rate then so low as to not contribute at all? (I'd think about Arrhenius rate law, but no idea what the relevant constants would be. Nonetheless exponentials are pretty steep, so this might rule it out.)

  • $\begingroup$ What says they're storing it under inert gas? $\endgroup$ – Karl Oct 9 '17 at 7:12
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    $\begingroup$ I would think that no one here has enough data to answer this question. $\endgroup$ – Zhe Oct 9 '17 at 12:58
  • $\begingroup$ @Zhe Not enough data? en.wikipedia.org/wiki/… $\endgroup$ – Mithoron Oct 9 '17 at 19:34
  • $\begingroup$ @Mithoron "The reason for this drift has eluded physicists who have dedicated their careers to the SI unit of mass. No plausible mechanism has been proposed to explain either a steady decrease in the mass of the IPK, or an increase in that of its replicas dispersed throughout the world." I think they would say they need more data... $\endgroup$ – Zhe Oct 9 '17 at 20:43

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