The kilogram gets a little more massive

Some of you might have heard me on last weeks Naked Scientists show, talking about the fact that the standard kilogram is not so standard after all. If you didn’t hear it and would like to, you’ll find a link at the bottom of this piece. Here is a little article I wrote on the story:

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Le Grande K

On the western outskirts of Paris, in a lab buried deep within the International Bureau of Weights and Measures (BIPM), there resides a mysterious cylinder of metallic alloy. This is the international prototype kilogram (IPK), or Le Grande K as its known – and it doesn’t weigh a kilogram, it is the kilogram, against which all other kilograms are measured.

However a quiet battle is being fought. A battle against the bulge. Forty copies of the IPK were sent to the major national standards laboratories across the world, to act as the primary standard for that country. (Kilogram 18 is stored at the UK’s National Physical Laboratory, NPL). The problem is that since the late 1980s, there has been disparity between the masses of each of these kilograms, but because the mass measurements are relative, it was unclear if the replicas were getting heavier or the original was getting lighter.

But we now know that all of the replica kilograms are slowly gaining mass. Scientists at Newcastle University in the UK have shown that this is due to surface contamination by hydrocarbons and mercury, which are deposited at an average rate of tens of micrograms per decade. Even a disparity this tiny could cause a problem for international trade or when measuring things like radioactive materials.

Fortunately, the Newcastle group, led by Peter Cumpson, have gone further than simply quantifying the problem. The IPK and the replicas are made from the same alloy of platinum and iridium, and all 41 are stored identically, but there is one inconsistency. Currently, all of the kilograms are hand-cleaned by a skilled technician, using chamois leather dipped in alcohol. Because each lab will do this in a different way and at different times of the year, it’s impossible to know how much material is being removed from the kilograms each time they’re mechanically rubbed.

To go some way to quantifying this, BIPM and the other standards labs have, for many years, made detailed weighings of both the mass lost during cleaning and the mass re-growth in the months after cleaning/washing. Cumpson suspects that because the replicas are retrieved and handled more frequently than the IPK, more carbon-based contaminants have built up on them over time, rendering them heavier than the original. So the team proposed a new cleaning solution for the kilograms – no elbow grease required.

The technique exposes the kilograms to a combination of ultraviolet light and ozone gas, followed by a pure water rinse – a routine which had only previously been used to clean semiconductor surfaces in the electronics industry. This technique was developed by Cumpson while he worked at NPL, but using X-ray Photoelectron Spectroscopy (XPS), the team now based at Newcastle have confirmed that the UV-ozone combination provides a hands-off method to removing the hydrocarbon contamination that has built up on the kilogram’s metal surface; the result of living in an industrial society. However, this technique does not remove the mercury contamination, which is tightly bound to the platinum via a metallic bond – that problem is next on Cumpson’s hit list.

Eventually, the kilogram will be redefined using a fundamental natural constant rather than a variable block of metal. But in the meantime, we can rest easy knowing that a cleaner kilogram is a happy kilogram

The paper: http://iopscience.iop.org/0026-1394/50/1/27/pdf/0026-1394_50_1_27.pdf

The radio show: Naked Scientists 13 01 2013 13-01-13 (I’m on from the start until 7.11)