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Fatty French Kilogram needs a new-year diet, say Brit boffins

Methanol, Soxhlet and an ozone detox is what's needed

The smart choice: opportunity from uncertainty

The world's official kilogram has put on weight according to boffins who fear the mass needs a new-year diet.

Researchers at the University of Newcastle reckon the reference kilogram artefact, kept at the International Bureau of Weights and Measures (BIPM) in Paris and used to calibrate the world's weights, has put on less than 100 millionths of a gram since it was cast in 1879.

The increase is blamed on atmospheric mercury contamination and the growth of a carbonaceous layer. The platinum-iridium kilogram is sealed under a bell jar and is rarely removed from its resting place. Although other units, such as the metre, are defined mathematically using universal constants (such as the speed of light), the kilogram is defined by this one mass in Paris.

But the Brit boffins' findings aren’t conclusive because they are based on tiny changes to the UK’s kilogram prototype, which is a copy of the original kilo. However, they do lend weight to the lingering suspicion held by experts that the mass of the Paris kilogram has changed - and that it’s not reliable for calibrating the modern world’s highly precise scientific work.

And it reinforces the need to calculate the kilogram using a mathematical equation, such as using Planck’s constant, rather than measuring against a physical object. Work on that perfect theoretical kilogram, however, has hit an impasse: different ways of calculating the perfect kilogram produce different results. The deviation is barely minute but enough to ruin precise measurements.

With this in mind, the British university tried a cleaning process suggested by the BIPM and the UK's National Physical Laboratory, and successfully removed trace contaminants from material similar to their prototype. It measured the mass before and after the treatment and used the change to derive an estimated deviation in the official kilogram.

The academics tested a methanol solvent wash on platinum and gold surfaces in a Soxhlet extractor, exposed them to UV and ozone for 20 minutes and then heated the test metals in filtered air. The boffins said X-ray photoelectron spectroscopy proved the contamination was removed. Now they reckon the scrub could be used on other kilograms.

BIPM officials currently clean the kilogram mass using a combination of alcohol, ether and steam in a process that’s been adhered to for decades for no other reason than nobody wants to take the chance of ruining the kilogram’s mass.

“While all these data are directly relevant only to the UK national prototype, the general behaviour is quite similar to the mean behaviour of most prototypes,” stated the university's report in the journal Metrologica. ®

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