Kilogram Ready to Slim down for the New Year

UK scientists have developed a cleaning technique that could solve a long-standing puzzle in the field of metrology – how to return the standard kilogram, against which all others are measured, to its original mass.

The base unit of mass, the kilogram, is defined by the International Prototype Kilogram (IPK) – a cylinder of platinum and iridium that was cast in 1879 and stored in a vault in the International Bureau of Weights and Measures in Paris. Around 40 replicas were made and distributed around the world to use as a standard reference to produce weights and standardise mass. But following three periodic verifications, where copies were returned to Paris to check their mass against the original IPK, scientists discovered that their weights are drifting. 

These tiny changes – which are less than 100µg – are thought to be the result of surface chemistry increasing their mass due to atmospheric mercury contamination and the growth of a carbonaceous layer. It is also possible that nitrogen trapped when the prototypes were cast has since leached out.

However, there is another solution on the cards to solve the kilogram’s weight problem. Delegates at the 24th General Conference on Weights and Measures held in 2011 voted for a change in the definition of the kilogram. The idea is to define it in relation to the mass of a silicon atom or in terms of the Planck constant which, as far as experiments have revealed, are fixed.

The switchover is likely to happen in 2014 after more subatomic experiments. This would mean the primary standard would either be a silicon sphere or weight designed for use on a watt balance.

'In addition to platinum and gold alloys we are looking at several other materials with regards to next generation mass standards including nickel alloys, silicon, iridium, stainless steel and tungsten,' says Davidson. So even if the current IPK becomes obsolete, the new UV–ozone cleaning method could still prove useful for keeping the next generation of reference masses stable. 'The cleanliness of the standard is critical, particularly when transferring from air to vacuum where the change in the mass – mainly due to water sorption – will depend on how clean the surface is,' he adds.

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