Cleaning the Kilogram

Experts at Newcastle University, UK, announced that the original kilogram is likely to be tens of micrograms heavier than it was when the first standard was set in 1875.

The original kilogram – known as the International Prototype Kilogram or the IPK – is the standard against which all other measurements of mass are set. Stored in the International Bureau of Weights and Measures in Paris, forty official replicas of the IPK were made in 1884 and distributed around the world in order to standardise mass.

But despite efforts to protect the IPK and its duplicates, industrialization and modern living have taken their toll on the platinum-based weights and contaminants have built up on the surface. Now Professor Peter Cumpson and Dr Naoko Sano have used cutting-edge X-ray Photoelectron Spectroscopy (XPS) to analyse surfaces similar to the standard kilogram to assess the build-up of hydrocarbons – and how to remove them.

Publishing their findings this month in the journal of Metrologia, they reveal how giving the kilogram a suntan could be the answer to helping it lose weight.

“By exposing the surface to a mixture of UV and ozone we can remove the carbonaceous contamination and potentially bring prototype kilograms back to their ideal weight,” explains research lead Peter Cumpson, Professor of MicroElectroMechanical Systems (MEMS) at Newcastle University.

The kilogram is one of the seven SI base units from which all other units can be derived and is the only one which is measured against a physical object – the IPK – all others are standardized against known constants. The problem, says Cumpson, is that of the 40 IPK replicas around the world, there are slight differences and all are diverging from the original at different rates.

“But mass is such a fundamental unit that even this very small change is significant and the impact of a slight variation on a global scale is absolutely huge,” he says. “There are cases of international trade in high-value materials – or waste – where every last microgram must be accounted for.”

Work is underway internationally in several National Measurement Institutes to find an alternative to the IPK – a standardized value for the kilogram that is not based on a matchbox- sized piece of metal. But until then, the prototype kilograms are what the world relies on for its mass scale.

“If the kilogram does put on weight then it’s imperative that we understand exactly how the IPK is changing,” says Professor Cumpson.
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