Consistent Avogadro number a step nearer

15 February 2012

Chemical metrologists in Canada have made the most accurate measurement of silicon's molecular weight to date in a bid to derive a consistent and internationally acceptable figure for the Avogadro constant. The work forms a key part of the redefinition of the kilogram, agreed last year by the International Bureau of Weights and Measures.

Currently the kilogram is defined by the international prototype, a cylinder of platinum-iridium that was cast in 1879. The metrological community plans to redefine the kilogram in terms of the Planck constant. However, before the new definition can be adopted, international consensus on the value of the Planck constant is needed. There are two distinct experimental ways of measuring the number. One uses a special electronic balance - a watt balance. The other relies on the derivation of the Avogadro constant, which is directly related to the Planck Constant.

'When we change the definition of the kilogram in terms of the Planck constant, we need to ensure that the value we insert into that definition is the right one,' explains Ian Robinson of the National Physical Laboratory in the UK, who has worked on watt balances for thirty years. 'The results from these two completely different approaches must agree at a level where no one will see any change from now on. The aim is to ensure that the kilogram is stable.'

The internationally agreed way to determine the Avogadro constant is to count the number of silicon atoms in two 1 kg single crystal silicon spheres by exploiting their ordered arrangement in the crystal. The spheres are highly enriched with the 28 isotope of silicon. The Avogadro constant is based on the relationship between the density, molar mass and lattice parameters within the crystal. Multicollector inductively coupled plasma mass spectrometry is used to determine the molar mass with unprecedented accuracy.

Zoltán Mester and colleagues at the Institute for National Measurement Standards in Canada have made the latest calculation for the Avogadro constant, which while differing from the previous measurement remains, for the first time, overall within agreement.

The Canadian team's calculation for the atomic weight for silicon is 27.97696839 - the most recent result was 27.97697027. This gives a figure for the Avogadro constant of 6.02214040×10²³ mol^-1 .

'While this measurement and the previous measurement differ, they fall within a margin error of a few parts in 10⁸,' says Mester. 'This is the first time two independent estimates of Avogadro in the bigger picture are in agreement. The discrepancies are dwindling by an order of magnitude every decade.'

Source: Royal Society of Chemistry