Air is heavier than previously thought

Writing in the Institute of Physics journal Metrologia, a team
from the Korea Research Institute of Standards and Science (KRISS) and
the International Bureau of Weights and Measures (BIPM) in France,
report a new determination of the content of argon in air, the first
since 1969.

If asked to name the major chemical components of air, most of
us would list oxygen (about 21%), carbon dioxide (about 0.04%) and
water vapour (typically about 1%). The principal component of air is
nitrogen and the only other major component is argon (about 0.9 %).
Argon is chemically inert and its presence in the atmosphere poses no
problem to human well-being.

Old measurements dating from as early as
1903 gave the content (moles of argon/mole of dry air) as 0.934 %. The
most recent value available until now was lower (0.917 %) and was
thought to supersede the previous result. The work reported in
Metrologia gives a new figure (0.9332 +/- 0.0006)%, very close to the
measurement results of 100 years ago. The uncertainty in the new
measurement is given at the 95% confidence limit and is of
unprecedented accuracy.

The analysis was performed at KRISS using high precision mass
spectrometry. A set of air-like calibration gas mixtures was prepared
by very careful weighing of pure gases into high pressure cylinders.
Analysis of these synthetic air mixtures along with samples of natural
air contained in other high pressure cylinders yielded the result
reported in Metrologia.

Argon content is important to a small community of scientists
working on precision mass measurements. To understand why, think of the
old puzzle: which weighs more, a kilogram of feathers or a kilogram of
lead?

If it were possible to do the weighing on a very precise balance,
we would see that the balance readings are identical for the feathers
and the lead if the weighing is carried out in a vacuum. But the
feathers would produce a considerably lower balance reading for
measurements in air. This is because feathers are more buoyant in air
than is lead (Achimedes' Principle).

Mass metrologists use an equation to correct for the effect of
air buoyancy. The equation includes the air density which, in turn,
includes a parameter for the content of argon in the atmosphere. The
different historical values for argon content lead to a difference in
air density of just under 0.01%, or about 15 micrograms in the apparent
mass of one kilogram made of stainless steel (15 parts in 109). The
higher the argon content, the denser the air.

Even though the density of air is roughly 800 times smaller than water
density, the effects of air buoyancy are easily seen in precise
weighing. Thus the air density calculated from the new value of argon
content should agree with precise data obtained from the feathers and
lead experiment.

There is a stainless-steel cylinder on one side, which
is hollow inside, representing the low-density feathers. On the other,
a thick-walled tube is a solid piece of stainless steel, thereby
representing the high-density lead. The cylinder and tube have the same
surface area, which simplifies analysis of the experimental data.

The
results of measurements with several different sets of hollow and solid
objects are reported in a companion article in Metrologia, written by
scientists at the BIPM and the Physikalisch-Technische Bundesanstalt,
in Germany.

Michael Esler, from the Chemistry Section at BIPM, and one of
the authors, said: "The results confirm the new argon content and can
explain discrepancies that had already been observed using the
previously-accepted value dating from the mid-20th century. The new
determination of argon content was motivated by numerous mass
measurements which stubbornly failed to agree with the accepted formula
for air density. The new results should lead to improved coherence
among high precision mass measurements".

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