Sunday, February 24, 2013

The billionth of a kilogram effect

Early in the semester, my students invariably turn in a lab done in pencil, or worse yet, a lab corrected with correction fluid. My first reaction is to cringe, then I step back and remind myself that this is an introductory class- they don't know any better.

A lab done properly in pen (without correction fluid) presents data that is traceable. How else would Alexander Fleming have been able to trace his accidental discovery of  penicillin back to the substance he placed on the Petri dish a long time ago?

Accuracy and  reproducibility are two recurring themes throughout science. Another place this theme appeaars is in the Jan 12, 2013 edition of The Economist. Mass Effect describes the efforts of scientists around the world to clean and preserve the kilogram standard. Apparently, it is quite a challenge.

The central standard of the metric system (or Standard international or SI system) for mass is a platinum-iridium block that lives at the Bureau International des Poids et Mesures in Paris France. Unlike other standards of the SI system, it is not a reference to a constant of nature: it is a man-made metal that tends to attract pollutants and contaminants that affect its mass. Therefore, it must be constantly cleaned to keep it at its precise 1 kg mass- the standard of the entire system.

Two researchers in England have recently devised a new way of cleaning this standard to avoid the problems of over scrubbing (stripping away metal itself) and too lightly cleaning its surface (leaving added mass from contamination). They use a combination of ozone and ultraviolet light- apparently a process adapted from the chip-manufacturing industry.

The problem doesn't just apply to this one international standard housed in Paris, France. All over the world, countries keep a replica of the international kilogram standard. All of these standards must be cleaned in the same fashion as the official standard in Paris. And this presents its own little set of challenges of how to coordinate cleanings of all of these standards to keep them at the same mass as the international standard.

So the problem of accuracy and precision plagues us whether we teach introductory chemistry or whether we maintain the world's standard measure of mass. It is a central challenge in science.