16 October 2010

Calibrating a buret when there is nothing better to do

About 10 days ago I ordered a 50-ml buret from Ward's Natural Science. The catalog said that each buret was "individually calibrated". I couldn't have asked for more.

The buret arrived a few days ago carefully packaged in a tall box. I took out the buret and started examining it. I noticed the calibration temperature etched near the top: 27°C! Where on earth was this thing calibrated? I found the fine print on the cardboard sleeve: "Made in India". Images of un-air conditioned, hot and humid labs flashed before my eyes. Good thing it wasn't a record-breaking day when they worked on this particular buret.

The temperature in my basement is usually around 20-22°C and it never ever goes up to 27°C. The standard calibration temperature for laboratory glassware is 20°C. So I figured I needed to check the calibration of my Indian-made 27° buret.

An old book from my college days, Fundamentals of Analytical Chemistry by Skoog & West, provided the necessary instructions for buret calibration. It is really simple: you deliver with the buret several different volumes of distilled water, weigh each one, convert weights into volumes using the density of water at the lab temperature and finally, plot the calculated (or actual) volumes against the volumes read from the buret and derive the correction factors, if necessary.

Here are my results.

The largest volume I measured from the buret was 43.2 ml, while the calculated "actual" volume of it was 43.71 ml. Measurement errors notwithstanding, the difference of 0.51 ml is probably explainable by the differences in the volumes of water at 27°C during the original calibration and at 21.5°C in my basement. The volume difference at the smallest volume I measured, 9.8 ml, was 0.05 ml, which is almost negligible for my purposes.

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