The Subtleties Of the Volumetric Pipet

To the uninitiated, the volumetric pipet is just an oddly-shaped glass tube used to measure the volume of liquids. To researchers accustomed to fancy piston-driven micropipettes, the simple pipet is a throwback to a different age.

But although it continues to serve as a useful tool in preparing solutions and accurately measuring out volumes for titration experiments, the pipet is a great starting point to illustrate important analytical concepts and even some basic science.Why is it shaped the way it is?

By having a bulge in the midsection like some middle-aged man, it holds the bulk of its volume there. But where the bottom of the meniscus meets the volume-marker, it is extremely thin. This is deliberately done since any extra amount of liquid will have little room to spread horizontally and will rise instead. Errors will become obvious, and so this piece of glassware is far superior to a graduated cylinder, which has a wider diameter. 

A 50.00 ml pipet has an uncertainty of
±0.05 mL , introducing an error of only 0.1%. In a 50.0 ml graduated cylinder, the uncertainty is 8 times as large.

As with the use of a thermometer or burette, the meniscus has to be read at eye-level. Otherwise every other angle  would lead to a different perception. The pipet’s pointy tip is designed to trap the last drop, providing consistency. The desired volume delivered from the pipet is calibrated by taking that trapped volume into account, and so it should not be forced out.

The proper use of the pipet leads to precision. In science precision means that measurement can be reproduced. In other words, if 50.00 ml is measured repeatedly, it will deliver the same amount within ±0.05 mL. This could be verified by comparing the mass each time, and it will show consistency.

Does precision necessarily imply accuracy? Not necessarily. What if the manufacturer had placed the marker delineating the 50.00 ml spot in the wrong place? Then even though the corresponding mass would be obtained repeatedly, it would still not match the real mass of that perceived volume. If the temperature and density of the liquid are known to enough decimal places, then together with the mass, the volume can be calculated and the pipet’s supposed 50.00 ml can then be checked against the calculation for accuracy.

Finally how is the volume of liquid sucked into the pipet in the first place? In science, the word “sucks” sucks because it does not allow us to picture the behaviour of molecules.

Imagine a pipet immersed into a liquid with the bulb being squeezed and then released while it’s still attached to the pipette. Since the liquid is in the way, the only air that can reenter the pipette comes from the pipette itself. As the bulb regains its original shape, the total available volume for the air becomes greater than the original, lowering pressure inside the bulb and pipet. (the pressure drops because there are less molecular collisions in the more spacious volume).

The atmospheric pressure is now greater than the internal pressure of the bulb-pipette system, and so there is a net force that pushes the liquid into the pipette.

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