When Chemists Attack

A post over at Pooflingers Anonymous reminded me of a little rant I’ve been meaning to post for a while.

Consider this lovely Pasteur pipet.

The Pasteur pipet is truly a small wonder. It’s cheap, disposable, and can be used to transfer small amounts of liquid from one place to another with almost no mess. The suction from the bulb keeps the liquid inside from dripping. There’s also very little risk of contamination of sample, since the glass part is disposable – and is only used to transfer one solution.

What irritates me to no end in introductory labs, though, is one simple error that students will just keep on making over and over again – no matter how many times I point out the error and correct it. That error is …

… holding the pipet with the tip up.

Holding the pipet this way causes the liquid inside the pipet to run down into the rubber bulb. While the glass body of the pipet is essentially chemically inert, the rubber bulb is not. Strong acids attack the bulb, as do many solvents – and the products of these reactions get into the dispensed liquid. Any contaminant that’s present in the rubber bulb – usually caused by some other student holding the pipet with the tip up – will also get washed into the dispensed liquid.

All of these contaminants will screw up results. For students, that translates into bad grades. That, at least, is something that students should understand!