Re: How do the various types of pipette differ from each other in purpose?

Hi, Gemma – thanks for your question. It is a long time since I was personally using pipettes, but although things have moved on there are basically two types of pipettes, as you obviously realise.

Actually Pasteur pipettes can be made either of glass or plastic and are only used for transferring liquids where accurate volume dispensing is not required. The old fashioned eye dropper, made of glass with a rounded bulb at the tip, uses a rubber bulb on the top, which is squeezed to expel air, dipped in the required liquid and released; this draws liquid up into the pipette. Then dispensing can easily be done drop-wise, as into the eye, say for treating an infection. Plastic single-use versions that are designed as one piece are also common, and also use a bulb squeeze and release mechanism. In the lab, Pasteur pipettes would be used where, for example, you just need to add a drop (say an indicator in a titration) to a vessel. Otherwise most pipettes are volumetrically calibrated.

When I was a student, volumetric pipettes were all glass; they were either bulb pipettes intended to deliver fairly accurately one single volume (5ml, 10 ml, 25ml and so on), or they were graduated with a wider tube, no bulb and a calibrated scale up the side, so that you could dispense accurate intermediate volumes. One used mouth suction to draw the liquid up above the calibration line and then released with a moist finger tip on the top of the tube so the level aligned with the line. (We had to be careful – especially when dealing with toxic liquids!)

Then, rubber bulbs with built-in squeeze valves came in to avoid risk. (Most of us did not like these as they were harder to control accurately...)

Later, the kind of pipette in your link came in where the volume was controlled by the device to draw liquid up, often with the ability to dial in different values. Disposable plastic tips were attached to these, but often if using organic solvents this was not a good idea, as the solvent might either dissolve the plastic of some component from it (e.g. plasticiser).

Dispensing accurate volumes is not easy – temperature changes are problematic. Glass pipettes are calibrated at a specific temperature (25C if I remember correctly). The modern pipettes that rely on displacement of air (as in your link) might be at an advantage as the change in the volume of the piston movement may be able to be designed with less temperature variation than is the case with glass.

A long-winded answer to your question; the short answer is that Pasteur pipettes are not used for accurate volume dispensing, but there is a variety of types that are. Hope this answers you fully, and thanks again for the question.