A pipette is a piece of volumetric glassware used to transfer quantitatively a desired volume of solution from one container to another. Pipettes are calibrated at a specified temperature (usually 68°F [20°C] or 77°F [25°C]) either to contain (TC) or to deliver (TD) the stated volume indicated by the etched/painted markings on the pipette side. Pipettes that are marked TD generally deliver the desired volume with free drainage; whereas in the case of pipettes marked TC the last drop must be blown out or washed out with an appropriate solvent.
For high-accuracy chemical analysis and research work, a volumetric transfer pipette is preferred. Volumetric transfer pipettes are calibrated to deliver a fixed liquid volume with free drainage, and are available in sizes ranging from 0.5-200 mL. Class A pipettes with volumes greater than 5 mL have a tolerance of +/-0.2% or better. The accuracy and precision of the smaller Class A pipettes and of the Class B pipettes are less. The Ostwald-Folin pipette is similar to the volumetric transfer pipette, except that the last drop should be blown out. Mohr and serological pipettes have graduated volumetric markings, and are designed to deliver various volumes with an accuracy of +/- 0.5-1.0%. The volume of liquid transferred is the difference between the volumes indicated before and after delivery.
Serological pipettes are calibrated all the way to the tip, and the last drop should be blown out. The calibration markings on Mohr pipettes, on the other hand, begins well above the tip. Lambda pipettes are used to transfer very small liquid volumes down to 1 microliter. Dropping pipettes (i.e., medicine droppers) and Pasteur pipettes are usually uncalibrated, and are used to transfer liquids only when accurate quantification is not necessary.
Automatic dispensing pipettes and micropipettes are available commercially. Automatic dispensing pipettes, in sizes ranging from 1-2,000 mL, permit fast, repetitive delivery of a given volume of solution from a dispensing bottle. Micropipettes consist of a cylinder with a thumb-operated air-tight plunger. A disposable plastic tip attaches to the end of the cylinder, the plunger is depressed, and the plastic tip is immersed in the sample solution. The liquid enters the tip when the plunger is released. The solution never touches the plunger. Micropipettes generally have fixed volumes, however, some models have provisions for adjustable volume settings. Micropipettes are extremely useful in clinical and biochemical applications where errors of +/- 1% are acceptable, and where problems of contamination make disposable tips desirable.
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