Measuring pipettes are used for dispensing a variable volume of aqueous liquid. Pipettes are calibrated with distilled water. They can be used with non-aqueous liquids but they will not be as accurate. The form of the meniscus and the wetted volume remaining in the pipette will be different for other solvents.
Pipettes are usually used quantitively, though they are sometimes used as a convenient way of adding a known volume of solvent or reagent. The class used will determine the accuracy and traceability of the pipette. The type determines the way the pipette is graduated.
Class B pipettes should be used when accuracy of 1% is sufficient in uncontrolled environments.
Class A pipettes are used when an accuracy of up to 0.5% is required and in controlled environments.
Class A pipettes are calibrated to half the tolerance of Class B pipettes and each has a unique serial number for traceability. A certificate of accuracy traceable to national standards can be provided if required.
Type 1 pipettes are calibrated to deliver between two points with the zero at the top. The liquid is drawn up to the zero mark and the liquid allowed to run out until the desired volume has been delivered. It is not emptied completely. Not used as frequently as type 2 as it is difficult to stop the flow at exactly the desired point.
Type 2 pipettes are calibrated to deliver the measured volume. Total capacity is at the top. The most commonly used type. The desired volume is drawn up then allowed to run out completely. The operator then waits a further 3 seconds to ensure that all the liquid has drained out.
Type 3 pipettes are the same as type 2 but calibrated with total capacity marked at the outlet.
Type 3F are the same as type 3 but with a larger jet to allow rapid flow. Used where speed is more important than accuracy.
Type 4 are calibrated as type 1 but the last remaining liquid is “blown out” using a pipette bulb or dispenser rather than being allowed to drain. Generally used for less accurate work.