There are two types of pipettes: air displacement and positive displacement pipettes. Air displacement pipettes are meant for general use with aqueous solutions. Positive displacement pipettes are used for highly viscous and volatile liquids. Both pipette types have a piston that moves in a cylinder or capillary. In air displacement pipettes, a certain volume of air remains between the piston and the liquid. In positive displacement pipetting, the piston is in direct contact with the liquid.
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Image credits: Wytamma Wirth