|Pipettes are used to measure and transfer liquid accurately.
They function something like a straw; the liquid is sucked into the pipette
by mouth or with a pipette aid (e.g. pipet pump or pipet filler). There
are two types of pipettes, volumetric and serological. Volumetric pipettes
are more accurate than serological pipettes.
Volumetric pipettes are designed to measure and transfer a predetermined volume of liquid. For example a 5 ml. volumetric pipette has one marking on it. This marking measures exactly 5 ml. of liquid, no more, no less.
In contrast, serological pipettes can be used to measure and transfer various amounts of liquid. A 5 ml. serological pipette with one-tenth millimeter graduations can be used to obtain any desired volume of liquid in one-tenth millimeter increments between 0.1 ml to 5.0 ml.
Two kinds of serological pipettes are commonly employed: blow-out and drain-out. Blow-out pipettes are graduated to the tip and have etched or colored rings around the top. To obtain the desired amount, liquid in the tip of the pipette is "blown out". For example, to accurately transfer 10 ml. of liquid using a 10-ml. blow-out pipette, liquid drains from the "0" mark to as far as it will go on its own. The remaining drops of liquid are forced or "blown-out" and included in the aliquot being measured, resulting in exactly 10 ml.
Drain-out pipettes are not graduated to the tip and they lack colored or etched rings (although they may have a color-coded rectangle or square to indicate the size of the pipette). If you desire 10 ml. of liquid from a 10-ml. drain-out pipette, you allow the liquid to drain from the "0" mark to the 10-ml. mark and discard the rest of the liquid.
Pipette aids such as the pipet pump or pipet filler are used when mouth pipetting is risky, such as when pipetting hazardous or potentially infectious liquids.
Pasteur pipettes are
somewhat of a misnomer because they are merely droppers. Exact measures of liquid cannot be taken with a Pasteur pipette.