Graduated pipettes commonly come in 5, 10, 25 and 50 mL
volumes. To avoid accidental ingestion of potentially harmful substances, a
variety of propipetters have been developed, both entirely manual and
electrically assisted:
Originally pipettes were made of soda-lime glass, but
currently many are made of borosilicate glass; disposable graduated pipettes
are often made of polystyrene.
Graduated pipettes are often graduated in one of two
ways:
Backward or drain-out pipettes have a 0 mL mark just
above the end of the pipette. A volume is pulled into the pipette, sometimes to
the maximum volume, then the needed volume is aliquoted out.
Forward or blow-out pipettes have no 0ml mark as that
corresponds to an empty pipette. The volume needed is pulled into the pipette
and then dispensed.