Burettes are typically long glass tubes with a tap or valve at the bottom, used to dispense very specific amounts of liquid reagent. Unlike a measuring cylinder, the graduations measure from top to bottom. Therefore the difference between the starting and final volume is equal to the amount dispensed.
Burettes are commonly used in school titration experiments where accuracy in volume is essential. Routine maintenance can prolong the life of burettes and ensure they remain accurate and functional. There are several types of burette used in schools, the most common variation is in the design of the tap or valve.
Most modern burettes use a plastic tap which sits securely between the bottom of the glass tube and the tip. These types are usually fairly robust. Other older designs may incorporate an all-glass tap with a ground glass valve. Both types use the same method of allowing the liquid in the tube to pass through a small hole very gradually, the rate of flow adjustable by hand turning the tap.
Depending on the type of burette you have, maintenance can vary. Some will separate into a graduated tube, tap and pipette shaped tip. Each part should be cleaned separately in this case. The depth of cleaning required also will vary depending upon what liquid was last used in the burette. If dilute acids have been used, a simple rinse through may be sufficient. Alkalis are not often used in burettes but if they are, visible white scale may appear on the internal walls requiring a flush through with dilute hydrochloric acid. Long handled bottle brushes can also be used to remove stubborn stains.
The tap and pipette shaped tip may retain a small amount of liquid when in use but normally this is taken into account and the graduation measurements reflect this. If you are concerned at the amount retained or if you have recently changed a tap or tip, pour a specific amount of water into the tube, open the tap and re-measure (or weigh) the liquid that comes out.
Careful storage is important so that the burettes cannot easily fall and break. Racks are available though the majority of chemistry suppliers with room for several burettes. Single brackets that may be free standing or which attach to retort stands are also available. These can consist of sprung arms which grip the tube securely or have rubber bands which can be wrapped around the tube.
Maintenance of the tap will vary from model to model but most can be opened up with either a small screwdriver or may pop apart by hand. The insides should be rinsed thoroughly and a small amount of Vaseline can be applied to any parts that rub together. Ensure no Vaseline can get into the system and affect the outcome of any experiment. If in doubt, refer to the manufacturer’s guidelines on cleaning and maintenance.