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An automatic burette, auto-fill burette or self-zeroing burette is a type of burette which can be easily filled from a reservoir attached to the burette.
An automatic burette consists of a thick flask with ground glass joint, where a specially designed burette is positioned. This type of burette consists of a lower concentric glass tube. The upper part of the burette contains a short lateral tube with two openings, one open and the other connected to a rubber bulb pump, through which air is pumped inside the burette flask. A capillary tube carries the solution from the flask up to the scaled burette tube, filling it from top-down. A small outward tube fitted with a needle-valve ground glass or PTFE stopcock allows the draining of the burette. The solution level inside the burette is kept a fixed point (which corresponds to the number 0 from the scale) thanks to the capillary tube end being slightly above the 0 line. Some burettes have a second stopcock, which allows you to drain the solution back inside the flask directly, without having to remove it by hand from the flask and empty it through the main tap.
Automatic burettes are made of plastic or glass, transparent or amber.
How to operate an automatic burette
To fill a self-zeroing burette, follow the steps below:
- Remove the burette from the flask and carefully place it on a table.
- Fill the flask with the desired solution, but make sure the liquid fills at least 1/4 of the flask.
- Place the burette back on the flask.
- Close the release valve stopcock.
- Grab both the burette and flask neck with one hand, while covering the vent tube with the index finger or thumb (depending on the burette model) from the same hand.
- With the other hand take the rubber bulb and begin squeezing it to pump air inside the burette flask.
- If you followed the above steps correctly, the solution should begin to climb up through the capillary tube and pour inside the scaled burette and begin to fill it.
- When the level of the solution from the burette tube is above the capillary exit, slowly remove the finger and release the pressure from the flask. This will cause the solution from the capillary to drain back in the flask, which removes the excess liquid, leaving the level of the solution to match the first point of the scale (which is 0).
- Drain the solution from the tube to remove the air from the outward tube and needle-valve, then repeat the filling.
To drain the burette, you must:
- Open the stopcock and drain the solution from the burette tube in a beaker, then, when the tube is empty, pour the solution back in the flask.
- If the solution you're pouring is light sensitive or has an unpleasant odor, remove the burette tube from the flask, and then pour the solution directly inside the flask through the main release needle-valve. When the last drop has fallen, place the burette back on the flask.
- Some burettes have a secondary stopcock which allows you to drain the content of the burette directly in the flask, without having to move it.
- Do not pump air too fast as too much pressure inside the flask may cause the burette to jump from its position and the solution from the flask may splash.
- If during filling air bubbles get trapped in the burette, stop filling it and wait to see if they rise. If they remain stuck on the tube, drain the solution and fill it again. While it sometimes works, it's recommended to avoid gentle tapping the burette tube to break the trapped air bubbles, as there's a chance you might crack the glass tube. If air gets trapped on the outward draining tube, open the stopcock and drain the solution until the air is removed.
- When filling the burette with dense liquids, wear gloves and try to hold the burette tighter than usually, but not too tight as you might break it. Gently pump the liquid inside the burette and make sure you don't increase the pressure too much inside the flask.
Automatic burettes can be purchased from lab suppliers or online. They're not very cheap, but they're very useful.
Make sure the stopcock is well greased to prevent it from freezing.
When not used, the stopcock of the burettes should always be in the closed position.
Burettes with a volume of 10 ml or below, which have a small tube diameter, are often prone to clogging, especially when working with viscous solutions.