A buret is a piece of laboratory equipment used for measuring liquid when accuracy is critical. The buret, along with the piston assembly with which it's almost always used, looks like this:

         _____   (A)
          | |  
          +_+
         |   |
         |   |\
         |   |||
         |   |/  (B)
         |   |
         |   |
         |___|
          | |
          | |
          |-|
          | |
          |-|
          | |    (C)
          |-|
          | |
          |-|
          | |
          |-|
          | |
          |-|
          | |
          | |
         -+-+-   (D)
           ||
            |    (E)

The opening at the bottom (E) is dipped into a flask of fluid, and the fluid is introduced into the buret by either pulling up on the piston for speed (A) or rotating the wheel (B), which moves the piston in smaller increments, for accuracy. The fluid enters the main part of the buret (C), which is graduated, usually to an accuracy of 0.1 mL. When the desired volume is reached, the stopcock (D) is turned to the "closed" position"—perpendicular to the buret. When dispensing the fluid, the stopcock is opened and the piston depressed until all fluid has been dispensed. (Note: If the buret is being used to dispense the unknown volume in a titration, no depression of the piston is necessary; open the stopcock evacuate the necessary volume by gravity-feed, and calculate the dispensed volume.)

Burets are used for high-accuracy experiments, such as titration, as well as calibration of other instruments. Burets are made of glass, or a glass-like borosilicate (Pyrex). They come in a variety of volumes (10, 25, and 50 mL are common), but since the piston assembly is removable, one piston can be used for multiple burets.

While in use, the buret is supported by a buret stand, a nifty little device that's extremely hard to describe. A rough diagram:


 B
 B___/____|
 /  /A    |
OB /      |
 BO       |
 B        | C
 B        |
 B        |
 BO       |
OB \      |
 \__\_____|
 B  A\
 B

The points labeled "A" are the tension points for two clamps, the ends of which are round rubber pieces represented by "O"s. The buret is that line of "B"s; were the buret not there, the pairs of Os would be touching. When the buret is in place, the rubber pieces provide enough friction to keep the buret from falling out. C is just the support, which extends down to the base that sits on the lab table.

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