Burets are special peices of equipment that are designed to deliver (TD) a qualtity of a liquid. Most burets are 50 mL burets, meaning that they can deliver, to the nearest 0.01 mL, 50 mL of a liquid to another container such as a beaker or an Erlenmyer flask. They are therefore, marked in tenths of a mililiter, and the user then estimates to the nearest hundredth of a mililiter. Burets are read from top to bottom. When they are "filled" to the top line the user records the level as being 0.00 mL. This is the "initial level". The level is then noted after the desired amount of liquid has been released out of the buret. This is considered the "final level" and the difference between the two levels (final level - initial level) is the volume of liquid that has been delivered to the other container.
One of the main uses of burets is for the purpose of titration. The stopcock at the bottom of the buret can be turned in such a way that the flow of liquid out of the buret can be controlled. That coupled with the very small tip at the bottom of the buret lets the user have good control of just how much liquid is delivered at any one time.
The notes and pictures below show some of the proper ways to use a buret.
The buret is securely held in a special buret clamp. The tip of the buret should be slightly below the lip of the Erylemeyer flask. Notice that the flask is sitting on a white piece of paper. This helps to see the endpoint of the titration more easily than if it was against the dark background of the buret stand base. Sometimes the flask is placed on a magnetic stirrer with a stir bar to do the stirring as the titration proceeds. This is especially helpful when only one person is handling the addition of the titrant (the solution in the buret) and trying to swirl the flask at the same time. A close look at the liquid level in a buret shows very clearly where the meniscus is located. The meniscus is the curve in the liquid level. The lowermost portion of the meniscus is the what is recorded as the "buret level". Almost all burets are markin tenths of a milliliter, therefore all levels are recorded to the nearest 0.01 mL. Many people often place a white or colored index card behind the buret to help them see the meniscus better. This is the endpoint you will see when you are titrating a weak acid, such as acetic acid, with a strong base, such as sodium hydroxide, and are using phenolphthalein as the indicator. The color should be light pink and last for at least 30 seconds while being stirred.
Send questions, comments or suggestions to
Gwen Sibert, at the
Roanoke Valley Governor's School
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