Learning Objective: A buret is used to deliver a liquid or solution
in precisely measured, variable volumes. Frequently it is used in titrations,
where a reactant solution is delivered carefully to accurately reach the
end point. Here you will learn how to use a buret.
|Volumetric glassware is used for
precise and accurate volumes. The types of volumetric glassware you will
use are volumetric flasks, volumetric pipets and burets. Flasks and burets
are in your locker while burets are located at your workbench or in one
of the fume hoods.
To read the liquid level in all of these, find the bottom of the meniscus and read accurately from the graduations on the vessel. When you do this, make certain that your eye is at the same level of the meniscus to avoid error due to parallax.
Be sure to wear gloves when rinsing the buret with the solution.
Always handle burets with great care. First we will clean the buret with deonized water. Make sure the stopcock is in the closed (horizontal) position.
Pour some deionized water into a beaker and then put 10-15 mL into the
buret. Holding the buret in your hand, place your index finger over the
top and tilt it horizontally. Roll it and gently tip the buret so that
the water reaches all of the inside surface. Open the stopcock over the
sink and drain the water through the tip to wash it as well. Repeat this
step again with deionized water. Be sure the
stopcock is closed once you've emptied the buret.
The next step entails rinsing the buret with the solution the buret will contain. Transfer some solution to a beaker and then pour about 10-15 mL into the buret, like we did with the deonized water. Since we are rinsing with a solution, you should wear gloves to protect your skin.
Close off the top of the buret with your finger and then turn the buret horizontally. Roll and tilt the buret so the solution wets the entire inside surface. Since we're using a solution, we have to discard it into a waste container (beaker). Drain the solution into the beaker. Repeat this step with the solution. Remember to close the stopcock once you're finished.
Fill the buret with solution above the 0.00 mark. Since the volume measured by a buret is determined by the difference between the initial volume and the final volume, don't waste time adjusting the initial level to the zero mark. (In fact, if you try to adjust the initial reading to 0.00 mL, you are introducing a bias in your measurement.)
Notice an air gap at the bottom of the buret. The quick way to get rid of the air gap is to open and close the stopcock quickly, forcing most of the air out. Since a few small bubbles may remain, you may have to use the alternate slow method to get rid of them.
Tilt the buret on an angle simultaneously forcing the liquid to the bottom
of the buret and the air bubbles to the end with the stopcock. Position
yourself so that the stopcock end of the buret is over the waste container
and gently tilt the buret so the stopcock end comes down. Open the stopcock
and then gently ease your finger off the top end of the buret. The air
bubble will be forced out. You can now close the stopcock.
Locate and read the meniscus. Always try to read it to the one hundredth of a mL (0.00). Record the initial reading and then clamp the buret to the stand and wait 30-60 seconds before checking the reading again. This step is important to ensure the buret is not leaking. Compare the two readings; if they're the same, there are no leaks. If there are leaks, you should get a new buret.
Return the buret to the ring stand and then transfer the desired solution to a beaker. Open the stopcock partially to control the flow of the solution. Once you've transferred the desired amount, close the stopcock. Remove the buret from the stand and read the level again.
Never subtract a buret reading from 50.00
(if using a 50 mL buret) and enter the result in your lab data. Just record the actual initial buret reading, and the
actual final buret reading (which is read
once you deliver volume from the buret). The difference between these
two buret readings is the volume of solution that has been delivered from
Before starting you should check to make sure liquid will flow smoothly through the stopcock. Add some water to the buret and then open the stopcock and check the flow. If the liquid flows slowly or if the tip is clogged, ask for another buret. Also check for any leaks from the stopcock barrel. If there is one, try adjusting the lock nut on the side of the Teflon stopcock. If this does not stop the leak, replace the buret.
When you have finished titrating, empty the buret(s) into a waste container
(usually a waste beaker at your work station) and rinse well with deionized
water. Never use soap to clean a buret! Never place a buret in the sink!
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