Eliminate any air bubbles in the buret tip by open ing the stopcock completely

Eliminate any air bubbles in the buret tip by open

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Eliminate any air bubbles in the buret tip by open- ing the stopcock completely for a second or two, in or- der to quickly release a little solution and any bubbles into the discard container. Then drain more solution until the bottom of the meniscus aligns with, or is slightly below, the 0.00-mL mark. It is not necessary to exactly align the liquid level with the 0.00-mL mark, as long as you read and record the liquid level at this point, before you proceed further. Finally, touch the buret tip to a wet glass surface to remove the hanging drop. Note: Buret calibrations increase in value from top to bottom, the reverse of most volumetric ware calibration scales. A 50-mL buret is calibrated in 0.1-mL units, but one-fifth of a unit can be reproducibly estimated. Therefore, you should always estimate the liquid level if it is between calibration marks, and record ev- ery reading to the nearest 0.02 mL. Read the liquid level in the buret with great care. To facilitate more accurate reading, use a white card with a dark stripe on it. Hold the card directly behind the buret, with the top of the black stripe slightly be- low the meniscus, as shown in Figure 12 on the next page. Burets with glass stopcocks . Burets with glass stopcocks must be manipulated carefully, to TECH 511/Laboratory Techniques: Measuring the Volume of Liquids 9 © 1999 Chemical Education Resources
avoid loosening the stopcock plug and causing a leak. Using your right hand to operate a stopcock handle oriented on the right side of the buret usually loosens the stopcock. Figure 13 shows the preferred method. In the figure, note that, although the stop- cock handle is on the right, it is being manipulated with the left hand. While turning the handle, you should apply light pressure toward the buret with your left hand, to prevent the plug from slipping out of the barrel. However, be careful not to touch your left palm to the stopcock or the plug could loosen. This procedure leaves the right hand free to swirl the solu- tion in the receiving flask. Burets with Teflon stopcocks. When using burets with Teflon stopcocks, you can operate the stopcock with either hand, without any risk of the plug slipping out. You should drain solution from a buret slowly so that the film of solution on the buret wall will drain reproducibly. After you have drained the desired vol- ume, touch a glass stirring rod to the buret tip, to re- move the hanging drop. Use a wash bottle to rinse the stirring rod with distilled water, allowing the rinses to run into the receiving flask. After a brief pause, re- cord the final buret reading. When you are finished using the buret, drain the excess solution and dispose of it as instructed. Rinse the buret thoroughly with tap water, then with distilled water. Clamp it to a ring stand with the stopcock open, and let it drain dry.

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