7# f :d~d~d~ddd.eeee& e0 e:e:zexef, fLf*fd.ffd'fff1ffffffExperiment I - Appendix 3 VOLUMETRIC MEASUREMENT OF LIQUIDS Volumetric measurements of liquids are made with graduated cylinders, burets or transfer pipets. The graduated cylinder (Figure 1) is usually used to measure approximate volumes of liquids. Aqueous solutions wet the glass walls forming a concave meniscus. The bottom of the meniscus is used to indicate the volume of liquid. To avoid parallax error (caused by change of observational position), your eye should always be level with the meniscus when you are making a reading. The volume is estimated to one-tenth of the smallest division. The graduated cylinder is calibrated to deliver (TD) the volume that is read, and it actually contains slightly more than the volume read, thus compensating for the thin film of liquid left on the walls when the contents are poured out. The buret is used for more precise volumetric work and for titrations. If it is clean, the solution will leave an unbroken film when it drains. If drops of solution adhere to the inside of the buret, it should be cleaned with a brush, hot water, and detergent until it drains properly. Absolute cleanliness is important because the volume of a 25- or 50-ml buret can ordinarily be estimated to the nearest 0.02 ml, and the error caused by a single large drop adhering to the inside of a buret  Figure 1: The proper method of reading a meniscus to avoid parallax error. causes an error of about 0.05 ml. The presence of several drops would obviously result in poor measurement of the volume delivered. Transfer pipets are designed to deliver a single fixed volume of liquid. The graduation mark is located on the narrow part of the pipet to assure precision. They come in various sizes varying from less than 1 ml to 100 ml. They are calibrated to deliver (TD) the specific volume if they are handled in the prescribed manner. Pipets are ordinarily calibrated at room temperature or close to it. In very careful work, temperature corrections are necessary if the solution temperature is markedly different from the calibration temperature of the pipet. Fill the pipet by placing the tip in the flask of the solution and using a suction bulb to draw the liquid up past the calibration mark (Figure 2). Then slip off the bulb and, with the forefinger, quickly seal the top of the pipet before the solution drops below the calibration mark. Wipe the outside of the pipet with tissue or a clean towel, and then allow the liquid to flow out until the bottom of the meniscus is just at the calibration ring. to pick off the last drop adhering to the outside of the tip, touch the tip to the side of the flask. Then withdraw the pipet from the flask and hold it over the vessel into which the liquid is to be transferred. Allow the pipet to drain in a vertical position, with the tip against the side of the vessel. Allow 15 to 20 seconds for drainage after it appears that most of the liquid has drained out. The tip of the pipet will still contain some liquid. Ordinarily, this has been accounted for in the calibration and should not be blown out (Certain types of pipets are calibrated to be blown out--most of these will have a sandblasted ring at the top of the pipet.). Like the buret, the pipet must be scrupulously clean if precise results are to be obtained. If the pipet is still wet from cleaning, rinse with several portions of the solution to be pipetted, using the same procedure as you would for rinsing a buret. Figure 2: The procedure for using a transfer pipet. The abbreviation TD designates glassware that is calibrated to deliver the volume specified; for example, a 50-ml TD pipet.  Taken from: Student Handbook, Julian L. Roberts, Jr., and James B. Ifft, Pages 14 thru 18. -- CH 141 Lab: Expt. I CH 141 Lab: Expt. I -- !*?!?! 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