TECHNICAL INFORMATION
406
Care and Handling of Volumetric Glassware
FILLING AND EMPTYING The results obtained with volumetric apparatus depend not only on the accuracy with which the ware is calibrated, but also on the method of use. Insofar as possible, users should manipulate the instruments in exactly the same way as the manufacturer does when locating the calibration lines. The instructions given here apply particularly to apparatus used with a liquid which wets the glass and must be followed in every respect to obtain the most accurate results when such apparatus is to be calibrated. Since mercury does not wet glass, the factors of importance are cleanliness, the use of an ascending meniscus when possible, and the tapping of the vessel before reading the meniscus. The following rules have been suggested by NIST. (These requirements may seem at times to be unduly detailed, but they are based on exact knowledge of the behavior of the various types of apparatus as determined experimentally.) Burets Burets should be held in a vertical position and filled to about 10 mm above the 0 line. Any liquid on the outside of the tip is removed at this time with clean filter paper. The setting to the 0 line is made by slowly emptying the excess liquid into a beaker or other receptacle with the side touching the edge of the tip. Any liquid remaining at the tip after setting has been made is touched-off against the wet side of the receptacle. When delivery is started, stopcocks should be completely open, even though it is necessary to slow the delivery near the end of a titration. If an exact quantity is desired, the liquid is slowed down when it is about 10 mm above the final line in order to set the meniscus accurately at this line. As soon as delivery has been completed the tip is touched to the side of the receiver and the latter removed from contact. Measurements made in laboratories with burets ordinarily are from the 0 mark. Other initial points may be used on precision apparatus without serious error, but this is not advised with burets having very rapid delivery. Cylinders In filling a cylinder the liquid is allowed to flow down one side only. However, the entire wall is wetted for about 10 mm above the meniscus, by a gentle rotation of the cylinder or, preferably, by tipping it slightly in several directions. Here, a drainage time of about 2 minutes is allowed before completing the setting. The delivery of the contents of a cylinder is accomplished in the same way as described for a flask. ( NOTE: These directions apply when highly accurate deliveries are wanted. In ordinary laboratory work where measurements are of approximate volumes only, no special precautions in filling and emptying cylinders are necessary.) You should bear in mind, however, that the accuracy you achieve is dependent on your method. Good method, consistently applied, will yield good results. Poor or sloppy methods will yield poor results. Flasks When a solution is made up to a definite strength in a volumetric flask, the entire neck usually is wet at the time the volume is adjusted, due to the mixing process. Consequently, in filling flasks the entire neck is wetted by the distilled water. A drainage time of about 2 minutes is allowed before completing the setting. If a flask is calibrated to deliver its indicated capacity, the delivery is made by gradually inclining the flask, avoiding, as much as possible, agitation of the contents and sloshing around of the liquid. The process should take half a minute. At the end of the emptying the instrument should be nearly vertical and should be held so for another half minute. Then the drop adhering to the lip should be touched off against the wet surface of the receiving vessel.
Pipets Pipets also are held in a vertical position and filled to about 10 mm above the 0 line. The technique of setting to this line is the same as used for burets. The rate of outflow at this point is controlled by slight pressure of the forefinger on the top. With MEASURING and SEROLOGICAL PIPETS, delivery is unrestricted, unless the liquid descends so rapidly that it would be impossible to stop at the desired place. In this case, however, delivery should be as fast as possible while retaining control with a finger. The tip is touched to the side of the receiver when delivery is completed and then removed immediately, except for certain types of serological pipets. These are “calibrated for blowout”, i.e., to deliver the indicated capacity when the small amount remaining in the tip after free delivery has ceased is blown out (with tip in contact with the wet side of the receiver, if possible) and added to the initial volume. As mentioned before, all blowout pipets are marked with either a wide opaque ring, two narrower opaque rings near the top, or two printed rings near the top. ( NOTE: With measuring pipets of small capacity, there may be a tendency for free delivery to cease before the liquid reaches the lowest graduation lines. If this is so, the tip should be kept in contact with the receiving vessel during the delivery, and not just to touch off the last drop when delivery is finished.) VOLUMETRIC PIPETS are held in a vertical position and outflow is unrestricted. The tip is touched to the wet surface of the receiving vessel and kept in contact with it until the water has ceased to flow. A count of two is made and the tip then withdrawn horizontally from contact with the receiver. The water remaining in the tip is not blown out, except in the case of Ostwald pipets used in biochemical work. These pipets are usually “calibrated for blowout” and are marked also with an opaque ring near the top.
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