TECHNICAL INFORMATION
409
Care and Handling of Volumetric Glassware
DRAINAGE TIMES FOR VOLUMETRIC GLASSWARE Burets and Measuring Pipets
Fig. 4 Volume of after-drainage in 50 mL buret for various delivery times and after-drainage times.
Amount of Drainage from Interval 0 mL to 50 mL
When a liquid which wets glass is delivered from a buret or measuring pipet, some of the liquid is retained on the inner wall. Unless delivery is extremely slow, the volume held back is greater than the amount needed to form a wetting film. The excess eventually drains down. It is customary to calibrate burets and measuring pipets for unrestricted delivery and without any waiting period after delivery before a reading is made. Other methods of calibration, such as delivery with the stopcock partially closed or the prescription of a waiting period before reading, are not practical. On the other hand, in actual titrations, a considerable time in excess of the free delivery time may elapse due to the necessity for slowing down when nearing the end-point. During this time, some of the liquid left on the wall drains and rejoins the main column. This results in a rise in the position of the meniscus and an apparent decrease in the volume delivered. For accurate work, therefore, a buret should be used which has a free delivery time long enough that after-drainage is very small. NIST has specified minimum times for tubes of different graduated lengths which meet this requirement. Kimble® burets and measuring pipets of Class A grade comply. Many chemists object to these times as being too long. However, if more rapid times are used, some sacrifice in accuracy must be accepted in many titrations. For example, the behavior of 50 mL burets calibrated for different free delivery times is shown in Figure 4. The time of 106 seconds approximates to the minimum requirement of NIST. The time of 56 seconds is approximately the average for Kimble® burets other than Class A. It can be seen that appreciable errors due to after-drainage may occur in titrations when burets are calibrated for deliveries much faster than this. One more point concerning the delivery time of burets deserves attention. Sometimes the tip or stopcock of a buret is damaged and a repair is made. Unless the original delivery time is duplicated rather closely, the accuracy of the buret may be impaired, due to a change in the delivery characteristics. Volumetric Pipets The amount of liquid retained on the wall of a volumetric pipet also depends on the delivery time. NIST has established minimum delivery times for this type of instrument, so that slight changes in technique will have the least possible effect on accuracy and reproducibility of deliveries from pipets of various capacities. To overcome uncertainty concerning the volume delivered by large tips, some authorities have advocated a waiting period before considering that delivery has been completed. However, if a certain total time is to be allowed for the delivery, more complete and consequently more reproducible deliveries will be obtained by using this time for the initial delivery and eliminating the waiting time. This is illustrated in Figure 5, which shows the total volumes delivered by a 100 mL pipet when various drainage periods are allowed after different free delivery times. The maximum delivery time allowed by NIST for a 100 mL pipet is 60 seconds. If all of this time is used in the initial delivery, 99.94 mL will be obtained from this particular pipet. To get this much with a free delivery of 30 seconds, a drainage time of 140 seconds also must be allowed, or a total time of 170 seconds. Sometimes, repairs to the tips of pipets of this type are attempted. Since the capacity is to the tip, it may be changed by the repair, even though the delivery time has not been changed. A recalibration should be made before reusing the pipet.
Drainage Time (seconds)
(V. Stott. J. Soc. GI. Tech., 169-198 (1928))
Fig. 5 Volumes delivered by 100 ML volumetric pipet for various delivery times plus various drainage periods.
Volume Delivered in ML.
Drainage Time (seconds)
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