TECHNICAL INFORMATION
396
Calibration of Volumetric Ware
TECHNICAL INFO.
VOLUMETRIC CALIBRATION & VOLUMETRIC STANDARDS Just as in gravimetric calibration, cleanliness of all vessels to be calibrated and of the standards used is necessary for accuracy in volumetric calibration. It was mentioned before that burets and pipets should be calibrated gravimetrically when accurate results are wanted. The occasional small flask also can be handled best by a gravimetric method. If large quantities of flasks or similar items are to be checked, vacuum standards similar to those developed by NIST will be useful. Figure 4 illustrates this type of standard. The bulb of the standard is calibrated gravimetrically to deliver from tip to tip slightly less than the nominal volume desired. To fill it the tube at the left is connected to a vacuum line with both stopcocks closed. The delivery tip is immersed in a container of distilled water and the left hand stopcock opened. As soon as the water fills the standard to overflow, this stopcock is closed and the container of water withdrawn. The delivery tip is wiped clean, the flask to be calibrated is brought under the tip so that the tip extends into the neck about 10 mm and the right hand stopcock is opened. The water then flows out into the flask. Any water in the cup at the top can be drawn off through the right hand tube. To complete the filling of the flask a buret with a smaller diameter than the neck of the flask is used. The capacity of the flask is the sum of the volume of the standard and the volume delivered by the buret. Burets may be used to calibrate small cylinders, centrifuge tubes, Babcock test bottles, and similar instruments. However, the buret must be selected with due regard for the precision of reading wanted. To illustrate this, suppose the 100 mL oil centrifuge tube shown in Figure 5 is to be calibrated. The accuracy specified by the American Society for Testing Materials for it at the various points is:
Limit of
Limit of
Range
Error
Range
Error
0 to
Above 3 to 5 mL
0.1 mL
0.02 mL
0.2 mL
Above 0.1
Above 5
to 0.3
0.03 mL
to 10
0.5 mL
Above 0.3
Above 10
to 1.0
0.05 mL
to 100
1.0 mL
Above 1 to 3
0.1 mL
A 50 mL or 100 mL buret is satisfactory for the upper part of the scale, but it is obvious that the first few milliliters at the bottom of the tube should be checked by a buret of about 5 mL capacity subdivided to 0.01 mL or 0.02 mL. If a large cylinder is to be calibrated at one or more points, the most convenient and accurate method for the average laboratory consists in delivering quantities from volumetric pipets of as large a size as possible, finishing up with a measuring pipet. The differences in diameters and tolerances at the calibration marks between pipets and cylinders are so great that no appreciable error will occur with the multiple deliveries necessary from the pipets to calibrate a cylinder.
Fig. 4
Fig. 5
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