Tolerance buret

It is possible to check the calibration of a pipet, flask, or buret. The process involves weighing with a calibrated analytical balance. The volume of water (temperature noted) delivered or contained by the glassware is weighed. Then the analyst converts this weight to volume (using the density of water at the temperature noted), corrects the result to 20°C (the usual temperature of the factory calibration), and compares it to the factory calibration. If the difference is not tolerable, the piece of glassware is either not used for accurate work or a correction factor is applied. It should be pointed out that the thermometers used must be properly calibrated and that the timer used to measure the delivery time for the burets and pipets must also be calibrated. 

Pipets deliver known volumes of liquid. The transfer pipet in Figure 2-1 la is calibrated to deliver one fixed volume. The last drop does not drain out of the pipet and should not be blown out. The measuring pipet in Figure 2-1 lb is calibrated like a buret. It is used to deliver a variable volume, such as 5.6 mL, by starting delivery at the 1.0-mL mark and terminating at the 6.6-mL mark. The transfer pipet is more accurate, with tolerances listed in Table 2-4. 

Another systematic error arises from an uncalibrated buret. The manufacturer s tolerance for a Class A 50-mL buret is 0.05 mL. When you think you have delivered 29.43 mL, the real volume could be anywhere from 29.38 to 29.48 mL and still be within tolerance. One way to correct for an error of this type is to construct a calibration curve, such as that in Figure 3-3, by the procedure on page 38. To do this, deliver distilled water from the buret into a flask and weigh it. Determine the volume of water from its mass by using Table 2-7. Figure 3-3 tells us to apply a correction factor of —0.03 mL to the measured value of 29.43 mL. The actual volume delivered is 29.43 — 0.03 = 29.40 mL. 

Burettes are very seldom tip- or end-heat strengthened as are pipettes, and they are therefore more prone to chipping or cracking. Burettes with removable tips and/or stopcocks can be useful for salvaging burettes that otherwise would be thrown away. Because the burette s calibration is exclusively on the column, removable tips and stopcocks have no effect on the buret s tolerance quality, nor should they imply the level of quality. 

Standards of Accuracy The capacity tolerances for volumetric flasks, transfer pipets, and burets are those accepted by the National Institute of Standards and Technology (Class A),1 as indicated in the accompanying tables. Use Class A volumetric apparatus unless otherwise specified in the individual monograph. For plastic volumetric apparatus, the accepted capacity tolerances are Class B.2... 

Also calibrate the entire apparatus. Put 400 mL of dry (0.02 % water maximum) xylene in the apparatus and test in accordance with Section 9. When complete, discard the contents of the trap and add 1.00 0.01 mL of distilled water from the buret or micro-pipet, at 20 C, directly to the distillation flask and test in accordance with Section 9. Repeat 8.1.2 and add 4.50 0.01 mL directly to the flask.The assembly of the apparatus is satisfactory only if trap readings are within the tolerances specified here ... 

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