Calibration of Volumetric Glassware
Calibration is the process by which a stated measure such as the volume of a container is checked for accuracy. In general, measurements of mass can be determined more precisely and accurately than measurements of volume. Therefore, the mass of the liquid contained or dispensed by the glassware will be measured and the corresponding volume calculated using the density of the liquid. However, a relatively small change in temperature causes a change in the liquid’s volume and thus its density.
To Contain vs To Deliver
Volumetric glassware is calibrated either to contain (TC) or to deliver (TD) the stated volume. Beakers and graduated cylinders are generally calibrated to contain while most pipettes and burettes are calibrated to deliver.
The Analytical Balance
The basic measuring device in the laboratory is the analytical balance. The accuracy of the counterweights inside the balance is much better than one part per thousand and the balances are serviced and calibrated at regular intervals to ensure their accuracy.
In the most accurate work two corrections are required. One is to correct for difference between an object weighed in air and the same object weighed in vacuum. According Archimedes’ principle an object is buoyed up by a force equal to the weight of air it displaces. Second, is the fact that glass expands with increasing temperature, so the volume of a container also increases. By convention, volumetric glassware is always calibrated at 20°C. Since the temperature at which the calibration is done may be somewhat different there is a small correction for the cubic coefficient of expansion of glass. Fortunately the correction is very small within a few degrees of 20°C and can be neglected in ordinary work.
Volumetric glassware is calibrated at a specific temperature, usually 20°C, but quite often it is used to deliver or contain volumes at a different temperature. The temperature variations make it necessary to adjust samples and/or standards to the calibration temperature before measurement, or to apply temperature corrections to the volumes measured. Glassware that is designed to deliver specific volumes also may have specific drain time associated with its calibrated volume and must be scrupulously cleaned to drain properly. Therefore it can be seen that variances in volumetric measurements can be a major, if not the chief, source of error in an analytical laboratory.
There are three general methods commonly employed to calibrate glassware:
Direct, absolute calibration
Indirect, absolute calibration
A volume of water delivered by a burette or contained in a volumetric flask is obtained directly from the weight of the water and its density. For example, if at 25°C, a 20.00 ml pipette delivered 19.970 g of water then the volume delivered at 25°C would be 19.970 g x 1.0040 ml/g = 20.05 ml. At 20°C the volume would be 19.970g x 1.0037 ml/g = 20.04mL. (See table in Section 3.0 on page 3 for temperature specific values for water density)
Volumetric glassware can be calibrated by comparison of the mass of water it contains or delivers at a particular temperature with that of another vessel which had been calibrated directly. The volumes are directly related to the masses of water. This method is convenient if many pieces of glassware are to be calibrated.
It is often necessary to know only the volumetric relationship between two items of glassware without knowing the absolute volume of either. The situation arises, for example, in taking an aliquot portion of a solution. Suppose that it is desired to titrate one-fifth of an unknown sample, the unknown sample might be dissolved and diluted to volume in a 250 ml volumetric flak. A 50...
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