calibration
1.0 Calibration of Volumetric Glassware
1.1 Introduction:
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.
1.2 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.
1.3 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.
1.4 Volumetric Glassware
Volumetric glassware is calibrated at a specific temperature, usually 20°C, but quite often it is used to deliver or
1.1 Introduction:
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.
1.2 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.
1.3 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.
1.4 Volumetric Glassware
Volumetric glassware is calibrated at a specific temperature, usually 20°C, but quite often it is used to deliver or