For volumetric instruments, such as bulb pipettes and graduated measuring pipettes, used for delivery of a liquid, the volume delivered is always less than the volume contained, due to the film of liquid left on the inner walls of the volumetric instrument. Most pipettes are calibrated ‘to deliver’ (TD, Ex); i.e. the delivered (not the contained) quantity of liquid corresponds to the value printed on the pipette.
Volumetric instruments – like any other measuring instrument – have a limited accuracy. Various sources of error are inherent in calibration and use. When the greatest possible accuracy is desired, the pipette should be used as closely as possible to the manner in which it has been calibrated. Methods for the testing, calibration and use of volumetric instruments made from glass are specified in the international standard ISO 4787:2010. The shown handling of a pipette including the angle of about 30° is part of the standardized calibration methods:
The pipette shall be held in a vertical position. Delivery shall be made into another glass vessel (the receiving vessel), with the receiving vessel held inclined at an angle of about 30°, so that the tip of the jet is in contact with the inner surface of the receiving vessel, above the level of any collected liquid, but without movement of one against the other throughout the delivery period, and finally drawing the tip over a distance of about 10 mm.
The resulting capillary effects influence the time required for the pipette to deliver its nominal volume and the volume of the film of liquid that is left on the inner walls of the pipette. Therefore, the volume delivered depends on the manner in which the pipette is used. Since capillary effects depend considerably on the material on which the liquid runs down, it is important that a receiving vessel made of glass is used.