TECHNICAL INFORMATION
393
Calibration of Volumetric Ware
TECHNICAL INFO.
GRAVIMETRIC CALIBRATION When temperature corrections are not made to the results obtained in volumetric work, it is seldom necessary to know the capacity of a vessel more closely than the guaranteed tolerances. However, more precise values can be obtained in the laboratory by careful calibration, and directions are given on the following pages for determination of capacity to this higher degree of precision. The liquid regularly used by the National Institute of Standards and Technology (NIST) for calibration of apparatus is distilled water. Accordingly, manufacturers base their testing methods also on distilled water, except where some particular liquid such as mercury for calibration is described. Calibration by weighing the quantity of water delivered is recommended for apparatus made to deliver the contents through a tip such as burets and pipets. In the case of volumetric flasks, cylinders, and similar containers, gravimetric calibration is preferred, although volumetric methods may be used when quantities of glassware are to be checked. The balance must be mounted in such a way as to damp out external vibrations. The temperature of the liquid must be known accurately enough to eliminate errors from this cause. The precision needed in the temperature measurement depends on the volume of the vessel. For small articles, the nearest 1 °C is usually sufficient, while for a 1 liter flask, it may be necessary to know the temperature to the nearest 0.1 °C. The empty object to be calibrated, or the receiver into which water from the instrument being calibrated is to be delivered, is placed on one pan of the balance and weighed. The container is filled and reweighed. The difference is the apparent weight of the liquid. This apparent weight must be corrected in order to obtain the volume at 20 °C. The method of doing so will be described later. The best type of receiver for water delivered from a buret or pipet is a flat-bottomed flask. Use a tight fitting rubber stopper from which all loose particles have been removed by scrubbing with 1% sodium pyrophosphate and rinsing well. When the balance has hooks at the top of the bows, it will be found convenient to hang the vessel, such as a “to contain” (IN) pipet, from a hook. If there is no hook, then it may be necessary to rest the vessel or pipet on a saddle placed on the pan. Care must be exercised in handling the receivers to prevent moisture or grease from the fingers from being deposited on the outside. Wiping of the outside with a cloth is sometimes necessary, but in doing so surface moisture may be removed or an electrical charge may be produced on the surface. Usually about 30 minutes are required to restore the surface moisture so that it is in equilibrium with the room. Static electricity will dissipate itself in time, and frequently can be discharged by touching the glass to a clean dry conductor. With experience, settings of menisci of water at a line can be made easily to within one-tenth of a millimeter, without using anything to assist the eye. However, many observers prefer to have some magnification. Small telescopes with short focal lengths are available which can be clamped to the glass. An ordinary reading glass of 4x or 6x magnification will also serve.
Fig. 2
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