TECHNICAL INFORMATION
394
Calibration of Volumetric Ware
TECHNICAL INFO.
GRAVIMETRIC CALIBRATION CALIBRATION WITH WATER Burets
When determining the capacity of a buret, the water should always be run out from the 0 mark to the line to be checked. This is the method used by manufacturers in producing burets. Values are obtained for at least five points (every 10 mL on a 50 mL), and many laboratories calibrate at ten points to further assure accuracy. The buret and a plain glass tube of about the same length are clamped vertically on a stand as shown in Figure 2. A thermometer is placed in the plain tube, whose diameter should be somewhat greater than that of the buret so that it will hold about the same volume of water when the thermometer is immersed in it. A water reservoir, which can be made from a cylindrical separatory funnel, is useful. By keeping a supply of distilled water in it, the water will be near the temperature of the room when needed. Rubber hose is attached to the stem of the funnel and a bent glass tube is inserted into the free end of the hose. Water is easily conveyed to the two tubes in this way. For occasional calibration, a beaker of distilled water will serve. The first operation is the filling of the two tubes several times and emptying through the stopcocks, in order to bring the tubes to the temperature of water and room. They are both refilled and allowed to stand for about 30 minutes to check for leakage, not only at the tip, but also around the stopcock plug. Then both are filled again, with the level of water in the buret about 10 mm above the 0 mark. Any water on the outside of the buret tip is removed with clean filter paper. Then the water in the buret is lowered slowly to a setting at the 0 line. After setting has been made, any water on the outside of the tip is touched-off against the wet side of the container. The temperature of the water is observed just after setting the water at the 0 line. The receiver, which is kept stoppered except when actually delivering water into it, is brought under the buret tip and delivery started. The stopcock should be completely open. Keep water from splashing up into the neck or onto the tip of the buret by inclining the receiver at an angle of about 20° and touching the receiver with the buret tip, being sure that no water touches the side of the neck in the region covered by the stopper. Otherwise an error may be introduced in subsequent determinations made with the same receiver due to evaporation from the stopper when it is removed to allow another delivery to be made. When the water has descended to a few mm above the line to be checked, delivery is slowed down in order to set the meniscus accurately. After setting is completed, the tip is removed horizontally from the receiver to secure any water on the outside of the tip, and the receiver removed and stoppered. As soon as a delivery has been made, empty buret and thermometer tube and refill both, with water level in buret above 0 mark. Balance receiver, record weight, obtain volume as directed later on, and proceed to deliver next volume into receiver, using the same technique as given above. Note that the thermometer tube is handled in the same manner as the buret, to insure that water of the same temperature will be in both. With burets of sufficient diameter, such as the standard 50 mL and 100 mL types, the thermometer tube may be dispensed with and the thermometer inserted in the top of the buret itself. If this is done, the thermometer is read just before delivery and removed, more water being added if necessary to raise the level in the buret above the 0 mark. Then the procedure of setting to 0 and delivering into the receiver is the same as described originally.
Fig. 3
Powered by FlippingBook