In the Laboratory
A Manageable Measurement Method for Sodium Mario Branca and Steven Loiselle Dipartimento di Chimica, Università di Sassari, Via Vienna 2, 07100 Sassari, Italy
The use of sodium in chemical research is hampered by its reactivity in moist air, which leads to difficult handling and measurement methods. Shriver (1) and King (2) have described two possible methods, the latter using capillary glass tubing. We describe a much simpler method based on the same principle but designed for smaller quantities. In our method, we use standard disposable microcapillary pipets that have been sealed at one end (Fig. 1). These glass micropipets are commonly available at low cost and can be found in sizes from 0.25 µL to 100 µL. Using proper hand and eye protection, the disposable micropipet is end-sealed using a minitorch. INERT GAS
VACUUM
MICRO-PIPETTES
SODIUM
Figure 1. Micropipet filling system for sodium.
The approximate quantity of sodium is removed from its storage container and washed with dry light petroleum ether to remove the oil in which it is stored. The cleaned and superficially oxidized piece of sodium, dried and handled under a stream of inert gas (nitrogen or argon), is loaded into a pyrex test tube with our specially prepared micropipets, open end down, as can be seen in the figure. The jointed test tube is closed with a three-way stopcock and flushed with inert gas. We evacuate to 20 mbar
with clean liquid sodium and the oxidized sodium is using a standard vacuum pump (do not use a water vacuum) and carefully melt the sodium (mp = 97.8 °C) using a heat gun. After liquefaction, we inject a quantity of inert gas, bringing the test-tube pressure to atmospheric. The pressure increase over the sodium causes the liquid to fill the evacuated micropipet completely. The micropipet is filled left behind in the test tube. As the sodium in the test tube solidifies, the test tube can now be opened and the micropipets removed. The solidified sodium on the exterior of the micropipet should now be carefully removed and treated along with the remaining sodium in the test tube using isopropanol. The solid sodium in the micropipets can now be used as desired. The open end of the micropipet has been closed with a thin layer of oxidized sodium. For longer storage the open end of the pipet can be submerged in oil. As these pipets are made of thin glass, storage should protect the tubes from rupture. As the volume of liquid is a function of tube length, a required volume and therefore a required mass (density = 0.97 g/mL) can be obtained by using a specific tube length. Tube lengths can be measured exactly using a microscope and internal diameters are supplied by the manufacturer. The measured quantity of sodium can now be used by fracturing the pipet within the reaction container using the magnet-hammer method or by sublimation. As with all procedures using glass, proper hand and eye protection should be used. Literature Cited 1. Shiver, D.F. The Manipulation of Air Sensitive Compounds; McGraw-Hill: New York, 1969; pp 98–100. 2. King, B. J. Chem. Educ. 1982, 59, 867.
Vol. 73 No. 9 September 1996 • Journal of Chemical Education
857
