An Alkali Metal Sampler The alkali metals, especially sodium and potassium, are used extensively in chemical research as powerful reducing agents. Because the metals react vigorously with moist air, special handling methods are necessary in order t o prepare clean, pure, and accurate quantities of metal. Shriver' has descrihed a method for introducine a weiehed auantitv of oure metal into thin-walled bulbs. Another convenient approach is to draw the molten metal up into capillary glass tuhing and cut measured lengths of the metal-in-glass tubing. The metal is then sublimed or dissolved out. Here we describe in detail haw pure potassium or sodium may he drawn up into glass capillary tuhing. Potassium and sodium are usually stored under paraffin oil or other inert hydrocarhon. Chunks of metal are cut and washed-two or three times with petroleum ether or ligroin to remove adhering oil. Any oxidized surface layer is scraped off with a knife. The chunks are charged into the bulb B of the sampler while it is flushed with nitrogen or argon. The unit is sealed off a t A and the whole system evacuated, with stopcock D open. When a high vacuum is attained (better than l O W torr), the bulbB is flamed gently with a heat gun or immersed in an oil bath todegasand melt the metal, which then runs down and enters the capillary tube. The melt is pumped on until a high vacuum is again attained. The sampler is then isolated from the vacuum and filled with argon or nitrogen (with stopcock D still open) to a pressure which is determined hy the Length of capillary tubing. (For K metal and a length of 500 mm, the pressure is calculated as 30 to&.) Stopcock D is then closed, the capillary tuhing F is heated (to prevent premature cooling of the melt), and a vacuum is gently aoolied. Pure..clean . metal rises into the caoillam. -. leavine the dross behind in the bulb. (Bulb E serves as a trap for molten metal if it should rise too far.) The unit is then cwled slowly to r w m temperature, the vacuum is shut off, stopcock D is opened, and inert gas is admitted to a pressure of 1atm. The capillary tuhing is then sealed off a t C while the compression fitting a t H is disconnected. The metal-in-glass tubing is severed a t G and the end protected from the atmosphere with a rubber septum. The rest of the unit is made free of alkali metal by "killing" the metal with isopropanol. ~~~~~
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Shriver, D. F., "The Manipulation ofAir-Sensitive Compounds,"McGraw-Hill, New York, 1969, pp. 98-100. The relationship between the pressure difference (AP) across the two arms of the unit and the height (h) of molten metal is given by the usual manometer equation.
AP g x h -=P gc where g = acceleration due to gravity, g, = gravitational constant, and p = density of molten metal. This equation reduces to: A P = 0.073 ph where AP is in t o n , p is i n g cm+, and h in mm. Baldwin King Drew Universiw Madison, NJ 07940
Volume 59
Number 10 October 1982
867
