MARCH, 1952
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AN APPARATUS FOR STORING AND MEASURING SOLUTIONS OF LITHIUM ALUMINUM HYDRIDE' CLYDE R. DILLARD Tennessee A & I State College, Nashville, Tennessee '
LITHIUM aluminum hydride has assumed an increasingly important role as a chemical reducing agent. More than 200 papersz concerning its reactions and those of related alkali-metal borohydrides have been published since Schlesinger and his co-workers announced their first results less than five years ago.% Lithium aluminum hydride is prepared by the reaction of lithium hydride with aluminum chloride according to the equation: 4LiH
+ AlCL
-
3LiCl
+ LiAIH6
I t is available commercially as an ether-soluble grayPresented before the Division of Organic Chemistry at the Southwide Chemical Conference at Wilson Dam, Alabama, October 19,1951. Chem. Em.News.29. 1203 (19511. . ' FINHOLT,"A.E., A. C: BOND,AND H. I. SCHLESINGER, J. Am. Chem. Soc., 69,1199 (1947).
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white powder. The solubility of this compound in ethyl ether is 25 to 30 g. per 100 g. of pure solvent; however, it is usually desirable to use more dilute solutions. The chief drawback to the use of lithium aluminum hydride is its extreme reactivity. Operations such as weighing, filtering, or transfer of solutions must be done in an inert atmosphere or under vacuum. Moreover it is inconvenient to store large amounts of solution or to measure out exact aliquots from a stock solution. In several instances violent explosions have occurred when reactions involving lithium aluminum hydride were carried out in ether ~olution.~. I t is the purpose of this paper to describe an appara-
' BARBARAS. G. K.. G. BARBARAB. A. E.
FINHOLT.AND H. I. m . ' ~ h e Soe., m . 70; 877 (1948). Chem. Eng. News,29,3042 (1951).
SCHLESINUER, J. ~
JOURNAL OF CHEMICAL EDUCATION
cocks, S-1 and 8-4;one T-bored three-way stop-cock, 8-2;and one oblique-bore three-way stopcock, S-8.
tus in which lithium aluminum hydride solutions may be stored for long periods of time without decomposition, and from which exact aliquots may be conveniently withdrawn. The apparatus is shown in the figure. Its chief parts are a liter storage flask, A; a 25-ml. buret, B; an ether reservoir, C; two 1-mm. straight-bore stop-
The buret is made from a 25-ml. pyrex graduated pipet and is attached to the storage flask by means of a 24/40 3 joint. The joint and stopcocks are lubricated with ether-resistant silicone grease. (Stopcock 8-3 should be lubricated regularly once a week.) For purposes of safety, the apparatus is set in a container of dry sand and the nitrogen inlet is connected by means of a Ttube to a mercu~y-filledpressure regulator (not shown in the figure). The ether solution of lithium aluminum hydride is filtered directly into flask A,under a stream of nitrogen. Then the buret, B, is put in place with stopcock S-1 open and stopcocks S-2and S-S turned so that nitrogen flushes out the buret. 8-1 is then closed and S-3 is turned to communicate with the storage flask, A. A solution thus stored a t room temperature under an atmosphere of nitrogen will remain clear and practically unchanged in composition for several months. The procedure for withdrawing aliquots is as follows: The ether reservoir, C, is filled with dry ether and closed with a CaCL tube. Stopcock 8-2is turned to connect the buret with the atmosphere and nitrogen is admitted through S-1 forcing solution up through S-Sinto the buret. 8-1and S-3are closed in that order. A sufficient quantity of solution is withdrawn through SS to fill the capillary delivery tube and the initial buret reading is taken. The desired aliquot is withdrawn into a receiver which may be simultaneously flushed with a slow stream of nitrogen. Then the final buret reading is taken and the excess solution is drained back into flask A through S-3. 8-5 is closed and a small amount of flush ether is admitted through 8-4 and withdrawn through the capillary delivery tube. This procedure is repeated several times to wash out the buret completely. Then, with 8-4 closed, dry nitrogen is blown through S-1,8-2,and out of 8-3. Finally, S-S is turned to communicate with the storage flask, A, S-1 is closed, and the solution is again left under an inert atmosphere.
