VOL. 6,NO.10
AMMONIUM AMALGAM DEMONSTRATION
1767
LECTURE DEMONSTRATION OF AMMONIUM AMALGAM J. H. REEDY, UNIVERSITY OP ILLINOIS, URBANA, ILLINOIS
The most impressive evidence of the metallic nature of the ammonium radical is the formation of its mercury alloy, ammonium amalgam. A lecture experiment in frequent use, based on Sir Humphry Davy's method for making ammonium amalgam, consists in the addition of concentrated ammonium chloride solution to sodium amalgam in a tall cylinder. Upon stirring, the mercury swells up in a curious fashion, forming a spongy mass of about the consistency of butter. Whether the lecturer says so or not, the student usually assumes that this metallic froth is ammonium amalgam. This, however, is a debatable inference. According to Moissan,' ammonium amalgam prepared a t low temperatures is about as dense as mercury. This suggests that the buoyancy of the product is due to gases enmeshed + in the mercury. Such a view is supCwbm" ported by the work of Seely,%ho studied the compressibility of the voluminous product, and found that under pressure it behaved like a foam. For h,,4sQ,&"m reasons such as these, the frothy product is not conclusive evidence of the existMerw,J ence of ammonium amalgam. Moreover, the whole phenomenon might b e explained as due to a surface reaction between the sodium amalgam and tk ammonium chloride solution, which would be favored by the stirring and effervescence. A more convincing experiment is the transfer of the ammonium radical through mercury by means of an electric current. A convenient form of apparatus (see Figure) consists of a beaker divided into two compartments by a tube dipping into a layer of mercury a t the bottom. Ammonium sulfate solution is placed in the outer compartment, and sodium sulfate in the inner. Carbon electrodes are used, the anode being in the outer compartment containing the ammonium sulfate. A direct current, with a potential of about 10 volts, is used. After electrolyzing for five to ten minutes, the presence of ammonium in the inner compartment can be demonstrated by transferring a portion of the liquid to a beaker and adding sodium hydroxide solution. Now the ammonium group can reach the cathode only by diffusing through the mercury in the form of a dissolved metal, since ammonia and ammonium salts are quite insoluble in mercury. There may be considerable inflation of the mercury due to the decomposition of some of the ammonium groups in transit, but this would never account for the appearance of the radical in the cathode compartment. Compl. rend., 133,806 (1901). a
C k m . N m s , 21,265 (1870).
