The decomposition of ammonium amalgam

In extending our study of chemical kinetics, we sought systems as far removed as possible from ordinary aqueous and organic solutions. The decomposition of ammonium amalgam 

It was found impossible to measure the rate of decomposition by the evolution of gases because the release of these gas bubbles is very slow and erratic. The course of the reaction was followed by analyzing samples for the ammonium ion. Small amounts of the decomposing amalgam were forced through a capillary tube into a chilled solution of an iodate. The ammonium reacted with iodate ion to give iodide ion. The solution was then acidified with acetic acid and the iodine distilled out, collected and titrated with sodium thiosulfate. The method was checked with samples 

The facts are readily explained on the theory that decomposition of the ammonium amalgam occurs very slowly as an homogeneous reaction at temperatures below zero and that a rapid, heterogeneous reaction takes place at the surface of little droplets of liquid ammonia which are dispersed throughout the mercury. The surface tension of the mercury is sufficient to cause liquefaction of the first ammonia which is liberated. The autocatalytic effect occurs when the reaction has had time to build up these droplets of liquid ammonia. 

It was found that this heterogeneous reaction can be stopped by the addition of small amounts of lithium. It is likely that lithium with its higher electrode potential is able to prevent the self-ionization of liquid ammonia, and the heterogeneous reaction between ammonium in solution and liquid ammonia is inhibited. When this heterogeneous reaction is prevented, the decomposition follows a perfectly satisfactory second-order reaction, and the temperature must be raised up to the range from zero to 20° in order to obtain a measurable reaction rate. The reaction is satisfactorily explained on the assumption that ammonium dissociates in mercury giving ammonium ions and free electrons NH4— NH4+ -be . The older view that the ammonium exists as a free radical, NH4, seems less likely. Such a radical would be unstable and it would not be expected to have such a long life. Ammonium ions, however, are more stable. 

All the observed facts in this reaction can be explained on the assumption of ammonium ions and free electrons in the mercury solution. In a metal such as mercury there is probably a large number of free electrons, but the fact that the reaction is second order is thought to show that the reaction is 

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