The exchange of silver ions between the (111), (100), (110), and (311) surfaces of single-crystal silver and silver nitrate solutions has been studied, using Ag 110 as tracer. The initial step is very rapid and is probably largely adsorption, since much of the radioactivity acquired during the first few seconds can be removed by soaking in distilled water. After about 15 rain the ratecontrolling process appears to be diffusion in the solid. In 0.01M AgNO~ solution the radioactivity uptake continues steadily if the surfaces are frequently desorbed with distilled water, but if the adsorbed ions are allowed to remain, the rate of uptake is reduced. In 0.1M AgNO8 solution the radioactivity on the surfaces reaches constant values after 4 days, whether or not desorption is carried out. The penetration profile and diffusion coefficient in the metal were obtained by "chemical sectioning." It was found that diffusion occurred by means of "dislocation pipes," and resembled grain boundary diffusion rather than lattice diffusion by a vacancy mechanism which occurs at high temperatures.The exchange of silver ions between polycrystalline silver metal surfaces and aqueous solutions of silver salts has been investigated by a number of workers using radioactive silver isotopes as tracers (1-11). In general, all agree that a rapid exchange involving only a few atom layers takes place in the first few seconds, followed by a slower process which is affected by the concentration of the solution, the state of the surface, and the temperature. This slower uptake of ions from the solution is held to be controlled by diffusion in the solid. King and Levy (2) found that surfaces that were removed from the solution, and washed frequently, underwent more exchange than surfaces that were left undisturbed; they attributed this to the removal of adsorbed salt that slowed down the exchange. However, King and McKinney (3) concluded that the increased exchange was due, more likely, to increased diffusion caused by a rise in temperature due to the hot water used in the washing. They calculated a grain boundary diffusion coefficient of the order of 10 -21 cm2/sec. Sandor (8), who removed the radioactivity by electroetching, obtained a value of 1.37 x 10 -20 cm2/sec for the grain boundary diffusion coefficient.The present work was undertaken to ascertain the behavior of specific single-crystal surfaces with respect to the exchange of silver ions with aqueous silver nitrate solutions, and to obtain a value for the room temperature volume diffusion coefficient.
Experimental DetailsThe single crystals of silver were grown in a horizontal furnace by the method of Chalmers (12), using 99.999% pure silver obtained from Johnson, Matthey and Mallory Limited, Toronto. The crystals were then sectioned and oriented along the desired direction, and the resulting surfaces were electroetched and/or chemically etched and polished, as described below. Most cuts were made using a spark cutter, but two specimens were cut by means of an "acid saw." The orientation of ...