The exchange reactions between silver and its ions in solution have been investigated by the use of radioactive tracers. Silver of highest purity was cleaned carefully and immersed into a radioactive silver nitrate solution. The course of the early absorption process and subsequent diffusion of the ions into the interior of the metal was followed in detail, and the diffusion coefficient a t room temperature was calculated. The penetration depth was determined by controlled electropolishing of the metal, and the reverse movement of active ions from the metal into inactive solutions was also observed. INTRODUCTIONThe exchange reaction between metallic silver and its ions in solution has been investigated by several authors. This metal is of particular interest because it is available in high purity, is chemically inert, and has a convenient radioactive isotope with a half-life of 270 days.Rollin (1) found that metallic silver, shaken with a silver nitrate solution containing radioactive silver, acquired an activity correspondiilg to more than 100 apparent atomic layers of silver. Baerg and Winkler (2) found that "abraded" and etched foil exchanged to different depths. They measured the "true" area by the Bowden-Rideal method (3) and tried to correlate surface area and exchange, but concluded that "the metal surfaces were poorly reproducible in respect to both exchange and area". Gerischer and Vielstich (4) observed that after a rapid initial exchange, which depended on the surface treatment, a second, slower increase in observed activity took place, and they attributed this increase to solid-state diffusion.Tingley, Henderson, and Coffin ( 5 ) concluded from their work that generally there are two mechanisms acting in silver exchange reactions, one "kinetic" and the other a "true exchange"; which one of these was acting depended on the state of the surface.King and iMcKinney (6) recently also found evidence that solid-state diffusion occurs in silver, accompanying the exchange reaction. A theoretical study of the heterogeneous reaction was done by Zimeils (7). He devised equations relating the progress of the solidliquid exchange with time, assuming that one of the followiilg processes is the ratedetermining one: ( I ) diffusion across the interface, (2) chemical reaction a t the phase boundary, and (3) diffusion of the exchanging atoms into the interior of the solid. The rate laws for these processes are for~nally similar and are usually of an exponential nature.I t was the purpose of the present experiments to study the exchange between silver and its ions, following Zimens' model. 'Manuscript
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The exchange reaction between zinc metal and zinc ions in solution has been studied by means of radioactive zinc-65. Polycrystalline zinc and bicrystals of zinc were used under conditions where the radioactive material was incorporated either in the solid or the liquid phase. Strain-free clean surfaces were obtained and autoradiographs taken.The results of the experiments using bicrystals were interpreted on the basis that a local cell action sets up an electrochemical potential between liquid and solid a t the beginning of the reaction. Exchange was found to take place in both directions, but the reaction did not follow a simple logarithmic law. INTRODUCTIONThe extensive use of radioisotopes has made possible the study of many exchange reactions. In 1915, Hevesy ( I ) showed that in the system Pb++/Pb(N03)2 a deep exchange talres place in a very short period of time, i.e. more than one monolayer exchanges, and this was found later to happen in other metals, too. Various mechanisms have been suggested for such exchange reactions, but no conclusive explanation has yet been p u t forward. In the general case one expects more or less rapid exchange in the surface layer, followed by slow penetration to the interior of the metal a t a speed controlled by the diffusion coefficient and orientation of the crystals.The heterogeneous exchange reaction between zinc and .its ions has been studied by Rollin (2) in zinc dust, by Gaudin and Vincent (3), Haenney and Mivelaz (4), Matsura (5), and King and Evans (6), in both zinc dust and zinc foil; and Bushmanov and Vozdvizhenlry (7) in zinc single crystals. There are collsiderable variations in the results obtained by the different authors in this field, due to the fact that some factors, such a s surface preparation, oxidation, corrosion, are not always fully considered.In this investigation, zinc metal was dipped in different solutions of zinc salt to show that the exchange reaction ZnO,,,,, @ Zn++,,l takes place in both directions. This was done by labeling either the metal or the zinc solution with radioactive zinc-65, and following the change in activity for different times of immersion. The influence of oxide or hydroxide on the exchange was also investigated. Finally, the exchange reaction between zinc bicrystals and carrier-free solutions of ZnG was studied.The following factors were considered of importance in this study: (1) preparing undistorted zinc surfaces; (2) working in an oxygen-free atmosphere; (3) minimizing of corrosion and adsorption. I t is known that zinc is a soft metal, easily worlred, and t h a t the depth of distorted metal after mechanical polishing may commollly extend to a t 'Manuscript received M a y I S , 1960.
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