Exchange Reactions Between Zinc and Its Ions

Abstract

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.