Cations of Cu, Pb and Cd were added to tabular AgBr emulsions when the emulsions were coated. Although Pb
++ and Cd
++ did not show significant desensitization, Cu
++ and Cu
+ caused considerable decrease of both surface and internal speeds of the emulsions. Since the desensitization caused by Cu
++ was in competition with that caused by phenosafranine which is known to act as electron trape, it has been suggested that Cu
++ also act as electron traps at the grain surface. It has been suggested that the desensitization observed when Cu
+ was added to the emulsions result from the formation of Cu
++ as a result of oxidation of Cu
+ in the emulsions. By introducing Cu
++ to the emulsions before or after exposure by bathing the coated emulsions in Cu++ solutions, it has been found that the desensitization caused by Cu
++ arises from its retardations of both development and latent image formation. The action of Cu
++ as electron traps is supported by the fact that the redox potential of Cu
++ is the most positive among Cu
++, Cd
++ and Pb
++.
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