The Action of Ferricyanide on Sulphur-Sensitized Emulsions

Abstract

It is shown that sulphur-sensitized emulsions may be fogged or desensitized by ferricyanide, according to the concentration, provided that a sufficient supply of silver ions is made available. The distinction previously drawn betwsen this form of sensitization and silver-sensitization on the basis of response to ferricyanide is thus weakened.⁽⁷⁾ It is suggested that silver ions are adsorbed to sensitizing substance and promote adsorption of ferricyanide. Some results on the absorption of ferricyanide to unsensitized emulsion grains at various pAg levels are given. Slight effects can also be produced in unsensitized emulsions, pointing to the presence of foreign centres created during physical ripening. A small part of the sensitivity created by digestion, which resists the action of ferricyanide, is attributed to etching by the sensitizer. The amount of ferricyanide required to desensitize an emulsion at low pAg is slightly greater than that needed for latent image destruction. The response of digested emulsion to small amounts of ferricyanide (production of fog) would probably enable a very sensitive test for the latter to be worked out. In gensral, a parallel between the action of light and ferricyanide is noted.