The Site of Reaction in Direct Photographic Development

Abstract

The rate of the silver‐catalyzed reduction of silver ions derived from soluble silver salts is directly proportional to the concentration of the hydroquinone when the latter is the reducing agent. The kinetics thus give no evidence that adsorption of hydroquinone by silver is a pre‐requisite for reaction. Similar results are obtained with catechol as reducing agent. The rate of development of silver bromide grains, on the other hand, varies approximately with the 0.6 power of the hydroquinone concentration (pH 8.0–8.9) and the 0.56 power of the catechol concentration (pH 7.7), indicating adsorption by the silver bromide or by the silver‐silver bromide interface region prior to reaction. This result is in contradiction with the ``electrode'' mechanism of development, but is in accord with the concept of a direct reaction between the developing agent and silver halide at the triple interface.