Is Photographic Development an Autocatalytic Process?

Abstract

Photographic development is commonly assumed to be autocatalytic due to the growing developed silver surface. The parallel development of grains is shown to increase the silver mass as a simple linear function of time. Its constant term corresponds to the initial soaking period. After the smaller emulsion grains become completely developed, the linearity starts to be distorted. Its distortions, caused by lining a density scale instead of a silver mass scale, by high polydispersion of microcrystals and thickness of emulsion layer, have led to the wrong conclusion of first-order development kinetics. The granular development is shown to tend also to a linear kinetics with a larger constant depending on the replacement rate of pre-adsorbed bromide by developing species from a catalytic surface. The first-order expression seems formally closer to granular kinetics that is influenced by some deviations in the exponential size distribution of latent image centres. Evidence for the constant growth rate of silver filaments, the particular and permanent activity of their ends is discussed. The latent image nanoclusters, that can be stabilized by some interactions typical to ultradisperse metals in statu nascendi and polymers, suggest themselves to be the catalytic sites of particular and permanent structure on the surface of developing silver particles.