Simultaneous direct and physical development

Under the usual conditions of commercial practice, the development reaction does not occur entirely at the silver/silver halide interface. Some reduction of silver ions from solution takes place. Such reduction presumably can occur at any point on the silver/solution interface, and the mechanism should be the same as that for post-fixation physical development. The relative extent of the physical development in comparison with that at the silver/silver halide interface will depend upon the silver halide solvent action of the developing solution and upon the rate of the direct development. 

The silver halide solvent, sulfite ion, is always present in practical developing solutions which employ organic agents. The conventional solutions contain up to 100 g. sodium sulfite per liter. The action of the sulfite is manifold. It is added primarily to decrease the rate of loss of developing agent by aerial oxidation and to prevent the accumulation of quinone or quinonelike oxidation products of the developing agents, A third phase of its activity, the solvent action, is well-known but the extent to which it can alter the nature of the development process under proper conditions is often overlooked. 

A rather extreme example of the alteration of development by solvent action is given by some experiments with the hydroxylamine 

Effect of Sulfite upon Hydroxylamine Development NH2OH, 0.04 M pH, 10.8 excess Br, 0.00067 M 

To account for the sharp drop in rate at the higher sulfite concentrations, it was suggested that the solvent action of the sulfite can isolate the latent image nuclei from the main body of the grains, so that development can proceed only by the slower physical development process. 

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