Silver halide, developing agents

There are many chemical agents that will reduce silver halide to metallic silver, but developing agents are a special kind of reducing agent because they act only on silver halide that has been exposed to light. 

Dye release may relate either directly or inversely to the image-related reduction of silver halide. Release of the dye or dye precursor may be accomphshed or initiated by the oxidized developing agent or the unoxidized developing agent or by alkali or silver salts. 

Other detection modes in bright CL or BL reactions are multichannel detectors, which provide simultaneous detection of the dispersed radiation and produce a permanent image of a wide area. Photographic films or plates are emulsions that contain silver halide crystals in which incident photons produce stable clusters of silver atoms within the crystals. Internal amplification is provided in the development process by an electron donor that reduces the remaining silver ions to silver atoms within the exposed crystals. A complexing agent is used to remove the... 

Sheppard s mechanism was formulated primarily in reference to direct development (Sheppard, 3 Sheppard and Meyer, 3a) and he assumed that the complex was formed in the act of or as a result of adsorption of the developing agent by the silver halide. Since he did not specifically suggest application of the basic mechanism to physical development, consideration of his mechanism will be deferred until direct development is treated in detail. 

Several writers have expressed the view that the silver nucleus acts as an electrode which is charged by the developing agent. A cardinal postulate in such mechanisms is that the electrons can be transferred from the developing agent to the silver nucleus at any point where its surface is in contact with the developer solution. Silver ions are reduced primarily at the silver/silver halide interface. 

The suggested mechanisms differ in detail (Mott, 66, 67 Berg, 68 Anastasevich, 69 Frank-Kamenetskii, 70 Bagdasar yan, 17, 71) but all involve the idea that electrons can be transferred to silver much more readily than to a silver halide crystal. Each mechanism can be criticized on some detail (cf. Sheppard, 15 James, 72). As a general criticism, however, none of the mechanisms has explained the fact that the rate of development under simplified conditions varies with the square root of the hydroquinone and catechol concentrations, whereas the rate of reduction of silver ions from solution by the same agents varies as the first or somewhat higher power of the concentration. 

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. 

Internal latent image is a latent image formed inside the grains. It can be developed by a conventional developer to which is added a silver halide solvent or an agent that promotes recrystallization of the silver halide to allow the developer to make contact with the latent image centers. 

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