Latent image nuclei

Some physical development may result from a selective deposition upon the latent image nuclei of silver formed in the homogeneous reaction between silver ions and developing agent. Under the usual conditions of development, however, catalysis of the actual reduction of silver ions is the important factor. 

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. 

The solvent action of sulfite on silver bromide and the resultant tendency to isolate latent image nuclei from the grain accounts for the failure of sulfite itself to act as a direct developer in spite of the autocatalytic character of its reduction of silver ion. The active nuclei simply are isolated from the grain before development gets under way. The same phenomenon enters to prevent sulfite-containing hydroquinone solutions of low pH (e.g., 8.5) from developing readily even though the thermodynamic conditions are suitable for reaction and the hydroquinone develops readily at the same pH when sulfite is absent. 

The silver nuclei constitute an invisible latent image, which can be converted to a visible image by chemical action. The silver halide crystals that contain latent image nuclei can be reduced completely to silver by this chemical action (development) or silver ions from solution can be reduced at these nuclei. The amplification factor... 

The number of grains per unit area of the coating in which latent image nuclei are formed is related to the amount of light absorbed in that area. 

Physical development is another type of development, in which soluble and usually complexed silver ion is reduced from solution onto nuclei, which can be latent image nuclei, other suitable nuclei, or silver produced by chemical development. Chemical development and physical development are traditional terms derived... 

In the classical theory of Ostwald, Abegg, and Schaum [96] the homogeneous reduction of silver ion is assumed to be rapid and is followed by the physical deposition of silver on a latent image nucleus from a supersaturated solution of silver. The term physical development arises from this description and developers used at this time often deliberately contained soluble silver ion. It is now considered that physical and chemical development are both chemical, or electrochemical, processes in which silver ion reduction occurs at the latent image surface. 

If the lifetime of a single silver atom in or on the silver halide grain is only a small fraction of a second, as assumed in some detailed formulations of latent image growth, silver atoms could form, dissociate, re-form, and so on, many times before the final formation of silver at a growing nucleus site. If, however, the lifetime is of the order of seconds, the dissociation and re-formation sequence is less attractive as a means of transporting silver atoms on or in the crystal. An alternative is the migration of the undissociated silver atom... 

The latent image catalyzes the reduction of silver ion either from the solid silver halide phase, as in chemical development, or from a soluble source of silver ion, as in physical development (Figure 21). One view of chemical development is that interstitial silver ions move through the silver halide crystal and are reduced on the underside of the latent image speck. In purely physical development complexed silver ion moves through the solution and is reduced on the nucleus. In this sense physical development and the early stages of chemical development are similar. 

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