Photographic development, electron transfer

A description of the entire photographic process is clearly beyond the scope of this article, which concentrates on the chemical and physical aspects of the significant electron transfer reactions involved the three fundamental steps described above, i.e., image capture, development, and bleaching. Other aspects of photo-... 

Figure 10 Photographic development mechanism. The reduction potential, E"(Ag+/Agn), ofthe latent image clusters, when in contact with a solution. Increases with the number of atoms n. Therefore a nuclearity threshold for developmen t is created by the redox poten tial of the developer E°(CP/D). Above the critical nuclearity n, the potential E°(Ag yAg ) is higher than E°(CA/D), and alternate electron transfer toward A g, and Ag adsorption on Err allows the cluster to grow autocatalytically. On the contrary, when l"(Ag, /Agg is lower than E°(E>-/D), corrosion ofsubcritical clusters takes place by oxidizing molecules, such as D or Ox [7],...

Rate constants of the process and the nuclearity-redox potential correlation will be compared with corresponding data obtained in another environment, particularly when a surfactant or an associated ligand is present. The complete analysis of the autocatalytic transfer mechanism will also be compared with the photographic process of electron transfer from hydroquinone developer to clusters supported on silver bromide. 

Prussian blue is a mixed Fe(II)/Fe(III) complex polymeric species in which Fe(II) is octahedrally coordinated by C, and Fe(lll) is octahedrally coordinated by N, to give a structure containing Fe(II)-C-N-Fe(lll)-N-C-Fe(II)-linkages, in which the colour originates from electron transfer between the two metal oxidation states. It was discovered in 1704, and used in blueprints and also in the cyanotype photographic process developed by Herschel (see Chap. 11). The cyanotype process is made possible by the photochemical reduction of Fe(III) citrate (or oxalate) to Fe(II), which reacts with ferricyanide present in the coating formulation to give Prussian blue. A similar photoreduction of Fe(lII) oxalate to Fe(ll) is used in the ferrioxalate actinometer (see Chap. 14). 

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