Radiography in Modern Industry - Kodak

combines with an interstitial silver ion. In the development process, the electrons are supplied bya chemical electron-donor and combine with the silver ions of the crystal lattice.The physical shape of the developed silver need have little relation to the shape of the silverhalide grain from which it was derived. Very often the metallic silver has a tangled, filamentaryform, the outer boundaries of which can extend far beyond the limits of the original silver halidegrain (See Figure 134). The mechanism by which these filaments are formed is still in doubtalthough it is probably associated with that by which filamentary silver can be produced byvacuum deposition of the silver atoms from the vapor phase onto suitable nuclei.Figure 134: Electron micrograph of a developed silver bromide grain.The discussion of development has thus far been limited to the action of the developing agentalone. However, a practical photographic developer solution consists of much more than a merewater solution of a developing agent. The function of the other common components of a practicaldeveloper are the following:An AlkaliThe activity of developing agents depends on the alkalinity of the solution. The alkali should alsohave a strong buffering action to counteract the liberation of hydrogen ions--that is, a tendencytoward acidity--that accompanies the development process. Common alkalis are sodiumhydroxide, sodium carbonate, and certain borates.A PreservativeThis is usually a sulfite. One of its chief functions is to protect the developing agent from oxidationby air. It destroys certain reaction products of the oxidation of the developing agent that tend tocatalyze the oxidation reaction. Sulfite also reacts with the reaction products of the developmentprocess itself, thus tending to maintain the development rate and to prevent staining of thephotographic layer.A RestrainerA bromide, usually potassium bromide, is a common restrainer or antifoggant. Bromide ionsdecrease the possible concentration of silver ions in solution (by the common-ion effect) and also,by being adsorbed to the surface of the silver bromide grain, protect unexposed grains from theaction of the developer. Both of these actions tend to reduce the formation of fog.Commercial developers often contain other materials in addition to those listed above. Anexample would be the hardeners usually used in developers for automatic processors.Radiography in Modern Industry 208