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Chemistry Of Film

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Chemistry of Film Exposure and Development
Film is composed of an acetate base coated with a photographic emulsion layer. The base supports the emulsion, which consists of small silver halide crystals suspended in gelatin. The crystals are Silver-Iodobromide (silver bromide with added silver iodide). (Earle L. Kitts) When the emulsion is struck by light, the silver halide is converted to solid silver. (Janice Clifton) A crystal absorbs a photon of light, with energy equal to Planck’s constant times the frequency of the light, liberating an electron from the ion. This leads to the formation of a photoelectron and neutral bromine atom. (Earle L. Kitts) The photoelectron combines with a silver ion to form a neutral metallic silver atom. (Earle …show more content…

First, the unprocessed negatives are treated with developer, an agent that turns silver halide to solid silver. (Janice Clifton) The most common developing agents are phenidone and hydroquinone. The active agents have a high pH, causing the gelatin to swell and allowing for a more rapid penetration throughout the emulsion. (Earle L. Kitts) The bromide in the gelatin acts as a development restrainer, preventing the development of unexposed silver bromide grains that would cause fog. (Earle L. Kitts) Development is a chemical reaction, so time and temperature must be regulated. (Janice Clifton) After development, some photosensitive silver halide crystals remain. (Janice Clifton) A stop bath of Acetic acid neutralizes the developer with no effect on the silver halide. The developed negatives are treated with a fixer solution of high concentration sodium hyposulfite to dissolve the unused silver halide.(Earle L. Kitts)(Janice Clifton) Once the film is removed from the fixer, it can be exposed to regular light since only solid silver negative remains. A hypo-cleaning agent is then used to neutralize fixer on the film. This step is optional however the alternative is to run the film under water for an hour as opposed to 6 minutes. Photoflo, a wetting agent, is then added to break down the surface tension of water droplets so the film dries spot-free, similarly to dishwasher detergent with glass. (Janice

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