Reactions Involved in Photo Development
1. Ag+Br- (crystal) + hυ (radiation) --> Ag+ + Br + e-
A very small number of crystals in the film are oxidized. The electrons released from this oxidation reduce the Ag+ to silver metal in the surrounding crystal. The reaction above describes what happens when film is exposed to light. A light-sensitive emulsion layer containing silver halides, which is located on the top of the film, reacts with light when the lens is open. These silver halides undergo a chemical reaction occurs which changes them into pure, metallic silver to create a latent image. As the film receives more exposure, or bright light, more grains of silver halides are changed by the reaction.
(1) Ag+ is reduced to silver metal in the presence of a halide which can be oxidized (i.e., a photon kicks out an electron from the halide).
(2) Many complex ions of Ag+ (such as that formed with hyposulfite) do dissolve in water. Silver-based photography relies on oxidation-reduction reactions to capture the image.
End Result: "Invisible" Latent Image
2. AgBr+ HO-HO + 2 OH-->2 Ag(s)+ O-O + 2 H2O + 2Br-
The Ag metal atoms formed in the latent image act as a catalyst. It acts upon the surrounding halide salt so that, in the presence of a developer--which is a reducing agent--the sensitized AgBr is reduced, to produce black silver
metal in the areas that have been exposed to light.
End Result: Image is now visible.
3. Stop Bath
The reaction caused by the developer is then stopped by an acidic stop bath, which contains acetic acid. Silver grains in the film are reduced by the developer. This forms large amounts of more free silver, producing a dark area in that section of the film. When an exposed film is placed in a developer solution, the grains that contain silver nuclei are reduced much faster than those that do not. The greater the exposure of the grain or the more nuclei present, the darker image on the film. The extent of the development and intensity of the dark image created on the film is determined by factors like temperature, concentration of the developer, pH, and total number of nuclei in each grain.
End result: Reaction caused by developer is stopped.
4. 2AgBr + (NH4)2S2O3--> Ag2S2O3 + 2NH4Br
The Fixer acts to create a permanent image. Unexposed AgBr on the photography film is removed by complex formation with thiosulfate. The fixer agent reacts with excess chemicals to leave only the dark silver metal image.
End Result: Conventional "negative" image; image now permanent.