Many of the chemicals used in film processing can be toxic or hazardous to human health if used or disposed improperly. The discovery many years ago that silver atoms have unusual light-scattering properties when exposed to light led the revolution of photography. The field of photography has wide appeal among both amateurs and professionals. Many chemicals are used in processing film and making prints.
Photographic film is coated with an emulsion containing light-sensitive silver halide crystals.
Once film has been exposed, it must go through a series of chemical processes to bring out the image. The first step is known as development , and developers such as hydroquinone, catechols and aminophenols are used. Development is stopped by putting the film in a stop bath , which usually contains a solution of dilute acetic or muriatic acid. Next, fixers are used to remove any unreacted silver halide from the film. The primary ingredient in a fixer solution is usually either sodium or ammonium thiosulfate.