Photography: A Series of Redox Reactions
Photography is so common that most people never give a moment’s thought to how remarkable the process is. Ordinary black-and-white photographic film consists of a celluloid strip that has been coated with a gelatin emulsion containing very tiny crystals, or “grains,” of a silver halide, usually AgBr. There is a considerable amount of art as well as science to making the film, and the recipes used by major manufacturers are well-guarded secrets. When exposed to light, the surfaces of the AgBr grains turn dark because of a light-induced redox reaction in which Br- transfers an electron to Ag+, producing atoms of elemental silver and Br2 which reacts with the gelatin emulsion. Those areas of the film exposed to the brightest light have the largest number of silver atoms, and those areas exposed to the least light have the smallest number.
2Ag + Br2
Perhaps surprisingly in view of what a finished photograph looks like, only a few hundred out of many trillions of Ag+ ions in each grain are reduced to Ag atoms, and the latent image produced on the film is still invisible at this point. The key to silver halide photography is the developing process, in which the latent image is amplified.
By mechanisms still not understood in detail, the presence of a relatively tiny number of Ag atoms on the surface of an AgBr grain sensitizes the remaining Ag+ ions in the grain toward further reduction when the film is exposed to the organic reducing agent hydroquinone. Those grains that have been exposed to the strongest light—and thus have more Ag atoms—reduce and darken quickly, while those grains with fewer Ag atoms reduce and darken more slowly. By carefully monitoring the amount of time allowed for reduction of the AgBr grains with hydroquinone, it’s possible to amplify the latent image on the exposed film and make it visible.
Once the image is fully formed, the film is fixed by washing away the remaining unreduced AgBr so that the film is no longer sensitive to light. Although pure AgBr is insoluble in water, it is made soluble by reaction with a solution of sodium thiosulfate, Na2S2O2, called hypo by photographers.
AgBr (s) + 2S2O3- (aq) -> Ag(S2O3)23- (aq) + Br- (aq)
At this point, the film contains a negative image formed by a layer of black, finely divided silver metal, a layer that is denser and darker in those areas exposed to the most light but lighter in those areas exposed to the least light. To convert this negative image into the final printed photograph, the entire photographic procedure is repeated a second time. Light is passed through the negative image onto special photographic paper that is coated with the same kind of gelatin–AgBr emulsion used on the original film. Developing the photographic paper with hydroquinone and fixing the image with sodium thiosulfate reverses the negative image, and a final, positive image is produced. The whole process from film to print is carried out billions of times and consumes over 3 million pounds of silver each year.