What chemical reactions take place in photography?

Answer 1

Several different chemical processes have been used in Photography. Some of the earliest used silver salts that darkened on exposure to light. These salts were coated on metal plates and then had an image projected on them through a lens. Daguerreotypes used silvered copper plates that were exposed to iodine and bromine vapors to convert the surface silver to silver iodide and/or silver bromide. Once the image was created by exposure to light, the image was made permanent by exposing the plate to Mercury vapor which probably formed an amalgam with the sliver in the lighter areas that stuck to the plates. After development, the plates were washed with a hot salt solution which removed the silver from the darker areas where the mercury had interacted less with the silver.

More modern black and white photography uses grains of silver halides suspended in a gel. The chemistry of exposure is typically something like:

Ag+Br- (crystal) + hv (radiation) --> Ag+ + Br + e-

The silver ion can then combine with the electron to produce a silver atom.

Ag+ + e- --> Ag0

Association within the grains produces species such as Ag2+, Ag20, Ag3+, Ag30, Ag4+, and Ag40. The grains containing the free silver in the form of Ag4º are readily reduced by chemicals referred to as "developers" forming relatively large amounts of free silver; the deposit of free silver produces a dark area in that section of the film. The developer under the same conditions does not significantly affect the unexposed grains. Silver halide developers include:

p-Dihydroxybenze, Monomethylparaminophenol,1,2,3 Trihydroxybenzene, p-Aminophenol, Diamonophenol, and 1-Phenyl-3-pyrazolidone.

Once the exposed image has been developed to the desired degree, it is necessary to halt the chemical process quickly to prevent over-development and the production of fog. The solution used to that end is referred to as the "stop" bath. Since developers function a relatively high pH's, the typical stop bath is simply a solution of acetic or some other weak acid. The action of the acid is so rapid it usually requires only seconds for the process to be effectively halted.

The silver halides are only slightly soluble in water; therefore, to remove the material remaining after development it is necessary to convert it to soluble complexes which can he removed by washing. Sodium thiosulfate, commonly termed "hypo," has been used for this purpose since 1839.

The reactions in fixing can be written as follows:

AgBr + S2O3-2 --> AgS2O3- + Br- (adsorption complex)

which is followed by

AgS2O3- + S2O3-2 --> Ag(S203)2-3 (desorbed)

and by

Ag(S203)2-3 <--> AgS2O3- + S2O3 -2; AgS2O3 <--> Ag+ + S2O3-2

Recovery of the dissolved silver complexes is rather important in order to make this process affordable, so the used "hypo" is saved for later processing.

Answer 2

2AgCl ----> 2Ag + Cl2

Answer 3

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