Chemistry of Photography

The principle of photography is based on the photochemical reaction of light on certain silver halides. The photographic technique involves the following steps:
1) Preparation of sensitive plate or film :- It is an emulsion of AgBr in gelatin uniformly coated on a glass plate or a celluloid sheet (film) in a dark room.The process is done in dark because AgBr is highly light sensitive.
2) Exposure :- The sensitive plate or film is mounted on a camera and exposed for a few seconds to the image of a properly focused object. An invisible change occurs in that part of the emulsion on which light falls.The chemical reaction is that light reduces AgBr to Ag.
Br- + photon -------> electron + Br
Ag+ + electron -------> Ag
An image of the object is formed on the film which is not visible and therefore called latent image.
3) Development of the latent image :- To make the latent image visible, the film is passed through a reducing bath containing hydroquinone (an organic reducing agent). More AgBr is reduced and the rate of reduction depends on the intensity of illumination during exposure. Thus, parts of the film which were most strongly illuminated become the darkest.
2AgBr + C6H4(OH)2 --------> 2Ag + 2HBr + C6H4O2
This process is carried out in dark to prevent general darkening of the picture. Here the image becomes visible, but the shade is negative in relation to that of the object, ie, the brighten parts will be darker and vice-versa. Hence, it is called negative plate or film. But at this stage we cannot expose it to daylight.
4) fixing :- To make the negative film to be handled in daylight, the film is dipped in a solution of sodiumthiosulphate (hypo). It dissolves away the unreacted AgBr as a complex but leaves the metallic silver unchanged.
AgBr + 2 Na2S2O3 -------> Na3[Ag(S2O3)2] + NaBr
Since no AgBr remains in the film it can be handled in daylight.
5) Printing :- Here we get a positive print of the image. The negative film is placed over a piece of paper that has been coated with photographic emulsion. This paper is then exposed to light for a few seconds. The image gets reversed and we get a positive print. This is then developed as before and fixed using hypo.
6) Toning :- This is done to improve the appearance of the photograph. For this the print is dipped in gold chloride (AuCl3) or potassium chloro platinate (K2PtCl6) which respectively gives a purple or steel grey shade to the photograph. Here the Ag particles of the picture are partially replaced by gold or platinum.
AuCl3 + 3Ag -------> 3AgCl + Au

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