Modern photography is an application of the chemical behaviour of the silver halides which decomposes and turn black in light.
For example:
light
2AgBr ——————→ 2Ag + Br2
Following steps are involved in photography:
1. Preparation of sensitive plate or film
To a solution of NH4Br containing gelatin, amm. AgNO3 is added. Thus an emulsion of AgBr in gelatin is prepared
NH4Br + AgNO3 ————→ AgBr + NH4NO3
The mixture is allowed to stand is order for AgBr particles to grow in size. This process is called ripening of emulsion. Then the emulsion is solidified. It is washed with water to expel NH3NO3 and then melted and applied uniformly on a glass plate or celluloid film.
2. Exposure
The plate or film is fixed in camera. For a fraction of a second the film is exposed so that an impression of the object to be photographed is made on the film. The silver bromide which is affected by light gets reduced.
2AgBr ————→ 2Ag + Br2
An actual but inverted image of the object is thus formed on the plate which is not visible to the eye.
3. Developing
The exposed film is immersed in a solution of the developer which contains reducing agents like pyrogallol or quinol. This solution reacts with the exposed part of the film. After the developing process, those parts of the film are white which were originally dark in the object and those parts are black which were originally white. This is known as negative.
C6H4(OH)2 + 2AgBr ————→ 2Ag + C6H4O2 + 2HBr
quinol quinone
The process of developing is carried out in a dark room.
4. Fixing the negative
Fixing is the process of making the image permanent. This is done by placing the negative in the hypo which will dissolve the undecomposed silver bromide.
AgBr + 2Na2S2O3 ————→ Na3[Ag(S2O3)2] + NaBr
The negative plate can now be taken out of dark room into light.
5. Printing of the photograph
The positive paper is prepared in a similar manner as the negative with the only difference that it is less sensitive. Either the P.O.P (printing out paper) or bromide paper is generally used for the purpose. The POP has got a coating of AgCl and AgNO3. The paper is placed below the negative and then exposed to light. A negative of negative plate or positive with respect to the object is obtained on the print paper.
6. Toning
In order to impart a beautiful golden colour to the photograph, it is dipped in a dilute solution of gold chloride. The process is called toning. If a shining grey tinge is desired in the photograph, it is dipped in potassium chloroplatinate, K2PtCl4 solution.
The silver particles on the print are replaced by gold or platinum particles resulting in the toning up of the photograph.
AuCl3 + 3Ag ————→ 3AgCl + Au
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