MadSci Network: Chemistry

Re: What is the chemical equation when developing color film?

Date: Sun Oct 31 16:07:21 1999
Posted By: Harry Adam, Research Associate, Research Division, Kodak Limited
Area of science: Chemistry
ID: 941215407.Ch

Hello Lindsay – nice question! Dead right – developing film is all about 
oxidation/reduction reactions. In fact, most chemical reactions that take 
place involve oxidation and reduction – oxidation is removal of electrons 
from the substance being oxidised, and reduction is addition of electrons 
to the substance being reduced. So, as long as electrons are going from 
one species to another we have oxidation and reduction. 

Anyway – back to film -  which contains silver salts – halides like chloride, 
bromide and iodide. These are the light sensitive chemicals, and film has loads 
of tiny crystals of these salts spread in the layers. Light energy has the 
effect of reducing silver halide – turning positive silver ions (Ag+) into 
silver metal (Ag0). Photography depends on the fact that a tiny speck of 
silver (a few atoms) on a single silver-halide grain is enough to 
catalyse the development reaction so that the grain is completely 
developed before other grains which have no silver (unexposed) start to 
develop. That’s how we capture and then “develop” the image which is 
otherwise “hidden” or “latent” to use the technical jargon. It doesn’t 
matter whether it’s colour or black and white photography – the equations 
are much the same. The developing agents may be different – but are always 
reducing agents. In B&W, we use the developed silver for the image, 
whereas in colour, we then react the used developer to react with other 
chemicals (couplers) to form dyes. We then remove the silver – developed 
and undeveloped.

So, here are the equations that describe it all:

First – light reduces silver halide: light + AgX --> Ag + X. where X is 
Cl, Br or I, and X. is a halide radical – usually very reactive and very 
quickly removed by reaction with surrounding material (e.g. gelatine in 
the layer). 

Second – development: 2AgX + dev --> 2Ag0 + devox + X- where devox is 
oxidised developer and X- is halide ion – chloride, bromide or iodide. 
Most developers oxidise by taking on two electrons so one developer 
molecule can produce two atoms of silver.

And that’s it really – of course, in colour systems devox then reacts with 
couplers to form image dyes. (More oxidation/reduction…) But maybe that’s 
a step more than you need for now. If you do want more – don’t hesitate to 
e-mail me at

Thanks for the question!

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