|MadSci Network: Chemistry|
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 email@example.com Thanks for the question!
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