Photo Chemistry

A few weeks ago, my wife began taking a photography class at a local college. Although her real interest is in the area of digital photography, course prerequisites require her to take an initial course using real film. One of the things the course is designed to teach is how to develop your own film, so she's getting some hands-on experience in some basic chemistry. She may never use any of this knowledge, but she does seem to be enjoying the class.

In any case, she's reached the section of the textbook which covers the chemical aspects of film development and she's already asked me for help on her homework questions. Unfortunately, the questions turned out to be a little bit more difficult than I expected. This was due in part to the fact that the chemistry questions were written by someone who obviously was not a chemist, which always adds to the degree of difficulty. This meant I spent a lot of time trying to determine exactly what the instructor "thought" he was asking and what type of answer he was hoping for. (Now I don't mean to suggest that the non-chemist is always to blame in situations like this. Sometimes it's the chemist who is the problem. I recall trying to answer a Trivial Pursuit question many years ago which read "Glass is made out of what?" The answer, of course, was "sand", but all I could think of were answers like "silica" or other more esoteric chemical terms, despite being given the hint not to think like a chemist.)

The other difficulty arose when I came to the realization that I knew less about the chemistry of film than I thought. I knew that silver halide salts (the main ingredient in film) are light sensitive, decomposing to black silver metal upon exposure to light. After all, that's why silver salts are always shipped in dark brown bottles. And I had known about the role of sodium thiosulfate in the fixer (dissolving and removing unreacted silver halide) since I was a freshman. But what did the developer do? And what is a stop bath?

Apparently, the initial exposure to light only reduces a small fraction of the silver halide in the film -- not nearly enough to make a negative. The developer operates by magnifying the amount of silver reduced to the metallic state. The developer contains organic compounds (such as hydroquinone or p-aminophenol) which are good at reducing silver halides in the film, but only when catalyzed by the presence of small metallic silver clusters (such as Ag₄^o) which were formed during the initial exposure to light. The grain size of the silver halide particles are an important determiner of the amount of silver reduced by the developer, and this fact is used to produce films with different speeds.

And the stop bath? It's basically a solution of acid which lowers the pH of the developer and essentially halts the further reduction of silver halide.

At least my wife is experiencing the joys of using graduated cylinders in her lab. Maybe I'll make a chemist out of her yet!