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DIY stop bath.
Ok guys, so since the stop bath and fixer are offensively simple and easy to make I’ll have to pad these posts out with some theory. (which is synonymous with you actually having stopped reading before this sentence started, I guess) You can skip this post and be annoyed at the idiotic recipe that follows, claiming it’s bullshit or something.
Developers work in alkaline conditions (pH>7). Alkaline conditions make the gelatin our photosensitive stuff’s suspended in swell and open up to all the developer goodness penetrating it with its thick, throbbing reducing groups. Acids do the opposite. They denaturate the gelatin (make it harder, insoluble in water to a degree), closing off access to the silver halides et consortes and stop the developing action, also neutralizing the alkaline pH which is bad for film’s long-term condition.
So what we want is an excess (over the alkali left in our film from the dev) of very dilute (too much denaturation baaad) acid that’s inert towards the chems in our film – i.e. not nitric acid. It’s to stop the developing action dead, bring the pH of the film down and then get easily washed out during rinsing.
