Although the point is moving farther from the original queation, I don't quite agree with Steve. pH of acetic acid stop bath increases quite sharply during its use. Considering the strength of sodium carbonate typically used in paper developer, self-buffering effect of acetic acid is nothing. I recommend adding some sodium acetate to form a buffer if that's what you prefer. This will also make the stop bath act faster and last longer if mixed appropriatelly.(posted 8363 days ago)One could also buffer the fixer to reduce or remove those problems, whether it is made mildly alkaline, neutral or acidic. Buffering agents are cheaper than ammonium thiosulfate and it may be worthwhile experimenting to get maximum stable use out of same thiosulfate content. (I vaguely feel that amount of water is usually the limiting factor in fixer's processing capacity whereas the concentration of thiosulfate affects the processing time.)