Hi!
I've always used Kodak acetic stop with indicator and never once in my life had i seen it lose strength, i always discarded way before that happened.
But ever since i started using HC-110 a couple of weeks ago, it seems to change color very, very quickly.
Granted i'm doing a lot more printing, but that doesn't justify how quickly its losing strength.
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First, HC110 is film developer. I don't understand how your printing volume enters the discussion at all.
Indicator stop changes from yellow to purple when it is exhausted. Actually, the color is an indication of the pH of the stop. Developer is alkaline, while stop and fixer are acid. When the carry-over from the developer causes the fixer to change from acid to alkaline, it changes from yellow to purple.
I have never seen stop change color when processing film. Now, I will say that I use stop as a one shot - discarding it after a single use. Actually, in recent years I have not actually used acid/indicator stop - just a plain water rinse.
So if you are talking about film development with HC110, and you are seeing the stop change from yellow to purple, then either you are reusing stop (which I don't recommend), or you are not carrying over an enormous amount of developer into the fixer.