Whats the indicator in stop bath?

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Mike Lyons

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Hi to all. My usual supplier has told me that Ilford have discontinued "indicator" stop bath in favour of the odourless type.
I can't remember my High School chemistry enough to come up with a viable acid/base colour changing (cheap) additive to put into the new brew so that I can SEE when it is approaching exhaustion. Any suggestions?
Regards- Mike
 

clogz

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I think it is a dye, bromocresol, that turns blue when it is exhausted.
Regards
Hans
 

dr bob

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clogz said:
I think it is a dye, bromocresol, that turns blue when it is exhausted.
Regards
Hans
I think you are right. I remember an indicator "brom-cresol-green" which had a light greenish endpoint just before it turned (what I would call) purple - very much like indicator stop bath if reacted with excess developer. I never exhaust my stop (LOL), actually I do try to keep it fresh. After all it about the cheapest material in photography.
 

clogz

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It's a pity it doesn't work the same way with beer. No good drinking till you see green or turn purple there!
Regards
Hans
 

Ole

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Most of the time I don't use stop bath at all, just a brief dunk in water to get rid of most of the developer carry-over. When I do use stop (mostly for lith-printing when seconds really matter), I make new every time:

Go to a supermarket. Buy Citric acid. Dump about 8 grams (about one third of a 25-gram packet) of this in about one liter of water. Use until finished developing that day. Then dump the stop, and mix new the next time. No indicator needed. The capacity of this stop is far greater than acetic acid, the pH is higher (less risk of outgassing from carbonate in the devloper), and the smell is a lot nicer.

Hans (clogz) is probably right about the indicator used. Bromochreosol Purple (5',5"-Dibromo-o-creosolsulfone-phthalein) changes colour from yellow to purple in the pH range between 5.2 and 6.8. You can use it if you must, but I see no reason for it.
 

lee

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I went to a beer & wine making store and bought about 3 pounds of citric acid for $10 dollars. I use it for printing only. Water for film. I usually mix 2 tablespoons to a liter of water and dump when I am finished.

lee\c
 
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Mike Lyons

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Thanks to all. I'll probably go the citric acid route.If you ever get to the South of Western Australia, give me a shout.
Mike
 

jbj

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pH paper will also tell you when the buffering capacity of your stop bath has been exhausted.
 

ThomHarrop

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Bromocresol purple is also called "Eastman Organic Chemical No. 745" if you want to order if from Kodak. Although there are many other chemicals that change color with changes in pH, Kodak chose this one because they thought it was the easiest to view under safelight spectra.
 
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