At what pH is a stop bath considered exhausted?

markbarendt · Feb 27, 2014

Anon Ymous said:
The indicator is bromocresol purple and it doesn't "flip", it's a gradual transition. Have a look here. I toss it when it has that dirty colour, about pH 6. It's still acidic.

Great info Anon.

RattyMouse said:
Does the indicator flip at 7 or is it higher?

It's not a light switch.

Rick A · Feb 27, 2014

Water does not "stop" development, but it does seriously retard (slow) it. There is still some developer carry over when you use water between developing and fixing. This will shorten the life span of whatever fixer you use. If you use fixer as a single shot and dump, this is not a problem. If you reuse fixer, as many do, you will need to change fixer more often, using a two bath method is highly recommended.

RattyMouse · Feb 27, 2014

Thank you. I'll toss my home made brew at pH 6.0 then.

cliveh · Feb 27, 2014

I think an indicator stop bath is a good idea. If you are a visual person, as most photographers are, the change from yellow to purple means you need to replace it.

L Gebhardt · Feb 27, 2014

Using an indicator is an excellent way to determine if it's getting ready to toss. I don't have the reference at hand, but years ago I read that stop bath should be under pH 5.2 (which, I'm sure is not coincidentally the point where bromcresol purple starts turning from yellow).

I stopped using stop bath after getting some pinholes when developing film. Ruined a few nice shots. I found water worked fine, as many others have here. I eventually stopped for paper too when I started using a neutral fixer. I've never had a stained print, or other issue attributable to not using a stop bath.

cliveh · Feb 27, 2014

Tom1956 said:
Because it halts development dead, hardens the gelatin, and protects the longevity of the fixer. Just makes best sense.

As Tom points out, a stop bath makes sense.

RattyMouse · Feb 27, 2014

cliveh said:
I think an indicator stop bath is a good idea. If you are a visual person, as most photographers are, the change from yellow to purple means you need to replace it.

pH paper works for me. I have an infinite supply.

Sirius Glass · Feb 27, 2014

markbarendt said:
You do realize that plain old water will work as a stop bath, right?

The difference is mostly in how fast it stops development, water just takes a few seconds longer than fresh stop.

In addition to the above comments, using stop bath helps the hypo last longer.

Sirius Glass · Feb 27, 2014

RattyMouse said:
I'm not over thinking this. You guys are!

I'm looking for a number. At what pH is stop bath considered exhausted? Give me a number. That's all I want.

I do not have a pH meter, however since you want a number how about 5? Why? Because is you are doing a Suduko puzzle and you are not sure what number to put in a square, inserting 5 will on the average and the mean square error will provide the smallest probability of error. :crazy:

Photo Engineer · Feb 27, 2014

You will not get reticulation from a stop bath using today's films.

And, your question applies to both color and B&W.

And, that "squeak" test is quite applicable as you know being a chemist Ratty. Acids make gelatin less squeaky!

PE

Tom1956 · Feb 27, 2014

Photo Engineer said:
You will not get reticulation from a stop bath using today's films.

And, your question applies to both color and B&W.

And, that "squeak" test is quite applicable as you know being a chemist Ratty. Acids make gelatin less squeaky!

PE

RattyMouse · Feb 27, 2014

Photo Engineer said:
You will not get reticulation from a stop bath using today's films.

And, your question applies to both color and B&W.

And, that "squeak" test is quite applicable as you know being a chemist Ratty. Acids make gelatin less squeaky!

PE

PE, I work in such a specialized field that most of my general chemistry skills have long since evaporated away. Sadly, I would fail many basic level college chemistry tests if given with no prep!

Photo Engineer · Feb 27, 2014

Well, there is a typo there. Acids make gelatin MORE squeaky!!!!!

Sorry about that.

Bases make gelatin MORE squeaky.

And dear Rat, I raised the issue of you being a chemist because you did it earlier in a post about the safety of D76 vs Xtol. So, I figured...... Well....... Anyhow, sorry. This is how I made my living for about 50 years. Squeaking emulsion with my fingers!

PE

RattyMouse · Feb 27, 2014

Photo Engineer said:
Well, there is a typo there. Acids make gelatin MORE squeaky!!!!!

Sorry about that.

Bases make gelatin MORE squeaky.

And dear Rat, I raised the issue of you being a chemist because you did it earlier in a post about the safety of D76 vs Xtol. So, I figured...... Well....... Anyhow, sorry. This is how I made my living for about 50 years. Squeaking emulsion with my fingers!

PE

Oh I'm a chemist alright PE, no doubt about that. It's just that my experience in the last 20 years has been in a very specialized field and as a result, a lot of my basic chem knowledge has drifted away. Especially inorganic chemistry! No one should assume that I know everything. Or even anything.

Photo Engineer · Feb 27, 2014

Geez, what a night. Bases make gelatin LESS squeaky. I have no excuse for my errors. I've got too much on my mind tonight including the heavy snow and cold weather. I guess I have brain freeze. With wind chill it was -25F here today.

Sorry guys.

PE

Tom1956 · Feb 27, 2014

I know how you feel. It's 42°F out there right now and I've got 5 logs in the Ashley going on. Cold.

I'm glad to know I unwittingly glommed on to such a scientific term as squeaky. But I felt like fellows would know what that meant. Your thumb squeaks on it. Antonym of slimy.

Rick A · Feb 28, 2014

I have always tested my stop by dipping my fingers in the developer to make them slimy, then into the stop to see how fast it makes them squeak.

Rudeofus · Feb 28, 2014

Since RattyMouse asked for hard numbers, here are hard numbers:

All common development agents (except Amidol) need pH > 6.0 to become active ---> any bath at or below pH 6.0 will act as a stop bath.
Another factor to consider is buffering. Acetic Acid and Citric Acid, the two most commonly used acids in stop bathes, are decent buffers up to around pH 6.0, so the pH 6.0 threshold holds.

Photo Engineer · Feb 28, 2014

However, the higher the pH, the longer a stop takes to stop development. Just as the less buffer capacity decreases overall capacity.

PE

DREW WILEY · Feb 28, 2014

A film you have to be super careful about stop bath with is Efke 25. Just recently discontinued, so there still some around. An odd duck with
respect to the gelatin, but lots of people, including myself, found uses for it. I'd use indicator stop bath with just a tiny hint of yellow, nothing
more. Probably amounted to around 1/4% acetic acid at that point. But did the job.

Roger Cole · Feb 28, 2014

Rick A said:
Water does not "stop" development, but it does seriously retard (slow) it. There is still some developer carry over when you use water between developing and fixing. This will shorten the life span of whatever fixer you use. If you use fixer as a single shot and dump, this is not a problem. If you reuse fixer, as many do, you will need to change fixer more often, using a two bath method is highly recommended.

Depends on your fixer. I'm about to order some TF 5 to use with prints. It's alkaline so water stop is recommended.

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DREW WILEY · Feb 28, 2014

I use water for my amidol print developers, for which I use a citric acid preservative, and otherwise, for TF4, simply use an intermediate water
rinse between acid stop bath and the fixer itself. I mix stop bath weak, regardless, and always dump it at the end of the day, just like everything else. Don't take chances.

Photo Engineer · Feb 28, 2014

Michael is correct. TF-5 is just a tad acid so that there are no fumes,, but TF-4 is alkaline and has a slight bit of ammonia odor. I use a stop with both for safety sake and to preclude any chance of overpowering odor.

A water stop should never be used with prints or LF negatives as you can get severe non-uniformity problems.

Pinholes are not an issue with most all modern films.

PE

Roger Cole · Feb 28, 2014

Humm ok then. I've used both water and acid with prints and never had problems. I was planning to go to water for Foma even in 4x5 though I've not had problems before with (citric) acid. Maybe 1/2 strength?

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Photo Engineer · Feb 28, 2014

Roger, try making an 11x14 print with a water stop!

You will be lucky to get a good print if there are uniform gray areas in it such as a portrait.

PE

At what pH is a stop bath considered exhausted?

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