Reusable stop bath formula from vinegar and sodium hydroxide

Pixophrenic · Jan 16, 2018

Another thing I would like to share, a reusable stop bath. A high capacity stop bath used in the past in many processes contains 15 g sodium acetate and 25 ml of glacial acetic acid per liter. Now that there are restrictions on shipment of sodium acetate, or you just do not want to buy a separate chemical, here is what one could use.

For 1 liter
Water 280 ml
Sodium hydroxide 7.2 g
Vinegar* 720 ml
pH 4.0-4.5

*) supermarket vinegar, 5% acetic acid. Here in Canada, Longo’s White Vinegar.

I let sodium hydroxide dissolve before adding vinegar, but it may be unnecessary, any order would do. Optionally, one could add a few drops of indicator for stop baths (#124 from Anchell Cookbook), ~0.4% Bromocresol Purple in the amount imparting a clearly visible yellow color, which turns purple on exhaustion, but I understand that many people may have a problem sourcing it. One liter suffices for about 8-10 films. Also partly used bath keeps well for at least 3 months. Although there is even no consensus where a stop bath should be used or not, and, ahm, it smells like vinegar, but I hope someone would find this useful.

Mainecoonmaniac · Jan 16, 2018

I stopped using acetic acid years ago because of the smell. I use a tablespoon of citric acid per liter of water and toss it at the end of my print session. I use plain water as stop bath for processing film.

Pixophrenic · Jan 16, 2018

Thanks, Tom. My suggestion is not intended for everyone. The operational word here is "reusable". The 3Rs, you know?

Gerald C Koch · Jan 16, 2018

The strength of white vinegar can be anywhere from 4 to 5 percent, check the label. Considering the cost it really isn't economical to reuse stop bath as there is always the possibility of it failing during use..

DavidJRobertson · Jan 16, 2018

What does the addition of sodium acetate do above just using diluted vinegar?

Also, I reuse my stop bath (15g/l citric acid) but I do replace it reasonably often as it's pretty cheap. Cheap enough to use it once, but I don't think that's necessary. For this reason I see no value in adding an indicator - just replace often enough.

Gerald C Koch · Jan 16, 2018

DavidJRobertson said:
What does the addition of sodium acetate do above just using diluted vinegar?

Also, I reuse my stop bath (15g/l citric acid) but I do replace it reasonably often as it's pretty cheap. Cheap enough to use it once, but I don't think that's necessary. For this reason I see no value in adding an indicator - just replace often enough.

You guessed it as it's rather unnecessary for B&W.

Pixophrenic · Jan 16, 2018

Gerald C Koch said:
The strength of white vinegar can be anywhere from 4 to 5 percent, check the label. Considering the cost it really isn't economical to reuse stop bath as there is always the possibility of it failing during use..

Well, mine says 5% exactly, but hence the use of the indicator. Actually, how do you know your stop bath failed during use? It is the fixer that is in the danger of failing, but that can be controlled visually and it is gradual enough to be noticed. I understand the indicator use adds strength to the re-use idea, and I am currently looking into its accessible source.

john_s · Jan 17, 2018

There is an article by Ryuji Suzuki about the merit of acetic acid/sodium acetate stop baths. I could find it if anyone is interested. It describes a stop bath with not such a low pH as dilute acetic acid plus good stability over a long period of use. The not-so-low pH is supposed to be better for fibre paper if I remember correctly.

Martin Rickards · Jan 17, 2018

I fail to see the reason for substituting the relatively non-hazardous sodium acetate for the very corrosive sodium hydroxide. The paper usually comes ot of the developer with plenty of sodium ions anyway, which will also turn the acetic acid into sodium acetate and buffer the stop bath at a higher pH.

rpavich · Jan 17, 2018

Considering that Kodak Indicator stop bath is very cheap and mixed at a ratio of 64:1 I don't see the need to do this. A bottle of Kodak indicator stop bath is reusable and lasts a LONNNGGG time.

Pixophrenic · Jan 17, 2018

john_s said:
There is an article by Ryuji Suzuki about the merit of acetic acid/sodium acetate stop baths. I could find it if anyone is interested. It describes a stop bath with not such a low pH as dilute acetic acid plus good stability over a long period of use. The not-so-low pH is supposed to be better for fibre paper if I remember correctly.

Yes John, that would be a good idea. Also, for the sake of completeness, I am aware of other formulas in the same "family", like Kodak E4 process stop bath with 5.3 g sodium acetate and 32 ml glacial acetic acid (pH~3.5) or Agfa formula with 30 g/L sodium acetate and 10 ml glacial acetic acid (pH 5.0-5.5). BTW, the version I used for calculations is known to me under the name of Orwocolor 37 and it is somewhere in between those two. The existence of commercial indicator stop baths only shows that they have their use, but there is no obligation to use them and there are situations when they are not available for purchase. I am only offering an alternative that I found useful. It is not an attempt to undermine existing commercial formulations, or specifically save money. Also, I respectfully disagree with the argument about toxicity. Sodium hydroxide is sold in some countries as "drain opener" in supermarkets, along with vinegar. It is the restrictions on sodium acetate that as a chemist I do not understand. And finally, citric acid/sodium hydroxide as stop bath should be a higher capacity bath than just citric acid, I just have no interest to explore that at the moment.

Martin Rickards · Jan 17, 2018

What is the capacity of a stop bath? It just neutralises the alkalinity of the developer causing it to stop developing. Sodium acetate is itself alkaline, so it would be expected to have a negative effect on the process. The same goes for citric acid/sodium hydroxide; why add alkali when the purpose of a stop bath is to reduce the alkalinity.

Pixophrenic · Jan 17, 2018

As you rightfully mention, Martin, when a stop bath is just a dilute acid, as it is neutralized it loses its effectiveness, although still serves as a rinse. This acid solution should be sufficiently dilute to have a certain starting pH, as we can see from the examples above, from pH 3.5 to 5.5. Using a buffered acid instead allows to use a higher concentration of acid with the same starting pH as above. Hence more films can be stopped before the pH drifts into the area where a developer can still work.

Martin Rickards · Jan 17, 2018

Pixophrenic said:
As you rightfully mention, Martin, when a stop bath is just a dilute acid, as it is neutralized it loses its effectiveness, although still serves as a rinse. This acid solution should be sufficiently dilute to have a certain starting pH, as we can see from the examples above, from pH 3.5 to 5.5. Using a buffered acid instead allows to use a higher concentration of acid with the same starting pH as above. Hence more films can be stopped before the pH drifts into the area where a developer can still work.

That logic only works if you have more acetic/citric acid to start with. In any case, pure acid stop baths also receive an ample donation of sodium/potassium ions from the developer, buffering the stop bath.

Pixophrenic · Jan 17, 2018

Martin Rickards said:
That logic only works if you have more acetic/citric acid to start with. In any case, pure acid stop baths also receive an ample donation of sodium/potassium ions from the developer, buffering the stop bath.

This is correct. As you can see, typically pure acetic acid is diluted to one percent, but in those formulas it goes higher. It is also true that the acetic acid bath slowly turns into a sodium/potassium acetate buffer during use, but with a lower capacity than in those buffered formulas. Theoretically, a better idea would be to use not sodium hydroxide, but rather one of the so-called Good's buffers, unfortunately, they are uncommon in photography and expensive.

Gerald C Koch · Jan 17, 2018

Pixophrenic said:
Well, mine says 5% exactly, but hence the use of the indicator. Actually, how do you know your stop bath failed during use? It is the fixer that is in the danger of failing, but that can be controlled visually and it is gradual enough to be noticed. I understand the indicator use adds strength to the re-use idea, and I am currently looking into its accessible source.

The indicator used in indicator stop bath is expensive and not easy to obtain.

Acetic acid stop bath has its own 'built in' indicator -- its smell. No smell it's no longer effective.

Anon Ymous · Jan 17, 2018

rpavich said:
Considering that Kodak Indicator stop bath is very cheap and mixed at a ratio of 64:1 I don't see the need to do this. A bottle of Kodak indicator stop bath is reusable and lasts a LONNNGGG time.

Funnily enough, what used to be a very cheap product has become ridiculously expensive in Europe as far as I can tell. It's cheaper to buy acetic acid and the indicator dye itself and mix your own!

Gerald C Koch · Jan 17, 2018

If you look at the formulas for B&W stop baths by various companies; Agfa, Kodak, Ilford, etc you will not find any that use burrering. Why? because it serves no useful purpose.

Pixophrenic · Jan 17, 2018

Gerald C Koch said:
The indicator used in indicator stop bath is expensive and not easy to obtain.

Acetic acid stop bath has its own 'built in' indicator -- its smell. No smell it's no longer effective.

Gerald, acetic acid/acetate mix smells just as strongly. Addition of buffer does not quench the volatility of available acetic acid. As for the dye, as I said, I am looking into it. There are also alternatives to bromocresol purple, which may be easier to obtain, i.e. as food colors in the baking section.

Pixophrenic · Jan 17, 2018

Gerald C Koch said:
If you look at the formulas for B&W stop baths by various companies; Agfa, Kodak, Ilford, etc you will not find any that use burrering. Why? because it serves no useful purpose.

It may be just tradition or logistics, like in many other things offered for sale. It is interesting, though, that those people who buy the indicator stop bath do not complain about toxicity, while it is acetic acid in a dangerous concentration in that bottle. You can't be sure that there is no buffering, as you said before that they are not obligated to disclose all components, only the major ones. Besides, why offer a higher capacity bath, when there is no consensus if it has to be used at all? I am actually quite intrigued how much discussion my humble contribution initiated. It looks as if I am trying to force it on everyone.

Martin Rickards · Jan 17, 2018

Increased capacity is a myth. Adding alkali to a stop bath reduces its capacity to neutralise/acidify alkalies. This is a non-topic. Publicly available acetic acid is not toxic although glacial acetic acid (+96%) is considered corrosive..

Pixophrenic · Jan 17, 2018

Gerald C Koch said:
If you look at the formulas for B&W stop baths by various companies; Agfa, Kodak, Ilford, etc you will not find any that use burrering. Why? because it serves no useful purpose.

Well, Gerald, starting from here, I can say what useful purpose a commercial stop bath has, beside its intended one. It is to sell you a pH indicator for exorbitant price and include essentially free fairly concentrated acetic acid as a bonus. Hence little incentive to include a buffer, also since it cannot be diluted 1:50 or more for use. On the other hand, in many color reversal processes of the past a buffered stop bath after the first developer was often included.

Now I looked briefly into the issue of the indicators. First of all, forget about food colors, there are no candidates there. Also, there is nothing quite like Bromocresol Purple which is included in all, as far as I can see from the descriptions, commercial stop bath concentrates, including Fomacitro with citric acid. BP retails for around $40 for 5 grams here, which is substantially more than a lifetime supply. Further, if one looks up the chart of acid-base indicators from Wikipedia, one can see that while there is nothing as convenient as Bromocresol Purple, which changes color from yellow to purple around pH 6, there is another candidate which is cheaper by a factor of 2 to 4, and is very common at any decent University where they teach biology. I could find a local supplier who charges $13 for 5 g. This is Phenol Red, which is safe enough so that it is taken in by patients during some medical tests and also added to cell cultivation media for visual pH control. It starts to change color from yellow to orange a bit later than BP, when the bath is approaching neutral pH, and becomes a solid red when it is around pH 7.5. However, since most photographers are expected to have a good eye for color, a change from yellow to orange should be an early enough sign that the bath is approaching exhaustion. Importantly for me, one does not have to rely on smell and/or tactile perceptions, dipping fingers into it as described by late Ansel Adams in the Negative.

DavidJRobertson · Jan 17, 2018

Pixophrenic said:
Well, Gerald, starting from here, I can say what useful purpose a commercial stop bath has, beside its intended one. It is to sell you a pH indicator for exorbitant price and include essentially free fairly concentrated acetic acid as a bonus. Hence little incentive to include a buffer, also since it cannot be diluted 1:50 or more for use. On the other hand, in many color reversal processes of the past a buffered stop bath after the first developer was often included.

Now I looked briefly into the issue of the indicators. First of all, forget about food colors, there are no candidates there. Also, there is nothing quite like Bromocresol Purple which is included in all, as far as I can see from the descriptions, commercial stop bath concentrates, including Fomacitro with citric acid. BP retails for around $40 for 5 grams here, which is substantially more than a lifetime supply. Further, if one looks up the chart of acid-base indicators from Wikipedia, one can see that while there is nothing as convenient as Bromocresol Purple, which changes color from yellow to purple around pH 6, there is another candidate which is cheaper by a factor of 2 to 4, and is very common at any decent University where they teach biology. I could find a local supplier who charges $13 for 5 g. This is Phenol Red, which is safe enough so that it is taken in by patients during some medical tests and also added to cell cultivation media for visual pH control. It starts to change color from yellow to orange a bit later than BP, when the bath is approaching neutral pH, and becomes a solid red when it is around pH 7.5. However, since most photographers are expected to have a good eye for color, a change from yellow to orange should be an early enough sign that the bath is approaching exhaustion. Importantly for me, one does not have to rely on smell and/or tactile perceptions, dipping fingers into it as described by late Ansel Adams in the Negative.

Bromocresol purple is available from apcpure.com (UK), 10g for £12.75. However I do not see the point in using it anyway.

removed account4 · Jan 17, 2018

i stopped using stop bath completely in 1988 and haven't ever gone back
if i went back i would use a little citric acid, barely any, and thats it .. i can't imagine why someone
would want to reuse it over and over again or make it with toxic stuff or glacial acetic acid
to me it seems like over kill, when it costs pennies to make to begin with ..
its like photo flo, 1 small bottle lasted me nearly 40 years ,, and there are people who
use all sorts of stuff like dish soap or jet dry or whatever else they can find intead of the least expensive thing there is ...

faberryman · Jan 17, 2018

Concentrated stop bath is so cheap I use it once and throw it away. I don''t see the attraction of using white vinegar and additives, which is more expensive and more stuff to have to store.

Reusable stop bath formula from vinegar and sodium hydroxide

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