ECN-2/C-41 Homebrew Stop Bath Questions

Hello,

In a follow-up discussion to the stabilizer thread I recently posted, I now have some questions of the role of stop bath in homebrewing C-41/ECN-2 development.

I see for the ECN-2 official formula, one needs sulfuric acid to get to a pH with a range between 0.8 to 1.5. I've never handled sulfuric acid before but I'm aware of some of its dangers. Does sulfuric acid have a strong rotten eggs smell? I'm trying to avoid stinky chemicals if possible. I don't enjoy wearing a mask/respirator for them.

Another option I've heard is to use acetic acid instead. I've seen a formula of 10ml of glacial acetic acid and 10g of sodium sulfite in 1L as a very limited replacement. I've tried this formula and with my pH meter I got a pH of about 4.6, not nearly acidic enough but it seems others do adequately with using acetic acid knowing the limitations.

Then based on a previous suggestion made by Rudeofus in the aforementioned thread, he said I could try sodium bisulfite or amidosulfonic acid as possible stop bath replacements. Well, I have sodium metabisulfite and my understanding is that it is quite similar to sodium bisulfite. In my stop bath formula attempt, I tried some variations. One was 5g of sodium metabisulfite and 5g of sodium sulfite for a 500ml mix. The pH was about 6.2. Even if I added 10g more of the metabisulfite, the pH was still about 6.2. Without the sulfite and only 5g of metabisulfite, the pH was about 4.2.

I see that for a C-41 formula for a stop bath it also has 5ml of glacial acetic acid and 5g of sodium sulfite but I don't have a reference for the pH value. It seems it does not need to be as low as ECN-2 and I'm wondering if the metabisulfite stop bath would work properly for it compared to the ECN-2? Or is it even needed with a ferricyanide bleach?

I also referenced the 3rd edition of the darkroom cookbook and in it, Steve Anchell has a stop bath of 10g of sodium bisulfite per liter of water. He states it is a suitable replacement for an acetic stop bath. Is the conversion from bisulfite to metabisulfite something like only needing about 93% of metabisulfite to 100% of bisulfite?

When I developed the pictures with the metabisulfite stop bath without the sulfite, they came out pretty good. I wasn't sure what difference I can expect. I do have to adjust the colors and I sometimes see what looks like color crossover but I'm wondering if that has more to do with not having the proper pH of the developer?

I haven't tried the experiment yet with amidosulfonic acid yet and it appears it's a stronger acid. What would happen if I did a straight water bath as a stop bath for either ECN-2/C-41 while using ferricyanide bleach? Would the film just continue to slowly develop in the water bath and add slightly more density or would the colors shift as well?

Thanks for any help!

You don't need a stop bath with C-41 unless you are worried about the bleach being harmed. In any case, if you want to use stop I would look at vinegar. Concentrated acid is expensive to buy and a real pain to handle and dispose of. If you want some potent acetic acid I'm sure Freestyle etc sells 28% acetic acid, it will still blind you but is easier to handle than glacial acetic. Sulfuric is dangerous stuff.

I only do C-41 and I can tell you that if you use a ferricyanide bleach, you absolutely must use a stop bath and water wash.

2% acetic acid, and 10 grams/liter of sulfite added is highly recommended, for a minute, followed by a water wash for a minute, before bleach. If any developer gets into the bleach when you use a ferricyanide bleach, you will stain the film. Metabisulfite may work as well, but I don't know a formula.

With the standard bleach, no stop bath is necessary but can be used. 1-2% acid, no sulfite--is okay. With any color film process, never go from developer to a water wash only, then to bleach, or you risk crossover and overdevelopment.

The sulfuric acid stop bath has no smell. True, it's a very strong acid, but just don't use a concentrated one as a source - it's not freely available anywhere anyway. The most readily available source is battery electrolyte - this is about 37% sulfuric acid in deionized water. And it must be handled carefully, but it is no longer so dangerous when diluted.
The smell of rotten eggs suggests the release of hydrogen sulfide - avoid such a source.
Glacial acetic acid is readily available (at least in my region), but... it stinks. It really stinks. Some (pseudo) commercial kits such as FFP contain it. But it stinks... And it's a weak acid, you can't get a pH below 1 with it. However, it is not clear if it is necessary - if the purpose of this step is to stop development immediately, or if there is another function.
A stop bath with metabisulfite (or just sulfite) would be interesting because it does not work by making the developer inactive by lowering the pH, but simply because the sulfite stops the release of dyes. It is likely that the development of silver will continue for a while, but it will eventually be phased out.
All of these options will work, the question is which one is the best…

RPC said:

With the standard bleach, no stop bath is necessary but can be used.

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I think you should always follow the manual for the kit you are using. There's definitely no stop bath or no water wash before bleach or blix and after developer in Fujihunt kit (c41 xpress) I am using. I have not seen it mentioned in the Tetenal Colortec kit I used to use, nor in Rollei Colorchem kit that is also available in my location.
I always use sulphuric acid stop bath in ECN2 process (homebrew). I make it from battery acid that is readily available (and cheap af) at any service station where I live. You really need a proper water wash after that, especially if you're using ferricyanide bleach, because if the bleach pH gets too low from the stop bath it can cause prussian blue forming on your film and that's impossible to get rid of from your emulsion.
I say cut no corners when developing color film and stick to the manual as much as you can (and no as much as you want to or are comfortable with) in order to get the best of the state-of-the-art color films that Kodak sells to you.

drp said:

What would happen if I did a straight water bath as a stop bath for either ECN-2/C-41 while using ferricyanide bleach?

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You'd risk (and quite likely get) problems with consistent development, but mostly problems with excess dye formation and fog due to leftover developer oxidizing instantly in the ferricyanide bleach and coupling with the nearest dye couplers to produce non-image density.

Here's something Photo Engineer once said to me about the sulfuric acid stop bath in ECN2 development:

Sulfuric acid is stronger than Acetic. If it were Acetic, I really think Kodak would let us know. And the reason is to extract all of the CD from the film with a strong acid bath which thus gives better dye stability.

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I admit I still use acetic acid regardless of his comment and find no problems with it, but I do use a regular PDTA C41 bleach with my ECN2 film. Previously I used the ferricyanide bleach, which also seemed to work OK, but it's been a long time since I last tried it and I don't see a good reason to go back to it. PDTA bleaches may be expensive, but they are extremely efficient and last a long time, so a little goes a long way.

Pelovsky said:

sulphuric acid stop bath in ECN2 process (homebrew). I make it from battery acid

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That's what I used to do and I still keep a bottle of battery acid at hand for whatever occasion requires sulfuric acid (mostly silver bleaches). It's odorless and easy to handle. Battery acid (37% sulfuric acid) is much less obnoxious than equal strength acetic acid!

Pelovsky said:

, because if the bleach pH gets too low from the stop bath it can cause prussian blue forming on your film and that's impossible to get rid of from your emulsion.

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That's not really a concern; I used to think this, but was corrected since there's no iron donor for Prussian blue to form in an ECN2 process. Moreover, even if you manage to get some Prussian blue to form, you can easily remove it by soaking the film briefly in a sodium carbonate bath, which will remove the Prussian blue. Anyone who has done any cyanotype toning will know how easy it is to overdo the bleaching and irreversibly lose the image!

The risks of acid carryover into the ferricyanide bath are really in terms of safety and the formation of cyanide gas. This is one of the reasons I stopped using a process that has a strong acid bath followed by a ferricyanide bath. Although it's perfectly safe as long as you prevent carryover of the first bath into the second, I still found the combination to be rather unfortunate. It only takes one wrong move in tilting the wrong bottle at the wrong moment in the wrong place to get into trouble.

PS: another reason I also left he ferricyanide bleaches is because they're suspect of degrading radical scavengers that may/probably are part of the C41 film emulsion; they'd basically be worn out instantly by the ferricyanide, leaving the dyes without protection against stuff like ozone. While this may not be a problem if the film is stored properly, and while I don't really know if this mechanism is really used in C41 film and even ECN2 film (although I do know it's a vital part of RA4 color paper technology), I don't see good reasons to use the ferricyanide sledgehammer approach if a more benign PDTA approach also works and is very cost effective.

koraks said:

I admit I still use acetic acid regardless of his comment and find no problems with it, but I do use a regular PDTA C41 bleach with my ECN2 film. Previously I used the ferricyanide bleach, which also seemed to work OK, but it's been a long time since I last tried it and I don't see a good reason to go back to it. PDTA bleaches may be expensive, but they are extremely efficient and last a long time, so a little goes a long way.

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I would use PDTA bleach, but I just don't have anywhere to get it. It's not sold here, nor are the ingredients available to make it at home. There used to be options for 5 liter tubes for minilabs, but I don't see that option anymore... So that leaves the option of ferricyanide bleach, or the crazy EDTA one that I would only use on E-6.

drp said:

Then based on a previous suggestion made by Rudeofus in the aforementioned thread, he said I could try sodium bisulfite or amidosulfonic acid as possible stop bath replacements. Well, I have sodium metabisulfite and my understanding is that it is quite similar to sodium bisulfite.

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Since nobody picked up on this issue: there is a big difference between Sodium Metabisulfite/Bisulfite and Sodium Bisulfate. Check the "i" vs. "a" here, it's really important!

If you make a bath of Sodium Metabisulfite (chances are minimal, that you can get your hands on actual Sodium Bisulfite, although many careless traders sell Sodium Metabisulfite as "Sodium Bisulfite"), it will smell like rotten eggs and can be quite pungent, especially if you add another acid to this brew. And yes, pH will not drop far below 4.

If you make this bath with Sodium Bisulfate (the one with "a"), you will get a mostly odorless stop bath with a pH somewhere around 2. This is the one I recommend for ECN-2. If you can get Sulfamic Acid, you can even create pH around 1.


Regarding C-41: the official procedures are geared for extremely fast processing, whatever the cost. Think one hour lab, think RA process. There is no time for stop bathes or intermediate washes, and if you buy chemistry in metric ton amounts, it's well cheap enough to reasonably trade chems for speed. This works, because bleach is quite acidic and at the same time mild enough to not cause trouble with carryover color developer. Normal C-41 process goes directly from CD to bleach bath.

This is, however, rarely the situation we amateurs are in. We try to conserve chems because they are sold to us at widely inflated prices. This also means: we should deviate from the official process as long as we don't sacrifice image quality and/or stability. We don't want to go directly from CD to bleach, because bleach is precious and we want to make it last as long as possible. We aren't sure, whether C-41 materials can handle pH below 4, there is nothing in the C-41 process chain below this pH, therefore our stop bath ought to stay at or above pH 4.

You can use a mild Acetic Acid based stop bath, something like Acetic Acid 1% should do the job just fine. You can go from there directly into bleach. Bleaching raises the pH of bleach, so the carryover Acetic Acid will actually make the bleach last longer. If you maintain bleach pH through other means, a few quick rinses between CD and bleach won't hurt. Watch out with the wash water temperature: there's no point to extend the life of our bleach while risking reticulation in the film we process!

Rudeofus said:

Since nobody picked up on this issue: there is a big difference between Sodium Metabisulfite/Bisulfite and Sodium Bisulfate. Check the "i" vs. "a" here, it's really important!

If you make a bath of Sodium Metabisulfite (chances are minimal, that you can get your hands on actual Sodium Bisulfite, although many careless traders sell Sodium Metabisulfite as "Sodium Bisulfite"), it will smell like rotten eggs and can be quite pungent, especially if you add another acid to this brew. And yes, pH will not drop far below 4.

If you make this bath with Sodium Bisulfate (the one with "a"), you will get a mostly odorless stop bath with a pH somewhere around 2. This is the one I recommend for ECN-2. If you can get Sulfamic Acid, you can even create pH around 1.


Regarding C-41: the official procedures are geared for extremely fast processing, whatever the cost. Think one hour lab, think RA process. There is no time for stop bathes or intermediate washes, and if you buy chemistry in metric ton amounts, it's well cheap enough to reasonably trade chems for speed. This works, because bleach is quite acidic and at the same time mild enough to not cause trouble with carryover color developer. Normal C-41 process goes directly from CD to bleach bath.

This is, however, rarely the situation we amateurs are in. We try to conserve chems because they are sold to us at widely inflated prices. This also means: we should deviate from the official process as long as we don't sacrifice image quality and/or stability. We don't want to go directly from CD to bleach, because bleach is precious and we want to make it last as long as possible. We aren't sure, whether C-41 materials can handle pH below 4, there is nothing in the C-41 process chain below this pH, therefore our stop bath ought to stay at or above pH 4.

You can use a mild Acetic Acid based stop bath, something like Acetic Acid 1% should do the job just fine. You can go from there directly into bleach. Bleaching raises the pH of bleach, so the carryover Acetic Acid will actually make the bleach last longer. If you maintain bleach pH through other means, a few quick rinses between CD and bleach won't hurt. Watch out with the wash water temperature: there's no point to extend the life of our bleach while risking reticulation in the film we process!

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Good clarification that there has indeed been confusion. I didn't know sodium hydrogen sulfate had such a low pH. Unfortunately, it is distributed only in sacks - apparently it is used exactly for such a purpose

lamerko said:

Good clarification that there has indeed been confusion. I didn't know sodium hydrogen sulfate had such a low pH. Unfortunately, it is distributed only in sacks - apparently it is used exactly for such a purpose

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Sodium Bisulfate is sold as "pH down" for adjusting swimming pool water. I've seen it in hardware stores in packs of 1 kg. This is still a lot more than you'll ever need for ECN-2 stop bathes, but you can also use it to adjust pH of other photographic bathes (e.g. instead of Acetic Acid) and it's cheap enough to not matter, if it rots away in your shelf.

Rudeofus,

Is sulfamic acid smelly or as corrosive as sulfuric acid? I will experiment with it and sodium bisulfate. When trying the sodium metabisulfite stop bath, that was much better than acetic acid. The powder has a slight sulfur smell but you don't need a respirator to use it. I think I'll stick to using metabisulfite over acetic acid. We'll see how the sulfamic acid and the bisulfate fare as a stop bath. Is there still a danger of releasing cyanide gas if I use the ferricyanide bleach and don't adequately wash in between the stop bath and the bleach when using the metabisulfite/sulfamic acid/bisulfate? Thanks!

drp said:

Is sulfamic acid smelly or as corrosive as sulfuric acid?

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Sulfamic Acid is a solid powder, much like Sodium Bisulfate. It is odorless, but you need to treat it respectfully, since it's a strong acid. I am not aware of a smelly dust problem. Use reasonable lab procedures like you would take with Metol or carbonate, and you will be safe.

I recommend you do not mix sulfite with strong acids of any kind, since the sulfite will release toxic Sulfur Dioxide gas. That's what you smell when you sniff Sodium Metabisulfite, and additional acid will release a lot more.

drp said:

Is there still a danger of releasing cyanide gas if I use the ferricyanide bleach and don't adequately wash in between the stop bath and the bleach when using the metabisulfite/sulfamic acid/bisulfate? Thanks!

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It probably depends on how much Sulfamic Acid or whatever gets into the ferricyanide, whether your bleach is buffered and so on. If you use a ferricyanide bleach, use a wash step between stop bath and bleach.

Hi Rudeofus,

You previously wrote in the other thread:

"Regarding concentration: I'd try to achieve the same concentration of H3O+, which means you need twice the molar amount of Sulfamic Acid or Sodium Bisulfate than you have Sulfuric Acid in the original stop bath. I doubt that the stop bath is very sensitive to changes in concentration."

Since I'm a total noob when it comes to chemistry, what would the actual numbers for this look like? For instance, the ECN-2 stop bath formula requires 50ml/L of sulfuric acid (7.0N). What is that converted for sulfamic acid and sodium bisulfate?

I've seen battery electrolyte at a 37% dilution of sulfuric acid. How much would that be if converted? I've seen 10ml/L of that suggested by others. I don't know if that's the correct conversion amount.

Thanks!

Kodak offer 10ml of concentrated sulfuric acid or 50ml of 7N or 35ml of 10N. Ready solutions like 7N are very expensive, they often refuse to sell them. Even if you find concentrated sulfuric acid somewhere, it is very dangerous. The most readily available option with battery electrolyte (37%) on simple ROUGH calculations should come in at around 27ml per litre, as long as I'm not lost in the simple calculations. Of course, this is a rough calculation, but for the purpose it is not so critical.
About the sodium bisulfate, I'd be interested too...

Battery acid: If you look at battery acid, according to this page it's somewhere between 4-5M. If ECN prescibes 35ml of 10N in one liter, then we need somewhere between 35-40ml of battery acid to get the original ECN-2 stop bath.

Sulfamic Acid: If we have 50ml of 7N Sulfuric Acid, then we have 0.5*0.05*7 = 0.175 moles of Sulfuric Acid. We should need about twice that, 0.35 mols of Sulfamic Acid to reach the roughly same buffering strength. With a molar weight of roughly 100g/mol we'd be somewhere around 35g Sulfamic Acid per liter ECN-2 stop bath.

Sodium Bisulfate: bla bla bla roughly 120g/mol (anhydride) or 140g/mol (monohydrate) bla bla 50g Sodium Bisulfate per liter ECN-2 stop bath.

These are quite concentrated soups, so once mixed they should last more or less forever, at least with current Cinestill prices.

If I'm not mistaken, sulfuric acid is a 2 N/M factor. Or if the battery electrolyte is 4.2M-5M, it should be 8.4-10N. But on this site they quote 29-32 percent electrolyte concentration, while in our region it is 37 percent...

37% is the industry standard for battery acid AFAIK.

koraks said:

37% is the industry standard for battery acid AFAIK.

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Maybe, but in any case it's very easy to measure the density of the solution and find out the exact concentration. Just bear in mind that temperature should also be measured.

If you have a bath at that concentration, ionic strength effects will throw all our molar calculations in disarray. My take home from this whole thing is: make a 50 g/l or 50ml/l solution of <strong acid> and be done with it. <strong acid> can be anything which pulls pH somewhere below 1.5 or 2.

And yes, @lamerko you are correct, a 5M Sulfuric Acid solution is indeed 10N and my calculations downstream are off by a factor of 2. Have just corrected my post, thanks for the heads up!

Anon Ymous said:

Maybe, but in any case it's very easy to measure the density of the solution and find out the exact concentration. Just bear in mind that temperature should also be measured.

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Yeah, true.
A few ml per liter will work fine as a stop bath, however, regardless of whatever concentration it turns out to be exactly.