B&W Reversal with ferricyanide bleach?

Donald Qualls · Dec 28, 2020

With all the B&W reversal threads running, I thought I'd add one.

I've read that silver chloride is FAR more soluble in ammonium hydroxide solution than other silver halides -- enough so that it's possible to bleach film with sequential baths of potassium ferricyanide/potassium chloride (rehal to chloride) and 3% household ammonia.

I have ferricyanide on hand, and clear ammonia is cheap at the supermarket. Obviously, one would want the exhaust fan on (or better, work under a fume hood) to control the ammonia fumes, but this seems like a cheap and relatively safe way to bleach image silver for reversal. The fact that ammonia is a foggant causes no trouble here.

Has anyone actually tried this?

Raghu Kuvempunagar · Dec 28, 2020

I tried ferricyanide-chloride bleach for reversal after reading about it in the context of @Athiril's copper sulphate bleach. It didn't work and I didn't try again. Will need to check my notes if I have any observations from my failed experiment. But it's very easy to try it and find out yourself.

Anon Ymous · Dec 28, 2020

I also tried a potassium ferricyanide - potassium chloride bleach, followed by ammonia, but it failed miserably.

Donald Qualls · Dec 28, 2020

Sounds like I need to test just the bleaching action -- I'll find a snip of exposed leader, dip in developer to develop a section, make up a ferricyanide bleach with potassium chloride, and verify it rehalogenates, then test whether it will dissolve in 3% ammonia. If it works, I'll shoot and develop a test without fixing to test with an image. If it doesn't, I'd be interested in what's missing. I might make up a little silver chloride directly from nitrate solution and test the solubility that way.

relistan · Dec 28, 2020

I'm definitely interested in your testing!

Raghu Kuvempunagar · Dec 29, 2020

Anon Ymous said:
I also tried a potassium ferricyanide - potassium chloride bleach, followed by ammonia, but it failed miserably.

Is there a reference for the rehalogenating properties of ferricyanide+chloride bleach?

I wonder if ferricyanide+chloride bleach is a true rehalogenating bleach like ferricyanide+bromide bleach in the sense that it converts most if not all silver ferrocyanide (formed by the reaction between silver and ferricyanide) to silver chloride. Could it be that the bleached image consists mainly of silver ferrocyanide without much silver chloride?

ludwighagelstein · Dec 29, 2020

Ferricyanide Bleach can´t work for bw reversal. It works well with CR though, since the image is formed by dyes and not silver in the end and all silver is removed by the fixer. The bleaching step is also at a different stage for that reason. The ferricyanide just oxidizes the metallic silver into silverhalides, and doesnt remove the silver. But that silver removal is prerequisite to bw reversal. Otherwise you´ll just get solarized negatives after redeveloping. And another thing: DO NOT MIX FERRICYANIDE WITH AN ACID! It will produce CYNANIDE GAS and you do not want to have that in your darkroom.

Anon Ymous · Dec 29, 2020

@Raghu Kuvempunagar It's a rehalogenating bleach as far as I know. It even gives warmer tone when used in sepia toning compared to bromide.

Ian Grant · Dec 29, 2020

Anon Ymous said:
@Raghu Kuvempunagar It's a rehalogenating bleach as far as I know. It even gives warmer tone when used in sepia toning compared to bromide.

Correct, you can't juse a rehalogenating bleach for Reversal processing. I have used it for toning though.

Ian

Donald Qualls · Dec 29, 2020

ludwighagelstein said:
Ferricyanide Bleach can´t work for bw reversal. It works well with CR though, since the image is formed by dyes and not silver in the end and all silver is removed by the fixer. The bleaching step is also at a different stage for that reason. The ferricyanide just oxidizes the metallic silver into silverhalides, and doesnt remove the silver. But that silver removal is prerequisite to bw reversal. Otherwise you´ll just get solarized negatives after redeveloping. And another thing: DO NOT MIX FERRICYANIDE WITH AN ACID! It will produce CYNANIDE GAS and you do not want to have that in your darkroom.

I think you're confused. The reason ferricyanide bleach works in C-41, ECN-2, and E-6 in place of the canonical EDTA or PDTA bleaches is because it rehalogenates, forming silver halides again which then fix away with thiosulfates or thiocyanates. The only reason it might not work in B&W reversal is if I misunderstood the solubility of silver chloride (formed by using a chloride donor in the bleach step) in ammonium hydroxide. What I recall reading is that silver chloride is and other silver halides aren't soluble in ammonium hydroxide solutions.

Easy to test -- we know ferricyanide bleach works, because it's used in bleach-redevelop processes. So, ferri bleach some developed film, until it returns to the milky undeveloped halide appearance (should be safe to do this under safelight without risk of printing out, because all the dyes for spectral sensitization were washed away in the original development), then soak the resulting film in ammonium hydroxide solution (couple bucks for half a gallon [= enough to cover 120 almost four times] at the supermarket, and ought to have some reuse capacity). I'll try to test it this upcoming long weekend, now that I'm done fighting with my computer. I'm pretty sure I have everything I need already in my darkroom -- if necessary, I can sub table salt for the potassium chloride.

ludwighagelstein · Dec 29, 2020

Donald Qualls said:
I think you're confused. The reason ferricyanide bleach works in C-41, ECN-2, and E-6 in place of the canonical EDTA or PDTA bleaches is because it rehalogenates, forming silver halides again which then fix away with thiosulfates or thiocyanates. The only reason it might not work in B&W reversal is if I misunderstood the solubility of silver chloride (formed by using a chloride donor in the bleach step) in ammonium hydroxide. What I recall reading is that silver chloride is and other silver halides aren't soluble in ammonium hydroxide solutions.

Easy to test -- we know ferricyanide bleach works, because it's used in bleach-redevelop processes. So, ferri bleach some developed film, until it returns to the milky undeveloped halide appearance (should be safe to do this under safelight without risk of printing out, because all the dyes for spectral sensitization were washed away in the original development), then soak the resulting film in ammonium hydroxide solution (couple bucks for half a gallon [= enough to cover 120 almost four times] at the supermarket, and ought to have some reuse capacity). I'll try to test it this upcoming long weekend, now that I'm done fighting with my computer. I'm pretty sure I have everything I need already in my darkroom -- if necessary, I can sub table salt for the potassium chloride.

Well, yes, you´ve put it in better terms. The result is the same though.

Raghu Kuvempunagar · Dec 29, 2020

Anon Ymous said:
@Raghu Kuvempunagar It's a rehalogenating bleach as far as I know. It even gives warmer tone when used in sepia toning compared to bromide.

Yes, that has also been my understanding. Hence, I'm puzzled by the result both of us got and want to read a reference that actually talks about the rehalogenating properties of ferricyanide+chloride. Interestingly, Haist makes no mention of ferricyanide+chloride as a rehalogenating bleach as far as I could find out. However, Haist does mention in page 128 of vol 2 something that's of relevance to this discussion:

"Four atoms of silver react with four molecules of potassium ferricyanide to form a molecule of silver ferrocyanide that is retained in place as the image. The three molecules of potassium ferrocyanide are soluble and go into the bleaching solution... All the ferrocyanide groups may be used to form metal ferrocyanides if, during the reaction of the ferricyanide with the metallic silver image, an ion is present that forms a silver compound less soluble than silver ferrocyanide. The following order of preferential formation of silver compounds is known: sulfide, iodide, bromide, ferrocyanide, chloride, ferricyanide."

Does this mean that silver chloride is not formed as ferrocyanide is present in the solution (as the product of the reaction between silver and ferricyanide) and chloride is more soluble than ferrocyanide?

Raghu Kuvempunagar · Dec 29, 2020

Donald Qualls said:
What I recall reading is that silver chloride is and other silver halides aren't soluble in ammonium hydroxide solutions.

This is correct. Silver chloride is more soluble than silver bromide which is more soluble than silver iodide in dilute ammonia solution.

Folks, it's incorrect to say that rehalogenating bleach doesn't work for reversal - it depends. Rehalogenating bleach that has bromide won't work for obvious reason. However, rehalogenating bleach that has chloride can work provided one uses ammonia solution to remove the chloride after bleaching the film. Copper sulphate + chloride bleach is a good example of a rehalogenating bleach that uses chloride and works very well for reversal - I've used it myself. As I mentioned earlier, the trick is to use ammonia solution to remove only silver chloride while retaining silver bromide. Read @Athiril's thread on the subject for full details of the process. Now, will all rehalogenating bleaches that have chloride work? Probably not and ferricyanide+chloride is one such bleach that doesn't work, it appears.

grainyvision · Dec 29, 2020

My understanding, and with my experience in testing, is that ferricyanide is too capable of bleaching silver on it's own, specifically to a silver-ferricyanide compound of some sort. You can try this yourself by using a simple solution of ferricyanide with no bromide. It seems that the silver prefers to go into this compound rather than silver chloride, but not into silver bromide/silver iodide. So, if you want to form silver chloride you should use a bleach which won't do this. The obvious candidate of course is a copper sulfate bleach with sodium chloride added

Donald Qualls · Dec 29, 2020

earlz said:
The obvious candidate of course is a copper sulfate bleach with sodium chloride added

That, however, defeats the purpose of a bleach with a low hazard level. Copper sulfate bleach requires the same level of acidity, created by the same sulfuric acid, as dichromate or permanganate. It's less environmentally hazardous than dichromate, but more so than permanganate. Ferricyanide breaks down to Prussian blue on oxidation, which is insoluble and largely inert.

Based on college chemistry (was it really in 1978?) I'd expect the least soluble species in an equilibrium like this to precipitate, unless the more soluble was much more active. There isn't anything in photography much more electronegative than Cl- ion (we don't use F- much), and none of the silver halides are significantly soluble in water at reasonable pH -- so in a reaction with ferricyanide and chloride, I'd expect silver to convert to chloride (more active, less or similarly soluble). If this doesn't happen, there must be an energy consideration -- or silver ferricyanide is even less soluble.

relistan · Dec 29, 2020

I am not sure if precipitating the silver is the right outcome since I am not sure where it will precipitate—ideally not back onto the film—but this might be interesting in the current context @Donald Qualls .

I found this old text https://nvlpubs.nist.gov/nistpubs/jres/7/jresv7n5p913_A2b.pdf (page 925):

"Precipitation of Silver —As sulphide— Silver can be precipitated by the addition of ammonium sulphide to the cyanide solution. This procedure separates it not only from the cyanide, but also from ferrocyanide and metallic impurities, excepting mercury which also precipitates as sulphide."

EDIT: hmm, that looks like nasty stuff: https://echa.europa.eu/substance-information/-/substanceinfo/100.032.009

I can't tell if that also applies to aqueous solutions in low concentrations.

grainyvision · Dec 29, 2020

relistan said:
I am not sure if precipitating the silver is the right outcome since I am not sure where it will precipitate—ideally not back onto the film—but this might be interesting in the current context @Donald Qualls .

I found this old text https://nvlpubs.nist.gov/nistpubs/jres/7/jresv7n5p913_A2b.pdf (page 925):

"Precipitation of Silver —As sulphide— Silver can be precipitated by the addition of ammonium sulphide to the cyanide solution. This procedure separates it not only from the cyanide, but also from ferrocyanide and metallic impurities, excepting mercury which also precipitates as sulphide."

EDIT: hmm, that looks like nasty stuff: https://echa.europa.eu/substance-information/-/substanceinfo/100.032.009

I can't tell if that also applies to aqueous solutions in low concentrations.

Ferricyanide turns insoluble silver into also insoluble silver ferricyanide. So I don't think this is what you'd want, much less the exact purpose of doing this is to use low toxicity chemicals and this looks like something that'd require a properly equipped lab (fumehood etc) to use

grainyvision · Dec 29, 2020

Donald Qualls said:
That, however, defeats the purpose of a bleach with a low hazard level. Copper sulfate bleach requires the same level of acidity, created by the same sulfuric acid, as dichromate or permanganate. It's less environmentally hazardous than dichromate, but more so than permanganate. Ferricyanide breaks down to Prussian blue on oxidation, which is insoluble and largely inert.

Based on college chemistry (was it really in 1978?) I'd expect the least soluble species in an equilibrium like this to precipitate, unless the more soluble was much more active. There isn't anything in photography much more electronegative than Cl- ion (we don't use F- much), and none of the silver halides are significantly soluble in water at reasonable pH -- so in a reaction with ferricyanide and chloride, I'd expect silver to convert to chloride (more active, less or similarly soluble). If this doesn't happen, there must be an energy consideration -- or silver ferricyanide is even less soluble.

Permanganate isn't too toxic (it's used for many medicinal purposes) but it is pretty hazardous, given it's extremely oxidizing nature. Copper sulfate is more environmentally toxic but also less stable. It'll react with free iron to produce ferrous sulfate and copper. If you're really worried about environmental toxicity, dispose of the bleach by placing some iron wool into the solution. The reaction will be exothermic and will produce ferrous sulfate (which is pretty natural) and precipitate copper. Pour the solution through a coffee filter to remove the copper. The iron sulfate could then be further precipitated into green iron hydroxide with a little carbonate which is messy but also basically what rust is and thus completely natural

mohmad khatab · Dec 29, 2020

Are you concerned about the environment with just a few grams of copper sulfate?
You must be careful to return to the International Convention of Minions in Paris. This is what will have the greatest impact on the environment.

Anhydrous copper sulfate is the best
Deionized water is best.
Sodium chloride (de-iodized) is the best.
Filtering this solution after mixing using a cotton swab, not a portafilter, will give you the best results.
Mixing this solution in water that has been heated in a microwave with a water temperature of no less than 52 ° C (is the right thing to do).
- Cleansing the emulsion from any developer residue with a solution of sodium sulfite and then splashing with water prior to bleaching is the correct practice.
- Yes, the ammonia solution after bleaching is really the difficult step, but wearing a good quality 3M mask with an air filter is a smart thing.

Raghu Kuvempunagar · Dec 30, 2020

earlz said:
My understanding, and with my experience in testing, is that ferricyanide is too capable of bleaching silver on it's own, specifically to a silver-ferricyanide compound of some sort.

Silver ferrocyanide actually. Let me try to explain based on what I understand. When a silver image is bleached in ferricyanide only bleach, the following reaction takes place:

4Ag (silver) + 4K3Fe(CN)6 (potassium ferricyanide) ---------> Ag4Fe(CN)6 (silver ferrocyanide) + 3 K4Fe(CN)6 (potassium ferrocyanide)

Silver ferrocyanide formed imagewise in the emulsion is insoluble whereas potassium ferrocyanide is soluble. Silver ferrocyanide is not very light sensitive but is redevelopable in a regular developer and gives a yellow-brown image on redevelopment. Also, silver ferrocyanide can be dissolved and removed by a fixer:
silver ferrocyanide (insoluble) + sodium thiosulphate -------> silver thiosulphate complex (soluble) + sodium ferrocyanide (soluble)

This is how a ferricyanide bleach of the non-rehalogenating type works.

When bromide is also present in ferricyanide bleach, it forms silver bromide by using up silver ferrocyanide in two steps:
4Ag (silver) + 4K3Fe(CN)6 (potassium ferricyanide) ---------> Ag4Fe(CN)6 (silver ferrocyanide) + 3 K4Fe(CN)6 (potassium ferrocyanide)
Ag4Fe(CN)6 (silver ferrocyanide) + 4KBr (potassium bromide) ------> 4AgBr (silver bromide) + 4 K4Fe(CN)6 (potassium ferrocyanide)

Silver bromide thus formed imagewise in the emulsion is insoluble whereas potassium ferrocyanide is soluble. Silver bromide is, obviously, light sensitive, and produces a silver image after light exposure and redevelopment. Further, silver bromide can be dissolved and removed by a fixer. This is how a rehalogenating ferricyanide bleach works.

Now, if chloride is used in a ferricyanide bleach instead of bromide, as @Donald Qualls wants to do, does the following reaction take place?
Ag4Fe(CN)6 (silver ferrocyanide) + 4KCl (potassium chloride) ------> 4AgCl (silver chloride) + 4 K4Fe(CN)6 (potassium ferrocyanide)

It's not clear to me whether such a reaction takes place. The two failed experiments reported by @Anon Ymous and I suggest that silver chloride is probably not formed when ferricyanide+chloride bleach is used. It would be nice to see some research study that discusses the rehalogenating properties of ferricyanide+chloride bleach. Does anyone know of a such a study?

Raghu Kuvempunagar · Dec 30, 2020

earlz said:
Permanganate isn't too toxic (it's used for many medicinal purposes) but it is pretty hazardous, given it's extremely oxidizing nature. Copper sulfate is more environmentally toxic but also less stable.

Copper sulphate bleach is definitely more stable than permanganate bleach and it can be reused several times. I've used copper sulphate bleach for over a month after mixing without any noticeable instability. The biggest complaint I've about copper sulphate bleach is that it is slow and takes 15-30 minutes in some cases.

Anon Ymous · Dec 30, 2020

Let's make a hypothetic scenario. You wash after FD and then use a sodium sulfide bath. You will then have a negative image made of metallic silver and a positive image made of silver sulfide. Wash again, then bleach in a potassium ferricyanide - potassium bromide bleach. This should convert the negative image to silver bromide, while leaving the positive unaffected. Wash again, fix, wash and you should have sepia toned positives. I know, a sepia toned image isn't evryone's cup of tea, but it seems to be the easier way around this. If you're only interested in scanning, then it doesn't matter at all anyway. As a side effect, your slides are more archival. That sasid, I don't know if the sulfide bath won't affect the negative image at all. It might have a slight toning effect on it, so I'd keep toning time to a minimum, which is almost instantaneous anyway.

Raghu Kuvempunagar · Dec 30, 2020

Anon Ymous said:
Let's make a hypothetic scenario. You wash after FD and then use a sodium sulfide bath. You will then have a negative image made of metallic silver and a positive image made of silver sulfide. Wash again, then bleach in a potassium ferricyanide - potassium bromide bleach. This should convert the negative image to silver bromide, while leaving the positive unaffected. Wash again, fix, wash and you should have sepia toned positives. I know, a sepia toned image isn't evryone's cup of tea, but it seems to be the easier way around this. If you're only interested in scanning, then it doesn't matter at all anyway. As a side effect, your slides are more archival. That sasid, I don't know if the sulfide bath won't affect the negative image at all. It might have a slight toning effect on it, so I'd keep toning time to a minimum, which is almost instantaneous anyway.

This is a known idea and previously discussed on this forum itself. It was said that the method results in severe loss of speed and some silver also gets converted to silver sulphide resulting in a muddy image. However, some people have used this idea for paper reversal and obtained nice results.

Anon Ymous · Dec 30, 2020

@Raghu Kuvempunagar Ah, so some toning does take place on the negative image, that's too bad.

Raghu Kuvempunagar · Dec 30, 2020

Raghu Kuvempunagar said:
I wonder if ferricyanide+chloride bleach is a true rehalogenating bleach like ferricyanide+bromide bleach in the sense that it converts most if not all silver ferrocyanide (formed by the reaction between silver and ferricyanide) to silver chloride. Could it be that the bleached image consists mainly of silver ferrocyanide without much silver chloride?

Raghu Kuvempunagar said:
Let me try to explain based on what I understand. When a silver image is bleached in ferricyanide only bleach, the following reaction takes place:

4Ag (silver) + 4K3Fe(CN)6 (potassium ferricyanide) ---------> Ag4Fe(CN)6 (silver ferrocyanide) + 3 K4Fe(CN)6 (potassium ferrocyanide)

Silver ferrocyanide formed imagewise in the emulsion is insoluble whereas potassium ferrocyanide is soluble. Silver ferrocyanide is not very light sensitive but is redevelopable in a regular developer and gives a yellow-brown image on redevelopment. Also, silver ferrocyanide can be dissolved and removed by a fixer:
silver ferrocyanide (insoluble) + sodium thiosulphate -------> silver thiosulphate complex (soluble) + sodium ferrocyanide (soluble)

This is how a ferricyanide bleach of the non-rehalogenating type works.

When bromide is also present in ferricyanide bleach, it forms silver bromide by using up silver ferrocyanide in two steps:
4Ag (silver) + 4K3Fe(CN)6 (potassium ferricyanide) ---------> Ag4Fe(CN)6 (silver ferrocyanide) + 3 K4Fe(CN)6 (potassium ferrocyanide)
Ag4Fe(CN)6 (silver ferrocyanide) + 4KBr (potassium bromide) ------> 4AgBr (silver bromide) + 4 K4Fe(CN)6 (potassium ferrocyanide)

Silver bromide thus formed imagewise in the emulsion is insoluble whereas potassium ferrocyanide is soluble. Silver bromide is, obviously, light sensitive, and produces a silver image after light exposure and redevelopment. Further, silver bromide can be dissolved and removed by a fixer. This is how a rehalogenating ferricyanide bleach works.

Now, if chloride is used in a ferricyanide bleach instead of bromide, as @Donald Qualls wants to do, does the following reaction take place?
Ag4Fe(CN)6 (silver ferrocyanide) + 4KCl (potassium chloride) ------> 4AgCl (silver chloride) + 4 K4Fe(CN)6 (potassium ferrocyanide)

It's not clear to me whether such a reaction takes place. The two failed experiments reported by @Anon Ymous and I suggest that silver chloride is probably not formed when ferricyanide+chloride bleach is used. It would be nice to see some research study that discusses the rehalogenating properties of ferricyanide+chloride bleach. Does anyone know of a such a study?

This nails it:

"Silver ferrocyanide is extremely insoluble in water, its solubility being intermediate to that of silver chloride and silver bromide. Thus, silver chloride dissolves in solutions of potassium ferrocyanide with precipitation of silver ferrocyanide, whereas silver bromide and iodide do not."

If any silver chloride were to be formed by the ferricyanide+chloride bleach's action on silver, then it will dissolve due to the presence of potassium ferrocyanide in the bleach and precipitate silver ferrocyanide again. In other words, no silver chloride is formed and ferricyanide+chloride is not a rehalogenating bleach.

B&W Reversal with ferricyanide bleach?

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