Copper Sulphate B&W Reversal Bleach

NedL · May 5, 2021

Raghu Kuvempunagar said:
"To discard ammoniacal silver solution safely, a sufficient volume of either a 20% solution of sodium chloride or a diluted solution of hydrochloric acid is added to convert the silver compounds to silver chloride, which is insoluble and immediately precipitates. An excess should be added to ensure complete precipitation of all the silver diaminohydroxide. This makes the solution non explosive."

Excellent! I'll read that carefully. I do this with left over silver nitrate containing waste: first salt to form silver chloride, which filters out easily ( and also sinks to the bottom of the container, so you can decant through a filter ), then somewhat later after that step I put in a small amount of polysulfide just to be sure any remaining silver is in an insoluble form.

Regarding ammonium chloride, if you make a very strong solution ( approaching saturation ) it will dissolve silver chloride fairly well. Not very economical, but a good trick to know if you are having any trouble with salt prints... wash wash wash them and then put them into a very strong ammonium chloride solution, prior to fixing in thiosulfate. If the combination of paper and process you are using is troublesome to clear perfectly, this can be very useful. I'm not sure how this relates to the chart Rudeofus posted above.

Also I did not know that silver citrate is soluble in sodium sulfite. That is very interesting -- and also would explain some things I've wondered about. The H2O2 bleach many of us played with converts silver to silver citrate, and I think the process can be sped up by adding sodium citrate ( I know for sure you can rehalogenate with chloride or bromide by adding sodium chloride or potassium bromide, I ran a series of tests and thought it might be useful to convert some papers to warmer tone or maybe even make them more lith develop-able. ) So now I'm also wondering if pot ferri with sodium citrate will work like pot ferri with sodium chloride or bromide? If that worked, and we could use sodium sulfite to remove the silver citrate, it might be an efficient way to do reversal.

This is an interesting thread.... thanks to everyone participating

Raghu Kuvempunagar · May 6, 2021

NedL said:
So now I'm also wondering if pot ferri with sodium citrate will work like pot ferri with sodium chloride or bromide? If that worked, and we could use sodium sulfite to remove the silver citrate, it might be an efficient way to do reversal.

I think this wouldn't work for the same reason Ferricyanide+Chloride doesn't work as Silver Citrate is more soluble than Silver Ferrocyanide. Somebody with chemistry expertise like @GLS or @alanrockwood might be able to throw more light on the subject.

relistan · May 6, 2021

NedL said:
Also I did not know that silver citrate is soluble in sodium sulfite. That is very interesting -- and also would explain some things I've wondered about. The H2O2 bleach many of us played with converts silver to silver citrate, and I think the process can be sped up by adding sodium citrate ( I know for sure you can rehalogenate with chloride or bromide by adding sodium chloride or potassium bromide, I ran a series of tests and thought it might be useful to convert some papers to warmer tone or maybe even make them more lith develop-able. ) So now I'm also wondering if pot ferri with sodium citrate will work like pot ferri with sodium chloride or bromide? If that worked, and we could use sodium sulfite to remove the silver citrate, it might be an efficient way to do reversal.

I don’t know how much silver citrate is soluble in a sodium sulfite solution (can’t find data), but I know that it’s enough at 20g/L sulfite to clear film from peroxide/citric acid/EDTA bleach like the one we formulated https://www.photrio.com/forum/resou...ach-—-hydrogen-peroxide-edta-citric-acid.448/

I really don’t know but I understand that potassium ferricyanide ended up forming some silver ferrocyanide as well as the desired silver complex. That’s at least one of the things I gleaned from this thread https://www.photrio.com/forum/threads/b-w-reversal-with-ferricyanide-bleach.180429/ . As far as I know that compound isn’t soluble in anything useful. So might end up dark even if the other reaction works. I vote for trying it out if you have the chemistry on hand.

Rudeofus · May 6, 2021

Raghu Kuvempunagar said:
Fantastic! I'll try your method some time this week to make just enough Copper Chloride to test my hypothesis.

If you want to test your hypothesis, you can not compare Copper Sulfate against Copper Chloride. You'd have to run Copper Sulfate against Copper Citrate or Copper Acetate, both of which would form soluble silver salts and can be made by the same procedure as Copper Chloride.

Donald Qualls · May 6, 2021

I saw something interesting go by since my last visit to this thread: silver citrate is soluble in sodium sulfite solution.

Copper sulfate with citric acid forms silver citrate, right? If we were to add sulfite to the solution, might we have a single-solution bleach that works without a follow-up bath the way dichromate and permanganate bleaches do? If so, who cares if it's slow -- pour the bleach and go do something else for a half hour.

Raghu Kuvempunagar · May 6, 2021

Donald Qualls said:
silver citrate is soluble in sodium sulfite solution.

pour the bleach and go do something else for a half hour.

The real challenge is in converting silver image from the first development to silver citrate. You can't possibly do it using a bleach comprising of Ferricyanide and Sodium Citrate.

Raghu Kuvempunagar · May 6, 2021

Rudeofus said:
If you want to test your hypothesis, you can not compare Copper Sulfate against Copper Chloride. You'd have to run Copper Sulfate against Copper Citrate or Copper Acetate, both of which would form soluble silver salts and can be made by the same procedure as Copper Chloride.

My hypothesis was that Copper Sulphate + Sodium Chloride bleach is slow because of Sulphate ions which act as anti-swelling agent. If I use a bleach comprising of Copper Chloride with no Sulphate ions and find that it does bleach relatively fast, I will have verified my hypothesis. Not sure why I need to compare against Copper Citrate/Acetate. Am I missing something?

Donald Qualls · May 6, 2021

Raghu Kuvempunagar said:
The real challenge is in converting silver image from the first development to silver citrate. You can't possibly do it using a bleach comprising of Ferricyanide and Sodium Citrate.

My comment was prompted by someone up thread seeming to say that copper sulfate bleach with citric acid does exactly that, if you don't add a chloride source. That's surely what the hydrogen peroxide/citric acid bleach does (with or without EDTA).

Should be easy enough for someone who's experimenting with this to mix up a batch and try it to see if it bleaches at all with citric acid, and then whether it'll do it in one stop with sulfite added.

Raghu Kuvempunagar · May 6, 2021

Donald Qualls said:
copper sulfate bleach with citric acid does exactly that, if you don't add a chloride source.

If this were the case then Copper Sulphate with Sulphuric Acid, a stronger acid than Citric Acid, would have converted silver into Silver Sulphate, a water soluble silver salt. But it doesn't. You need Chloride in the mix for Copper Sulphate to act as a bleach.

relistan · May 6, 2021

Donald Qualls said:
My comment was prompted by someone up thread seeming to say that copper sulfate bleach with citric acid does exactly that, if you don't add a chloride source. That's surely what the hydrogen peroxide/citric acid bleach does (with or without EDTA).

Should be easy enough for someone who's experimenting with this to mix up a batch and try it to see if it bleaches at all with citric acid, and then whether it'll do it in one stop with sulfite added.

Hey Donald, I suggested that but @Murray Kelly and @Raghu Kuvempunagar explained that the copper sulfate is actually getting converted into copper chloride in the bleach from the presence of NaCl and that this is actually the active oxidizer in the bleach. So my proposal won't work. However, I can tell you for (pretty) sure, from the one roll I ran this weekend, that silver citrate is indeed being formed in a fairly reasonable amount in this copper sulfate/sodium chloride/citric acid bleach. I know that because earlier impressions have made me think it was soluble in ammonia and I didn't run the clearing bath. The result: dark slide, with heavy silver citrate staining.

This is likely partly be inadequate bleaching, but I am pretty sure from the look and the color that a fair bit of this is silver citrate that wasn't cleared because I didn't use the sulfite bath. Not the best photo, but this should be the clear leader.

Donald Qualls · May 6, 2021

Hmm. When I'm at the Big Box store getting a paver to put my new 3D printer on, I'll have to grab a can of plumbing root killer (copper sulfate). I think I have citric acid, and I've got plenty of sulfite -- hence easy to test.

Raghu Kuvempunagar · May 6, 2021

NedL said:
Regarding ammonium chloride, if you make a very strong solution ( approaching saturation ) it will dissolve silver chloride fairly well.

@Rudeofus:
Solubility data of Silver Chloride suggests that 26.31% (or above) solution of Ammonium Chloride should be able to dissolve all Silver Chloride from the film bleached with copper sulphate bleach at 20C.

relistan · May 6, 2021

It may be of interest to folks on this thread that this article states:

"Bleaching time depends on density of the film and concentration of the bleach solution. Heavy densities increase bleaching time; more concentrated solutions reduce bleaching time." (page 739)

They are using double the strength of the bleach from Tim Rudman: http://www.quintpub.com/userhome/qi/qi_21_9_kaplan_8.pdf . This is x-ray film, but seems relevant.

Also, I read that silver citrate is soluble in citric acid solution. Which could be pertinent as well.

Murray Kelly · May 6, 2021

I am tempted to suggest that in the acid environment (citric) perhaps a meta bisulphite might be a better bet than plain sulphite (pH ~9.5)
Just a thought.

Raghu Kuvempunagar · May 6, 2021

Raghu Kuvempunagar said:
@Rudeofus:
Solubility data of Silver Chloride suggests that 26.31% (or above) solution of Ammonium Chloride should be able to dissolve all Silver Chloride from the film bleached with copper sulphate bleach at 20C.

@Rudeofus:
Wouldn't at as high concentrations as this, Ammonium Chloride act also as a Silver Bromide solvent and deplete a significant amount of Silver Bromide from the film rendering it less useful for reversal processing application?

relistan · May 7, 2021

Murray Kelly said:
I am tempted to suggest that in the acid environment (citric) perhaps a meta bisulphite might be a better bet than plain sulphite (pH ~9.5)
Just a thought.

This makes sense. I never tried it. For the peroxide bleach with citric acid I was washing between all the steps to make sure of low carryover but I will try to get some time this weekend to try your idea

Rudeofus · May 7, 2021

Raghu Kuvempunagar said:
My hypothesis was that Copper Sulphate + Sodium Chloride bleach is slow because of Sulphate ions which act as anti-swelling agent. If I use a bleach comprising of Copper Chloride with no Sulphate ions and find that it does bleach relatively fast, I will have verified my hypothesis. Not sure why I need to compare against Copper Citrate/Acetate. Am I missing something?

Have you looked at the redox potential of Cu2+ + e- <===> Cu+ vs. Ag + Cl- <===> AgCl + e- ? It's close to a miracle, that this reaction takes place at all!

Here is the explanation for the miracle:

Cu2+ + e- <===> Cu+	0.159
Ag <==> Ag+ + e-	0.7996
Ag + Cl-<==> AgCl + e-	0.22233
Ag + Br-<==> AgBr + e-	0.07133
Ag + I-<==> AgI + e-	-0.15224

As you can see, based on reduction potential the reaction should only take place with bromide and iodide. It will take place with cloride, if there is enough copper salt and chloride salt to tip the balance, but it will be slow at best - as you observed.

If you try to create a soluble silver salt (sulfate, citrate, acetate), then you will have to deal with the Ag <==> Ag+ + e- redox potential, which is way too high for Cu2+ + e- <===> Cu+ to overcome. If you want to see, whether the sulfate anion is a problem, you need to compare Copper Sulfate against Copper Citrate or Copper Acetate, not against Copper Chloride/Bromide/Iodide. Yes, Copper Chloride will bleach faster than Copper Sulfate, but not because of swelling.

Rudeofus · May 7, 2021

Raghu Kuvempunagar said:
@Rudeofus:
Wouldn't at as high concentrations as this, Ammonium Chloride act also as a Silver Bromide solvent and deplete a significant amount of Silver Bromide from the film rendering it less useful for reversal processing application?

We have an 1:1000 advantage with Silver Bromide over Silver Chloride. If we overdo it, it may happen, but I predict, that there is some leeway.

Rudeofus · May 7, 2021

Donald Qualls said:
Copper sulfate with citric acid forms silver citrate, right? If we were to add sulfite to the solution, might we have a single-solution bleach that works without a follow-up bath the way dichromate and permanganate bleaches do? If so, who cares if it's slow -- pour the bleach and go do something else for a half hour.

I do not know, how long the sulfite will remain sulfite in a copper sulfate solution.

Raghu Kuvempunagar · May 7, 2021

Rudeofus said:
We have an 1:1000 advantage with Silver Bromide over Silver Chloride. If we overdo it, it may happen, but I predict, that there is some leeway.

Definitely worth trying if the 26.31% Ammonium Chloride solution can be reused for bleaching. Otherwise it's not going to be economical.

relistan · May 7, 2021

Rudeofus said:
Have you looked at the redox potential of Cu2+ + e- <===> Cu+ vs. Ag + Cl- <===> AgCl + e- ? It's close to a miracle, that this reaction takes place at all!

Here is the explanation for the miracle:

Cu2+ + e- <===> Cu+ 0.159

Ag <==> Ag+ + e- 0.7996

Ag + Cl-<==> AgCl + e- 0.22233

Ag + Br-<==> AgBr + e- 0.07133

Ag + I-<==> AgI + e- -0.15224

As you can see, based on reduction potential the reaction should only take place with bromide and iodide. It will take place with cloride, if there is enough copper salt and chloride salt to tip the balance, but it will be slow at best - as you observed.

If you try to create a soluble silver salt (sulfate, citrate, acetate), then you will have to deal with the Ag <==> Ag+ + e- redox potential, which is way too high for Cu2+ + e- <===> Cu+ to overcome. If you want to see, whether the sulfate anion is a problem, you need to compare Copper Sulfate against Copper Citrate or Copper Acetate, not against Copper Chloride/Bromide/Iodide. Yes, Copper Chloride will bleach faster than Copper Sulfate, but not because of swelling.

That is super useful! I am still trying to understand exactly how this is calculated, but the numbers help me understand the scale of the difference quite well. I think this also explains why little bits of film bleached for me quite a lot better than a partial roll of film in only 3x the volume (same concentration). It makes me think that doubling the concentration of the copper sulfate and sodium chloride might actually have a noticeable impact on performance like that article I linked about the X-ray bleaching.

Murray Kelly · May 7, 2021

I have a question for Rudi. Would ammonium persulphate etch the silver but leave the salts?

After the first development but still in the tank use a persulphate bath and only after that, fog the film to activate the remaining silver salts for the second dev.

I don't have the chemistry.

Raghu Kuvempunagar · May 7, 2021

Murray Kelly said:
I have a question for Rudi. Would ammonium persulphate etch the silver but leave the salts?

After the first development but still in the tank use a persulphate bath and only after that, fog the film to activate the remaining silver salts for the second dev.

I don't have the chemistry.

I'm not Rudy but I've used Persulphate bleach. It's not strong enough to bleach the silver image produced by first development. It leaves back some density.

Another potentially useful bleach is the one used in holographic reversals, Philip's Safe Solvent Bleach. I dabbled with it but had emulsion peel off problem. Maybe it's worth revisiting.

https://books.google.co.in/books?id=W9vrCAAAQBAJ&pg=PA197&lpg=PA197&dq=pssb+bleach+holographic&

Raghu Kuvempunagar · May 7, 2021

relistan said:
doubling the concentration of the copper sulfate and sodium chloride might actually have a noticeable impact on performance

Increasing the temperature might also help. Maybe at 40C, the bleach is significantly faster in action than at 20C.

Donald Qualls · May 7, 2021

Rudeofus said:
I do not know, how long the sulfite will remain sulfite in a copper sulfate solution.

Assuming this works at all, we'd see that as the bleach slowing down or completely failing to clear the film, so it would be easy to detect (and wouldn't do anything we couldn't live with; you'd see remaining density in the highlight and rebleach with fresh solution).

Copper Sulphate B&W Reversal Bleach

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