Chemistry of Copper Bleach

RauschenOderKorn · Jul 8, 2019

What are the reaction equiations of the copper bleach? I have not found them in any of the books I have, and I am curious.

In some other thread on FB, a rather senior member claimed (with reference to a 2-pass lith process) without giving details that a copper halide is formed as by-product of the bleaching process, which will be developed during redevelopment and will form copper grains instead of silver.

Is that true? What are the equations?

Rudeofus · Jul 8, 2019

Copper is a noble metal, so there's a non-trivial chance that it gets reduced by developer. Another potential effect could be formation of insoluble Copper Oxide in high pH typically encountered in photographic developers. AFAIK the basic reaction between Cu2+ and Ag would be Cu2+ + Ag <===> Cu+ + Ag+, followed by 4 Cu+ + O2 + 4 H+ <===> 4 Cu2+ + 2 H2O.

RauschenOderKorn · Jul 16, 2019

Thanks Rudeofus. With that basic reaction, no copper should remain in the print after bleaching & washing?

The following ingredients are in the bleach (http://real-photographs.co.uk/formulae/toners/copper-sulfate-bleach/):

Water H20
Coppersulfate in H20: Cu2+ S042-
Sulfuric Acid in H2O: 2 H3O+ SO42-
Sodiumchloride in H20: Na+ Cl-

Furthermore, we have Ag in the picture which is to be bleached.

The Cu+ will not precipitate, as all substances which might form in the solution will remain in solution:

Cu2SO4 -> dissolves in water
CuCl -> dissolves in water

Copperoxide should not form in an acid environment and would stain immediately (which does not happen).
With no other substances present, what else could form?

With a standard process as follows:

1st development
Stop
Fix
Wash
Bleach
Wash
2nd development (lith)
Stop
Fix
Wash

no copper should be carried over to the second development.

Am I missing something here?

Rudeofus · Jul 16, 2019

Now that you bring it up: Copper I Iodide is quite insoluble, comparable to Silver Bromide. Obviously Copper I Iodide would not be affected by Sulfuric Acid. I wonder, however, whether sufficiently high amount of Copper II could oxidize all iodide to iodate before it could form insoluble salt with reduced copper ions.

Prof_Pixel · Jul 16, 2019

http://real-photographs.co.uk/formulae/toners/copper-sulfate-bleach/

Ag + Cu+2 => Cu+ + Ag+

RauschenOderKorn · Jul 17, 2019

Rudeofus said:
Now that you bring it up: Copper I Iodide is quite insoluble, comparable to Silver Bromide. Obviously Copper I Iodide would not be affected by Sulfuric Acid. I wonder, however, whether sufficiently high amount of Copper II could oxidize all iodide to iodate before it could form insoluble salt with reduced copper ions.

Yes, Copper(i)-Iodide is insoluable and Copper(i)Bromide does not dissolve well in water.

But we are talking about a paper emulsion which usually is not that rich in iodide, especially after development, fixing and washing. Furthermore, if there is iodide, it would be in the form of silveriodide which should not be affected by the bleach. The later goes for bromide, as the bleach is made with sodium chloride.

Rudeofus · Jul 18, 2019

My experience with Copper Sulfate bleach is not all that impressive. I used it for an E6 test, and the resulting slides had a distinct red cast that slides bleached with other bleaches didn't. Photographic paper bleached slowly with Copper Sulfate, but I could not detect any deficiency after bleaching was complete. Can't tell you, what the reddish stain in my E6 test strip was.

How would one distinguish developed silver from developed copper ?

RauschenOderKorn · Jul 19, 2019

The FB-thread I am refering to claimed that the reddish/brownish tones after the lith development were caused by copper having replaced the silver. But you can get the same tone with a one-pass lith, too, so I do not consider this a good indicator / evidence.

About E6: PE pointed out in another thread, that copper bleach was tested a Kodak and did not give satisfactory results. I guess you experienced an unrelated phenomenon, as lith gets it´s color from silver grain structure/defects and E6 has no silver grains left in the final image.
As far as I know, the only "commercial" use for copper bleach is two pass lith and copper intensifier, probably for a reason.

I will making print for the print exchanges anyway. I guess I will make an experiment testing if a non-lith redevelopment after copper bleach also gives the same reddish tone. If there was copper causing the tone, it should be visible with any developer.

Rudeofus · Jul 20, 2019

Regarding lith: the big advantage of Copper Sulfate plus Sodium Chloride bleach is that the resulting Silver Chloride appears to form smaller crystals and therefore more yellow/red colors after redevelopment.

I wonder whether one could verify presence of metallic copper in such prints after development by bleaching them with Sulfuric Acid and checking whether the used bleach turns deep blue with addition of Ammonia.

RauschenOderKorn · Jul 21, 2019

Rudeofus said:
I wonder whether one could verify presence of metallic copper in such prints after development by bleaching them with Sulfuric Acid and checking whether the used bleach turns deep blue with addition of Ammonia

I could do that. Do you have a formula/recipe to follow?

Rudeofus · Jul 21, 2019

No precise formula, but metallic copper is known to dissolve in Sulfuric Acid. I'd use battery acid, since it's cheap but concentrated, so we will not dilute the copper. Once a visible amount of image is bleached away, the acid should be already blue (Copper Sulfate), and turn deep blue if you add Ammonia.

PS: There's a non-trivial chance, that the copper is present as Copper Sulfide, which should be as stable as Silver Sulfide. No idea how to test for that.

RauschenOderKorn · Jul 22, 2019

Rudeofus said:
No precise formula, but metallic copper is known to dissolve in Sulfuric Acid. I'd use battery acid, since it's cheap but concentrated, so we will not dilute the copper. Once a visible amount of image is bleached away, the acid should be already blue (Copper Sulfate), and turn deep blue if you add Ammonia.

Give me a few days, I can try that.

Rudeofus said:
PS: There's a non-trivial chance, that the copper is present as Copper Sulfide, which should be as stable as Silver Sulfide. No idea how to test for that.

Where would the Sulfide come from?

Rudeofus · Jul 22, 2019

Any consecutive process step involving Thiosulfate could form Copper Sulfide, just like it does with silver. If you bleach a regular B&W print, that brown remaining image is Silver Sulfide formed during fixation.

Raghu Kuvempunagar · Jan 2, 2021

RauschenOderKorn said:
Give me a few days, I can try that.

@RauschenOderKorn: did you get a chance to do the test Rudy suggested? I'm curious to know if you reached any conclusion on the very interesting question you raised.

Chemistry of Copper Bleach

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