Copper Sulphate B&W Reversal Bleach

NedL · May 3, 2021

Raghu Kuvempunagar said:
I would like to understand the risk and would appreciate your insights into the subject. Does the risk reduce if 2% ammonia is used only once to clear the chloride from the bleached film and disposed right after the use?

Well, the AgCL will be dissolved in the ammonia, so it probably needs to go to the hazardous waste along with spent fixer, so that means I'd be storing the solution for a while. I'm a little worried about storing silver dissolved in ammonia ( the worry is silver nitride formation ). I agree if you can dispose of it right after use then there should be no worry. I'm not a chemist and I don't know how much danger there is here, I just remember PE saying to be careful with ammonia or ethanol with silver salts ( and when I worked with ammonio-nitrate of silver, I had a series of PMs with him describing my process and making sure I wasn't going to blow myself up! In fact there was some danger and it was important not to let the solution evaporate, even on the sides of a container... )

( And, yes, I understand we'd want silver bromide paper for this to work with paper negatives.

Raghu Kuvempunagar · May 3, 2021

Rudeofus said:
These 8.5% Ammonium Chloride, were these indeed 85 g/l ?

With Ammonia, you have an ingredient, which just complexes silver ions to form a soluble compound. With chloride and silver, you create a balance between insoluble Silver Chloride and soluble silver chloride complexes of the [AgCln+1]n- form. With little chloride in the mix, the balance will tilt towards insoluble Silver Chloride,

Would Ammonium Sulphate in place of Ammonium Chloride work well?

Raghu Kuvempunagar · May 4, 2021

NedL said:
I'm a little worried about storing silver dissolved in ammonia ( the worry is silver nitride formation ).

That's a serious concern indeed. Unless disposed right after use, one would be facing the risk of a possible explosion! Thanks for drawing my attention to this issue. Much appreciated.

I found a relevant discussion on stackexchange:
https://chemistry.stackexchange.com...ilver-halides-dissolved-in-ammonium-hydroxide

It's suggested to add nitric acid to the ammonia solution containing silver chloride to avoid the formation of silver nitride. Edit: however, there's a simpler alternative to this which is discussed in my subsequent post.

YoIaMoNwater · May 4, 2021

Can't say too much about silver nitride myself, but when I was in a chemistry lab years ago, I got to the lab and my professor was talking to another professor upstairs. Apparently someone in the lab upstairs blew his finger off because he was working with silver azide. Supposedly he was grinding it for some organic synthesis reaction, but the details weren't clear. Well, the professor upstairs said that when he arrived to the department there was a trail of blood on the staircase and fingers around in the lab... so yea, be careful with certain silver compounds.

Edit: What do ya know. If you search silver azide on Wikipedia there's also silver nitride on the "see also" section.

Raghu Kuvempunagar · May 4, 2021

YoIaMoNwater said:
someone in the lab upstairs blew his finger off because he was working with silver azide. Supposedly he was grinding it for some organic synthesis reaction, but the details weren't clear. Well, the professor upstairs said that when he arrived to the department there was a trail of blood on the staircase and fingers around in the lab... so yea, be careful with certain silver compounds.

That's both unfortunate and scary.

Coming back to the use of 2% Ammonia for clearing Silver Chloride, is it possible to derive a ball park estimate of the amount of Silver Nitride that is potentially formed after processing a 36 frames roll of 35mm film? That would help in understanding the severity of the threat as it applies to reversal processing.

@NedL: here is another recommended procedure for handling ammoniacal silver solutions. This appears to be a pretty simple process requiring no exotic or dangerous chemicals and easily doable every time we reversal process film.

"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."

Rudeofus · May 4, 2021

Don't waste money on extra Ammonium Chloride, it may simply not do the trick we hoped it would do. I will do a few experiments to shed some light on this. Maybe pH needs to be high, at which point the soup will smell anyway and one might as well use Ammonia ...

PS: When I started self mixing fixer, I bought 50 kg of Sodium Thiosulfate and 25 kg of Ammonium Chloride for a very good price. While I have worked my way through most of the Sodium Thiosulfate, the stash of Ammonium Chloride didn't shrink all that much. This is why I almost desperately search for applications for Ammonium Chloride. It may or may not work for this task.

Rudeofus · May 4, 2021

Raghu Kuvempunagar said:
T
"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."

This strongly suggests, that Ammonium Chloride will not quantitatively dissolve Silver Chloride. Bummer ...

relistan · May 4, 2021

Is there a chemical reason why using ammonia with silver by itself is dangerous but ammonium thiosulfate rapid fixer is not? Various ammonium compounds were also historically used in various developers. I would have thought the same reaction would occur? Or does the sulfate interfere?

Donald Qualls · May 4, 2021

In rapid fixer, the silver in solution is complexed with thiosulfate. After using ammonia to dissolve silver chloride, the silver in solution isn't bound that well, so it can react with the ammonium and hydroxide ions. You won't have nearly as much silver as you would in a mirror silvering solution, so the hazard is smaller, but it's non-zero (and silvering solutions have been known to explode when struck by sunlight -- presumably it's the UV that's responsible). I like the recommendation to add sodium chloride or hydrochloric acid to precipitate the silver (this also amounts to a form of silver recovery, as the precipitate can be filtered off and redissolved with thiosulfate). By analogy, it might also be possible to add glucose solution to cause the silver to plate out as metal (I haven't tried this, however).

Raghu Kuvempunagar · May 4, 2021

Raghu Kuvempunagar said:
Would Ammonium Sulphate in place of Ammonium Chloride work well?

It didn't at 10%.

relistan · May 4, 2021

Raghu Kuvempunagar said:
It didn't at 10%.

Good to know, thanks for trying it out!

Rudeofus · May 4, 2021

I poked a bit into this topic (Silver Chloride soluble in Ammonia but not in Ammonium Chloride), and discovered the reason for this effect: Silver will happily complex with NH3 but not with NH4+. The shift from the latter to the former happens somewhere between 8 and 10, and most likely neither the Ammonium Chloride, nor the Ammonium Sulfite bath had that high pH.

Since Ag+ has a different redox potential than Ag(NH3)2+, the shift from one species to the other can be easily seen in this chart.

If anyone wants to repeat the test with Ammonium Chloride or Ammonium Sulfite, then a high pH must be dialed in. Sodium Carbonate should do the trick. Since Ammonium Chloride brought up to pH 10 will have the same awful smell as Ammonia, this is really only worth it for people with a near infinite source of some ammonium salt.

Donald Qualls · May 4, 2021

Rudeofus said:
this is really only worth it for people with a near infinite source of some ammonium salt.

Near-infinite, or near free. Parson's Clear Ammonia (or generics -- must be clear, the cloudy has soap in it) from the cleaning aisle at the supermarket is 3% strength ammonium hydroxide, at or near technical purity; generics cost as little as a dollar per quart).

relistan · May 4, 2021

Rudeofus said:
I poked a bit into this topic (Silver Chloride soluble in Ammonia but not in Ammonium Chloride), and discovered the reason for this effect: Silver will happily complex with NH3 but not with NH4+. The shift from the latter to the former happens somewhere between 8 and 10, and most likely neither the Ammonium Chloride, nor the Ammonium Sulfite bath had that high pH.

View attachment 273670

Since Ag+ has a different redox potential than Ag(NH3)2+, the shift from one species to the other can be easily seen in this chart.

If anyone wants to repeat the test with Ammonium Chloride or Ammonium Sulfite, then a high pH must be dialed in. Sodium Carbonate should do the trick. Since Ammonium Chloride brought up to pH 10 will have the same awful smell as Ammonia, this is really only worth it for people with a near infinite source of some ammonium salt.

Thank you! I really appreciate this detail level, it helps a lot. The chart really helps. So indeed if I had not dumped out that solution I would just add some carbonate and see what happens. Next time. I am now personally reflecting on what the heck I would do with 25kg of ammonium chloride! Between that and the sodium thiosulfate you have a lifetime supply of good fixer.

relistan · May 5, 2021

So, I was thinking about this last night. We need the ammonia because the bleach rehalogenates to silver chloride. And the ammonia is there to exploit the solubility of silver chloride vs silver bromide and silver iodide. What if we don't need the chloride at all? Our goal is not actually to rehalogenate, that just happens to be a step on the way. As I understand it the copper sulfate is acting as the oxidizer here. In the hydrogen peroxide/EDTA/Citric acid bleach we cooked up in the thread about that, we were largely forming silver citrate (some was complexed by EDTA). Silver citrate is soluble in a sodium sulfite solution.

So could the solution be to try the bleach without the chloride at all, and with possibly more citric acid? Maybe it would be really slow, I have no idea. Just putting the idea out there. Along similar lines, silver acetate is quite soluble and acetic acid might work (though it blixes).

Raghu Kuvempunagar · May 5, 2021

Thinking about the reasons for the slowness of copper sulphate rehalogenating bleach, it struck to me that the bleach has a significant concentration of sulphate ions. As sulphate ions are known to act as anti-swelling agent, could their presence in copper sulphate bleach be the main reason for the slowness of the bleach? Further, when potassium hydrogen sulphate is used in place of sulphuric acid, the concentration of sulphate ions in the bleach goes up further. Maybe acetic acid or citric acid is better than potassium hydrogen sulphate in this respect. However, replacing potassium hydrogen sulphate with acetic/citric acid doesn't change the concentration of sulphate ions due to copper sulphate itself. Instead of using copper sulphate, I wonder if the bleach is prepared using copper chloride and acetic/citric acid, would it be considerably faster.

relistan · May 5, 2021

Raghu Kuvempunagar said:
Thinking about the reasons for the slowness of copper sulphate rehalogenating bleach, it struck to me that the bleach has a significant concentration of sulphate ions. As sulphate ions are known to act as anti-swelling agent, could their presence in copper sulphate bleach be the main reason for the slowness of the bleach? Further, when potassium hydrogen sulphate is used in place of sulphuric acid, the concentration of sulphate ions in the bleach goes up further. Maybe acetic acid or citric acid is better than potassium hydrogen sulphate in this respect. However, replacing potassium hydrogen sulphate with acetic/citric acid doesn't change the concentration of sulphate ions due to copper sulphate itself. Instead of using copper sulphate, I wonder if the bleach is prepared using copper chloride and acetic/citric acid, would it be considerably faster.

Interesting idea. But that still leaves you with the requirement to use ammonia to dissolve the AgCl? Related, this old article claims that Copper Chloride is a bleach on its own. Which makes sense I think. Presumably acidification would make it faster.

Raghu Kuvempunagar · May 5, 2021

relistan said:
Interesting idea. But that still leaves you with the requirement to use ammonia to dissolve the AgCl? Related, this old article claims that Copper Chloride is a bleach on its own. Which makes sense I think. Presumably acidification would make it faster.

1. I'm OK with Ammonia step if the addition of sodium chloride to used ammoniacal silver solution eliminates the threat of explosion. It gives a neat way to precipitate Silver Chloride and dispose the rest of the solution.
2. It's Copper Chloride that acts as a bleach and not Copper Sulphate. That's the reason why Sodium Chloride is needed with Copper Sulphate for the latter to act as a bleach. In fact, Haist recommends using 47g Copper Sulphate and 11g Sodium Chloride to form 25g of Copper Chloride (page 30, vol. 2).

Murray Kelly · May 5, 2021

I thought we all knew that Copper Chloride was what we are talking about. Ferric chloride is another old timer.
For interest I made CuCl2 by precipitating the CuSO4 with Na2CO3 until all the blue was gone. It settled over night and the supernatant was easy to separate. Added water and enough HCl to redisolve the precipitate yielding a lovely green solution.
I didn't persist as it was fiddly for not much advantage (if any) It did rehalogenate promptly. Any HCl excess would provide chloride ions to assist.

Rudeofus · May 5, 2021

Silver is more noble than Copper, therefore Copper ions will oxidize Silver only, if we remove the silver salts from the balance. We will not be able to bleach with Copper ions, if we want to keep free Silver ions in the liquid.

relistan · May 5, 2021

Raghu Kuvempunagar said:
1. I'm OK with Ammonia step if the addition of sodium chloride to used ammoniacal silver solution eliminates the threat of explosion. It gives a neat way to precipitate Silver Chloride and dispose the rest of the solution.
2. It's Copper Chloride that acts bleach and not Copper Sulphate. That's the reason why Sodium Chloride is needed with Copper Sulphate for the latter to act as a bleach. In fact, Haist recommends using 47g Copper Sulphate and 11g Sodium Chloride to form 25g of Copper Chloride (page 30, vol. 2).

Hmm, I assume you are talking about using the ammonia one-shot, then. It's expensive enough here that I'm definitely not excited about that.

Murray Kelly said:
I thought we all knew that Copper Chloride was what we are talking about. Ferric chloride is another old timer.
For interest I made CuCl2 by precipitating the CuSO4 with Na2CO3 until all the blue was gone. It settled over night and the supernatant was easy to separate. Added water and enough HCl to redisolve the precipitate yielding a lovely green solution.
I didn't persist as it was fiddly for not much advantage (if any) It did rehalogenate promptly. Any HCl excess would provide chloride ions to assist.

Well I didn't know. I don't have the chemistry background some of you guys do. And I do experiments and try stuff while I learn. So that sounds like my idea about not including the NaCl will not work.

Murray Kelly · May 5, 2021

OOPS! Sorry relistan. Nothing personal, I assure you. I have learned a lot over the last few years from the really knowledgeable members .

Raghu Kuvempunagar · May 5, 2021

relistan said:
Hmm, I assume you are talking about using the ammonia one-shot, then. It's expensive enough here that I'm definitely not excited about that.

500 ml of 2% ammonia is generally usable for two rolls after which it becomes muddy and I wouldn't recommend using it. Might not be a bad idea to process two rolls one after the other and discard the solution.

Raghu Kuvempunagar · May 5, 2021

Murray Kelly said:
For interest I made CuCl2 by precipitating the CuSO4 with Na2CO3 until all the blue was gone. It settled over night and the supernatant was easy to separate. Added water and enough HCl to redisolve the precipitate yielding a lovely green solution.

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

relistan · May 5, 2021

Murray Kelly said:
OOPS! Sorry relistan. Nothing personal, I assure you. I have learned a lot over the last few years from the really knowledgeable members .

Thanks Murray, I didn’t take it personally, just making it clear where I was coming from. I appreciate all your insight on this topic and many others.

Copper Sulphate B&W Reversal Bleach

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