Role of Sodium thiosulfate in Farmer's reducer

[Alex Benjamin]

Thanks for all the comprehensive and informative answers gentlemen!

Doremus

+1

 

[RalphLambrecht]

If you remove the thiosulfate, the ferricyanide bleaching action results in virtually colorless and invisible silver ferrocyanide complexes as highlighted above by @Raghu Kuvempunagar . So also without thiosulfate the action is immediately visible.


Arguably, bleaching and fixing separately gives more control because you can go back if you overdo the bleaching step, at least if you use a rehalogenating bleach.


This is factually incorrect as anyone can attest to who has done this. The resulting 'blix' remains active for quite some time and in practice more than long enough to work on a print or negative. While it is not long-term stable and it is indeed recommended to mix prior to use in the quantity needed for the session, the 'within minutes' qualification is overly pessimistic.

Farmer's Reducer(ferri+fixer)will perfprm exactly as I said earlier. You apparently need to try this to find out.
If you remove the thiosulfate, the ferricyanide bleaching action results in virtually colorless and invisible silver ferrocyanide complexes as highlighted above by @Raghu Kuvempunagar but this won't be visible unless they are removed by the fixer

Here's what happens when fixer is added to the potassium ferrocyanide solution:

Enhanced Bleaching Action: The fixer doesn't stop the bleaching—instead, it creates a more efficient and controlled process. The potassium ferricyanide (formed when ferrocyanide is oxidized) continues to bleach the metallic silver by converting it to silver ferricyanide, but now the fixer simultaneously dissolves these bleached silver compounds.

Continuous Process: Without fixer, the bleached silver compounds would remain in the emulsion as visible residue. With fixer present, these compounds are immediately dissolved and removed, allowing the bleaching action to continue more effectively. This prevents the formation of stains and allows for more complete reduction.

Controlled Reduction: The combination allows for better control over the bleaching process. You can monitor the reduction visually since the dissolved silver compounds are removed in real-time, making it easier to stop at the desired density.

pH Considerations: The fixer helps maintain a more stable pH environment, as the thiosulfate solution buffers the acidic byproducts of the bleaching reaction.

Practical Result: This combination creates a reducer that works faster and more efficiently than ferricyanide alone, with reduced risk of staining and improved control over the final result. It's particularly effective for local dodging and burning corrections in traditional darkroom work.

 

[koraks]

Farmer's Reducer(ferri+fixer)will perfprm exactly as I said earlier.

I didn't contest it did, Ralph. Perhaps you need to re-read my previous post; I think you may have misinterpreted it.

Here's what happens when fixer is added to the potassium ferrocyanide solution:

Am I correct in assuming you're relaying AI (e.g. ChatGPT) output now? Just checking. Either way, I notice you wrote 'ferrocyanice'. Farmer's reducer and regular 'ferri' bleaches are based on ferricyanide, not ferrocyanide. I suppose it's technically possible to start with the latter, but AFAIK it's uncommon. All commercial and DIY bleaches using this chemical approach that I've come across start with ferricyanide, which is readily available and cheap.

The fixer doesn't stop the bleaching—instead, it creates a more efficient and controlled process. The potassium ferricyanide (formed when ferrocyanide is oxidized) continues to bleach the metallic silver by converting it to silver ferricyanide, but now the fixer simultaneously dissolves these bleached silver compounds.

This bit of the AI output is a little confusing, IMO. While taken at face value, it seems plausible enough, at the detail level it's a bit odd. Firstly, there's the mention of fixer supposedly stopping the bleaching, which is a notion that AFAIK hasn't been put forth by anyone. So it's a little odd that the answer involves a response to that statement, which wasn't made. Secondly, there's the odd issue involving ferrocyanide vs. ferricyanide. I suspect that this may have originated in a typo in the prompt in the AI tool which might have erroneously involved 'ferrocyanide' instead of 'ferricyanide'. Perhaps you can confirm by going back to your prompt and checking for this typo. Thirdly, there's the somewhat vague phrase about "the fixer simultaneously [dissolving] these bleached silver compounds" - esp. the 'bleached silver compounds'. What are they, exactlty? For a good understanding of what's really going on from a chemical viewpoint, it makes a difference whether the fixer facilitates the removal/dissolution of a silver-ferrocyanide complex, or whether a silver-thiosulfate complex is dissolved. The practical implication is whether the bleach is being 'used up' in the reduction (chemically speaking: oxidation) process or not.

Without fixer, the bleached silver compounds would remain in the emulsion as visible residue.

This is what I was responding to specifically, and as also put forth by @Raghu Kuvempunagar above, the silver compounds that result from ferricyanide bleaching are not really visible on an emulsion on paper. They are visible (with some difficulty) on film, but usually only in those areas where bleaching is complete, as these subtle milky hues are readily overpowered by any remaining metallic silver image. I'm sure you have done a fair share of bleaching of film and paper, as have I, so you must be familiar with this effect. Again, this is what I was commenting on, and the nature of my comment was that Farmer's reducer is not unique in immediately showing the degree of bleaching. A non-fixing bleach shows the same. This is true not only for ferricyanide-based bleaches, but also for instance dichromate- and permanganate bleaches.

What I responded to was the 'better control', which is arguably not correct in my view. Farmer's reducer gives immediate feedback and when used judiciously, it gives good control. But arguably, better control would be awarded by a bleach that (1) is reversible so that any overbleaching can be corrected, and/or (2) a bleach that's more gentle/not so fast. As to (1), Farmer's reducer is at the disadvantage as it's irreversible. As to (2), this can of course be controlled by dilution, but practical experience (which I'm sure, again, you have as well as I do) shows that at the same concentration of ferricyanide, Farmer's reducer is considerably faster (and more rapidly spiraling out of control) than a plain ferricyanide bleach (without fixer).

The fixer helps maintain a more stable pH environment, as the thiosulfate solution buffers the acidic byproducts of the bleaching reaction.

I'd like to see chemical evidence of this. The thiosulfate ion is not a buffer to the best of my knowledge. Secondly, what are these supposed acidic byproducts of the bleaching reaction? If you look at post #15 where I posted the brief explanation of the ferricyanide bleaching mechanism as described by Glafkides, there's as far as I can tell no direct influence on pH. What am I missing, can you pinpoint this?

To be accurate, we may have to differentiate here between 'fixer' and 'thiosulfate'. The AI response appears to use them interchangeably, but this is not correct in this case - and I can see how things might change if we take 'fixer' to more literally mean just that: a fixer solution that contains thiosulfate, but possibly also other compounds. Most photographic fixers are indeed buffered (not by the thiosulfate itself; typically with an acetic acid/acetate buffer) to maintain a favorable pH. Adding such a fixer to a ferricyanide bleach will add a buffer action to the resulting reducer solution. But it's not due to the thiosulfate.

This combination creates a reducer that works faster and more efficiently than ferricyanide alone, with reduced risk of staining and improved control over the final result. It's particularly effective for local dodging and burning corrections in traditional darkroom work.

Interestingly, staining is mentioned here, but no explanation is provided in the preceding content. Perhaps you can ask your AI tool to expand on that part of the answer.
And to reiterate, I disagree with the 'improved control', partly because of the faster action that AI mentions itself, and partly due to the irreversible nature of the reducer vs. the simple bleach. Whether the reducer is more efficient, as AI states, is also an interesting detail that's not clear in the present form of the answer. It might be part of the first bit I highlighted.

Overall, I think the (assumed) AI response contains some interesting leads, but I'm also quite hesitant to take it at face value as you can see. Perhaps it's good enough if someone just wants to know if they can use Farmer's reducer on a print and they don't care how it works, whether there are alternatives and whether such alternatives might be suitable or even superior for their use case. But for those who do want to know what happens and who do want to make the best choice of their own work, I think the response is incomplete at best, and misleading or even incorrect at worst.

 

[RalphLambrecht]

I didn't contest it did, Ralph. Perhaps you need to re-read my previous post; I think you may have misinterpreted it.


Am I correct in assuming you're relaying AI (e.g. ChatGPT) output now? Just checking. Either way, I notice you wrote 'ferrocyanice'. Farmer's reducer and regular 'ferri' bleaches are based on ferricyanide, not ferrocyanide. I suppose it's technically possible to start with the latter, but AFAIK it's uncommon. All commercial and DIY bleaches using this chemical approach that I've come across start with ferricyanide, which is readily available and cheap.


This bit of the AI output is a little confusing, IMO. While taken at face value, it seems plausible enough, at the detail level it's a bit odd. Firstly, there's the mention of fixer supposedly stopping the bleaching, which is a notion that AFAIK hasn't been put forth by anyone. So it's a little odd that the answer involves a response to that statement, which wasn't made. Secondly, there's the odd issue involving ferrocyanide vs. ferricyanide. I suspect that this may have originated in a typo in the prompt in the AI tool which might have erroneously involved 'ferrocyanide' instead of 'ferricyanide'. Perhaps you can confirm by going back to your prompt and checking for this typo. Thirdly, there's the somewhat vague phrase about "the fixer simultaneously [dissolving] these bleached silver compounds" - esp. the 'bleached silver compounds'. What are they, exactlty? For a good understanding of what's really going on from a chemical viewpoint, it makes a difference whether the fixer facilitates the removal/dissolution of a silver-ferrocyanide complex, or whether a silver-thiosulfate complex is dissolved. The practical implication is whether the bleach is being 'used up' in the reduction (chemically speaking: oxidation) process or not.


This is what I was responding to specifically, and as also put forth by @Raghu Kuvempunagar above, the silver compounds that result from ferricyanide bleaching are not really visible on an emulsion on paper. They are visible (with some difficulty) on film, but usually only in those areas where bleaching is complete, as these subtle milky hues are readily overpowered by any remaining metallic silver image. I'm sure you have done a fair share of bleaching of film and paper, as have I, so you must be familiar with this effect. Again, this is what I was commenting on, and the nature of my comment was that Farmer's reducer is not unique in immediately showing the degree of bleaching. A non-fixing bleach shows the same. This is true not only for ferricyanide-based bleaches, but also for instance dichromate- and permanganate bleaches.

What I responded to was the 'better control', which is arguably not correct in my view. Farmer's reducer gives immediate feedback and when used judiciously, it gives good control. But arguably, better control would be awarded by a bleach that (1) is reversible so that any overbleaching can be corrected, and/or (2) a bleach that's more gentle/not so fast. As to (1), Farmer's reducer is at the disadvantage as it's irreversible. As to (2), this can of course be controlled by dilution, but practical experience (which I'm sure, again, you have as well as I do) shows that at the same concentration of ferricyanide, Farmer's reducer is considerably faster (and more rapidly spiraling out of control) than a plain ferricyanide bleach (without fixer).


I'd like to see chemical evidence of this. The thiosulfate ion is not a buffer to the best of my knowledge. Secondly, what are these supposed acidic byproducts of the bleaching reaction? If you look at post #15 where I posted the brief explanation of the ferricyanide bleaching mechanism as described by Glafkides, there's as far as I can tell no direct influence on pH. What am I missing, can you pinpoint this?

To be accurate, we may have to differentiate here between 'fixer' and 'thiosulfate'. The AI response appears to use them interchangeably, but this is not correct in this case - and I can see how things might change if we take 'fixer' to more literally mean just that: a fixer solution that contains thiosulfate, but possibly also other compounds. Most photographic fixers are indeed buffered (not by the thiosulfate itself; typically with an acetic acid/acetate buffer) to maintain a favorable pH. Adding such a fixer to a ferricyanide bleach will add a buffer action to the resulting reducer solution. But it's not due to the thiosulfate.


Interestingly, staining is mentioned here, but no explanation is provided in the preceding content. Perhaps you can ask your AI tool to expand on that part of the answer.
And to reiterate, I disagree with the 'improved control', partly because of the faster action that AI mentions itself, and partly due to the irreversible nature of the reducer vs. the simple bleach. Whether the reducer is more efficient, as AI states, is also an interesting detail that's not clear in the present form of the answer. It might be part of the first bit I highlighted.

Overall, I think the (assumed) AI response contains some interesting leads, but I'm also quite hesitant to take it at face value as you can see. Perhaps it's good enough if someone just wants to know if they can use Farmer's reducer on a print and they don't care how it works, whether there are alternatives and whether such alternatives might be suitable or even superior for their use case. But for those who do want to know what happens and who do want to make the best choice of their own work, I think the response is incomplete at best, and misleading or even incorrect at worst.

here is a link to an interesting Youtube video showing thbasics of working ith Farmers Reducer in which isalso mentioned BTW that the solution is only active for about 10 minutes.then a new solution hs to be prepared.

 

[koraks]

I'm aware of common lore @RalphLambrecht . Like most online sources and your earlier AI response, the content is eerily thin in chemical theory. And repeated assertions don't necessarily make truth.

 

[Alex Benjamin]

here is a link to an interesting Youtube video showing thbasics of working ith Farmers Reducer in which isalso mentioned BTW that the solution is only active for about 10 minutes.then a new solution hs to be prepared

I'm aware of common lore @RalphLambrecht . Like most online sources and your earlier AI response, the content is eerily thin in chemical theory. And repeated assertions don't necessarily make truth.

A few pre-AI (not the Nikon type ) repeated assertions:

"Your solution of ferri and fixer will get paler and become inactive after 10-15 minutes, depending on its concentration and the strength and freshness of the fixer."
— Tim Rudman: The Photographer's Master Printing Course

"You can mix your own Farmer's reducer from ferricyanide, hytpo and water... Dissolve it just before using: once dissolved, it doesn't last long."
— David Vestal: The Craft of Photography (updated edition)

"Farmer's Reducer consists of separate potassium ferricyanide and sodium thiosulfate stock solutions. These are combined just before use. Once combined, they interact destructively and have a working life of only a few minutes."
— Arnold Gassian: Exploring Black and White Photography

"The best known and moxt widely used subtractive reducer is Farmer's solution. It is almost always kept in the form of two stock solutions... The stock solutions keep indefinitely, the mixed [potassium ferricyanide and hypo] solution has very little keeping power."
— C. I. Jacobson & R. E. Jacobson: Developing (1976 edition)

They may be short on chemical explications — "they interact destructively" being the closest we have — but coming from both chemists and experience users, they seem to point to a certain truth about the duration of the mix.

 

[koraks]

I never contested it's short-lived, but "minutes" as alleged earlier is overly pessimistic. I did a quick test this afternoon. The reducer, which I mixed on the weak side to begin with, was still happily reducing (as well as actually fixing) after about 15 minutes. I then had to go out and only came back several hours later, at which point it was dead, unsurprsingly. However, in the interim, substantial further bleaching action had occurred on strips left in it, so its useful lifespan was at least more than 15 minutes, but less than 4 hours. The solution does get noticeably pale quite early on compared to a control solution of just ferricyanide, but it doesn't stop working right at that point.

It's easy enough to try.

I'm still awaiting another brilliant YouTube video or AI response on the underlying chemistry. I'm afraid I'll have to resort to good old fashioned lit research for that, however.