Role of Sodium thiosulfate in Farmer's reducer

Alex Benjamin

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Found many answers to my questions regarding bleach in various Phototrio threads, except those regarding sodium thiosulfate.

I do understand the role of the potassium ferricyanide, of course, and also understand that the addition of potassium bromide makes the bleach a rehalogenating bleach, meaning I can re-develop the print.

What I'm not sure I understand is the role played by sodium thiosulfate in Farmer's reducer. What, exactly is its impact? Can I still re-develop if my solution contains both potassium bromide and sodium thiosulfate, or is that cancelled because of the sodium thiosulfate? Do I still need to fix the print afterwards?

I use this formula from the latest edition of The Darkroom Cookbook:

 

revdoc

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Sodium thiosulfate is fixer.

The reducer does this:

Ferricyanide and potassium bromide convert silver to insoluble silver bromide.
Sodium thiosulfate forms complexes with silver bromide and effectively dissolves it, removing it from the emulsion.

The second step is just what fixer does with film and paper normally, and makes this process irreversible.

You shouldn't have to fix after reducing, but I guess a lot of people will, anyway.
 

koraks

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The thiosulfate does two things: (1) it accelerates the bleaching action and (2) it renders it irreversible. The thiosulfate acts as a fixer just like in a regular B&W process. What's fixed out, cannot be redeveloped later. Hence, no fixing is needed after Farmer's reducer, no redevelopment is possible, but the print of course should be washed at the end of the process.
 
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Alex Benjamin

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MattKing

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FWIW, I always fix after using Farmer's Reducer, because I always fix after using a re-halogenating bleach.
Just in case the amount of thiosulfate concentration in the Farmer's reducer is off.
And to be sure.
Of course, sometimes I wear a belt and suspenders too.
Does anyone know what the role of Potassium bromide is in Farmer's Reducer?
 

reddesert

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I am not a photographic chemist, but ... you need a source of halide, right? Development is a reducing reaction. Bleach undoes development, it's an oxidizing reaction. You have a print with metallic silver, the ferricyanide oxidizes some of the silver to silver ion, which can then react with the bromide to make silver bromide. This is an (insoluble) silver halide like undeveloped silver halide, and the sodium thiosulfate reacts with it like fixer (it is fixer) to make a soluble complex that can then be washed out.
 

MattKing

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That makes sense!
 
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Alex Benjamin

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MattKing said:
Does anyone know what the role of Potassium bromide is in Farmer's Reducer?
Click to expand...

In addition to reddesert's excellent explanation, here's Tim Rudman's:

 

john_s

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But Farmer's Reducer works without bromide. See, for example:

Will the real Farmer's Reducer formula please stand up?

The versions of this formula in the Darkroom Cookbook (both 3rd and 4th editions) lists 60.0 grams of Potassium ferricyanide and 30.0 grams of Potassium bromide in water to make 250ml for Solution A. Most of the internet sources I've looked at recently call for 64.0 grams of Potassium...
www.photrio.com
 
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Alex Benjamin

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Yes, but these versions, Kodak R-4a or the Ansco formula Ian Grant gives in post #5, are also used for negatives.

It does work with prints, but without the bromide, you cannot re-develop. The Potassium bromide is essential if you are doing sepia or lith toning, or if you want to re-develop in a different developer than the first one.
 

john_s

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Farmer's works on negatives without bromide. I think people find it convenient to have the ferri with bromide as it is multi purpose, so one bottle not two. If you use that bleach mixed with thiosulphate my understanding is that the bromide has no effect.
 

Raghu Kuvempunagar

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MattKing said:
Does anyone know what the role of Potassium bromide is in Farmer's Reducer?
Click to expand...

"The presence of potassium bromide in Farmer's reducer has been said to increase the length of the activity by about 30%. The rapidity of action is decreased by the presence of the bromide, but the slower action is beneficial for the treatment of photographic prints."

Grant Haist Modern Photographic Processing Vol2 Page 58.
 

koraks

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Alex Benjamin said:
In addition to reddesert's excellent explanation, here's Tim Rudman's:
Click to expand...
Which is not the most fortunate excerpt, in all honesty. One of the things he mentions in relation to fixing is that "...the fix converts everything to plain metallic silver". This is not the case! He continues: 'the bromide bubbles off into solution' - which is not technically correct, although less problematic from a practical viewpoint than the other mishap. I don't doubt Rudman's understanding of what does happen, but the way he formulated it in that specific bit is really unfortunate. He also stats that "at this point [after fixing] we can put the lights on", which of course is true, but it doesn't mean you can't turn the lights on earlier if you use a stop bath.
 

Doremus Scudder

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A more interesting question for me is how potassium ferricyanide bleaches a silver image when used alone, without bromide or thiosulfate.

Doremus
 

Raghu Kuvempunagar

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Doremus Scudder said:
A more interesting question for me is how potassium ferricyanide bleaches a silver image when used alone, without bromide or thiosulfate.
Click to expand...

Potassium Ferricyanide converts Silver to Silver Ferrocyanide which is a pale yellow insoluble compound.

4Ag + 4K3[Fe(CN)6] ⟶ Ag4[Fe(CN)6] + 3K4[Fe(CN)6]

So when only Potassium Ferricyanide is used as bleach, Silver isn't removed from the print/film, it is transformed to Silver Ferrocyanide which can be subsequently removed using Thiosulphate/Fixer.
 

reddesert

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It had never occurred to me until this thread, that it is a little ironic that "Farmer's reducer" is an oxidizing agent, not a reducing agent.
 

Raghu Kuvempunagar

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reddesert said:
it is a little ironic that "Farmer's reducer" is an oxidizing agent, not a reducing agent.
Click to expand...

"Photographic reduction is a chemical oxidation process, a somewhat confusing situation, because chemical reduction is not the principal reaction when the term "reduction" is used. Reduction refers to the lessening of the amount of metallic silver in the image, not the chemical reaction involved in the solubilization of the image silver."

Grant Haist in Modern Photographic Processing Vol 2.
 

F4U

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Doremus Scudder said:
A more interesting question for me is how potassium ferricyanide bleaches a silver image when used alone, without bromide or thiosulfate.

Doremus
Click to expand...

Yes it does. but it works harshly. Not being photographic chemist, off hand I'd say the the hypo works as a sort of buffer, for lack of a better term in my vocabulary.
 

koraks

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reddesert said:
It had never occurred to me until this thread, that it is a little ironic that "Farmer's reducer" is an oxidizing agent, not a reducing agent.
Click to expand...
Yeah, it's funny, isn't it?

F4U said:
Yes it does. but it works harshly. Not being photographic chemist, off hand I'd say the the hypo works as a sort of buffer, for lack of a better term in my vocabulary.
Click to expand...
You'll see that the question was answered twice above in more chemically sound terms. More importantly, Doremus was inquiring after the situation where hypo is not present, and only a solution of ferricyanide is applied. Concerning the term 'buffer', in a chemical sense this has a particular meaning; see e.g. here: https://en.wikipedia.org/wiki/Buffer_solution It does not apply here, although technically speaking, a bleach can be (and in commercial chemistry often is) buffered.
 

RalphLambrecht

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the role of the thiosulfate is to dissolve the bleached silver salts right away so the effect of bleaching is immediately visible during bleaching, thereby having more control over the process If you bleach and then fix,the bleaching process is a bit of a guessing game. However, mixing bleach with thiosulfate is not recommended because the mixture loses activity within minutes. If at all only mix just prior to use. or better yet, fix after bleaching.
 

koraks

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RalphLambrecht said:
so the effect of bleaching is immediately visible during bleaching
Click to expand...
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.

RalphLambrecht said:
thereby having more control over the process If you bleach and then fix
Click to expand...
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.

RalphLambrecht said:
mixing bleach with thiosulfate is not recommended because the mixture loses activity within minutes
Click to expand...
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.
 
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Alex Benjamin

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Thanks for bringing this up, koraks. Regarding the mix's active life, I've read and heard from "a few minutes" to "about 1/2 hour under certain circumstances" before it dies. Of course, the last thing you want to do is work fast when bleaching, especially if you're bleaching small, specific sections of your print.
 

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