Bleach Bath Formula

[Vlad Soare]

Hello,

I'd like to understand the way a bleach bath works, and how the differences between different formulae affect the final, toned image. I'm interested in the usual, ferricyanide-based bleaches. I believe there are other compounds that can bleach a silver image, too (I remember stumbling upon a formula based on potassium dichromate some time ago), but I'll leave these out for the time being.
OK, all formulae use potassium ferricyanide. This is what actually does the bleaching job, I believe. So far, so good.
Most formulae include potassium bromide. At first glance, this seems to be used as a source of bromine - if you want to turn silver into silver bromide, then bromine must come from somewhere.
The formula of the Agfa (or Ansco) 221 bleach also calls for 20g of sodium carbonate monohydrate.

My first question regards the ferricyanide and how it works. I know that it can bleach a print all by itself. Just ferricyanide and water, and nothing else, will bleach a silver print. Obviously, it cannot turn silver into silver bromide (or any other halide) simply because there's no bromine (or any other halogen) available. But even if the resulting compound is no silver halide but a silver salt of another type (probably some sort of cyanide), it can still be redeveloped in an ordinary paper developer; I've tried this myself.
Can anybody tell me what happens, chemistry-wise, when you insert a print into a plain ferricyanide solution? What are the actual reactions?

Second, I remember reading somewhere that the amount of potassium bromide only affects the bleaching time, and nothing else. However, I'm not quite sure about this. If the ferricyanide turns silver into a silver salt, and then potassium bromide turns this salt into silver bromide, then this could mean that the bleached image will be made of a combination of these two salts (bromide and the other one), in a ratio dependent on the amount of potassium bromide (or rather on the ratio of potassium bromide to ferricyanide). So the amount of potassium bromide could theoretically influence the final tone of the toned print.
Is my reasoning correct? Is this what actually happens?

Third, what does the sodium carbonate do? Is it there merely for adjusting the pH, or does it also serve a purpose in the bleaching process itself (and maybe later on, during the toning)? I believe the more basic the bleach bath, the faster it works. Does the increased pH affect anything else beside the speed of the reaction?

OK, I admit that I could just mix a known formula (or buy one already mixed), use it, enjoy it, and forget all about this baloney.
But I'm a curious kind of guy and I like to know how things work.

Thank you.

 

[Angelo di Mango]

Unfortunately I cannot give you an answer but i am asking myself the same questions...

 

[Photo Engineer]

If you wish to tone an image, then you must use a Potassium Ferricyanide bleach bath. For best results, you must use some bromide in the solution along with the Ferricyanide. And, no cyanide salts of silver are formed. You actually reform the Silver halide which can be redeveloped and toned. So, you must fix before this type of bleach.

Do not use Dichromate bleaches if you wish to tone or redevelop your image in any way. The Dichromate/Sulfate bleaches also remove a good percentage of the silver, and render the rest unusable for toning.

PE

 

[Vlad Soare]

And, no cyanide salts of silver are formed. You actually reform the Silver halide

But how? We have silver and potassium ferricyanide (which consists of potassium, iron, nitrogen and carbon). How can it form a halide, when no halogen is present?
I'm talking about a bleach made of ferricyanide alone, with no bromide or other components. Just ferricyanide and water.

 

[Photo Engineer]

K3Fe(CN)6 + KBr + Ag ---> K4Fe(CN)6 + AgBr. And, I said for best results you should have a halide ion present.

If you do not, then you form K3Ag(CN)6 which is generally unsuitable for redevelopment or toning. It also can dissolve in the water IIRC, thereby ruining the image quality you are trying to work with.

PE

 

[georgegrosu]

Chemistry of bleaching process was presented by the PE.
Details you can find in the "Reţetar pentru laboratorul foto-film" by Pivniceru and Mioc.
When I did turn picture in sepia I used a dilute solution of bleach for color negative.
Image not totally disappears. I get a wash and a solution of sodium sulphide.
George

 

[CBG]

I am interested in what bleaches are available, and what their properties are. Digging around I see references to various formulae with more different constituents than I had known existed for bleaching.

I see what I assume are bleaching agents :Cupric Sulfate, Ammonium Persulfate, Iodine, Potassium Permanganate, Potassium Persulfate, Potassium Iodide, Ammonium Ferric EDTA solution, Potassium Dichromate, and given your comments, that all leave stains or remove silver and thus are not useful before toning or are not useful for prints in general.

 

[Photo Engineer]

All leave stains? No, they do not. Some do and some do not. Some stains can be removed and some cannot. It depends.

This is why I have contended for years here on APUG and on PN that bleaching is as complex a science as development or fixing but it has been dreadfully overlooked. I could probably give a week long course in bleaches and another on blixes.

PE

 

[CBG]

I have also got the sense that bleaches are kind of a forgotten corner of the world of photography, and that a lot of the rudimentary information lurks in hidden corners. Bleaching possibly lacks the appeal, at first glance, of other parts of the process, but the more I look, the more I want to know about bleaching, especially around local controls for BW printing, for two part toning and for contrast controls like the Sterry process.

 

[Iwagoshi]

Ron,
A course in bleaching and fixing? Sign me up. I've just spent two weeks trying to figure out why my "accidental" use of E-6 fixer (as fixer) bleached out my silver print. If "Ammonium Ferric EDTA" is indeed a bleaching solutions, I finally have an answer (thanks CBG).
Quite coincidently I'm also on the toning and bleaching chapter in Barry Thorton's book. So I have been fiddling around with bleaching back--with used E-6 fixer--selenium toned prints. Also thanks for "K3Fe(CN)6 + KBr + Ag ---> K4Fe(CN)6 + AgBr." Bleaching with straight K3Fe(CN)6 was on my agenda for today's fiddling, instead I'll head into town to buy some KBr.

Terry

 

[Iwagoshi]

I have also got the sense that bleaches are kind of a forgotten corner of the world of photography, and that a lot of the rudimentary information lurks in hidden corners...

I think part of the problem is Ansel Adams' statement on bleaching.
"I do not enjoy this process as it is uncertain and can produce strange tonal relationships. In addition, some scientist have questions about the archival effects of bleaching, although I have not personally been aware of any problems." (Ansel Adams, 1983, The Print, page 137)

As a novice I was shying away from bleaching, that is until it accidently kicked me in the head.

 

[Photo Engineer]

Well, bleaching does not affect final image stability in any way. It turns Ag into AgX which is then turned into "something else" for the toning in a B&W system. In color, Bleaching is the precursor to removal of all silver in color systems.

Ferric EDTA salts and Ferric NTA salts will bleach a silver image but there is none in E6 or C41 fix. It is in the bleach however. The combination of Ammonia and Ferric EDTA forms a mild blix, but it is not strong enough to do the job alone so it is just considered a bleach.

Yes, I could do a week course on Fixing, Bleaching and Blixing each. I could also do one on Development. That is 4 weeks of instruction. Hey, let make it a full course for credit..... Yeah, as if that would happen.

PE

 

[Iwagoshi]

... In color, Bleaching is the precursor to removal of all silver in color systems.

Ferric EDTA salts and Ferric NTA salts will bleach a silver image but there is none in E6 or C41 fix. It is in the bleach however. The combination of Ammonia and Ferric EDTA forms a mild blix, but it is not strong enough to do the job alone so it is just considered a bleach...

Ron,
My E-6 work flow does not include a wash between the bleach and fix stage, so I am assuming that my used (waste) E-6 Fixer--Ammonium Thiosulfate--is contaminated by Ammonium Ferric EDTA from the E-6 Bleach, which is beginning to sound like a bleaching "bleach." Am I on the right track? And yes I (shamefully) was using wasted, single-use film fixer for paper.

 

[Photo Engineer]

Yes, you are on the right track. You are also on the right track to ruining the fix. Carryover of bleach into fix will "exhaust" the fix about 2 - 10x faster depending on carryover. Below a certain amount, the "blixing effect" becomes almost zero though.

PE

 

[Iwagoshi]

I don't process enough E-6 to reuse the fixer, so it was a one shot deal. I was holding the waste E-6 fixer for proper disposal, that is until I had a wild hair to try it on B/W paper.

 

[Vlad Soare]

K3Fe(CN)6 + KBr + Ag ---> K4Fe(CN)6 + AgBr. And, I said for best results you should have a halide ion present.

If you do not, then you form K3Ag(CN)6

Thank you very much. That's exactly what I wanted to know.
So, if you have enough potassium bromide then you only get silver bromide and potassium ferrocyanide. But if the amount of potassium bromide is lower, then I guess you end up with a combination of silver bromide and K3Ag(CN)6 (how is this called, by the way?). So the ratio of potassium bromide to ferricyanide should dictate the ratio of silver bromide to K3Ag(CN)6, and ultimately the tone of the final image. Am I right?

which is generally unsuitable for redevelopment or toning. It also can dissolve in the water IIRC, thereby ruining the image quality you are trying to work with.

I've tried this once, using bleach A (potassium ferricyanide only) and redevelopers 1+2. Although I didn't get the purple-brown I expected, the image did redevelop normally.
Admittedly I only did this once, so I don't know how reliable and repeatable this procedure actually is.

I could probably give a week long course in bleaches and another on blixes.

That would be great. I'm in!

 

[Photo Engineer]

Actually, I'm surprised no one caught my typo.

It is really K3AgFe(CN)6.

Fe+2, K3 +3, Ag+1 = +6 and CN6 -6 balance.

Wow, did I goof. Anyhow, this mess is probably not a complex per se and it would further degrade into silver and Ferro-Ferri cyanide salts similar to cyanotype. I actually never went down this path in experimentation as I knew it would end up in some sort of a mixed brew so I don't know what you would end up with in the solution.

PE

 

[Vlad Soare]

Actually, it did seem strange to me that the iron had disappeared, but I had no time to think too much about it, as I was at work at that time (and still am). I wrote down the equation and thought I'd study it later, when I get home.