Purpose of Bromide in bleaching?

Just curious - could anyone give me a layman (non-chemist) level answer?

We can bleach and redevelop using only ferri, but most recipes call for equal parts ferri and bromide? What are the differences of bleaching with each? What does the bromide bring to the party? What reasons would one choose just ferri vs. ferri+bromide?

When Ferricyanide oxidized Silver, some counter ion must be found to form a silver salt. If no other counter ion is present, typically Silver Hydroxide or Silver Oxide is formed. These salts are not terribly stable, and they are not very insoluble either. If you want to form somewhat stable and insoluble silver salts for further processing, an anion has to be provided by the bleach to form one. Bromide is bar far the cheapest and most available anion to serve this purpose.

Therefore if you just want to get rid of image silver (e.g. in a consecutive fixing step), a plain Ferricyanide bleach will likely do the job. If you want to sulfur tone or possibly redevelop some of the bleached silver, I'd use a bleach with Bromide.

M Carter said:

We can bleach and redevelop using only ferri

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Echoing Rudeofus, I'm not sure you can bleach and redevelop using only ferricyanide.

MattKing said:

Echoing Rudeofus, I'm not sure you can bleach and redevelop using only ferricyanide.

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Y'know, I may never have done that, but I seem to recall some toning articles where it's used without Bromide, but I may be mistaken. I think there's a Rudman technique where you use ferri and then selenium (no Bromide) but that's not really "redeveloping" of course.

M Carter said:

Y'know, I may never have done that, but I seem to recall some toning articles where it's used without Bromide, but I may be mistaken. I think there's a Rudman technique where you use ferri and then selenium (no Bromide) but that's not really "redeveloping" of course.

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Selenium to protect the shadows, then a rehal bleach (or ferri if you want) can be a very effective way to tweak highlights/ change print colours quite radically. Been around for a long, long time. Changing the halide can also affect things too. Have mainly used selenium followed by a ferri+bromide bleach - if you take a darker print, tone it quite strongly, stick it in strongish rehal bleach, you can get some odd colours. Or less toning, but fully bleach the unprotected areas & redevelop in the same print developer & that'll quite radically alter print tone too.

Lachlan Young said:

Selenium to protect the shadows, then a rehal bleach (or ferri if you want) can be a very effective way to tweak highlights/ change print colours quite radically. Been around for a long, long time. Changing the halide can also affect things too. Have mainly used selenium followed by a ferri+bromide bleach - if you take a darker print, tone it quite strongly, stick it in strongish rehal bleach, you can get some odd colours. Or less toning, but fully bleach the unprotected areas & redevelop in the same print developer & that'll quite radically alter print tone too.

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Yes, I've been doing all of the above for some time; I had it in my head that ferri alone would rehal. though, not sure now if that's true. (Still trying to get "china" bleaching figured out though, using Iodine post Selenium).

There are bleaches just using Ferricyanide followed by a toner, these give paler tones than a re-halogenating bleach, choice iof halide and the amount also makes a significant difference.

Ian

M Carter said:

I had it in my head that ferri alone would rehal. though, not sure now if that's true.

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No, it won't. When you originally develop the film, the halogens, mostly bromide in the case of film, are released into the developing solution. So when you get to the bleach, if you have not specifically added any (such as in the form of KBr) it cannot rehalogenate.

Would chloride work, rather than bromide?

barzune said:

Would chloride work, rather than bromide?

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Chloride works well but gives warmer tones than Bromide, Iodide gives colder tones.

Ian

From my understanding, you can look at it from the perspective of equilibrium kinetics. Solubility constants of various silver salts are in the order of:

ferricyanide > chloride > ferrocyanide > bromide > iodide

Silver ferrocyanide having finite solubility, the extent of its formation by oxidation of silver by potassium ferricyanide is limited due to equilibrium. In order to move the reaction to the right to achieve greater conversion, KBr is added. Silver bromide solubility is much lower (order of magnitude) than silver ferrocyanide. So the silver ferrocyanide is converted to silver bromide carrying the reaction forward and to completion.

:Niranjan.

You have to be a bit careful with Chloride, because if too much is in solution it turns into a silver solvent. The problem with Iodide is that the Ferricyanide will quickly turn it into Iodate, which has much higher solubility product with silver.