Substituting salt for potassium bromide in bleaching

quiver

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I was wondering if anyone here has substituted sodium chloride for the potassium bromide in the rehalogenating ferricyanide bleach that is often used in home development of color films? Would it be inadvisable to use iodized salt as a substitute? I do have potassium bromide, but I would rather conserve on this salt if possible, and ferricyanide bleach requires a fairly large amount of it.
 

Donald Qualls

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Seems to me I recall that ferricyanide won't rehalogenate to silver chloride -- reaction potentials or solubility limits, as I recall. The iodide in the salt becomes your only active halogen source. Potassium iodide isn't hard to get, perhaps try that instead (require rapid fixer instead of plain hypo, but otherwise it's fine).
 

koraks

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quiver said:
I do have potassium bromide, but I would rather conserve on this salt if possible
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KBr is pretty cheap. No need to preserve it. Just get some more.

NaCl (kitchen salt) will work for a bleach, but it's not optimal. Bleaching may not be entirely complete, it may be slow (due to Na instead of K) and ultimately may not fix out entirely or rapidly enough.

So again, just get some more KBr. The cheap stuff is perfectly fine for this; doesn't have to be expensive lab grade or anything.
 

nmp

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Something to do with solution kinetics.

Solubility constants 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. Notice chloride is on the other side so it will move the conversion in the opposite direction.

Or you can hear what PE had to say about the subject:

https://www.photrio.com/forum/threads/rehalogenation-with-a-chloride.27451/

:Niranjan.
 
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quiver

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I miss PE.

It looks like the unsuitability for rehalating paper is an endorsement for a color process bleach. Iodized salt could only help from what I see.

Cost isn't so much of an issue, but availability is. I need to order out for KBr, for salt I only need to visit the store.
 

Donald Qualls

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If you have a source for it, you could add some elemental iodine (drug store tincture should work) to the solution with table salt. Enough of the iodine should ionize to give that push to the right in the solution kinetics list above (I think). I'm not a chemist, I just understand bits and pieces of this stuff. A trace of lye might be needed to push the iodine into its ionic state, but you can still buy lye at supermarkets or home improvement stores, at least some places.
 

Raghu Kuvempunagar

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quiver

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Right, so rehalating with chloride probably won't happen in the presence of ferrocyanide, but will probably have a blixing action instead. In the end I don't see this as a disadvantage in a color process were the goal is ultimately the complete removal of the silver image. I wonder how long it would take for a ferricyanide-sodium chloride blix to completely clear a silver image? Still need a fix for the portion that was undeveloped, but it might give a little bit more life to the fixer.

I think this will require experimentation.
 

Raghu Kuvempunagar

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Can't you use Ferricyanide+Thiosulphate reducer followed by a fixing step?
 

Ian Grant

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Sodium Chloride works in a rehalogenating bleach, it's main use is to achieve warmer tones when sepia toningcompared to a Ferricyanide/Bromide bleach. Potassium Iode can also be used as an alternative and that gives more purplish tones.

Ian
 
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quiver

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Raghu Kuvempunagar said:
Can't you use Ferricyanide+Thiosulphate reducer followed by a fixing step?
Click to expand...

That would greatly reduce the lifespan of the solution.

Ian Grant said:
Sodium Chloride works in a rehalogenating bleach, it's main use is to achieve warmer tones when sepia toningcompared to a Ferricyanide/Bromide bleach. Potassium Iode can also be used as an alternative and that gives more purplish tones.

Ian
Click to expand...

That's interesting to see, but do I assume correctly that this is with paper? Have you any experience with these variations with film?
 
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