CP-5 bleach?

Mike Té · Feb 6, 2023

Hi all.
I came across these cans of Kodak CP-5 bleach, containing
- sodium nitrate
- pot ferricyanide
- pot bromide
Proportions unknown. The Cookbook doesn't mention this combo, although I may have missed it.
I think I remember that the CP series of chemicals are/were for colour processing (and usually machine processing?).
Anyone know if this combo would be useful for some B&W bleaching, as for pre sepia, etc.?
The quantity is a bit much, having to mix up a gallon's worth.
Thanks!

koraks · Feb 7, 2023

Mike Té said:
Anyone know if this combo would be useful for some B&W bleaching, as for pre sepia, etc.?

Looks like it. I'd give it a try. I don't think the additional nitrate is going to be much of a problem, and the pot. ferri. vs. KBr ratio isn't very critical usually.

Mike Té said:
The quantity is a bit much, having to mix up a gallon's worth.

Quite a bit more I suspect, because this bleach will likely be quite concentrated and fast if it's intended for color processing. You'll probably have to dilute it back a lot more for decent control over the extend of bleaching.

Don_ih · Feb 7, 2023

What is the sodium nitrate for?

relistan · Feb 7, 2023

Don Heisz said:
What is the sodium nitrate for?

I don't know for sure, but I think it's for forming silver nitrate alongside the silver bromide. Given how soluble silver nitrate is, I wonder if this is suitable for B&W at all. I think it might actually solubilize most of the silver and is not a normal rehalogenating bleach. This would make sense for a color process.

koraks · Feb 7, 2023

relistan said:
I don't know for sure, but I think it's for forming silver nitrate alongside the silver bromide.

Don't think so. I'm not a chemist, but it seems to me that energetically this would be opposite to what can be expected in a mixture like this. Perhaps one of the resident chemists can go into this?
My guess it's there for buffering purposes, possibly to manage pH in the bleach bath in the face of carry-over from a preceding acidic stop bath.

Don_ih · Feb 7, 2023

relistan said:
I wonder if this is suitable for B&W at all.

I'm gonna find out, because I just ordered a can. It was cheap.
I find it interesting that it's the first ingredient listed and wonder if it makes up the bulk of what's in the can.

koraks · Feb 7, 2023

relistan said:
. I think it might actually solubilize most of the silver and is not a normal rehalogenating bleach.

If that were true:
* Why is it a bleach and not a blix?
* Why would we bother with much more problematic blix formulas that are inherently unstable?
* Why would there be any potassium bromide in it if it doesn't have to rehalogenate?
You see, it doesn't add up.

relistan · Feb 7, 2023

koraks said:
If that were true:
* Why is it a bleach and not a blix?
* Why would we bother with much more problematic blix formulas that are inherently unstable?
* Why would there be any potassium bromide in it if it doesn't have to rehalogenate?
You see, it doesn't add up.

I did not say it's a blix. I said that I think you will get some developed silver turned into silver nitrate and dissolved into solution. What the full implication is of that, I cannot say. But I would not expect it to be a fully rehalogenating bleach.

Anon Ymous · Feb 7, 2023

FWIW, there's some ammonium nitrate in E6 bleaches and it's supposedly there to limit corrosion in metal parts. This CP-5 bleach was used in colour paper processing.

koraks · Feb 7, 2023

relistan said:
I did not say it's a blix.

Alright, 'silver removal bath' then
Again, the assumption you're making is that metallic silver, silver ferricyanide or silver halides will form silver nitrate in the presence of nitrates in solution. Since I'm not a chemist, I can't verify this assumption, but it doesn't sound right to me. For instance, when making silver nitrate, you generally dissolve it in nitric acid. If nitrate would be capable of the same trick, making silver nitrate would be a whole lot safer than it is now.

Anon Ymous · Feb 7, 2023

Some interesting information here:

Kodak Camera Film

Kodak

www.photomemorabilia.co.uk

relistan · Feb 7, 2023

@Anon Ymous That makes sense. But color bleaches are full bleaches, meant to remove all silver. Better suited to reversal than what the OP asked for B&W paper work.

relistan · Feb 7, 2023

Looking up the reaction, it looks like the reaction will actually head toward silver bromide. Bromide ion is bonded with by silver in preference to nitrate. So it looks like it probably is a rehal bleach. Which seems weird for color.

Anon Ymous · Feb 7, 2023

relistan said:
@Anon Ymous That makes sense. But color bleaches are full bleaches, meant to remove all silver. Better suited to reversal than what the OP asked for B&W paper work.

It's a rehalogenating bleach and it doesn't remove any silver. It converts it to AgBr, which can be removed by fixer. In order to remove metallic silver in a single step, a blix is required. With the exception of RA4, all other colour processes have separate bleach and fixing baths. Some simplified amateur kits come with blixes though.

relistan · Feb 7, 2023

Ah, yeah, you are right. Apologies... shows how much time I spend on color stuff.

Mike Té · Feb 7, 2023

Thanks for all the replies. Lots of food for thought here.
I'll give it a go, of course, will test on some reject prints at original dilution and then higher dilution.

For anyone interested, he still has quite a few cans for sale:

Kodak Bleach CP-5 Darkroom Developing Chemicals Tin Can Makes 1 US Gallon | eBay

Find many great new & used options and get the best deals for Kodak Bleach CP-5 Darkroom Developing Chemicals Tin Can Makes 1 US Gallon at the best online prices at eBay! Free shipping for many products!

www.ebay.ca

Also, quite a collection of interesting photo things among his other items.

Don_ih · Feb 15, 2023

First: the contents of the can are pretty much a solid clump. A bit hard to deal with.
Second: a teaspoon of this in 500ml of water does .... nothing that I can see to rc Ilford paper. I'll test more some other time.

CP-5 bleach?

Mike Té

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