Colour bleach composition, why bromide?

[Anon Ymous]

I've been curious about it for some time. Every C41/E6 bleach formula I've seen uses a bromide salt, usually ammonium bromide. But why so? Ammonium chloride is cheaper and would be preferable from a financial standpoint, but clearly there's something wrong with it. What is it?

 

[Rudeofus]

There are two aspects which make a bleach powerful, as described in (there was a url link here which no longer exists) here on APUG:

The whole reaction can be thought of as a push/pull reaction: the oxidizer pushes the silver towards ionization, and the anion pulls the silver ions away from the pool. Together these two reactions convert metallic silver into a silver salt. A strong bleach needs a strong oxidizer and a compound that forms a highly insoluble salt.

Since Silver Chloride is a lot more soluble than Silver Bromide, any bleach based on Ammonium Ferric EDTA/PDTA and Ammonium Chloride would be a lot weaker than its counter part with Ammonium Bromide. Since the dyes already impose a severe limitation on oxidizer strength, careful choice of counter anion is the best way to make bleaching effective.

In case the question pops up: "Why not Ammonium Iodide then?": Yes, it would be even more active than its Ammonium Bromide based counter part, but Silver Iodide puts a very heavy burden on the consecutive fixing step, and Iodide is quickly oxidized to ineffective Iodate, either by aerial Oxygen, or by the oxidizer component of your bleach.

 

[Anon Ymous]

Oh, makes a lot more sense now, thanks for the clarification.

 

[trendland]

Oh, makes a lot more sense now, thanks for the clarification.

Coming to cheap workflow : in case of the use of ammonium bromide as a bleach compound like here :

- water. ..........................................800 ml
- ammonium iron EDTA
solution 50%................................150ml
- ammonium bromide...................150 g
- potassium nitrate. ........................40 g
- mercaptotriazole. ..........................0,5 g
- fill up with water.........................1000ml

the ammonium bromide ( 50g ) itself is
the replunisher - this fact makes is real
economical.

with regards

 

[Rudeofus]

Mercaptotriazole is good, but not at this high amount. I'd start with 0.1-0.15 g/l in bleach, or even better, put it into the prebleach.

 

[Anon Ymous]

...
the ammonium bromide ( 50g ) itself is
the replunisher - this fact makes is real
economical...

Yes, I've seen that you can add some ammonium bromide to an existing bleach and it will prolong its life, combined with some aeration that is. But there's got to be an upper limit of ammonium bromide that can be added a bleach solution for replenishment. Do you mean that 50g more is that limit?

 

[Rudeofus]

There was a thread here some time ago about the full chemical process taking place during bleaching and bleach aeration, and IIRC, pH goes up if used bleach is aerated. I would therefore assume, that Ammonium Bromide plus aeration is insufficient as replenishment regime. As you replenish, there will be a buildup of Ammonium plus whatever counter anion is added when pH is brought back down, and this buildup will eventually render the bleach ineffective. There will also be some water carryover from preceding wash cycle, diluting the bleach agent. A real replenisher will therefore contain Ammonium Bromide, some acid, and Ammonium Ferric EDTA/PDTA.

 

[Anon Ymous]

... A real replenisher will therefore contain Ammonium Bromide, some acid, and Ammonium Ferric EDTA/PDTA.

Besides, in a replenished system, some used bleach is discarded and there's no buildup like when adding ammonium bromide to a reused solution.

 

[trendland]

As you mentioned - in most cases the bleach is propper again when you are mixing/wiring oxigen into solution.

The next most case will be caused from
incorrect ph.
You just have to adjust the ph of the bleach and it will be fine again.

Comming to real exaustion - the replunishment is 50g ammonium bromide/ per replunishment.
Of cause you should check ph with this.

I would not like to say this method is good to 140.000 films (with 397 replunishments of the same solution )
We all could imagine that it is limited to some times .....
But you definitivly can replunish the bleach a couple of time with the need of all compounds and the addition of 150g
ammonium bromide (to 3 times replunishment) it is quite economical.

with regards

 

[trendland]

I would guess ( not as theory to chemical reactions ) just as a thought (
of logical concerns ) : at the time the bleach has a lost of reaktion to work quite well with your films (it doesn't bleach any more in correct way) and we
would like to say : "Honey I am sad ...
because my bleach solution is exhausted - just look to our nice holyday shots ....." at this time is no lost of full 150 g amonium bromide - am I right....!!
And the compounds in the solution should also have a rest of power!
Sure you have to make a complete new bleach to one day in the future.
But after a very great amound of bleached films - the very last film will show you : " Friend t his is the definitive END of replunishement : to avoid this you should make Tests before use"


Bon Chance