Nitric acid for reversal bleach?

[Raghu Kuvempunagar]

Persulphate is used in some super-proportional reducers IIRC and I have done some tests with them long time ago. Didn't notice any damage to the film. I've also used Ferric Nitrate bleach in some slide toning experiments and didn't notice any damage to the film. It's the combination, or at least as formulated in PSSB, that caused the damage.

Looking at the mechanism of PSSB, from my own notes, "Fe(III) catalyzes the generation of sulfate radicals from persulfate ions and gets reduced to Fe(II). When there is excess of persulfate ions, Fe(III) gets regenerated from Fe(II). And this cycle produces more and more sulfate radicals. Oxidation of silver is carried out by these sulfate radicals." I guess these radicals attack the film as well. So, as you said, reducing Persulphate might help.

 

[Yezishu]

Looking at the mechanism of PSSB, from my own notes.

Thank you for the information. I have some concerns about this mechanism. I don't doubt its bleaching ability, but controlling it may be challenging. The sulfate radical (2.7 V) is a much stronger oxidant than iron(III) (0.77 V), dichromate (1.33 V), cerium(IV) (1.44 V), or even persulfate(2.07 V) itself. Its oxidation potential is close to that of fluorine (2.87 V). It is commonly used in wastewater treatment to degrade almost any organic compound, which may make it unsuitable for gelatin. Additionally, sulfate radicals and iron(II) cannot coexist for long and iron(II) will soon be oxidized into iron(III), it means that the solution will decompose spontaneously even without a reductant, limiting its stability. You may have observed them. Efficiently triggering this mechanism should make it quite similar to peroxide bleaches that use hydroxyl radicals (2.8 V), but far from dichromate/cerium methods. These are just my initial thoughts, and I am not aware of specific ideas yet. I think if there is a way to avoid this mechanism to generate too much sulfate radicals, the performance of PSSB might be improved.