Potassium permanganate is an oxidizing agent. You do not want to use it with any of the sulfites which are reducing agents.
TL;DR – It's not the red/ox chemistry that makes a strait sulfite solution unsuitable, it's the acidity of the solution (and solubility of the oxidation products at the solution's pH).
True, KMnO4 is an oxidizing agent and sulfite is a reducing agent (Cr2O7 is also an oxidizing agent, albeit less powerful than MnO4). However, metabisulfite dissolves to bisulfite in solution, which is essentially a sulfite solution at lower pH ((there was a url link here which no longer exists) for more authoritative voices). So by using either a bisulfite or metabisulfite clearing bath, you are effectively using permanganate (oxidizer) with bisulfite (reducer). This is why the clearing bath is effective.
The reason bisulfite or metabisulfite are used with KMnO4 bleaches (rather than strait sulfite), is to produce an acidic solution. This means that Mn has an oxidation state of (II) and is soluble. If a strait sulfite solution (with no added acid) were used, the Mn would be left in the (IV) oxidation state, leaving insoluble MnO2 in the emulsion.
Note: the reaction does evolve SO2 gas however, so be sure to ventilate well.
Whenever there is a problem with reversal it is usually caused by people not following the instructions exactly whether formulas or methods. Stick with Ilford's strength recommendation for the bleach. Do ont exceed their time and temperature recommendations. If you can obtain potassium dichromate this is a better choice but hard to get with today's environmental regulations.
Once again you cannot sort of, maybe, sometimes, mostly, ... follow Ilford's instructions. Reversal is very different than ordinary development. There is essentially no latitude for variations in the method. That being said some films stand up better to reversal than others.
I really appreciate what you are saying, however it's widely reported by users that Ilford's bleach contains too much KMnO4, thereby softening the emulsion unnecessarily. I can fully bleach the film (by inspection) in about five minutes (Ilford's recommended time) using half of the KMnO4 (1g/L). I don't believe this is weakening the emulsion any more than Ilford's suggested 5min bath at 2g/L. If Ilford gave any supporting evidence/rationale for certain parts of their process, I'd be more inclined to follow it closely, however many people find that some of the practices outlined in their .pdf are not practically useable. I realize what I'm saying here may be contentious – I don't mean to provoke. I know doing anything outside of the specs is asking for trouble, and I accept full responsibility for failed results. None-the-less, I'm sure Ilford wouldn't suggest their canon process for this film, so I think some modifications may be appropriate. I'm asking here on APUG because I want to learn more chemistry and this seems to be one of the last places where people are pushing the envelope to exploit a broad array of truly magical possibilities of chemical photography. There's really something special to this place.
For the film in question try first using a hardening bath such as 3% chrome alum for five minutes normal agitation. Then followed by normal washing. I hope that none of the images are considered to be irreplaceable.
Thanks for this suggestion, I'll see if I can find some chrome alum. Since this was the first time doing this, it was a test roll with images that don't mean much to me.
Use a lower temperature (eg. 18°C).
Good advice.
Use sodium bisulfate as the acidifier of the bleach bath (don't use plain sulfuric acid).
I do use sodium bisulfate (as I can't source sulfuric acid), but I don't understand from a chemisty standpoint why this would make any difference.
Do not squegee. Use a chemical reversal bath (dithionite). Do not fix afterwards.
I agree on not squeegeeing. I imagine chem reversal (probably try Na2S) would help due to not having to load and unload the film on reels. Is there a reason why you suggest against fixing? I'm not sure I really understand the arguments for or against.
If the emulsion comes off in bits and pieces, then it could be the type of subbing used between the film base and the emulsion. The flakes are hardened emulsion lifting away from a "destroyed" subbing layer. In this case, there is probably nothing that can be done and indicates that the film in question cannot be processed by this method. IDK for sure, of course.
Fair enough. Thanks.
Switch to a dichromate bleach. It is so much more reliable and doesn't damage your film like Permanganate bleaches.
I currently can't source it here in Canada. Also, I'm a hack. As a college kid souping film in his bathroom, I'm not sure that I feel safe working with dichromate. I appreciate the suggestion though, dichromate is better in every way: more stable in solution, hardens gelatin, fast acting, and its exhaustion properties (easy to tell when it's no longer good because the exhausted Cr (III) is green).
I'm not sure I understand. Doesn't bioavailability (i.e. toxicity) depend on oxidation state? Cr (III) is in your multivitamin, Cr (VI) will kill you. A chemical’s speciation can change, so it can go from benign to deadly. Just like a child who eats lead paint chips. As we know, this can be very toxic. But if you give the child enough cola (with phosphoric acid), the lead will precipitate out as pyromorphite, and pass through their GI tract harmlessly.
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Things I learned from this thread (thanks to all of you):
1. Emulsion damage was in the sprocket area, so chem fogging (not removing/reloading films onto reels) might help.
2. Careful temperature control, processing at a lower temperature and careful agitation could help reduce the toll taken on the emulsion.
3. Despite my best efforts, the film may not be compatible with my process (KMnO4).
Some questions moving forward:
I fundamentally do not understand hardening of films, and I really want to learn more. At what point in the process should it be carried out? How reversible is the hardening reaction? Does drying a film harden it, or does it become soft again when it is re-wet? Does putting the film in alkaline solutions negate a hardener's effects? (My reversal process has a lot of changes in pH: alkaline FD, acid bleach, mild acid clear, alkaline 2nd dev). Any way of hardening with formaldehyde (I don't have access to chrome alum currently.)
Thanks as always for your responses. We really have a special community here that I treasure greatly.
