Potassium Dichromate and Sodium Bisulfate Bleach - Reversal process paper

I’ve seen some mention of using sodium bisulfate as a replacement for sulfuric acid in the traditional potassium dichromate based bleach for reversal. Can this be used for reversal processing black and white paper (making direct positives) and if so, can anyone who’s done this successfully post their formula (full process, not just the bleach) and example images?

Side note: I’m aware that there are several other reversal formulas of varying safety (permanganate, hydrogen peroxide + citric acid, etc). I’m trying many formulas so not looking for alternatives to using dichromate in this case. Looking for specific details on this particular formulation (if it works for paper).

Hello @Fragomeni: some time ago I dabbled with reversal processing of paper and made some notes based on my findings and reading. Sharing the same here. YMMV.

Raghu Kuvempunagar said:

Hello @Fragomeni: some time ago I dabbled with reversal processing of paper and made some notes based on my findings and reading. Sharing the same here. YMMV.

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Thanks Raghu, this is helpful. I notice in your notes that for the bleach you’re referring to Potassium Bisulfate rather than Sodium Bisulfate being used as a replacement for sulfuric acid. Do you recall why you referenced one rather than the other or both? I think I’ve only seen Potassium Bisulfate referenced a few times including in Ilford’s reversal process for film (but it’s only referred to in the clearing bath) and is treating interchangeably with Sodium Bisulfate. What I’m getting at is do you know if the volume and behavior for Sodium Bisulfate would be the same as the Potassium Bisulfate referenced in your notes? I believe they would achieve the same end with the same volume used (they are just being used as alternative acidifies here and I think would just produce different additional byproducts re potassium vs sodium).

Also, in your notes, are you referring to concentrated sulfuric acid or one of the more commonly found dilutions like 35% battery acid etc?

You should be able to use Sodium hydrogen sulphate as the substitute for Sulphuric acid in place of Potassium hydrogen sulphate. I use the Potassium salt as it is more easily available to me in the local chemical stores. I think the substitution formula for Sodium salt is 4.4g of the salt for every ml of concentrated Sulphuric acid. So 12ml comes to around 50g.

Raghu Kuvempunagar said:

You should be able to use Sodium hydrogen sulphate as the substitute for Sulphuric acid in place of Potassium hydrogen sulphate. I use the Potassium salt as it is more easily available to me in the local chemical stores. I think the substitution formula for Sodium salt is 4.4g of the salt for every ml of concentrated Sulphuric acid. So 12ml comes to around 50g.

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Thanks very much, this is very helpful. I thought I had sodium bisulfate in my darkroom but it turned out to be sodium bisulfite which wont work so I've placed an order for the sulfate and will do some tests when it arrives. Thanks for sharing your notes!

It works for paper, but it's been a while since I used it for that, and I have no prints to show. I still use the reversal bleach for lith film reversal. The formula isn't that critical. My usual mix is 10ml of a 13% potassium dichromate solution, about a teaspoon of sodium bisulfate, and water to 250ml. I re-use the bleach until it takes over 90 seconds to bleach all traces of black.

For permanganate, I used 250ml of a 1% permanganate solution and about 1 teaspoon of bisulfate. The issue with permanganate is that it softens the paper surface. Dichromate doesn't do that.

You probably know that dichromate bleach can be cleared with sodium sulfite, but permanganate has to be cleared with bisulfite or metabisulfite. The clearing bath can actually remove undeveloped halide, so it helps to keep it to a minimum.