Questions about Potassium Thiocyanate and Ilford black and white reversal

In the near future I will be making a developer that will contain Potassium Thiocyanate in it. After development can a stop bath be used or should I resort to plain old washing for about a minute? According to one place that sells thiocyanate there is only one mention of extreme caution with nitric acid, Decomposes out to NO and HCN. Would this apply to any acid or just certain ones that would create HCN?

The only warning I've seen is for D-94 that it should not be combined with permanganate bleach. Unfortunately, there is no explanation why, but the only qualitative difference with D-94A is precisely in the thiocyanate. At the same time, the reversible processes of Kodak and ORWO do not use an acid stop bath, I have no idea if it is related to thiocyanate.
If a parallel can be drawn with potassium ferricyanide (I don't know if it is appropriate), there the danger of cyanide release is related only to mineral acids, not to organic ones.

Thyocinate can produce hydrogen cyanide in permangante bleach (permanganate and sulphuric acid). Highly toxic stuff you dont want to be near of.

I have developed color slides in the past. This would be my first time with doing black and white reversal. I do have the Ilford document for the reversal steps. I feel a little edgy about adding hypo to a developer but the measurements look minuscule enough not to cause much of an issue.

Originally I was going to use D-19 for the developer. Would a longer wash remove most of the thiocyanate before the bleaching step? If not I can go with the ID-62 formula/PQ formula and the Ilford instructions. I have all of the ingredients for that developer except for the benzotriazole.

Personally I'd start with thiosulfate as a silver solvent and only move towards thiocyanate if there's a compelling reason to do so. It's one more chemical to stock - not that it's particularly expensive or difficult to get hold of, but since you probably have some thiosulfate around anyway, might as well use that.

koraks said:

Personally I'd start with thiosulfate as a silver solvent and only move towards thiocyanate if there's a compelling reason to do so. It's one more chemical to stock - not that it's particularly expensive or difficult to get hold of, but since you probably have some thiosulfate around anyway, might as well use that.

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Is there a well-founded comparison for quantitative substitution anywhere? While the formulas with thiocyanate or DTOD have exact amounts, Ilford gives floating values, for which it is not clear whether they are adequate with other formulas...

I'm no expert in this field and I'd defer to e.g. @Ivo Stunga when it comes to B&W reversal. But as far as I can tell, B&W reversal formulas will always have to be tailored to the type of film for optimal results.

Having challenged myself to reverse most (if not all) films, so far I can do everything I need with sodium thiosulfate, have used just that and cannot say anything about analogues.

Most films need just a tiny amount of it (0 to a gram), but some - 5g per 500ml developer (like Delta 100), so yes - it's film dependent and the logic is to reduce density (if needed) enough to have a balanced slide with great DMIN and DMAX.
And I can make a good fixer out of the same raw chemical (stabilizing it with a chem for Clear), making hypo a no-brainer.

Potassium Thiocyanate has been the preferred halide solvent in professional B&W reversal processes including Kodak, Agfa Scala and Orwo. Dr5 is an exception, but they don't use a halide solvent in their first developer apparently and nobody has complained.


Haist, Vol 2, page 319.

I will be placing an order for hypo soon. What is cool is that you can get the hypo in buckets. This stuff has many uses. I was looking at some of the recipes for making up a rapid fixer from hypo. Might even give that a go. My default fixer for a long time has been Hypam.

I will grab a small bit of thiocyanate just to have it on hand.

I'm also looking into sodium bisulfate instead of sulfuric acid. I see a lot of it on Amazon in buckets but I am unsure about the purity. Many places have bisulfate but in very small quantites. The only place I have found bisulfate in a one pound quantity is at the Formulary but the shipping prices are prohibitively expensive.

I did find a place for 30% sulfuric acid, if I can't get the bisulfate. I'll mix it down to 10% when ready to use.

I have noticed two recipes for PQ: One is the ID-62 and the other being the PQ concentrated which is 2.5 times the strength. The instructions for Ilford reversal indicates to use PQ: at 1+5 for the first developer and 1+9 for the second. I am taking it this is from the stock mix that is from the 2.5 times concentrate recipe that gets diluted to those 1+5 and 1+9 dilutions or iis it from the ID-62 mix?

Both recipes use benzotriazole. I read a post from 15 years ago that benzotriazole was mainly for controlling image color. I am taking it this is concerning papers and not film? I knew of it as an antifog agent for both. For film developing does the benzotriazole need to stay in the recipe or can it be ommited?

I probably should have titled this thread: "Questions about Potassium Thiocyanate and Ilford black and white reversal". Update: Just did, lol.

What About Bob said:

Originally I was going to use D-19 for the developer. Would a longer wash remove most of the thiocyanate before the bleaching step?

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At the recommended addition of 2 g Thiocyanate per litre of FD, assuming one is processing only one roll at a time, after the stop bath and thorough wash, if the film still has ~20 ml of FD in it, that would be 2 X (20/1000) = 0.04 g of Thiocyanate, most of which would have already diffused out from the emulsion into the wash water. Another round of thorough wash, preferably in alkaline water to swell the emulsion can help further. You can do a test for the presence of residual Thiocyanate in the wash water as decribed here and be convinced:


To convince myself that the test works well in low concentrations of Thiocyanate, I prepared 0.5% solution of Thiocyanate (i.e. 0.5 g in 100 ml water) and added a couple of drops of this solution to 1% solution of Ferric Nitrate.

Befor adding Thiocyanate:




After adding a couple of drops of 0.5% Thiocyanate:



The colour change is immediate and easy to notice.

Raghu Kuvempunagar said:

Another round of thorough wash, preferably in alkaline water to swell the emulsion can help further.

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Would a touch of sodium sulfite or carbonate in a first wash after development be a good idea followed by a few regular washes?

Thanks for posting your test for thiocyanate.