Halide solvents

[Hans Borjes]

Before I start the next iteration in reversal processing, I had to order another chemical, a halide solvent for the first developer.

As I am using Rodinal I thought it would be a good idea to use potassium thiocyanate as halide solvent, because it supposedly decreases grain size.

I know that potassium/sodium thiosulphate (aka hypo, i.e. fixer crystals) is commonly used for that purpose.

Does anyone know whether there is a difference in halide solvent activity between sodium thiocyanate and potassium thiocyanate, or between sodium thiosulphate and potassium thiosulphate, or between potassium thiocyanate and potassium thiosulphate? Maybe a strange question, but important to understand the recipes in the web.

Even more interesting would be: has anyone experienced potassium thiocyanate in Rodinal? I hope this will work okay and not conflict with this substance (MSDS says "Incompatible with acids, strong bases and strong oxidizing agents.").

 

[Gerald Koch]

There is no difference in the action of sodium thiocyanate and potassium thiocyanate. Same for the thiosulfates. Avoid ammonium salts as they should not be added to developers. There is no point in purchasing the potassium salts as they are usually more expensive. Typical formulas suggest that you can sustitute thiosulfate for thiocyanate by using twice as much. Potassium thiocyanate is extremely hygroscopic and will quickly absorb enough water to dissolve. It is better to buy the sodium salt or purchase potassium thiocyanate as a percentage solution.

 

[Gerald Koch]

While the thiocyanates are silver halide solvents, their addition to developers can cause dichroic fog and their use is not recommended except in the first developers for B&W and color reversal.

 

[Jordan]

I think Hans is looking to use these for B&W reversal, if I'm not mistaken.

As Gerald indicated, the thiocyanates are significantly more potent as silver halide solvents in reversal processing, but the thiosulfates are easier to handle. Sodium thiosulfate at a rate of something like 8-12 g/L of working solution, depending on the activity of your first dev, should probably work.

 

[lowellh]

Thiocyanates are used to reduce image or "fog" in the toe of the curve. If you are trying to produce a continuous tone image do not use this silver solvents in the First developer. It will increase contrast and reduce the amount of information in the final result.

 

[Hans Borjes]

Thiocyanates are used to reduce image or "fog" in the toe of the curve. If you are trying to produce a continuous tone image do not use this silver solvents in the First developer. It will increase contrast and reduce the amount of information in the final result.

My intention is to remove a yellowish base tone from the slides. If this is caused by fog that remains after bleaching because the first developer is not strong enough to develop it, then this could be the solution for me. I don't know it yet, but it doesn't harm to try it.

 

[Hans Borjes]

@Gerald and Jordan,

thanks for the info regarding activity of thiocyanate versus thiosulphate. By studying the different recipes I can now at least calculate where to start with my experiments. I found a cookbook recipe for 20ml Rodinal (exactly the same amount that I use) and 0,5g sodium thiosulphate for 500ml solution (I use 260ml).

Is there an additional emulsion softening or hardening effect to be considered?

How long will it take for the potassium thiocyanate to absorb enough water to dissolve when stored in a tightly closed container?

 

[lowellh]

The "yellowish" stain is a result of the bleach. You need a stronger clearing bath to remove that stain.

 

[Gerald Koch]

Thiocyanates can cause severe softening of the emulsion and thiosulfates may be a better choice.

Keep the solid well sealed in a cool, dry place. But even so it will eventually turn to liquid. I would advise that if you buy the solid that you make up a 25% solution using part of the solid. This will lesson the number of times the solid is exposed to the air.

 

[Photo Engineer]

Thiocyanates have long been used as the preferred silver halide solvent in reversal first developers. At the level used, they give no dichroic fog, nor do they soften the emulsion.

PE

 

[Hans Borjes]

The "yellowish" stain is a result of the bleach. You need a stronger clearing bath to remove that stain.

I was also considering this as the cause. If the halide solvent will remove detail from the highlights without giving a crystal clear base then I will try this for sure.

The only thing that seems to point into the other direction is that this yellow stain is dependent on the intensity of fogging before/during second development, and it actually turns into brown if you fog/develop long enough after bleach.

So it may be the same yellow effect that appears if a normal paper print is not fixed and exposed to light?

 

[Photo Engineer]

If you don't use a good clearing bath and wash after the bleach, you often get a yellow fog from the bleach chemistry.

PE

 

[Jordan]

I'm going to chime in with P.E. and say that I think that Kodak's "standard" B&W reversal first developer formula is essentially Kodak D-19 (which you can still buy pre-mixed in a yellow envelope) with added sodium thiocyanate.

I also remember reading that reversal first developers with thiocyanate as the silver solvent keep better than those that use thiosulfate. I always mix one-shot, so it hasn't been an issue for me.

I think you'll find sodium thiosulfate much easier to handle than sodium thiocyanate.

 

[Hans Borjes]

I would say that KSCN does the job. It came in an 80g container and due to its hygroscopic behaviour there were big chunks inside - no other choice than creating a 25% solution first, part of it went into a 2,5% solution. 10ml of that 2,5% solution for the first developer, i.e. 250mg KSCN per film.

I kept all other parameters constant to see what KSCN does, and yes, it removes yellow stain and creates a whiter and lighter image. Now it appears that I need to increase the time for the fogging developer because I see a little unevenness in the dark parts of the slides and they lost some Dmax.

That may enable the "fog and redevelop til completion" approach.

Thanks for your help.

 

[Hans Borjes]

The effect of KSCN on grain size is quite nice. We have been reversing a 35mm film yesterday and it turns out that Rodinal is becoming better suitable for that format than expected. It was okay for 120 film already, but now it is even better.