Need recommendations for KOH for rodinal compounding

[eli griggs]

I am looking for recommendations for potassium hydroxide or KOH for compounding developers, substituting sodium hydroxide, particularly for Rodinal or Parrodinal.

God Speed to All,
Eli

 

[Donald Qualls]

The formula I have for Parodinal (which I've made at least a dozen times between 2005 and 2007) used sodium hydroxide in the form of Red Devil Lye from the supermarket. Potassium hydroxide should work just about the same, buy you need a little more (potassium is heavier than sodium, mole for mole). The difference in pH is negligible, the difference in solubility won't matter in this formula.

It's easy enough to calculate the molecular weights (about 56 for KOH, about 44 for NaOH) for mole-for-mole substitution, which will work in most formulae. I've read that you need KOH specficially for actual Rodinal, but given Parodinal uses easily sourced ingredients and is much simpler to mix, for the same result, I've never tried to make Rodinal.

 

[Ian Grant]

I just use pellets of Potassium Hydroxide, I get them from a chemical supplier. Bostick & Sullivan should have some in the US.

Ian

 

[eli griggs]

The main advantage I am looking for comes out of an old post, explaining that KOH results in "clearer" and "cleaner" results in Parrodinal.

I have the correct Rodinal chemicals, once I have the KOH, BUT, I want to make the Parrodinal I've used before using NaHO, changing to KOH.

Cheers

 

[koraks]

What's the kind of recommendation you're looking for? Just calculate the equivalent amount of KOH using the molar mass of KOH vs NaOH and go. Or forget about the conversion and replace 1:1, it'll likely work OK anyway.
You also need potassium sulfite.

 

[Donald Qualls]

Just use 56/40 times as much KOH as the NaOH the Parodinal formula calls for.

Be sure to do the negative comparisons "blind" -- that is, in such a way that you can't tell until afterward which negative is from which developer. And despite the pH being almost the same, there's a possibility the KOH version may be very slightly more active; if you really want to compare you may need to adjust development time to get the same contrast index or gamma (whichever you prefer to use).

Or don't worry about it, like normal darkroom workers.

 

[hol571]

My experience is that I didn't really see any difference in the film, but switching to an all Potassium based pararodinal did improve the solution solubility by a decent amount. This means using KOH, and potassium metabisulfite, and potassium sulfite. Just calculate the molar equivalents needed (see also Tube 2 formula in the linked thread). When I mixed KOH with sodium metabisulfite / sulfite, overall solubility wasn't any different than an all sodium based version. I haven't done any experiments with it for a few years, switched to pyrocat-HD. However, some of my notes/formulas are in this older thread: https://www.photrio.com/forum/threa...e-looking-for-help-with-the-chemistry.147303/

If you are just looking for a source of KOH, I use ACS grade pellets from Sigma-Aldrich (mainly because I have it available at work and probably overkill for this)

 

[koraks]

When I mixed KOH with sodium metabisulfite / sulfite, overall solubility wasn't any different than an all sodium based version.

Yeah, I would have expected this. Rodinal is very heavy on the sulfite to begin with, so if you stick with sodium sulfite you're still left with a whole lot of sodium ions that will reduce solubility of the combined set of salts.

 

[Raghu Kuvempunagar]

Rodinal is very heavy on the sulfite to begin with

For home use, it should be possible to use only as much Sodium sulphite as it goes into the concentrate and compensate for the remainder at the time of preparing the working solution. Possibly a little inconvenient but not too much.

 

[koraks]

Yeah, that would work OK. It might affect its keeping properties a bit, but that's quite a gamble to begin with.

 

[Donald Qualls]

I don't recall having sulfite that wouldn't go into solution when I've mixed Parodinal with the easily sourced sodium species. You may see crystals on the bottom of the storage bottle after the solution (warmed by dissolving the alkali) cools, however. The old legend was that (for both Parodinal and original Agfa Rodinal) one needed to ensure at least some crystals stayed with the solution when decanting (to ensure the solution remained saturated with whatever that was).