Crystallising Potassium Oxalate

[Ian Leake]

I use hot Potassium Oxalate for developing my platinum and platinum/palladium prints. I've noticed that when the developer cools it tends to form crystals in the bottom of the jug or bottle. Sometimes the crystals dissolve when I heat them gently, but not always. I'd like to stop them forming.

Does anyone have any experience of this? If so, how do you deal with them?

Thanks in advance.

 

[Vaughn]

You probably are having water evaporate from the developer and thus concentrating it and forming a super-saturated solution. You might try adding some distilled water after your printing session to bring the concentration down.

What I have done is pour the Potassium oxalate into my warming-container, leaving the crystals behind. Then add the distilled water to the crystals, warm it up and when they are dissolved, pour it in with the rest of the developer.

 

[Gerald C Koch]

It's hard to make any conclusions about the problem without the specific strength of the solution and your darkroom temperature. The solubility of potassium oxalate is 31.0 g/l at 10C, 36.4 at 20C and 38.9 at 30C. As an example, if your working solution contains more thaqn 38.9 g/l and cools below 20C you will see some crystalization.

 

[Ian Leake]

Thanks Vaughn and Gerald, I think you've solved my problem (well, it's not really a problem – it's more of an oddity really).

I make up 33% a Potassium Oxalate solution as standard. Certainly some evaporates while I'm working, although not that much because I keep it in a cafetière which tends to recycle the evaporated water. But my darkroom gets cold at night. I'm not sure how low the temperature gets, but sometimes it's probably below 10°C.

I guess I could eliminate the crystallisation by using a weaker solution, but I'm not sure that I want to do that. So I need to either live with it or keep my developer warm at night.

Thanks again,

Ian.