20% Ferric Oxalate solution

I'm attempting to get a 20% solution of Ferric Oxalte solution to dissolve in distilled water but for some reason all of it will not dissolve. I've heated it, stirred it, shook it, etc. The bottle has now been sitting for over 24 hours but still looks cloudy and has a layer of undissolved FO at the bottom of the dropper bottle.

Should I dump it and start over?

I'm mixing this for kallitype printing.

donbga said:

I'm attempting to get a 20% solution of Ferric Oxalte solution to dissolve in distilled water but for some reason all of it will not dissolve. I've heated it, stirred it, shook it, etc. The bottle has now been sitting for over 24 hours but still looks cloudy and has a layer of undissolved FO at the bottom of the dropper bottle.

Should I dump it and start over?

I'm mixing this for kallitype printing.

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I would leave it until tomorrow. If the solution is not clear by then, add about 20% more water, heat it again, and see what happens.

Sandy

Ferric Oxalate Not Dissolving Completely

The ferric oxalate not completely dissolving is a function of the particular batch of ferric oxalate you have. I have experienced this off and on for 16 years. Some batches of powder have a precipitate or undissolved FeOX no matter how much you heat or shake it. Some batches like the current one I'm working with disssolve completely. I mixed 100ml tonight and it dissolved completely.

Filter the solution through a medium to coarse filter paper and use it. It will be fine. Coffee filters are not sufficient for this operation. You should use a laboratory grade medium to coarse filter paper. (I use "coarse"...I'm a little impatient.) It will filter out the sediment. I do not reccomend adding water. The material may still not dissolve and then you have a diluted sensitizer.

Out of curiousity, from which supplier did you purchase the ferric oxalate?

bobherbst said:

The ferric oxalate not completely dissolving is a function of the particular batch of ferric oxalate you have. ?

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Thanks for the tip. Unfortunately I have no filter paper and you are correct a coffee filter doesn't work.

bobherbst said:

Out of curiousity, from which supplier did you purchase the ferric oxalate?

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B&S.

The water should be about 170 F to dissolve the FO. I usually heat more water than is required for my solution and then use that as a hot water bath to keep the little brown bottle hot for a longer time. That and some agitation from time to time reduces the over all time to about....12 hours.

Joe Lipka said:

The water should be about 170 F to dissolve the FO. I usually heat more water than is required for my solution and then use that as a hot water bath to keep the little brown bottle hot for a longer time. That and some agitation from time to time reduces the over all time to about....12 hours.

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Joe,
I'm a bit wary of heating FO in a closed bottle since I had a dropper bottle explode once. I wasn't happy nor was my darkroom.

Since yesterday I added 5 ml to the 50 ml solution and a lot of the FO dissolved over night. However there still remains a small quantity that didn't dissolve. As a result I added 5 more ml of water and placed the bottled in a water bath heated gently on my darkroom gridle. Hopefully it will all dissolve but I won't have a 20 % solution - 16.6 % instead.

Heating Ferric Oxalate

donbga said:

Joe,
I'm a bit wary of heating FO in a closed bottle since I had a dropper bottle explode once. I wasn't happy nor was my darkroom.

Since yesterday I added 5 ml to the 50 ml solution and a lot of the FO dissolved over night. However there still remains a small quantity that didn't dissolve. As a result I added 5 more ml of water and placed the bottled in a water bath heated gently on my darkroom gridle. Hopefully it will all dissolve but I won't have a 20 % solution - 16.6 % instead.

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I have weighed the resulting undissolved ferric oxalate after drying the filter paper and subtracting the weight of an unused piece of filter paper and the actual weight is a very small amount. It just looks like a lot.

I use an isopro camp stove in my darkroom for heating up to 1 liter of solution. I mix and heat ferric oxalate in a boiling flask over the flame. The round profile improves heat transfer. Place a rubber stopper LOOSELY on the top of the bottle or flask. This serves as a pressure valve similar to the weight used on a pressure cooker. The stopper also reduces evaporation of liquid which can be significant if heating over a flame. I heat up to the boiling threshold but do not boil. You can feel it with your fingers as you hold the flask. 170 degrees might do it but I suspect slightly higher heat will improve the speed of solubility.

I have a darkroom ceiling covered with ferric oxalate stains from an eruption the first time I mixed my own from powder. I had pressed the stopper into the flask. Fortunately I was wearing goggles and a dust mask and the glass didn't break. I still had to take an immediate shower to wash ferric oxalate out of my hair.

Now you understand why I suggested against adding water. You now have a 16.6% solution. Compensate with your Pt/Pd accordingly so you don't waste metal.

In the grand scheme of things, don't worry about a little undissolved powder. There are a million variables in this process so minor variations from the small amount of undissolved powder won't make enough difference to matter.

That's my 2 cents...time to get the last print out of the wash.

Bob Herbst

I would be wary of heating the ferric oxalate solution more than once to disolve it. In the tests we've done at B&S, we've found that the second heating degrades the ferric oxalate and causes grain to be introduced into the image.

Don, if the ferric oxalate came from B&S, please send us an email or give us a call, and we'll replace the stuff you have, free of charge.

Making FO solutions seems like one of those situations that stirrer-hotplates are ideal for. Heat, turn on stirring, walk away for a while...

Ferric salts tend to hydrolysis in water solution with the eventual formation of hydrated ferric oxide. This hydrolysis occurs in solutions with a pH higher than 3.2 and is accelerated by heat. My suggestion would be to not heat the solution unduly during solution and to filter out any sediment. As the solution ages more sediment may fe formed.

I've found that intermittent agitation can help a stubborn-to-dissolve batch of FO. I have a magnetic stirrer which I will let run for 10 minutes or so. I then turn it off and let the solution sit for several hours, then repeat as necessary. In the end, if there's some solids left, I simply filter them out and use the solution as is. I agree that too much heat can cause more problems than a solution that is at a slightly lower concentration. Analytical filtration units like this with fine filter paper (0.45 micron) and a vacuum pump make filtering quick and very effective.

Kerik said:

I've found that intermittent agitation can help a stubborn-to-dissolve batch of FO. I have a magnetic stirrer which I will let run for 10 minutes or so. I then turn it off and let the solution sit for several hours, then repeat as necessary. In the end, if there's some solids left, I simply filter them out and use the solution as is. I agree that too much heat can cause more problems than a solution that is at a slightly lower concentration. Analytical filtration units like this with fine filter paper (0.45 micron) and a vacuum pump make filtering quick and very effective.

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Kerik,

Thanks very much for the link to the analytical filtration units. I have been looking for something just like this to filter the pigmented gelatin solutions I use to make carbon tissue.

Sandy