Ferric oxalate question

revdoc said:

I read somewhere that free oxalic acid has no adverse effect in platinum printing, but for kallitypes you want none. Or at most, a very small excess of oxalic acid.

You can probably eliminate the acid by adding an alkali.

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An excess of oxalic acid is very good for platinum/palladium. However, according to Dick Stevens, an excess of oxalic acid is bad for Kallitypes. I don't have direct experience of this because I haven't made many Kallitypes.

You can reduce excess oxalic acid by adding a small amount of ferric nitrate. This reacts with oxalic acid to make ferric oxalate and nitric acid. Alternatively you can add a small amount of hydrogen peroxide, which over a few days will react with oxalic acid to make water and CO2.

Edit: yes, silver nitrate reacts with oxalic acid to produce a white precipitate of silver chloride oxalate (correction from @nmp below)

Ian Leake said:

Edit: yes, silver nitrate reacts with oxalic acid to produce a white precipitate of silver chloride.

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You mean silver oxalate.

:Niranjan.

nmp said:

You mean silver oxalate.

:Niranjan.

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You are right. It's too early in the morning for me to think straight

Ian Leake said:

You are right. It's too early in the morning for me to think straight

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I know how that goes. But you put the finger on the crux of the problem. They (like B&S) have separate FO for kallitype and Pt/Pd for this same reason, I suspect. The former contains little or no excess oxalic acid while latter does. On account of seeing white precipitates, it looks like OP's FO has excess oxalic acid that results in white silver oxalate. It's not your typical cloudy or cruddy mixture.

:Niranjan.

Ian Leake said:

An excess of oxalic acid is very good for platinum/palladium. However, according to Dick Stevens, an excess of oxalic acid is bad for Kallitypes. I don't have direct experience of this because I haven't made many Kallitypes.

You can reduce excess oxalic acid by adding a small amount of ferric nitrate. This reacts with oxalic acid to make ferric oxalate and nitric acid. Alternatively you can add a small amount of hydrogen peroxide, which over a few days will react with oxalic acid to make water and CO2.

Edit: yes, silver nitrate reacts with oxalic acid to produce a white precipitate of silver chloride oxalate (correction from @nmp below)

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Interesting! What kind of volume/concentration of hydrogen peroxide qualifies as "small" in this context?

t5SQ said:

Interesting! What kind of volume/concentration of hydrogen peroxide qualifies as "small" in this context?

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That's tricky for me to give a simple answer because I make ferric oxalate for platinum/palladium not Kallitypes. I try to finish with lots of oxalic acid rather than a minimum. Also, the amount of hydrogen peroxide will depend upon how much ferric oxalate solution you have and how much oxalic acid is in it.

Here are a couple of pointers that may help:

• I use 30% hydrogen peroxide when making ferric oxalate. For me this is a reasonable balance of concentration and safety (remember it is a strong oxidiser – read the MSDS).
• When I'm finishing up my ferric oxalate, for 50-100ml of solution, I add between a few drops and no more than 1ml of 30% H2O2 at a time. The amount depends upon the reaction I see when I add the first drops. This is for another purpose than removing oxalic acid (remember I want to keep my oxalic acid), but it's probably a reasonable starting point for further experiments
• After I think I've finished, I leave the ferric oxalate to mature in the dark for a week. The hydrogen peroxide will likely produce CO2. It's good to let that gas escape before putting the solution in a sealed bottle.

You'd have to experiment. I would start by taking some emerald green ferric oxalate solution, add 1ml of 30% H2O2 then leave it to stand for a week. Does the colour change? Does silver nitrate produce white precipitate? Try adding some more, etc. You won't want to add too much because you're also adding water which will dilute the ferric oxalate, although if need be that could be corrected afterwards through evaporation.

Personally, I don't think this is worthwhile unless you're making your own ferric oxalate and want to understand the parameters you're working within. Given the price of commercially available ferric oxalate, it's probably better to throw it away and buy the Kallitype stuff.

I'd be interested to hear from Kallitype printers who make their own ferric oxalate. I assume they'll have lots of techniques that I don't know.

I have had success using the yellow-colored B&S ferric oxalate for Kallitypes.

Out of curiosity, has anyone ever tried dissolving ferric oxalate in propylene glycol? If that's even possible, would it theoretically lead to a longer shelf-life for the 20% solution used in Sandy King's Kallitype recipe?

BHuij said:

Out of curiosity, has anyone ever tried dissolving ferric oxalate in propylene glycol? If that's even possible, would it theoretically lead to a longer shelf-life for the 20% solution used in Sandy King's Kallitype recipe?

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I think this would be counterproductive. If I've understood it correctly, propylene glycol is used in some developer recipes to reduce aerial oxidisation, hence extending developer shelf life.

With ferric oxalate, you want to facilitate oxidisation rather than inhibit it. As ferric oxalate solutions age, some of the iron(iii) naturally reduces to iron(ii), which is why old ferric oxalate tends to produce black spots. Oxygen in the water can re-oxidise this unwanted iron(ii) and, if there's enough free oxalate floating around, turn it back into ferric oxalate. In other words, oxygen plus oxalic acid preserves ferric oxalate.