Why Oxalic Acid Combats Black Spots

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Over the last few days I’ve been reading, writing and thinking about black spots. There’s also been a big discussion about this on Facebook in the Platinum Palladium Printing group.

A key question in my mind has been: “Why does adding oxalic acid to the sensitiser stop black spots from appearing?” Now I think that I have worked out the answer to this question.

Firstly, a short recap.

Ferric oxalate is more properly called iron(iii) oxalate. Iron(iii) oxalate can be reduced to iron(ii) oxalate by light, heat or chemical reactions. We want this to happen during UV exposure because that’s how our magnificent platinum/ palladium image is formed, but we absolutely don’t want it to happen at other times because that causes fogging and black spots.

Almost any accidental exposure of our ferric oxalate solution will cause it to contain a little bit of unwanted iron(ii) oxalate. Accidental exposure could happen during manufacture, when preparing the ferric oxalate solution, when mixing the sensitiser, or even when coating and drying the paper. Heat can also reduce iron(iii) oxalate to iron(ii) oxalate. It’s very common to heat the coated paper to dry it more quickly, and quite a lot of heat can be generated during UV exposure too. Finally, chemical contaminants in the paper (or even substances that aren’t normally considered as contaminants) can also reduce the iron(iii) oxalate to iron(ii) oxalate: for example aldehydes used to harden gelatin size, or sulphites used in wood pulp treatment (Platinomicon by Mike Ware, p. 206). It is therefore almost inevitable that the sensitised, unexposed paper contains some iron(ii) oxalate.

Here’s the chemical equation for the reduction by light of iron(iii) oxalate to iron(ii) oxalate:

iron(iii) oxalate + photon  iron(ii) oxalate + carbon dioxide
Fe2(C2O4)3 hv 2FeC2O4 2CO2

Note that this reaction results in the permanent loss of an oxalate molecule which is converted into carbon dioxide gas.

Iron(ii) oxalate has low solubility in water (~0.097% wbv at 25ºC according to Wikipedia). This means that trace levels of contamination will stay dissolved, perhaps causing slight fogging of the highlights but not causing black spots. However once you have higher levels of contamination then it is inevitable that your sensitiser will contain tiny particles of iron(ii) oxalate. When the print is developed, typically (but not always) in hot potassium oxalate, these iron(ii) oxalate particles gradually dissolve and reduce the neighbouring platinum/palladium salts to form a black spot.

Last year I discovered that adding a few drops of 10% oxalic acid to my sensitiser magically made the black spots disappear. I think I have worked out why this happens.

Iron(ii) oxalate will happily oxidise back to iron(iii) oxalate if it has access to oxygen (to trigger the oxidation) and free oxalate (to replace the oxalate that was lost to CO2 in the previous reduction reaction). When mixing, coating and drying the sensitiser there is sufficient oxygen from the air to satisfy the first requirement, and adding a few drops of fresh oxalic acid (C2H2O4) ensures there’s an excess of free oxalate (C2O4) to satisfy the second requirement.

This allows the following reaction to take place:

iron(ii) oxalate + free oxalate  iron(iii) oxalate
2FeC2O4 C2O4 Fe2(C2O4)3

Note that the hydrogen in the oxalic acid plus the oxidising oxygen end up combined together as water, so, although essential for the reaction, for simplicity I haven’t shown them in the above equation.

As if by magic, our unwanted iron(ii) oxalate particles have now been converted back to valuable iron(iii) oxalate, and those pesky black spots have gone. Eureka!

I should stress that right now this is my working hypothesis, not a proven solution. It works for me, and I think the chemical explanation is valid. However, printing practices vary so widely that there may well be circumstances in which it doesn’t work, for example chemical (or metallic) contamination of paper, unusual combinations of sensitiser chemistry, or contaminated developer. Also, I doubt it will resolve problems caused by high levels of iron(ii) oxalate contamination, for example from using dirty brushes. But if you are experiencing black spots then it’s worth a try.

Please let me know whether this works for you.

Update: In my initial version of this post I messed up my chemical equations. Sorry for any confusion this may have caused.