Updated: 14 December 2024 – improved manufacturing process for larger quantities, improved chemistry, improved testing, improved labelling.

There are quite a few ways to make ferric oxalate. In my previous article I explained how to make it with the historical process used by Pizzighelli and others. This article explains a better, modernised method for quickly and easily making high quality ferric oxalate.

This method is based upon US Patent 1,899,674: “Process for the production of ferric oxalate” filed by Leo Curtin in 1933 and now expired.

A more complicated version of this method was published by Dick Stevens in Making Kallitypes, p.52. Stevens’ version produces sulphuric acid as a by-product, so should be avoided.

Part 1. Equipment

Not all of these items are essential, but they will make it easier to make the ferric oxalate successfully.

• Magnetic stirrer
• High precision scales
• Micropipettes accurate for 10ml and 1ml
• Glass beakers (200ml), measuring cylinder (100ml) and dishes
• Glass rods for stirring
• Plastic spoons for handling small quantities of chemicals
• Small glass or plastic funnel
• Filter paper

Part 2. Ingredients

The naming of chemical substances can be confusing, especially if you are reading historical sources (from times when different names were used) or photographic literature (where authors often use incorrect or ambiguous names for substances).

For example, the modern name for ‘ferric oxalate’ is iron(iii) oxalate. Its chemical formula is Fe₂(C₂O₄)₃. Be careful, though, because the modern name for ‘ferrous oxalate’ is iron(ii) oxalate. The difference between the ‘iii’ and the ‘ii’ is profound.

In this article, I normally use modern chemical names, indicating alternative photographic names where there is possible ambiguity.

You will need the following ingredients to manufacture ferric oxalate.

When the substance is commonly available in both anhydrous and hydrated forms, I specify which of these I use. This is important because it affects the quantity (mass) of the chemical and the concentration of the final product.

Iron(ii) oxalate dihydrate – FeC₂O₄∙2H₂O: a fine yellow powder. It is also known as ferrous oxalate.

Picture credit: Benjah-bmm27

Anhydrous oxalic acid – C₂H₂O₄: a white powder. You can also use oxalic acid dihydrate but will have to add about 40% more to allow for the extra water contained in it. The extra water will also leave you with a lower concentration of your final product. Handle oxalic acid with care because it is a strong acid.

Picture credit: Leiem

30% hydrogen peroxide solution – H₂O₂: a colourless liquid. Handle hydrogen peroxide with great care because it is a powerful oxidiser and will oxidise you if you touch it. In certain circumstances it can explode. Always wear suitable gloves, eye protection and an apron when handling hydrogen peroxide. Store it in a cool place.

Part 3. The Manufacturing Process

This recipe produces about 100ml of 20% ferric oxalate solution optimised for platinum/palladium printing. It produces no unpleasant by-products: just water, oxygen and a little carbon dioxide. The first few steps can be done in bright light, but the latter steps should be done in dim light, for example under a red darkroom safelight.

1. Add 5g of anhydrous oxalic acid to 45ml of distilled water in a 200ml glass beaker. Stir without heating until dissolved.

2. Weigh exactly 18g of iron(ii) oxalate dihydrate and gradually add it to the oxalic acid solution, stirring continuously.

3. Place the beaker in an ice-water bath to keep the solution cool during the following steps. You may need to add fresh ice during the next steps.

4. If you have a magnetic stirrer then start it stirring now. The iron(ii) oxalate will not dissolve, but should move around freely in the water. Do not heat it.

5. Dim the lights, because from this point onwards the solution is light-sensitive. However, make sure that the light is bright enough for you to safely handle the hydrogen peroxide.

6. This is the most difficult step. Slowly add 18-20ml of 30% hydrogen peroxide, two to three drops at a time, with constant stirring. The mixture will react vigorously, with bubbling and heat. After adding the hydrogen peroxide, wait until the fizzing subsides before adding a few drops more. Towards the end, the reaction will become less and less vigorous. Continue until you can add 0.5ml of hydrogen peroxide without triggering vigorous bubbling. The solution should now be a clear, dark brown colour, containing many fine bubbles. If it seems like you need to keep adding more and more hydrogen peroxide, then you have probably overcooked the solution and are now effectively just adding water: start over again, taking greater care to add the hydrogen peroxide slowly.

7. You can now remove the beaker from the ice-water bath, because the hot part of the manufacturing process is complete.

8. Repeat this step until the solution is a clear emerald green — add 0.5g anhydrous oxalic acid, stirring until it dissolves. The solution’s colour should change quite quickly.
   

   

9. Do a ‘Prussian Blue’ test (see Part 5 below), which will probably fail.

10. Repeat this step until the ‘Prussian Blue’ test passes — add 0.5g of anhydrous oxalic acid, stirring until it dissolves, followed by 0.6ml of 30% hydrogen peroxide. Stir for a few minutes, then do the ‘Prussian Blue' test again.

11. Once the solution passes the ‘Prussian Blue’ test, add a final 1g anhydrous oxalic acid, stir until it has dissolved, then leave the solution to stand in the dark for a few days.

12. Filter the solution into a clean glass beaker using Whatman #6 (3µm) filter paper. You may see some crystals of oxalic acid in the bottom of the solution. These can be washed down the drain.

13. The final test is to make a print. If the print has any black spots, then there is still some iron(ii) in the solution, so go back to step 10 and repeat.

Assuming the print has no black spots, then you have made a good ferric oxalate solution. However, your manufacturing is not yet completed because you need to determine and adjust its concentration. You can do this by measuring its specific gravity (see Part 5 below).

This seems like a good time to talk about the chemical reactions that you've just seen.

Part 4. The Chemical Reactions

The chemical equation for this manufacturing process is deceptively simple: iron(ii) oxalate + hydrogen peroxide + oxalic acid → iron(iii) oxalate + water.

2FeC₂O₄ + H₂O₂ + H₂C₂O₄ → Fe₂(C₂O₄)₃ + 2H₂O

As is so often the case with chemistry, a lot of things are happening to make this simple equation work. The heavy lifting is done through three near-simultaneous reactions, but other reactions are also involved.

Reaction 1: in the first reaction, the hydrogen peroxide oxidises the iron(ii) into iron(iii). In doing this, the reaction releases hydroxide ions (OH^-) which immediately trigger reaction two.

1) 2FeC₂O₄ + H₂O₂ → 2Fe³⁺ + 2C₂O₄^2- + 2OH^-

Reaction 1 requires an acidic environment, which is provided by the oxalic acid.

Reactions 2 and 3: the hydroxide ions grab hydrogen atoms from nearby oxalic acid molecules in order to form water. This creates enough free oxalate ions to make iron(iii) oxalate (ferric oxalate) in reaction three.

2) H₂C₂O₄ + 2OH^- → C₂O₄^2- + 2H₂O

3) 2Fe³⁺ + 3C₂O₄^2- → Fe₂(C₂O₄)₃

These three reactions give off a lot of heat, which causes some of the hydrogen peroxide to decompose into water and oxygen gas. This is what causes the vigourous bubbling seen during manufacture.

4) 2H₂O₂ + heat → 2H₂O + O₂↑

Most of the oxygen floats away into the air, but some may hang around long enough to facilitate another beneficial reaction.

5) 4FeC₂O₄ + 2H₂C₂O₄ + O₂ → 2Fe₂(C₂O₄)₃ + 2H₂O

Again, this is actually another series of near-simultaneous reactions:

6) 4FeC₂O₄ + O₂ → 4Fe³⁺ + 4C₂O₄ + 2O^2-

7) H₂C₂O₄ + O^2- → C₂O₄^2- + H₂O

8) 2Fe³⁺ + 3C₂O₄^2- → Fe₂(C₂O₄)₃

Finally, when everything has calmed down, the remaining hydrogen peroxide will slowly react with excess oxalic acid to remove both of them from the ferric oxalate solution. This reaction continues over days.

H₂C₂O₄ + H₂O₂ → 2H₂O + 2CO₂

This elegant series of reactions produces incredibly pure ferric oxalate.

Part 5. Testing

The simplest way to test your freshly made ferric oxalate is:

1. ‘Prussian Blue’ test for purity
2. Specific gravity for solution concentration
3. Printing

Prussian Blue Test: tests for the presence of iron(ii) in the solution.

In dim light, put one drop of ferric oxalate solution into a white dish. Add five drops of distilled water to make it easier to see colour changes. Add one drop of 0.5% potassium ferricyanide. Pure ferric oxalate may darken a little, but stays green. If there is any ferrous material in the solution then it will take on a distinct blue colour

Inspecting the test results under bright light will, of course, convert some of the iron(iii) back into iron(ii) and trigger the blue result. I find that the quickest way to check the test is to have a sheet of white kitchen tissue ready, then to wipe the dish with it right after adding the ferric oxalate. It is easier to see the blue colour when the test solution is absorbed in the tissue. 

Specific Gravity: determines the concentration (strength) of the ferric oxalate solution.

Weigh the mass of exactly 1ml of solution. The mass in grams is equal to the solution's specific gravity.

A specific gravity of 1.17 or 1.18 is ‘standard’ for platinum/palladium printing (about 20% wbv or 0.5M). Following this recipe, I usually get a specific gravity of about 1.2 (about 0.6M), which is easily adjusted to the 'standard' by adding some distilled water.

If the specific gravity is too low, then leave the solution to stand in the dark for a few days, allowing some water to evaporate. You can accelerate the evaporation by placing the solution under a fan.

The following chart can be used to convert the specific gravity to a percent concentration.

Printing: of course, the final test is the quality of your prints.

Part 6. Labelling

Once you are happy with the concentration of your ferric oxalate solution, filter it into a brown glass bottle. Label the bottle with the date and specific gravity. It is now ready to use.

Happy printing!