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
• Micropipette accurate for 6ml of solution
• Micropipette accurate for 1ml of solution
• High precision scales
• Glass beakers and dishes
• 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.

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 28ml of 20% ferric oxalate solution. It produces only two by-products: water and 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. Weigh exactly 6g of iron(ii) oxalate dihydrate and put it in a 100ml glass beaker.
2. Add 20g of distilled water. Weighing 20g on scales will often be more precise than trying to measure 20ml by eye.
3. 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.
4. Weigh exactly 6g of anhydrous oxalic acid. Crush any lumps in the powder, then add it slowly to the iron/water mixture. Stir until the oxalic acid has dissolved.
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. Draw 6ml of 30% hydrogen peroxide into a pipette, then add it to the mixture two to three drops at a time, with constant stirring. The mixture will react vigorously, with fizzing and heat. After adding the hydrogen peroxide, wait until the fizzing subsides before adding more. If you add it too quickly then you will ruin the product. Continue until all the hydrogen peroxide has been added. The hydrogen peroxide will dissolve the iron(ii) oxalate. Initially it will dissolve into a clear brown solution, but by the end of the process it will should be a clear apple-green solution. Towards the end, the reaction will become less vigorous, possibly stopping completely.
7. Let the solution stand in the dark for twenty to thirty minutes.
8. Weigh-out exactly 2g of anhydrous oxalic acid, and slowly add it to the solution, with constant stirring. You may see some fine bubbles in the solution. Wait until they disappear. By this time, the solution should be a clear apple-green colour.
9. Add a few drops of 30% hydrogen peroxide. If any bubbles appear in the solution, then add a few more. Repeat until no more bubbles are produced. You should need very little extra hydrogen peroxide, if any.
10. Let the solution stand overnight in the dark.
11. Test the solution as described below.
12. Leave the solution to mature in the dark for a week.
13. Retest the solution, then filter it into a brown glass bottle. Label the bottle with the date and specific gravity. It is now ready to use.

Part 4. The Chemical Reactions

The chemical formula in this process is: iron(ii) oxalate + oxalic acid + hydrogen peroxide → iron(iii) oxalate + water + carbon dioxide.

This happens via two near-simultaneous chemical reactions, followed by a third slower reaction.

Reaction 1: in the first reaction, the hydrogen peroxide converts the iron(ii) oxalate into iron(iii) hydroxide. It requires an acidic environment to do this, which is provided by the oxalic acid.

2(FeC₂O₄・2H₂O) + 3H₂O₂ → 2Fe(OH)₃ + 2H₂O + 4CO₂↑

Reaction 2: as the iron(iii) hydroxide is created, it is converted by the oxalic acid into iron(iii) oxalate (ferric oxalate).

2Fe(OH)₃ + 3H₂C₂O₄ → Fe₂(C₂O₄)₃ + 6H₂O

Once the reaction completes, all the iron(ii) oxalate has been converted to iron(iii) oxalate, but there may still be some excess hydrogen peroxide or oxalic acid.

Reaction 3: adding additional oxalic acid ensures that the iron conversion is complete, and that any excess hydrogen peroxide is removed from the solution.

H₂C₂O₄ + H₂O₂ → 2H₂O + 2CO₂

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: 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.

Specific Gravity: 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. If the specific gravity is too low, then leave the solution to stand for a few days, thus allowing some water to evaporate. If the specific gravity is too high, then add a little distilled water to dilute the solution.

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

Happy printing!