All About Ferric Oxalate - How to make etc.

Ferric oxalate is traditionally given the formula of Fe2(C2O4)3.5H2O. However, according to Dr. Michael Ware ferric oxalate is an “ill defined” compound. It is more in line with ferric ammonium citrate, which does not have a formula in the Merck Index.. Ware, however, does say that the double oxalate salts, (ammonium ferric oxalate, sodium ferric oxalate, etc.) are “well defined.” Ware refers to ferric oxalate as an oxalato-ferric complex.

Ferric oxalate is also known to form polymers. Tests have shown that different precipitation rates produce differing solubility’s from the same batch. Free oxalic acid in the ferric oxalate is an issue, especially if one wishes to use it for Kallitype printing where the free oxalic will form an insoluble silver oxalate [Ag2C2O4], sludge that will interfere with the Kallitype process.

The B+S powdered grade was made with a trace amount of free oxalic acid until 1996. At that time we changed to a new manufacturing procedure which allowed us to make an exceptionally pure product. Around this time we also began supplying it to a chemical wholesaler who was supplying it to labs and universities who were using it for purposes other than platinum printing and who did not want or need the free oxalic acid that was in the product.

Ferric oxalate and oxalic acid and color of the solution
Ferric oxalate will vary in color according to the amount of oxalic acid present. With little of no oxalic acid it will be a golden amber color. With moderate mounts of oxalic acid it will be a yellow, with large amounts it will turn fluorescent green.

The following are procedures for testing and making ferric oxalate.

Testing for free oxalic acid:
Add 1 drop of a 10% to 20% silver nitrate to a 2 ml sample of the ferric oxalate solution. If a white precipitate is formed, this is silver oxalate, and indicates that there is free oxalic acid present. One can verify this test by adding a drop of 10% oxalic acid to the sample and then adding the silver nitrate again. Note that the ferric oxalate solution should be at room temperature when the test is made.

Testing for ferrous oxalate
Ferrous oxalate is the result of the decay by light or age of ferric oxalate. This causes the precipitation of the platinum or palladium metal that forms the image in a platinum print. Your ferric oxalate should be relatively free of ferrous for good printmaking.

There is a traditional test for the presence of ferrous oxalate. This involves testing with potassium ferricyanide. This test is unreliable in that it gives false positives when EDTA has been added. It also can produce a cyan color when the material is still acceptable for making prints.

A better test is to make a fog test by coating a piece of paper and developing without any exposure. If there is a gray tint to the coated part of the paper after clearing, the ferric oxalate has turned bad.

Removing free oxalic acid.
You can add small quantities of ferric nitrate with stirring. Re-test after each addition of ferric nitrate. This method will leave a minute amount of nitric acid in the solution. It is harmless health wise but it is a oxidizer like chlorate, etc., that it could in sufficient quantities make the ferric oxalate contrastier.

Dissolving ferric oxalate
There are several ways to do this:
1. Add water and stir. Now stir every few hours until it is dissolved. This may take as long as 12 to 24 hours.
2. Zap it in the micro wave. Loosen cap and give the bottle 10 seconds zaps with shaking between zaps. Zap it until it is hot to hold.
3. If you are lucky enough to have a laboratory hotplate stirrer, this will work dandy. Set the temp to about 140 Deg F.
Older texts warn of using heat as it will cause the generation of ferrous oxalate. This may have due to the impurities in older versions of the product. We have found that it will take quite a bit of heat with decaying into ferrous.

The double ferric oxalates -- For Ziatypes
Sodium ferric oxalate

ferric oxalate 6.7 gm
sodium oxalate 5.9 gm
water 40.00 ml

The solution will turn cold and may need to be heated to dissolve and clear the liquor. If still too cloudy cautiously add small quantities of water.

Sodium ferric oxalate method 2
Sodium oxalate can be made in solution, so another way to make the sodium ferric oxalate is:

oxalic acid 3.9 gm
sodium carbonate anhydrous 4.6 gm
water 40.0 ml

Dissolve the oxalic acid and slowly add the carbonate
(Lots of harmless bubbling and fizzing goes on so do this in a 200 ml beaker otherwise it will look like and explosion in a root beer factory.)
You now have 5.3 gm of sodium oxalate in solution

Now add

ferric oxalate powder 6.7 gm

Stir and gently heat until dissolved and is bright green. It is ready to use. If it is dirty looking and not a bright green, add tiny amounts of oxalic acid until it turns a nice green.

For the rest of these I will just cite the quantities and chemicals. Follow the procedure as above.
Lithium ferric oxalate:
This is a very interesting creature. Makes a sharper cooler brown which is a lot closer to the standard palladium brown obtained by the developing out procedure. It is also at least one paper grade higher in contrast than ammonium ferric oxalate. It takes longer to print. I made a print and added 1 drop of 2% AD and it went crazy, right off the map in contrast and lots and lots of grain and the color was blue-black. It looks like you need a .1 % to a .5% AD for this puppy.

ferric oxalate 6.7 gm
lithium oxalate 4.4 gm
water 40.0 ml

Lithium ferric oxalate method 2.
Lithium oxalate can be made in solution thusly:

lithium carbonate* 4.0 gm
oxalic acid 5.6 gm
water 40.0 ml

You can now add

ferric oxalate powder 6.7 gm

and proceed as above.
(*Lithium carbonate is expensive from chemical supply houses but can be gotten cheap from ceramic supply houses. It is used as a flux in glazes. I bought 5 lbs for $20.00, Spectrum wants $50.00 per 500 gm for practical grade which is the same as the ceramic supply house.)

Ammonium ferric oxalate:

ammonium oxalate 5.4 gm
ferric oxalate 6.7 gm
water 40.0 ml

Ammonium oxalate
To make: neutralize oxalic acid with strong ammonium or ammonium carbonate, filter and dry the crystals.

Procedure for making ferric oxalate*

This procedure requires a modest amount of chemical skills and prudent safety precautions should be observed.

Take 200 gm of ferric ammonium sulfate [FeCH4NO8S2] and dissolve in 300 ml water in a 500 ml beaker.

Add 100 ml of strong ammonium hydroxide [880 Ammonia] and stir. (Caution the ammonia is strong and ghastly, do this in a well-ventilated area.) When the ammonia is added, a mud-like gelatinous precipitate will be formed. This is iron hydrate, which is actually just iron in water.

Keeping adding the ammonia until no new precipitate is formed. Too much ammonia won’t hurt.
This next step is to wash the gel. You will want to get out the excess ammonia and sulfates from the gel. This is a little tricky.

First let the mud settle to the bottom of your beaker. This may take an hour or so. Carefully pour off the clear solution on top.

Now add more water to bring it to the approximate level as before decanting. Do this 3 more times. On the last time do not add more water.

Now take a 1 foot square piece of an old tee shirt or bed sheet and drape it over another beaker forming a pocket. Attach the fabric around the beaker with a large heavy rubber band.

Make sure you have your gloves on at this point!

Now pour and scoop out the washed mud from the beaker into the pocket.

Remove the rubber band and form a pouch of the fabric with the mud inside.

With gloved hands take the pouch and twist the ends wringing out any water from the mud. Of course do it over a sink. The juice squeezed out can go down the drain. Be careful if the fabric is old as it can rip or burst.

Replace the pouch back to an empty beaker, fastening with the rubber band as you had originally done.

Pour @ 400 ml of water through the mud in the pouch. The pouch will now be suspended in the water.

Remove and squeeze as before.

Replace in the beaker with the rubber band as before. You are now ready to make your ferric oxalate solution.

Dissolving the mud
You don't need to measure the oxalic acid, you will be doing this by inspection.

Slowly begin adding the dry oxalic acid to the mud in the pouch. You should soon see some of the “mud” begin to dissolve. The resulting liquid is ferric oxalate.

Keep adding oxalic acid and gently stirring with a plastic spoon. As you add it, you will continue to see the mud go into solution. Eventually it will begin to drip through the cloth into the beaker.

Continue until you have used up about 90% of your original “mud.” You don’t want to go any further or you risk putting in excess oxalic acid.

Remove and discard the pouch.

The liquid should be a bright yellow green. This is your ferric oxalate. It should be approximately 27%.

If is it cloudy, take a small sample and add a tiny quantity of oxalic to the sample. If it clears up, the cloudiness is ferric hydrate and you can proceed to add and stir in small quantities of oxalic acid to the solution in main beaker until it has cleared up. Alternately you can filter the hydrate out with a filter and filter paper.

You may now want to test for free oxalic acid as shown above.

Now clean up your room – it’s messy! There’ll be red yukky all over the place.

I’ve published various versions of this paper over the last 20 years. This is the latest go-round. I printed several 50 print Harry Smith portfolios using this method of making ferric oxalate back in the early 80’s. With care you can make an excellent grade ferric oxalate in one afternoon.

Now go play with your ferric oxalate.

*If you are lucky enough to have a Buchner filter, vacuum flask, and an aspirator this will help quite a bit. Probably worth the approximate $100.00 investment if you plan to use ferric oxalate regularly. Any good chemistry lab techniques textbook will explain its use.

-- Dick Sullivan