Making a Ferric Oxalate Solution

1)  Weigh out 200 grams of Ferrous Ammonium Sulfate.  With constant stirring, add it to 500ml of rather warm distilled water - 125F to 150F.  I use a 1000ml flask to mix the solution in with an electric stirrer.  After the ferrous ammonium sulfate (FAS) has completely dissolved, add to it 65g of Oxalic  Acid.  You will see a yellow precipitate form.  Continue to carefully stir the solution for 5 minutes.

2)  Allow the solution to sit for at least an hour.  You will notice that the yellow precipitate has settled to the bottom of the mixing vessel  Carefully pour off the clear liquid.  This is a strong acid and should be handled with care.  You are trying to keep the yellow precipatate in the vessel. 

3)  Add distilled water to the precipitate and allow it to settle again. You will repeat this step several times.  I try to continue rinsing the precipitate until the rinse water has a pH of 6.0 or higher. 

4)  Weigh out  55g Oxalic Acid and add it to 125ml  water @ 125 F to 150 F.  After the Acid has dissolved completely, add the solution to the yellow precipitate  and stir for several minutes.

5)  Using a buret , carefully add the H2O2 to the precipitate and oxalic acid solution.  This will be a very violent reaction which creates much heat.  You will add approximately  80ml to reach final reaction.  DO NOT ADD IT QUICKLY.  You may add several milleliters at once.  You must however wait a little while afterwards , to allow the solution to mix and cool. I would recommend that you start by adding it a ½ ml at a time with constant stirring until you feel very confident in your process.   I use a cool water bath, 45F during this step  to prevent the solution from becoming too hot. 

6)  As the reaction reaches it conclusion, the addition of H₂O₂ will produce little effervescence.  If the solution has been keep relatively cool, about 80F during the addition of the H₂O₂, it will appear a greenish brown color.

7)  You should begin testing for the presence of Ferrous ions left in solution.  To do this, mix a small very dilute solution of Potassium  Ferricyanide.  Place a small sample of this solution into a 50ml beaker or other small vessel.  Add a drop or two of the ferric oxalate solution to it.  There should be a brown precipitate form. If a blue precipitate forms there are still ferrous ions in solution. 

8)  To eliminate the ferrous ions,  add small amounts of oxalic acid to the solution ; a ½ gram amount at a time.  Allow some time for this to get into solution and retest for the ferrous ions with the  potassium  ferricyanide and a few drops of you ferric solution.  There will be a color shift as you get closer to eliminating the ferrous ions;  the solution will get greener with less of a yellow brown quality.

9)  You can add a few drops of  H₂O₂ to the solution until a slight effervescence returns. It is believed in some circles that the addition of the H₂O₂ at this stage, will convert excess oxalic acid to CO₂.

10)  If you do not have a positive test for ferrous ion, it is time to let the solution set for a while (This may be several hours or overnight) and then test the SpGr of your ferric.  The SpGr is affected by all the components that you have used, so it is possible to have a slight range in SpGr.  The reason to determine the SpGr of your solution is to allow you to help judge future batches.  I produce a solution that has a SpGr of 1.190. 

11)  You are done when you get a brown result using the Potassium  Ferricyanide test. 

12)  If you plan to use the ferric oxalate for other iron processes such as Kallitype, you may want to make further alteration to this formula. This may include the addition of ferric nitrate.  For a more complete look at this step, refer to Dick Stevens Making Kallitype – A Definitive Guide.