I find mixing chemicals fun, so the time is of no problem. Here's the recipe I have.
You need the following substances to make ferric oxalate:
ferrous ammonium sulfate crystals 1 pound
oxalic acid crystals 1 pound
hydrogen peroxide 30% solution 1 pint
potassium ferricyanide crystals 1 pound
distilled water 1 gallon
You also need the following equipment:
Scale (0.1g to 100g is fine)
Graduate 150 ml.
2 Glass Beakers 500 ml.
Brown glass bottle 500 ml.
Glass or plastic funnel (about 1 pint)
Filter paper
Acid test paper or pH meter
Brown glass or plastic bottles 4 ounces or 100 ml.
Use the following procedure for making ferric oxalate:
1. Begin by combining 125.1 g of ferrous ammonium sulfate (do not use ferric) with 300 ml of distilled water in a 500 ml glass beaker. Heat the water to facilitate dissolving the chemical. About 150°F will do. Stir until all the chemical is dissolved.
2. When the ferrous ammonium sulfate is completely dissolved, add 40.2 g of oxalic acid crystals to the hot solution. Stir continuously until a yellow precipitate, ferrous oxalate, forms. This will take about 3 minutes.
3. When the solution is uniform, set the beaker containing the ferrous oxalate aside and allow the precipitate to settle. The ferrous oxalate will slowly sink to the bottom, leaving a relatively clear liquid at the top. The liquid is very acidic; it contains sulfuric acid in sufficient quantity to measure between pH 1 and 2. Pour the liquid off into a separate container, taking care to avoid the loss of any of the yellow precipitate. Neutralize the acid in the liquid by adding a cheap alkali such as sodium bicarbonate and dispose of it.
4. Wash the ferrous oxalate of any adhering chemicals. Do this by adding distilled or deionized water to the beaker and stirring. Tap water will do if that's all you have, but it's better not to use it, as it contaminates the ferrous oxalate with undesirable compounds such as chloride. After the precipitate settles, pour off the liquid. If the liquid is still strongly acidic, neutralize it before disposing of it.
5. Repeat the process of adding water, stirring, letting the precipitate settle, and pouring off the liquid four or five times until the pH of the washing liquid rises to at least 2.5. You may wish to repeat the process until the pH of the ferrous oxalate reaches 7 (about ten washes), but that much washing is not necessary.
When you have washed the precipitate at least four times, drain the ferrous oxalate, taking care to lose as little product as possible. Make sure the ferrous oxalate is in a 500 ml glass beaker.
6. Combine, in another vessel, 20.1 g of oxalic acid crystals and 60 ml of distilled water. Stir until the crystals are dissolved. Heat the water to hasten dissolution.
7. When the oxalic acid is dissolved, add it to the ferrous oxalate. Stir thoroughly in the 500 ml glass beaker. There will be little visible change, although the acid solution will thin the ferrous oxalate.
8. This step requires care and restraint. Make sure you wear safety glasses, rubber gloves, and a protective apron. Carefully measure, without spilling or touching, 50 ml of hydrogen peroxide 30% solution in a graduated cylinder. With a dropper, carefully add the hydrogen peroxide, 1 to 2 ml at a time, to the ferrous oxalateoxalic acid mixture. The reaction may start slowly but will become quite active as it proceeds. Continue to add the peroxide very slowly, drop by drop, to control the heat and steam producing reaction. If you add more than a few drops of peroxide, the mixture will sputter and boil. Stir the solution with a long-handled glass rod after each
addition of peroxide.
After you have added approximately 20 to 30 ml of peroxide, the reaction will produce a clear brown solution (it looks like root beer). The amount of hydrogen peroxide needed for complete clarification depends on the amount of acid present in the ferrous oxalate.
9. Continue adding what remains of the 50 ml of peroxide, until the ferrous oxalateoxalic acid mixture becomes a clear, homogeneous dark brown solution. It is possible that more than 50 ml will be needed to complete the reaction and clarify the solution. No harm will be done if you add a bit too much. You may have to add as much as 65 ml of hydrogen peroxide before the solution clears completely.
As the end of the reaction approaches, it becomes progressively less violent. You'll know the reaction is over when the addition of another milliliter or two of peroxide causes little or no, effervescence. The resulting ferric oxalate solution will be quite hot.
10. Test the solution for the presence of ferrous oxalate by using the dilute ferricyanide test (at the end). It is likely that some ferrous oxalate will be present in the solution, and you should expect the test to show a blue response. The ferrous ion is present because the ferric oxalate was made with as little oxalic acid as possible. If the test shows a blue precipitate, add a small amount of oxalic acid. As long as a blue precipitate forms when you add drops of the ferric oxalate solution to the ferricyanide solution, you must add more oxalic acid to the ferric oxalate. Try to add as little oxalic acid as possible to eliminate the remaining ferrous ion. Finally, add a milliliter or two of hydrogen peroxide to oxidize the ferrous oxalate to ferric oxalate.
11. To eliminate all the ferrous ion, continue adding oxalic acid to the ferric oxalate solution at a rate of 0.25 to 0.33 g at a time. After each addition of oxalic acid, add a drop or two of hydrogen peroxide until the effervescence reappears. It may be necessary to add as much as 1 to 2 g of oxalic acid to convert all the ferrous oxalate to ferric oxalate. After each addition of the two chemicals, repeat the ferricyanide test with fresh ferricyanide solution. Continue adding slight increments of oxalic acid and hydrogen peroxide until the addition of ferric oxalate solution to fresh
ferricyanide solution no longer produces a blue precipitate but only a yellow-brown response. Then the solution no longer contains any ferrous ion, and the process of making the ferric oxalate is finished. You may wish to add a few milliliters of hydrogen peroxide (until effervescence occurs) to ensure that the ferric oxalate solution has been completely oxidized. Effervescence occurs when all the ferrous oxalate has been
converted to ferric oxalate and the oxidizer begins converting excess oxalic acid to carbon dioxide.
12. Cool the ferric oxalate solution by putting the beaker in a larger vessel containing water and ice and stirring. When the solution has cooled to 60°F, measure its specific gravity 1.115 = 18% solution and pH (approx. 2). After you have taken these measurements, pour the solution into a dark brown bottle with an airtight seal.
Testing for Ferrous Ion: Dissolve a small amount (1/10 teaspoon) pf potassium
ferricyanide in 1 ounce of water, Stur until you have a uniform solution. Add 1 drop of the ferric oxalate solution to be tested in the ferricyanide solution. If no blue precipitate appears, the ferric oxalate is devoid of ferrous ion.
Have fun!
Steve
