Process in brief
Preparation of the solution of Ferric Oxalate.
Chemical formula: Fe2(C2O4)3.5H2O.
A compound chemically indefinable. The best approximation is "a complex of Ferric Oxalate ".
It can easily create different polymers. For this reason, many different forms of crystallization, that are created during the preparation, could influence the solvability of the product.
In the preparation it is usually present a small excess of oxalic acid to improve the conservation of the product. This excess, irrelevant for the technique of Platinum / Palladium, makes it unusable for Kallitype.
This procedure is applied to the majority of Ferric Oxalate commercially available.
To get a solution of about 27-30%, proceed as follows: Weigh 30g of crystals and add them to 100-110ml of demineralized water, heated not more than 50°C, stir it with a glass or plastic rod. This operation is very long and can last for several minutes. It would be really helpful to have an heating magnetic stirrer. A residual turbidity is eliminated by filtering it on a filter paper, the color of the final solution varies depending on the amount of free oxalic acid present:
A golden amber color indicates the absence of free oxalic acid. (This Solution is obtained with the LabOldTech Ferric Oxalate that melts in maximum 30 minutes).
A yellowish color is an indicator of small amounts of free oxalic acid.
A green fluorescent staining indicates a greater amount of free acid.
To use it in the process of Kallitype, where the presence of free acid can give impurities that interfere negatively, it’s necessary to eliminate this excess. You can verify the presence of free acid in it with a 10% solution of silver nitrate. If you add a few drops of the solution in a couple of milliliters of Ferric Oxalate, the presence of turbidity is an indication of the presence of oxalic acid. To remove the free oxalic acid, add small amounts of Ferric Nitrate waving and repeating the test until the disappearance of turbidity.