For C41 and E6 bleach we need Ammonium Ferric EDTA, which is often not easily available, especially at a good price. The most easy to obtain EDTA compound is Na2-EDTA, since EDTA is frequently used as water softener and the Disodium salt is much more water soluble than the free acid. If we want to make Ammonium Ferric EDTA ourselves, we would need to find a method to remove Sodium from Disodium EDTA. This is an important task as PhotoEngineer is quite clear that Sodium ions do significantly slow down the bleach process.

Since Sodium ions are very hard to precipitate out of a solution, the easier course of action would be precipitating the EDTA out of solution instead. The difference in solubility between Na2-EDTA (100 g/l) and EDTA free acid (0.5 g/l) tells me that only deprotonated EDTA is soluble, and the easiest way to give ionized EDTA its protons back is by lowering pH. By adding mineral acids (or NaHSO4 for those who don't like nasty liquids) to a concentrated Sodium EDTA solution we should be able to precipitate EDTA free acid. Wikipedia says it works that way, United States Patent 2162971 says it works that way, but none of these resources give specific instructions how to do this. I wish some of the chemistry experts here could provide some answers:

  • What's the best pH to precipitate EDTA? We know that if pH is too low, the amino groups can take on up to two protons which ionize the whole compound again, possibly putting it back into solution. Or doesn't it?
  • How can I be sure that what precipitates is the free acid and not Na-EDTA or Na2-EDTA ?
  • Is this a trivial task or do we have to worry about exact process parameters plus pixie dust for getting as little Sodium left in our EDTA as possible?
  • Wikipedia says that EDTA free base is soluble in ethanol while Sodium Sulfate isn't. Would this be a way to clean the precipitate or is washing with distilled water the preferred way?