The effectiveness of Ammonium Ferric EDTA is diminished in both bleach and blix baths by the presence of sodium Ion.
In any event, the question was about making the ammonium salt, not the sodium salt which is what would form otherwise. You would get Disodium ammonium Ferric EDTA AAMOF, which is not very active compared to the ammonium salt sold as an ~60% solution.
That particular method, adding disodium EDTA to Ferric Ammonium Chloride or Sulfate was how I made the first blix for Kodak. We improved it mainly through elimination of sodium.