Na2 EDTA - AmmEDTA

infinit · Nov 13, 2014

Does anyone know the purpose of Disodium EDTA in bleaches & fixers? Buffering?
Is there any reason why prefer Disodium EDTA instead of Ammonium EDTA?
Taking into account that sodium ions will slow bleaching.

Gerald C Koch · Nov 13, 2014

The various salts of EDTA are used as chelating agents to bind various metal ions and prevent them from causing trouble. They are seldom used in concentrations high enough to be significant as a buffering agent. The ammonium ion decomposes in alkaline solution to form gaseous ammonia. The small amount of sedum ion that they contribute is not enough to slow the rate of fixation.

An exception occurs in an ferric EDTA bleach where EDTA is used in high concentration where it prevents hydrolysis of the ferric ion.

infinit · Nov 13, 2014

Thanks Gerald!!!!

Photo Engineer · Nov 13, 2014

In Ferric EDTA bleaches, I must add to Gerry's comment that Ammonium ion has a powerful effect on rate of bleaching, and also that excess EDTA is needed to prevent stain from forming.

PE

Rudeofus · Nov 14, 2014

Gerald C Koch said:
An exception occurs in an ferric EDTA bleach where EDTA is used in high concentration where it prevents hydrolysis of the ferric ion.

The EDTA is not just there to keep Fe(OH)3 or whatever from precipitating, it's main purpose is to tone down the oxidative power of Fe3+. You could, in theory, make a bleach from FeCl3 alone, but unlike Ammonium Ferric EDTA it could destroy your emulsion in the process.

infinit · Nov 18, 2014

Let's say I need 10% excess of EDTA out of the FeAmmEDTA. I have EDTA acid and Disodium EDTA, and let's say I could get some AmmEDTA.
How do you order it by preference? AmmEDTA will increase the rate, Na2 will give some Na ions slowing down the process, what about the EDTA acid?

Photo Engineer · Nov 18, 2014

If you have EDTA acid, you can make either the Sodium or Ammonium salt, but not without precautions due to the chemical reaction which will take place, and not without knowledge of how much of each to use and how to do it.

I suggest not even trying!

PE

infinit · Nov 19, 2014

My question is, taking into account that I have the three of them, which one will work best with FeAmmEDTA? Na2EDTA, AmmEDTA or plain EDTA acid?

Photo Engineer · Nov 19, 2014

AmmEDTA or EDTA Acid depending on pH.

Rudeofus · Nov 20, 2014

infinit said:
My question is, taking into account that I have the three of them, which one will work best with FeAmmEDTA? Na2EDTA, AmmEDTA or plain EDTA acid?

EDTA will not dissolve as acid, only as H2-EDTA--/H-EDTA---/EDTA---- anions. If you add it to Ammonium Ferric EDTA, you would have to add NaOH, or preferably NH4OH until pH goes above about 4.

If you have a pH meter, you can use EDTA and Ammonia, otherwise I would recommend using Na2-EDTA. (there was a url link here which no longer exists) also uses Na2-EDTA and Ammonium Ferric EDTA.

Na2 EDTA - AmmEDTA

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