Which EDTA for what purpose?

pdeeh · Feb 11, 2015

I quite often read discussions of photographic chemistry here at APUG where EDTA is referred to quite generically, but there are a variety of forms, and it's not always clear (to those of us without a background in chemistry) from the context which form is being referred to and why.

Here's a list of the EDTAs available from Silverprint (UK), as listed on their website ... and the question is, which EDTA is used for what purpose in photographic chemistry?

EDTA Acid
EDTA Tetra Sodium
EDTA NA FE
EDTA disodium

Just to be clear, I don't have a particular need or desire to buy or use EDTA, I just want to know in order to stretch my knowledge out a bit further.

Gerald C Koch · Feb 11, 2015

The iron (Fe) salt is used in bleaches for color film. The others are used as chelating agents for calcium and other metal ions. Which one that is used depends on the pH of the solution. The free EDTA acid produces the lowest pH and the tetrasodium salt the highest.

pdeeh · Feb 12, 2015

Thank you

john_s · Feb 13, 2015

Thank you from me too. Would it be a reasonable assumption that the tetra sodium salt could be viewed as a "general purpose" chelating agent, given that developers are generally rather alkaline?

I made up some ID-78 stock, extra concentrated, using Dimezone-S instead of Phenidone, and potassium carbonate instead of sodium. I added a bit of EDTA-4Na because I was trying to duplicate Agfa Neutol-WA which had been my favourite print developer, and its MSDS listed it. I was surprised that the typical pink colour instantly vanished. The developer was excellent and lasted well. I'm about to make up another batch. Would the EDTA-4Na have helped it last so well?

Gerald C Koch · Feb 13, 2015

In developers the EDTA is used to prevent calcium carbonate from precipitating when tap water is. The tetrasodium salt is usually used for this purpose. The only developing agent that requires a chelating agent to help preserve it is ascorbic acid. However in this case EDTA cannot be used. DTPA is usually used instead.

swmcl · Feb 13, 2015

Can we assume that DTPA is http://en.wikipedia.org/wiki/Pentetic_acid ??

Rudeofus · Feb 13, 2015

swmcl said:
Can we assume that DTPA is http://en.wikipedia.org/wiki/Pentetic_acid ??

Yes. Xtol would be an example of a developer which uses this compound to prevent early decomposition of Ascorbate.

Rudeofus · Feb 13, 2015

Gerald C Koch said:
The only developing agent that requires a chelating agent to help preserve it is ascorbic acid. However in this case EDTA cannot be used. DTPA is usually used instead.

I have reads statements in photographic literature claiming that HQ and some other development agents also decompose faster if traces of free Iron ions are around. The effect is, however, a lot less pronounced than with Ascorbate.

Rudeofus · Feb 13, 2015

@pdeeh: The chelating agent is frequently the first compound being dissolved during mixing of a developer, so the very soluble Tetrasodium EDTA is preferable to the quite soluble Disodium EDTA, which in turn is preferable to the sparingly soluble EDTA. All three forms will affect the final pH of your developer, so be careful with dilute or poorly buffered film developers.

pdeeh · Feb 13, 2015

I always use hexametaphosphate as otherwise my tap water throws calcium precipitate the moment any sulfite or carbonate goes in, when I'm making d23/d76 or mq paper developer.

I might try edta when my calgoo runs out, if it's not more expensive
Never bother when using commercial developers.

It had not occurred to me to consider pH when using calgoo

Rudeofus · Feb 13, 2015

pdeeh said:
I always use hexametaphosphate as otherwise my tap water throws calcium precipitate the moment any sulfite or carbonate goes in, when I'm making d23/d76 or mq paper developer.

I might try edta when my calgoo runs out, if it's not more expensive
Never bother when using commercial developers.

It had not occurred to me to consider pH when using calgoo

Calgon does not affect pH much, which is nice, but it hydrolyzes over time. If you want long term stability, EDTA is better.

pdeeh · Feb 13, 2015

All good info, thanks.

Any developers I make up don't sit around for more than two or three months so long term stability isn't something that concerns me.

Vaughn · Feb 13, 2015

EDTA disodium

I just read in some literature that recommended this as the first clearing bath for platinum printing as the lower pH keeps some hard to wash-out compounds from forming. Sorry for being so non-specific. I did find it more difficult to go into solution than EDTA tetra sodium.

I use citric acid as a first clearing bath, followed by two baths of Kodak HCA/EDTA (tetra sodium).

pdeeh · Feb 17, 2015

As a further MOI ... I've determined I need about 1.5g of calgon per litre to avoid precipitation.

What would be a suitable equivalent were I to use edta (Di- or tetra-) ?

doughowk · Feb 17, 2015

I had been using HCA in my 2nd bath for Palladium prints, but noticed a bleaching effect. Dick Arentz, in his book, says that HCA can bleach pure Palladium prints . Sandy King has said that the ph of the developer is very important for removal of FO - should be 6.0 to 6.4

Which EDTA for what purpose?

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