Whats the difference and use of EDTA and DTPA (Pentetic acid)

[pkr1979]

Hi all,

Im curious to know the purpose and difference between these two chemicals for use in developers, and why one would use one of them instead of the other? And, are other chemicals more common instead of those two?

Cheers
Peter

 

[relistan]

Hi all,

Im curious to know the purpose and difference between these two chemicals for use in developers, and why one would use one of them instead of the other? And, are other chemicals more common instead of those two?

Cheers
Peter

They are both chelating agents that chemically sequester certain ions in the solution that might otherwise interfere with the photographic development (or fixing) process. Calcium, iron, copper, and other ions can interfere with developing agents, cause precipitation of scum onto the film, or catalyze the destruction of the developing agents in storage.

So these two substances and others (notably sodium hexametaphosphate “calgon” or etidronic acid “fotoplex”) are used to isolate those ions into harmless, soluble compounds.

The extent to which the chelating agents chemically bond with different ions, which ions you are trying to isolate, and the developing agents in the developer would affect the choices you make.

Notably, ascorbic acid and phenols can break down in a reaction known as a Fenton reaction (“sudden death”) that is catalyzed by iron or copper ions in solution. EDTA is a good chelating agent but does not isolate the iron enough to prevent this catalyzing behavior. In fact it appears to exacerbate it. So in those cases you see DTPA used, or Ryuji Suzuki used TEA and salicylic acid.

 

[pkr1979]

Thanks @relistan ! As far as I understand this isnt really used in all developers. So... does this mean that they (EDTA/DTPA) is more useful in some developers than others? If so, what kind of developers?

 

[relistan]

If you use distilled water and have very pure chemicals you can get away without (e.g. sodium carbonate might contain iron in very tiny amounts). You might be fine with tap water if you have soft water without a lot of other components. If you are making stock solutions to last a long time or using more fragile chemistry like phenols or ascorbic acid, then you probably want a chelating agent. If you are using hard tap water then you almost certainly want one.

Another strategy is to put the developing agents into a solution containing no water, like HC-110 or PC-TEA, etc

 

[pkr1979]

Thanks for this man - much appreciated

 

[Rudeofus]

As far as I understand this isnt really used in all developers. So... does this mean that they (EDTA/DTPA) is more useful in some developers than others? If so, what kind of developers?

While few published formulas report EDTA or DTPA, there's a good chance, that commercial formulas contain one or both of them. We know, that E-6 kits contain DTPA and phosphonic acids, and we assume similar ingredients in C-41 chems. There's also also DTPA in XTol.

Here is a decision list, whether to add these chems or not:

1. Is this for your own use, or for commercial use/distribution? You can accept much more risk, if you are the only user. As soon as you owe other people results, you should add decent sequestering agents as insurance against premature developer decomposition
2. Is your developer very alkaline and do you use tap water to mix it? If so, the addition of EDTA may control effects from water hardness.
3. Do you always mix your developer fresh, or do you want long storage life?
4. Is your developer cheap, or does it contain expensive ingredients?

 

[pkr1979]

I can probably do without... havent had any issues so far at least. But if EDTA or DTPA is added - how much would you use?

 

[relistan]

I can probably do without... havent had any issues so far at least. But if EDTA or DTPA is added - how much would you use?

They are usually small amounts. 1g/liter for EDTA in whichever form you get it is probably fine. I haven’t used DTPA. If you are doing fine without it you might not need it for your application. Note that if you get the free acid version of EDTA, it will affect your pH enough to matter.

 

[Anon Ymous]

They are usually small amounts. 1g/liter for EDTA in whichever form you get it is probably fine. I haven’t used DTPA. If you are doing fine without it you might not need it for your application. Note that if you get the free acid version of EDTA, it will affect your pH enough to matter.

Another choice when you don't want pH to be affected is sodium hexametaphosphate. It used to be the most common agent used, but was phased out because of environmental concerns (eutrophication). It's not the most effective one at high pH. I've seen it fail miserably in E6 CD.

 

[albada]

Here's a rule:

If your developer contains ascorbate (includes ascorbic acid) and you want it to last a while, then add DTPA.
Otherwise, if you are using tap water and won't use it within 20 minutes after mixing, then add EDTA.
(For film developers, precipitates form after 20 minutes or so).

I've used disodium EDTA in a paper developer (pH around 10.3) at 1 g/liter with good results with the hard water here.

Mark

 

[pkr1979]

Thanks for the feedback guys. I have to admit I have never really considered this. How come it isnt listed in recipes, at least not for most of those Ive been looking through or used?