Making EDTA from Na2-EDTA

[Rudeofus]

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 (there was a url link here which no longer exists).

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?

 

[sehrgut]

From my lab work, I can say that differential solubility is generally preferable to simple washing. Denatured alcohol (if you're in a country that allows it to have only methanol as its denaturant, like the US) can be used for this.

As far as precipitation pH, I'd titrate it with HCl, and note the pH with maximum turbidity.

 

[nworth]

Another question would be whether the sodium actually hurts the process. If it does not, it may be possible to add the appropriate amounts of sodium and ferric ion to the mix. Some fine tuning might be required, and the choice of anions may be important.

 

[Rudeofus]

Another question would be whether the sodium actually hurts the process.

See the link I provided. I don't think that trace amounts of Sodium will hurt, but 2 moles Sodium per mole Ferric EDTA are claimed to substantially slow down the bleach process.

 

[Rudeofus]

Just an update for all those who care: I prepared 5% Na2-EDTA solution and used 50% NaHSO4 stock solution to incrementally lower the pH. Starting at a pH of about 4.3 something started to precipitate at pH of about 3. The behavior was a bit odd, because every time I added NaHSO4 stock solution pH dropped to 2.6 and then went back up to about 3.3 or 3.4 within a few minutes. My interpretation is that pH rose back as EDTA free acid crystallized.

After a lot more NaHSO4 stock solution pH finally stayed stable below 2.5 and went down smoothly from then on until pH 0.5. Given the amount of NaHSO4 stock solution needed to reach pH 0.5 I ended the test run at this level. The EDTA did not settle but had to be filtered.

Subsequent washing with distilled water did not reduce the amount of compound in the filter. The pH at which the compound precipitated from solution would point at the EDTA2- ---> EDTA- transition. Another data point would be this book where Na-EDTA is shown to have pH 3.5 while EDTA free acid supposedly has pH 2.0, which would tell me that I precipitate EDTA free base when crossing the pH 3 line downwards. Both Na-EDTA and EDTA free acid are described as poorly soluble ...

I will try to find some way of determining whether I precipitated EDTA free acid or Na-EDTA or a mixture thereof with my method.

 

[Photo Engineer]

Ok, assume you have EDTA, what next?

Add Ammonia? The most concentrated you can get is 28% and so that will limit you to the final concentration of Ammonium EDTA, and how many Ammonium ions do you need? NH4EDTA, (NH4)2EDTA, (NH4)3EDTA and (NH4)4EDTA all exist. So, lets assume you want (and get) NH4EDTA, what next. It will be very dilute so you will have to purify it. Next, ow do you react it with Ferric Ion to get pure (NH4)FeEDTA. Note that the amount of Ammonium ion is really essential to balance the charge on the final ion!

This is really a thankless task!

Now, as to the essential need of using Ammonium Ferric EDTA. Well, we made the first bleaches and blixes for C41 and E6 and Ektaprint 3 using Sodium salts and there was no way that we could go to completion in a reasonable time. As we went to more Ammonium ion, the bleaches and bliexes sped up. So consider that EK recommends 6' at 100F (about 40C0 with an essentially all Ammonia bleach, then figure that it might take 12 or more minutes to do it with all Sodium. Then the presence of other cations would fall somewhere in between those extremes. It is not a straight line though.

PE

 

[sehrgut]

12+ minutes is indeed an unreasonable time for a commercial lab, but given that we're used to stand-developing in Rodinal one to a bazillion for hours at a time, 12-minute bleach times don't seem terribly inconvenient to me. Would the 12-minute bleach time have detrimental effects on the image or emulsion, or is it simply a matter of time convenience?

 

[Photo Engineer]

I have no idea what 12+ minutes would do, and I have no way of telling you what fix times to use under those conditions. Why don't you test it with some valuable pix? Just kidding. It is true though that I don't know at all and some rather sophisticated experiments would be in order.

Sorry.

PE

 

[Rudeofus]

Add Ammonia? The most concentrated you can get is 28% and so that will limit you to the final concentration of Ammonium EDTA

28% Ammonium EDTA means 33% Ammonium Fe-III EDTA which is not bad at all. Note that Fuji's E6 formula calls for 12%, so whatever we can make with these primitive methods is over twice as concentrated as the final working solution.

and how many Ammonium ions do you need? NH4EDTA, (NH4)2EDTA, (NH4)3EDTA and (NH4)4EDTA all exist. So, lets assume you want (and get) NH4EDTA, what next. It will be very dilute so you will have to purify it. Next, ow do you react it with Ferric Ion to get pure (NH4)FeEDTA. Note that the amount of Ammonium ion is really essential to balance the charge on the final ion!

Since the EDTA chelates the Ferric ion I would assume we end up with (FeIII-EDTA)- which will take on as many Ammonium ions as it has room for: one. The pH can be used to fine tune this.

This is really a thankless task!

I could not agree more, especially if one considers that over 100g of EDTA free acid are needed for making one liter of bleach. Ideally one gets the Ammonium Ferric EDTA from a company that has the know how and the means to make it in bulk quantity and good quality. This may not always be an option for everyone, though, and the method described here might be a work around for very determined people.

 

[sehrgut]

Why don't you test it with some valuable pix? Just kidding. It is true though that I don't know at all and some rather sophisticated experiments would be in order.

I may try it clips from a roll of test charts. I'd be interested to know if cheaper chemistry could produce similar results with only inconvenience to pay.

My only problem with experiments like this is I don't have a densitometer, and scanning things without SOME automatic compensation is difficult with any software. Makes it difficult to detect subtle differences like half-stop speed losses and such.

 

[Rudeofus]

Just wanted to follow up with a short update:

I dissolved 5 g of Na2-EDTA in 100ml distilled water and added enough NaHSO4 to lower pH below 1. A precipitate quickly formed which could be filtered and washed with distilled water. After letting the filter plus its contents dry the filter weighed 4 grams more than a fresh filter from the same box. Since I wanted to know whether I created Na-EDTA, EDTA or EDTA anhydride with this method, I did the math to find out how much NaOH is needed to make 1g of each compound soluble as Na2-EDTA in 40 ml water. According to my calculations it takes 0.127g NaOH for Na-EDTA, 0.274g NaOH for EDTA, and 0.312g NaOH for EDTA anhydride.

A quick test confirmed that 0.186g NaOH (it comes in pellets, hence the odd numbers) could not, but that 0.289g could slowly dissolve the 1g of unknown powder in 40ml water, which tells me that most likely EDTA free acid was formed in the reaction with NaHSO4. I also did some pH measurements which seem to confirm this:

1.35 g Na2-EDTA in 40 ml water: pH = 4.28
0.186g NaOH + 1g test powder in 40 ml water: pH = 3.60
0.289g NaOH + 1g test powder in 40 ml water: pH = 4.35

This tells me that with only slight excess NaOH (0.289g instead of the calculated 0.274g for EDTA) we get only a slight rise in pH.

With the evidence at hand I would conclude that I produced about 4 grams of EDTA free acid from 5 grams of Na2-EDTA and quite a bit of NaHSO4. Availability and cost will be determining factors whether this is a good starting point for making Ammonium Ferric EDTA, but from a technical standpoint it works.

 

[Photo Engineer]

Rudi;

With this method, you stand a chance of getting mixed salts too easily. Make a saturated Na2EDTA solution and then just add some automobile battery acid which is about 37% H2SO4. The EDTA precipitates out easily then.

Or, just buy EDTA acid. It is very inexpensive.

PE

 

[Rudeofus]

I'm sure there is a gazillion ways to do this better than I did, and even I most likely got the compound I was after with reasonable yield. And no, EDTA is not cheap, in fact it is about twice as expensive and not nearly as available as Na2-EDTA. And yes, making EDTA is only the first tricky step towards NH4-FeIII[/SUB]-EDTA, and the FeBr3 needed to get there is also hard to obtain.

 

[NolanGalbreath]

I remember looking for the free acid form of EDTA a while ago; cheapest I was able to find it was $100 for the smallest quantity. Now I can't even easily find it from a place that will ship to individuals.