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  1. #271
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    When you add salicylic acid to an alkaline solution, the solubility goes up! So, there may be some leeway here.

    PE

  2. #272

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    Quote Originally Posted by Rudeofus View Post
    If you look at the numbers, you'll see that Salicylic chelates FeII better than Citrate (6.55 vs. 3.2), and it also chelates FeIII better than Citrate (16.35 vs. 11.85). If you have Salicylic Acid in your dev, you won't need Citric Acid any more. Note that chelate stability does not tell the whole story since EDTA, which is a strong chelating agent for both FeII and FeIII, is known to accelerate the Fenton reaction.
    My goof! I swapped the FeII and FeIII numbers for Salicylic. So you're right; Salicylic beats Citric for both types of Fe ions.

    I've heard before that EDTA accelerates the Fenton reaction, and that this is why Kodak uses DTPA in XTOL instead. But here's a report saying that DTPA also accelerates the Fenton reaction:

    From http://www.efchemicalconsulting.co.u...cal-damage.pdf
    other aminocarboxylate chelating agents such as NTA
    (nitrilotriacetic acid) and DTPA, in addition to EDTA,[7] all greatly accelerate the
    Fenton reaction.

    My question is: Has anyone verified that EDTA indeed destroys ascorbate developers? Or is this a hypothesis based on studies not related to developers?

    Mark Overton

  3. #273

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    Keep working! But for the time being I have a 2-year supply of XTOL tucked away.

  4. #274
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    Quote Originally Posted by albada View Post
    My question is: Has anyone verified that EDTA indeed destroys ascorbate developers? Or is this a hypothesis based on studies not related to developers?
    A brief search revealed the following:
    • EDTA slows down the Fenton reaction, but less so than other chelating agents. Note that in the experiments described in this paper, an Ascorbate solution lost 3% of its Ascorbate after 15 minutes in a phosphate buffer (pH = 7.0) "protected" by EDTA. Other chelating agents appeared to protect the Ascorbate much better, DETAPAC (which we commonly refer to as DTPA) is 10 times as effective at pH 7. Without chelating agent 30% of the Ascorbate was gone after 15 minutes!
    • The same paper also claims that the effectiveness of a chelating agent is strongly dependent on pH. What this paper postulates for pH 7 is completely different in acidic environment, and may again be different at the pH of your dev. Note that DTPA gets better and better compared to EDTA at higher pH and that Xtol is known to be stable for about half a year.
    • Another experiment discovered that EDTA becomes less effective after 24 hours in solution, strongly dependent on which other ions are in solution.

    If you ask me, given these numbers, I would put a good iron chelating agent into your soup, at least during testing. Note that you pondered about much smaller changes in AA amount than 30% when you formulated and tested your dev, yet I do not think that you always tested your dev within 1 minute of mixing it.

    PS: Please note that what I wrote above is no peer reviewed literature survey from an expert in the field, it's just an internet forum posting after a brief online search. Other papers I am unaware of may well state very different results and with my limited level of knowledge in the field I wouldn't be able to judge which one was right. So take these with a grain of salt, maybe more knowledgeable people in the forum could share their opinion on this.
    Trying to be the best of whatever I am, even if what I am is no good.

  5. #275

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    Quote Originally Posted by Rudeofus View Post
    If you ask me, given these numbers, I would put a good iron chelating agent into your soup, at least during testing. Note that you pondered about much smaller changes in AA amount than 30% when you formulated and tested your dev, yet I do not think that you always tested your dev within 1 minute of mixing it.
    I'll need to run some tests to see if Fe(II) and Fe(III) are a problem. It hasn't *appeared* to be an issue with my mix-to-use times of under an hour, but I won't know for certain until I've run tests. Do you know of an easy way to get a known molarity of these Fe ions in solution? Something better than a rusty nail? In addition, I have some salicylic acid, so I can run with/without tests to see how much difference that makes.

    Your link shows that citric acid is good at sequestering Fe(III) but poor with Fe(II). So what else can be used for Fe(II)? How about histamine?

    Mark Overton

    EDIT: I still haven't seen any data showing that EDTA accelerates the Fenton reaction. I think I'll buy some and try it.
    Last edited by albada; 10-24-2012 at 05:30 PM. Click to view previous post history.

  6. #276

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    Whether a particular chelating agent is useful in protecting ascorbate is rather complicated. It depends on the stability constant for a particular species like iron III but also on the redox potential of the complex. Ryuji did a lot of research on the net and recommentded that EDTA not be used. One of the problems he encountered was in finding all the redox potentials for chelating agents. The data is rather sparse. He settled on salicylic acid because it is readily available and cheap. BTW, the TEA is to complex any copper II which also catalyses the Fenton reaction.

    As a source of Fe III ions may I suggest ferric ammonium sulfate and for iron II ferrous ammonium sulfate. Another posibility would be ferric nitrate. These salts are quite stable and can be obtained in high purity. Other ferric salts tend to decompose to various hydrated oxides in water. For the amounts needed for a test the ammonium ion will not be a problem.
    Last edited by Gerald C Koch; 10-24-2012 at 07:29 PM. Click to view previous post history.
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  7. #277
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    Quote Originally Posted by albada View Post
    I'll need to run some tests to see if Fe(II) and Fe(III) are a problem. It hasn't *appeared* to be an issue with my mix-to-use times of under an hour, but I won't know for certain until I've run tests.
    You are unlikely to see much effect since the activity vs. AA content curve is very flat in the region you are looking at. AA decay may explain some of your characteristic curve anomalies you described, though. If you look at Pat Gainer's data plot, 30% less AA is not a big issue as long as you are not concerned with 1/10 of a stop.
    Quote Originally Posted by albada View Post
    Do you know of an easy way to get a known molarity of these Fe ions in solution? Something better than a rusty nail? In addition, I have some salicylic acid, so I can run with/without tests to see how much difference that makes.
    The electrical engineer in me would look at FeCl3 as source of FeIII, but chemists might know better. The trace quantities of FeIII you are going to add make it unlikely that the Cl- has any effect on your developer's properties.
    Quote Originally Posted by albada View Post
    Your link shows that citric acid is good at sequestering Fe(III) but poor with Fe(II). So what else can be used for Fe(II)? How about histamine?
    You could still use DTPA like Xtol, or Dequest as suggested by PhotoEngineer ...
    Quote Originally Posted by albada View Post
    EDIT: I still haven't seen any data showing that EDTA accelerates the Fenton reaction. I think I'll buy some and try it.
    From the second paper I linked to:
    EDTA stored in buffer appears not only to lose to some degree its antioxidant potential (arising from chelation of traces of transition metal ions), but in some model systems even to acquire pro-oxidant activity.
    Trying to be the best of whatever I am, even if what I am is no good.

  8. #278

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    TEA vs METABORATE

    Alan: Your statement that 99% TEA is difficult to obtain in the UK made me re-think the use of TEA. Another disadvantage of TEA is that because the pH-versus-temperature curve differs from metaborate, the developer can't use a fixed-factor of XTOL's times for temperatures above 20C. It would need its own time-temperature table. So I've decided to pursue the previous baseline dev based on sodium metaborate that I posted here: http://www.apug.org/forums/viewpost.php?p=1397796

    The main purpose of TEA was to provide strong buffering in order to minimise the effect of inaccurate measurement of sulfite. A good solution is to measure sulfite by weight instead of volume. I mentioned earlier in this thread that electronic scales are cheap. One with .1g resolution is only US$10. That's the same cost as a packet of XTOL! At that price, there is no excuse for folks to not have a scale. Search for "scale .1g" or "scale .01g" in amazon.com.

    I ran another strip on the metaborate-based baseline dev last evening, but adding propylene glycol as if it came from a concentrate (and boosting metaborate by .1g to adjust pH). The PG made the dev-time increase to 13.1 minutes (it was 12.25 without the PG), and the results look the same: a hair finer grain than XTOL. Here are scans from XTOL and this soup:

    XTOL: Click image for larger version. 

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ID:	58875, Test-soup: Click image for larger version. 

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    FENTON

    Rudi: The only question remaining is the rate of the Fenton reaction. One informal test I did a few weeks ago using the baseline dev above was: Develop a strip, let the soup sit for five hours, and dev a 2nd strip with the same soup. I got a 5% density-loss (on a log scale), whereas I expected about a 1% loss from using the dev twice. That 1% loss per hour is fine for a one-shot dev that is specified to be used within 30-45 minutes after mixing. But I need to test to see if much faster loss can occur. And as you point out, the AA-density curve is nearly flat there, so an X % density-loss means there was a much greater ascorbate-loss. So tests should be run in a steeper part of the curve. An AA/Phenidone ratio of 20 (instead of my 90) would be appropriate. The PC-Sulfite developer that I posted in January would work well for this testing.

    Jerry: Thanks for the info about ferric and ferrous ammonium sulfate. I'll put them to use.

    After all these months, I'm trying to finish this project. I've settled on that baseline developer. Now I need to insure that Fenton-destruction isn't a problem, and then run some tests on various kinds of film.

    Mark Overton

  9. #279

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    Quote Originally Posted by Rudeofus View Post
    The electrical engineer in me would look at FeCl3 as source of FeIII, but chemists might know better. The trace quantities of FeIII you are going to add make it unlikely that the Cl- has any effect on your developer's properties.
    Ferric chloride is a poor choice for making a solution of known ferric ion concentration. It is one of those compounds of indefinite composition. It hydrolyses in moist air and loses chlorine. It is also deliquescent. So there is no exact composition. The three compounds I suggested in a previous post are all stable and have definite compositions.
    A rock pile ceases to be a rock pile the moment a single man contemplates it, bearing within him the image of a cathedral.

    ~Antoine de Saint-Exupery

  10. #280

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    Jerry mentioned that copper will catalyze the Fenton reaction also.
    What form of copper would chemists recommend for getting a known concentration of Cu ions in solution?
    Would copper sulfate be okay for this?

    TIA,
    Mark Overton



 

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