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  1. #291
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    Mark;

    I would say that these rolls (Xtol vs your developer) differ in contrast. If your developer had shorter time or Xtol longer, they would match more closely. I think that this may be the situation.

    PE

  2. #292

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    Quote Originally Posted by albada View Post
    I'll guess that the by-products of the phenidone and AA accelerate development. I remember reading about this, but this is the first time I've seen it.

    Mark Overton
    Not according to Pat Gainer, see post#20.The phenidone is not used up, ascorbate is converted to the more acidic dehydroascorbate.
    http://www.apug.org/forums/forum37/1...-pc-tea-1.html
    It's hard to find the original source for this information.

  3. #293
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    Quote Originally Posted by Alan Johnson View Post
    Not according to Pat Gainer, see post#20.The phenidone is not used up, ascorbate is converted to the more acidic dehydroascorbate.
    There is plenty of published literature available online, and it suggests that dehydroascorbate is not the end of the story, especially in alkaline solution. Also, at pH of 8 it doesn't matter much whether some acid has pKa of 1 or 5, it will be fully protonized anyway. The question remains how many protons are produced or consumed in the various redox reactions of Ascorbic Acid and how many protons the resulting compounds will let go at pH around 8.

    A pH measurement before and after development sounds easier ...
    Trying to be the best of whatever I am, even if what I am is no good.

  4. #294

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    Rudi: pH before and after development were 8.17 and 8.13, a drop of .04.

    PE: You suggested reducing dev-time. I ran another roll this evening (and got the above numbers for Rudi), but with the shorter dev-time of 12:15 minutes. The graphs of XTOL vs the baseline developer:

    Click image for larger version. 

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    It's an excellent match with XTOL, and with slightly better performance near the shoulder. Leader densities are 2.70 for XTOL, and 2.75 with the latest roll. Grain in the baseline dev looks slightly finer than XTOL in loupes.

    The remaining mystery is: Why does developing an entire roll require less time than just a test-strip to achieve the same densities? The pH drops instead of rises during development, so rising pH from by-products isn't the reason. XTOL doesn't need less time, and the only substantial chemistry differences are (1) the presence of propylene glycol, and (2) ascorbic acid instead of ascorbate.

    Alan suggested that the pH would drop, and it did. I wish I had a deep enough understanding of the underlying chemistry to figure out this stuff. I have books, and plan to re-read the O-chem chapter.

    Mark Overton

  5. #295
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    Mark,

    it would be worthwhile looking at whether a pH drop of 0.04 has that much effect on contrast. You could try a few test clips all with the same time but with pH varied through 8.1, 8.14, 8.18, 8.22.

    If you see a big difference, it might be worth investigating what Xtol does differently. You could also try and compare buffer strength of Xtol vs. your dev, which can be done by preparing a small quantity of both, measuring their pH, then putting same amount of acid into the liquids, then measuring pH again.
    Trying to be the best of whatever I am, even if what I am is no good.

  6. #296
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    IMHO, that pH drop would be insignificant in the face of the actual results.

    I am still at a loss to explain it. It is known that some polyols affect development. I don't think that is the explanation though.

    PE

  7. #297

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    Quote Originally Posted by albada View Post

    The remaining mystery is: Why does developing an entire roll require less time than just a test-strip to achieve the same densities?

    Mark Overton
    Are you still using the test strip set-up described in post#110?
    http://www.apug.org/forums/viewpost.php?p=1375472
    Perhaps the flow across the emulsion is different for test strip and roll.

  8. #298

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    It's dangerous to make conclusions from a single data point. Does this always happen that a full roll develops faster than a test strip?
    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

  9. #299

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    Alan, I'm using a different baffle now, but the same concern is true of it: Flow will be different compared to a roll. The odd thing is: With XTOL, the graphs for strip and roll match. But with my dev, a strip needs more time.

    Quote Originally Posted by Gerald C Koch View Post
    It's dangerous to make conclusions from a single data point. Does this always happen that a full roll develops faster than a test strip?
    Three rolls have confirmed this. I wondered if I'd made a mistake with the first roll, so I ran a second -- with identical results. Last night, I ran a third roll with a shorter dev-time, and it matched XTOL's graph well. Here's a summary of what I've run (with and without adding propylene glycol):

    XTOL strips and roll -- gave nearly identical graphs.
    Strips with no added PG: 12.25 min is correct (where "correct" means "matches XTOL's graph").
    Strips with added PG: 13.1 min is correct.
    Rolls with added PG: 12.25 min is correct.

    It's acting as if having a low fluid-to-film ratio quickly neutralizes the retarding effect of the PG, causing development to proceed at its normal (higher) rate.

    Mark

  10. #300

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    With Xtol they are the same because the high ascorbate concentration is enough to regenerate phenidone "oxide" adsorbed on silver.
    With Xtol concentrate working solution the strip is not getting enough ascorbate to regenerate adsorbed phenidone "oxide".The lower concentration of ascorbate in the concentrate working solution means its diffusion is more affected by the difference in flow patterns.



 

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