Amidol developer actions

Discussion in 'B&W: Film, Paper, Chemistry' started by DWGrant, Jan 6, 2005.

  1. DWGrant

    DWGrant Member

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    What is the validity that an amidol-based developer acts on an emulsion from the "back", or substrate-side, to the surface of the emulsion? If this is true, what attribute(s) of amidol permit this action versus other developers acting on the emulsion from the surface to the back?
     
  2. Ole

    Ole Moderator Staff Member Moderator

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    Amidol works even in slightly acidic solutions. So as it diffuses through the emulsion, it starts working faster from the bottom where pH will be a little higher initially.

    It's said to be true :smile:
     
  3. DWGrant

    DWGrant Member

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    pH profile of emulsion

    Thanks Ole...however, why is there a pH gradient within the emulsion?
     
  4. Ole

    Ole Moderator Staff Member Moderator

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    Because the developer solution is more acidic than the emulsion.
     
  5. DWGrant

    DWGrant Member

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    :confused: This doesn't explain why the development initiates at the substrate...the developer encounters the surface of the emulsion first...???
     
  6. Ole

    Ole Moderator Staff Member Moderator

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    It's got nothing to do with the substrate. But even if it works in slightly acidic solution, it works faster with higher pH. So if the solution has lower pH than the emulsion, there will be a gradient. So the inner bits, where the pH is highest, will develop fastest.
     
  7. DWGrant

    DWGrant Member

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    Ole...

    ...I wasn't suggesting that the substrate itself had an effect...I should have stated "near the substrate"...

    ...so, as the developer diffuses into the emulsion the pH of the developer gradually increase and the pH becomes "higher" NEAR the substrate than at the surface, therefore becoming more active near the substrate than at the surface...???

    ...the fact that amidol works in the absence of an alkali is the reason that an amidol developer begins development near the substrate...???

    Thanks again...:smile:
     
  8. David A. Goldfarb

    David A. Goldfarb Moderator Staff Member Moderator

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    That's how I understand it. Pretty cool, eh? That's why water bath development works so well with amidol.
     
  9. Ole

    Ole Moderator Staff Member Moderator

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    Eh - not quite.

    The emulsion has a pH too, and that is initially HIGHER than the developer solution. The pH doesn't "become higher" near the substrate, it becomes lower more slowly near the substrate.
    Remember that gelatin isn't a solid, it's a gel containing lots of water and other stuff. Even when dry.
     
  10. DWGrant

    DWGrant Member

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    OK...however the pH of the developer "becomes higher" as it diffuses throught the emulsion towards the substrate, making the developer more active near the substrate...correct?...
     
  11. Ole

    Ole Moderator Staff Member Moderator

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    Yes!
     
  12. DWGrant

    DWGrant Member

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    Thanks again... :smile:
    ...it amazes me how little information there is in the technical literature concerning amidol...!!!