Amidol developer actions

DWGrant · Jan 6, 2005

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?

Ole · Jan 6, 2005

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

DWGrant · Jan 6, 2005

pH profile of emulsion

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

Ole · Jan 6, 2005

DWGrant said:
Thanks Ole...however, why is there a pH gradient within the emulsion?

Because the developer solution is more acidic than the emulsion.

DWGrant · Jan 6, 2005

Ole said:
Because the developer solution is more acidic than the emulsion.

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

Ole · Jan 6, 2005

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

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.

DWGrant · Jan 6, 2005

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...

David A. Goldfarb · Jan 6, 2005

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

Ole · Jan 6, 2005

DWGrant said:
...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...???

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.

DWGrant · Jan 6, 2005

Ole said:
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.

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?...

Ole · Jan 6, 2005

DWGrant · Jan 6, 2005

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

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