From: Peter Marshall (petermarshall@cix.co.uk)
Date: 05/22/01-02:34:39 AM Z
> Katharine Thayer wrote:
>
> >And that brings up the question of why we talk about dichromated
> colloid
> >processes as if they were all the same, when the colloids are
> different
> >molecular beings? It seems to me that there might be somewhat different
> >chemical mechanisms for the hardening of each.
> >
> >
> >Katharine Thayer
>
> Perhaps. However, Kosar appears to indicate that different colloids
> react
> in a very similar way as regards sensitivity and contrast.
>
> Sandy King
>
>
>
>
I thought the hardening was simply a matter of effect of the high charge
density of the small Cr3+ ion producing a higher degree of ordering in the
water molecules which where formerly a part of the open colloid structure.
I thought these are essentially similar structures involving a lot of
linking by hydrogen bonded water molecules, and the chromium ion comes and
nicks these, leading to a collapse of their open structure.
In case anyone is still confused, chromate is CrO4-, dichromate Cr2O7=
(both Chromium (VI)) and the species that does what we want to the
colloid is Cr3+ (Chromium (III)).
As Liam has said you need to supply something that is readily oxidised to
get the reduction from Chromium (VI) to Chromium (III) to occur, and
photographic developer (normally used to reduce silver (I) to silver (0))
fits the bill. Being alkaline it also precipitates the chromium(III)
making it less likely to be a disposal problem.
Peter Marshall
Photography Guide at About http://photography.about.com/
email: photography.guide@about.com
_________________________________________________________________
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Also on Fixing Shadows: http://www.people.virginia.edu/~ds8s
and elsewhere......
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