From: Katharine Thayer (kthayer@pacifier.com)
Date: 05/20/01-11:44:38 AM Z
Sandy King wrote:
>
>
>
> I wonder if you might comment on the following. In carbon printing my usual
> practice is to add liquid ammonia to the potassium dichromate solution to
> the point where it changes in color from reddish/orange to a pale yellow
> and takes on a distinctly ammoniacal smell. At this point the potassium
> dichromate has changed from a dichromate to the double chromate salt,
> potassium ammonium chromate. It is in fact no longer a dichromate but a
> chromate, but of course chromates can tan and harden gelatin just as well
> as dichromates. If this procedure could be adapted to gum would it not
> eliminate the gum green problem?
>
It makes sense that chromates can harden colloids, because according to
the physical chemist I consulted, both the dichromate and the chromate
ion are hexavalent. Since it's the change from 6 to 3 chromium that
gives you the color change and also the reduction-reaction you need, you
still have to end up with the green +3 chromium in the end. But I might
argue with the statement that chromates can harden colloids "just as
well" as dichromates, since they are slower.
As for adding ammonia to the gum mixture on principle, I can't think of
a reason I'd want to do that. Raising the pH slows the reaction; ammonia
stinks; ammonia is what we use to remove hardened gum where we don't
want it, when we've accidentally overexposed....
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
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