Re: Tanning theory of dichromated colloids

From: MARTINM <martinm@SoftHome.net>
Subject: Re: Tanning theory of dichromated colloids
Date: Sun, 09 May 2004 12:16:43 +0200

> "...Incidentally, primary alcohols, alkenes and alkynes can also be
> oxidized to carboxylic acid with strong oxidizing agents like
> permanganate or chromium (VI)."

> That has been interesting me for quite a while. Do you know what
> kind of condition (pH/temperature/time) is required to initiate that
> reaction?

I don't really have a direct experience with this type of reactions
except for a few special cases. But it's a typical organic chemistry
textbook matters (nothing advanced) so you should be able to find a
good deal of info there. Generally high temp and high pH are preferred.

Alcohols can be oxidized directly to carboxylic acids with strong
oxidizing agents, but even with mild oxidizing agents, the reaction
can happen with intermediate steps, typically in aldehyde form. This
latter type of reaction is common in many places... ethanol can be
oxidized by aerial oxygen to acetaldehyde while aging wine and
whisk(e)y.

Aldehydes can be readily oxidized by many things as well, one of the
reasons why formaldehyde solution doesn't keep well. But the real
problem for silver gelatin processors is that monoaldehydes as well as
aldoses (sugars containing aldehyde) can be oxidized by
silver-ammonium complex and in this reaction silver gets reduced to
metallic silver. This is one (if not the) reason why photographic
gelatin is highly purified to remove sugars and aldehydes (among many
other things), because, otherwise, silver gelatin emulsions get fogged
(useless!).

> "So one possible test might be to oxidize gum with suitable
> oxidizing agents and see if chrome alum can harden it better. But
> the amount of oxidizing agent must be suitably chosen, because you
> don't want gum molecules to break down at once before they get
> hardened."

> What would be an appropriate oxidizing agent? Thinking of some
> special bleaches occasionally used for holographic silver-halide
> materials and involving Cr bleaching, copper sulfate or potassium
> persulfate come to my mind...

What are you trying to do?

From: MARTINM <martinm@SoftHome.net>
Subject: Re: Tanning theory of dichromated colloids (was gelatin
Date: Sun, 09 May 2004 11:45:17 +0200

> I assume that's the Canadian group (Lessard, Couture, Changkakoti, Bolte,
> Solano, Capollo etc.)...

That's them. The paper we are actually talking about is:

G. Mannivannan, R. Changkakoti, R. Lessard, G. Mailhot, and M. Bolte,
J. Phys. Chem. 97, 7228 (1993).

There's also similar study on primary photochemical process of
dichromated gelatin by Lanford et al (2000), Opt. Eng. vol 39, no 3,
610-615. There are a couple others I read but they're in my office
and no reading note in my computer. I think the recent one used sol
preparation instead of typical dried films.

--
Ryuji Suzuki
"You have to realize that junk is not the problem in and of itself.
Junk is the symptom, not the problem."
(Bob Dylan 1971; source: No Direction Home by Robert Shelton)