Re: Tanning theory of dichromated colloids (was gelatin

MARTINM wrote:
>
>
> I guess the EDTA thing goes back to Oster & Oster who pointed out that EDTA
> interfered with the crosslinking action of Cr(III). That was in the context
> of
> dye sensitized DCG.

So in this case they knew they already had Cr(III) but the EDTA
inhibited the crosslinking?

>
> "Duncan & Dunn also suggest that their observations rule out the
> possibility of free radical (polymer) intermediates, but Mannivannan et
> al show clearly the presence of such an intermediate."
>
> That's interesting. I wonder whether its (the free radical's) action would
> be sufficient for certain monomers to initiate polymerization.

My chemist friend, after reading Mannivannan et al last weekend, said
that the formation of a polymer radical suggests to him that once the
radical is formed (by transferring electron to chromium) then it could
start a series of reactions among the gum molecules to effect the
hardening without further chromium being required. I didn't completely
understand what he was saying so I haven't incorporated it into my
thinking. But is this what you're talking about?

> Incidentally, Sasaki, Honda, Kichuchi (Studies on photosensitive dichromated
> materials, Tokyo 1979)

This is a book? Is it in English?

suggest possible intermediate Cr(V) or Cr(IV)
> formation upon photoreduction of HCrO4 to Cr(III).

Mannivannan et al (1993) say Cr(V) but not Cr(IV), in their
investigation of dichromated PVA.

Thanks for the dialogue,
Katharine

> ----- Original Message -----
> From: "Katharine Thayer" <kthayer@pacifier.com>
> To: <alt-photo-process-l@sask.usask.ca>
> Sent: Friday, May 14, 2004 9:05 AM
> Subject: Re: Tanning theory of dichromated colloids (was gelatin
>
> > Martin M wrote:
> >
> > Katharine Thayer wrote:
> >
> > Duncalf and Dunn, which I am waiting to read myself
> > >> before I draw any conclusions about its utility, but which according to
> > >> Manivannan et al doesn't establish whether the chromium is bonded to
> the
> > >> gum. It's hard to see how this study could address that question
> anyway,
> > >> since it's pretty obvious even from the brief summaries that it's a
> > >> qualitative study. There's always going to be some trivalent chromium
> > >> there; the question is whether it's bonded to the colloid or not.
> >
> > >I assume that's the Canadian group (Lessard, Couture, Changkakoti, Bolte,
> > >Solano, Capollo etc.)...
> >
> > Yes, that's right. Perhaps Ryuji answered this already, but I haven't
> > read his posts in this thread past the one where he accused me of having
> > an "anti-scientific mind" because after going through another crisis
> > about my father, I don't have any emotional energy left to deal with the
> > sneering and jeering, and besides it seems to me that if Ryuji had
> > something more persuasive to offer on the subject than Mannivannan et
> > al, Duncalf and Dunn, the hardening of gelatin with chrome alum, and
> > invisible chromium, he's had ample time to suggest it by now.
> >
> > I do have Duncalf and Dunn in my hands now. The qualitative analysis
> > that I'd seen summarized elsewhere is only a part of the research; most
> > of the work is UV spectrometry. The qualitative analysis is just as I
> > thought:
> >
> > "Insoluble film from which residual dichromate had been extracted was
> > dissolved in 0.1N hydrochloric acid and reprecipitated by running the
> > solution into acetone. The precipitate was insoluble in water and
> > contained chromium."
> >
> > Since, if we can trust the authors, residual dichromate but not residual
> > reduced chromium was removed from the hardened PVA, it goes without
> > saying that chromium would be found, just as I said above. This would be
> > true whether the chromium was coordinated to the PVA or not coordinated
> > to the PVA; there would have to be some reduced chromium present for the
> > crosslinking to take place, whether the chromium is part of the matrix
> > or not.
> >
> > The abstract of the paper states that "ultraviolet spectrometry shows
> > that secondary hydroxyl groups are oxidized to ketone groups, but
> > insolubilization is attributed to crosslinking of polymer chains by
> > coordination of alcohold groups to "nascent" chromic ions formed by
> > reduction of the dichromate." Unfortunately, just as Mannivannan et al
> > commented, there is no direct proof to back up the attribution of the
> > crosslinking to this purported coordination. The attribution is based
> > on these two observations: (1) "insolubilization of the film on exposure
> > to light could be prevented by incorporating at least enough
> > ethylenediaminetraacetic acid to combine with 75% of the chromium
> > present." The assumption of the authors seems to be that the reason
> > crosslinking is prevented by doing this is that the chromium becomes
> > unavailable to coordinate with the polymer. But it seems more reasonable
> > to me, if I were speculating something, to speculate that the reason the
> > crosslinking doesn't occur is because the chromium, by bonding with the
> > reagent, has been rendered incapable of participating in the electron
> > transfer that makes the crosslinking possible. (2) insoluble films could
> > be dissolved with the same reagent. This observation does suggest
> > chromium coordination to me, but it doesn't constitute anything like
> > proof.
> >
> > I'm inclined to agree with Mannivannan et al in their assessment of this
> > research "...photoreaction of dichromate in PVA films was studied by
> > Duncalf and Dunn, and they suggested that the insolubilization of PVA
> > was caused by the complex formation between PVA and Cr(III) even though
> > no clear evidence was shown for the involvement of Cr(III)."
> >
> > Duncan & Dunn also suggest that their observations rule out the
> > possibility of free radical (polymer) intermediates, but Mannivannan et
> > al show clearly the presence of such an intermediate.
> >
> > P.S. One of my guests last weekend was a chemist; he saw the pile of
> > books about chromium chemistry etc and was curious. I told him I was
> > trying to figure out the chemistry of the gum process in the absence of
> > any research on the topic, and that I was especially interested in the
> > question of what happens to the chromium, whether it attaches to the
> > matrix or not. He was intrigued and read all the material I have, and
> > when he was done I asked, "So what happens to the chromium?" He said,
> > "It looks to me like no one knows what happens to the chromium." Which
> > is what I've been saying for five years. He says that the chromium could
> > just as easily stand by to participate in the electron transfer without
> > actually coordinating with the colloid.
> >
> > Those of you who are quick to jeer for little reason should understand
> > that I've never said that the chromium DOESN"T attach to the matrix.
> > All I'm saying is that no one knows whether it does or not. The argument
> > that it "must" do so because that's the kind of thing chromium likes to
> > do, is theoretical and no matter how much sense it makes logically, it
> > has to be demonstrated empirically; so far such empirical demonstration
> > seems hard to come by.
> > Katharine Thayer.
Received on Sun May 16 16:03:31 2004