Re: Tanning theory of dichromated colloids (was gelatin

"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 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.

"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.
Incidentally, Sasaki, Honda, Kichuchi (Studies on photosensitive dichromated
materials, Tokyo 1979) suggest possible intermediate Cr(V) or Cr(IV)
formation upon photoreduction of HCrO4 to Cr(III).

Martin

----- 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 12:03:54 2004