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 Fri May 14 10:04:34 2004