Re: dichromated colloids

From: Katharine Thayer <kthayer@pacifier.com>
Subject: Re: dichromated colloids
Date: Fri, 21 May 2004 05:18:33 +0000

> Never mind, I do see; at least I see why it would make sense that you
> wouldn't see Cr(III) absorptions right away if Cr(V) is stable in PVA.
> What I don't understand now is why you attribute the discrepancy to
> impurities if Mannivanan pay attention to purity; in that case I would
> think you would be more inclined to credit Mannivannan's result than
> less inclined. I'm not finding fault, I'm just trying to understand.

I don't think it's a matter of who gets the credit. It's the matter of
reconciling disagreeing data in literature.

First of all, it is very difficult to detect signal from Cr(III) even
if it is present, using absorption spectroscopy because signals from
Cr(VI) and Cr(V) are much stronger. But signals from Cr(III) is more
redily detected with other colloids reported than with PVA from some
groups. Some PVA reports do show the absorption of Cr(III) but only
after postexposure heat treatment or some sort of treatments. I'm a
bit reluctant to say more than this until doing searches for recent
papers, but given hugely varying degree of Cr(V) stability in
different media, and looking at what kinds of changes/manipulations
seem to increase Cr(III) signal, I think that big difference among PVA
studies could be understood if some small part of PVA's contained
carboxyl group or a redox catalyst was present. Cr(V) is sensitive to
different wavelength than Cr(VI) as well.

By the way, I'd expect density increase of 0.05 or less at 570-580nm
region after thorough UV irradiation, with ordinary amount and coating
thickness of dichromate used for printing. That's the "color" of
chromium you are talking about. Absorptions of Cr(V) and Cr(VI) are
much stronger so this kind of signal is easily masked if they are
present even in small quantity. At least one study added Cr(III) to
PVA and they observed very strong complexing, though PVA wasn't
previously oxidized by Cr(VI), in which case the complexing is thought
to be at -OH groups of PVA but they didn't establish that.

--
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)