Re: Dichromates ???

From: Yves Gauvreau <gauvreau-yves@sympatico.ca>
Subject: Re: Dichromates ???
Date: Wed, 09 Nov 2005 16:29:07 -0500

> Could there be a point then at which the dichromate crystals
> themselves or in solution, would become unusable because of exposure
> to heat, UV and whatever else might affect the dichromate?

Not that I know of, but this one is more of an empirical question than
theoretical one.

Water is a very very weak reducing agent (standard potential = 1.23 -
0.059pH Volts; usually viewed more as a weak oxidizing agent) and it
can only reduce very strong oxidizing agents. Dichromate's standard
potential is 1.38V so theoretically the reaction may occur but the
difference is small considering the normal range of overpotential
required (0.6V or on that order), and if there's a redox reaction, it
would be very slow.

Besides water, for alcohols, see Mytych, Karocki, and Stasicka,
2003. Mechanism of photochemical reduction of chromium(VI) by alcohols
and its environmental aspects. J. Photochem. Photobio, A, 160,
163--170.

> Would you happen to know the frequency or range of frequencies that
> dichromnates are most sensitive to?
> I assume this could vary with the various dichromates.

The peak is in 350 to 370nm range but there are multiple peaks. This
is well known and frequently published. For example, see Grimm, Hilke
and Scharrer, 1983. The mechanism o fthe cross linking of poly(vinyl
alcohol) by ammonium dichromate with U.V.-light. J. Electrochem. Soc.,
130, 1767--1771.
Received on Sat Nov 12 03:30:18 2005