Katharine Thayer wrote:
>
> Martin Angerman wrote:
> >
> > The sulfite acts as a reducing agent. It clears the dichromate by reducing
> > it to an inactive form.
>
> Hmm. I've always wondered just what a clearing agent does, chemically,
> but this explanation doesn't work very well for me, since what is called
> "dichromate stain" in my experience almost always consists of Cr+3 not
> Cr+6.
I'm not sure I posed my question clearly here: what perplexed me was
that while sulfite does of course act as a reducing agent, then why is
it effective on chromium that's already reduced? I got a helpful answer
this morning: apparently the sulfate that results when sulfite is
oxidized tends to pair easily with chromium, allowing it to be washed
out more easily. This seems a more compelling explanation to me than to
say that the sulfite "clears the dichromate by reducing it to an
inactive form." Since as I've said it's my understanding that the green
and green-brown material that comprise many "dichromate stains" is
largely composed of reduced chromium, it's obvious that the reduction by
itself doesn't explain the clearing effect.
Katharine Thayer
Received on Wed Aug 25 12:59:42 2004
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