Re: dichromate stain and chemistry question
On Jan 8, 2007, at 12:22 PM, Katharine Thayer wrote:
On Jan 7, 2007, at 7:29 AM, Christina Z. Anderson wrote:
To the chemists of the list, does this chemical explanation sound
plausible--it's Greek to me:
"Light acts the same as heat, and in the presence of organic
matter sets free the ammonia from the double salt, which then
becomes acid by the formation of chromic acid, which is reduced by
the organic matter to chromate or chromic oxide. If the washing
is continued this chromate is reduced to chromium oxyhydrate which
is a pale greenish color."
This doesn't look like Greek to me, but it's incomprehensible just
the same. There's no reference given, but from the sound of it I
would guess it dates back to the early days long before chemistry
advanced to the understanding of the photochemical process that we
know today to explain the process. Just the beginning statement,
"Light acts the same as heat" is incomprehensible in the light of
this knowledge. So I'm not sure how useful this information could
be to us, but since no chemists here have responded, I've forwarded
it to my physical chemist colleague for comment, and will report
back his comments when he replies, whatever they are.
His answer:
It was only in the 1880s that van't Hoff, Arrhenius and Ostwald
showed that osmotic pressures and electrical conductivities of
strong acids and salts were to be understood as arising from their
full dissociation into ions in aqueous solutions. Given that to be
the state of science in that period, it would not be appropriate to
read the statement you cited presuming a modern understanding of
chemistry or presuming current terminology. It would take some
careful historical research to determine what the author of the
statement knew and what he meant by his terminology. Almost
nothing was known about mechanism at that time. Again, one can't
use the statement to support current understandings of
photochemical processes.
Katharine
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