U of S | Mailing List Archive | alt-photo-process-l | Re: ferro vs ferri

Re: ferro vs ferri

I've already wrote about this in the past. Search the archive
for more info.

"Potassium hexacyanoferrate" is an incomplete description of
the compound. There is "potassium hexacyanoferrate (II)" and
"potassium hexacyanoferrate (III)." The old name for the
former is potassium ferrOcyanide and the latter potassium
ferrIcyanide. They have different oxidation numbers of the
iron and they behave differently. For all practical purposes,
regard them as different chemicals. The one that makes
prussian blue pigment when free ferric ion is present is the
former, hexacyanoferrate (II). The one that has strong
oxidizing power and often used as a bleach is the latter,
hexacyanoferrate (III).

I see no availability problem for these chemicals. It's just
that a lot of alternative process literatures continue to use
old nomenclature made obsolete in chemistry and the rest of
the world is moving forward.

Generally cyano ligands are tightly bound to the iron in these
compounds, but they can be liberated and give off toxic
cyanide gas when mixed with strong acids or exposed to UV
irradiation. This type of agents are removed from color film
and print processing, decades ago, and all emulsion
manufacturers in the western economy block claim that they are
also eliminated from commercial emulsion manufacturing,
although they are frequently used in emulsion research and
also described very heavily in patent literature. (The
quantity used in emulsion making is quite small anyway, not
like those used in color processing decades ago.)

I believe that the use of "potassium ferricyanide" or
potassium hexacyanoferrate (III) as a bleaching agent in
black-and-white darkroom work should be made obsolete,
especially because there are a number of alternative compounds
that work well as a b&w bleach and present much less
environmental damage. Actually there are a range of agents
that work from mild to strong bleach, depending on the
application. The only difference I can see is that ... they
cost a couple pennies more. However, the market for these is
quite small and no one (so far) wants to make a b&w bleach
(even after seeing the success of Silvergrain products) and
most authors are not updating their books to include new
recommendations... furthermore, my website is now gone and so
is my place to promote new technology... oh well.

Ryuji Suzuki

From: Trevor Cunningham <tr_cunningham@yahoo.com>
Subject: ferro vs ferri
Date: Wed, 29 Aug 2007 23:40:51 -0700 (PDT)

> Hey gang...trying to get back into things in a new location.
> I recently moved to Jakarta and have found a bulk chemical
> supplier.  I am able to find ferric ammonium citrate, but
> cannot find potassium ferricyanide.  Instead, they have
> hexacyanoferrate, or potassium FERROcyanide...wikipedia
> mentions that several suspicious Morrocans in Rome were
> arrested with the stuff and some waterway maps and also says
> it reacts quite unpleasantly with acid (which I find highly
> unusual that I can get it so easily).  However, the article
> does mention that potassium FERRIcyanide can be made from it
> with the introduction of chlorine (process completely
> unknown to me).  Availability seems to be an issue if I'm to
> make cyanotypes, but if I can make my own chemical, then the
> project factor seems almost irresistable.
>   Thoughts?
>   Trevor Cunningham
> "The optimist believes this is the best of all possible worlds.  The pessimist fears it's true"  - J Robert Oppenheimer
> ---------------------------------
> Ready for the edge of your seat? Check out tonight's top picks on Yahoo! TV. 

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