U of S | Mailing List Archive | alt-photo-process-l | Re: I have a question

Re: I have a question

From: BOB KISS <bobkiss@caribsurf.com>
Subject: RE: I have a question
Date: Sat, 20 Oct 2007 22:42:59 -0400

> You are the chemistry man so please suggest just one or two
> excellent silver bleach alternatives to potassium
> ferricyanide that would work and be more environmentally
> friendly (as well as regulation free), given standard silver
> gelatin processing procedures...yes, thiosulfate fix,
> washing, etc.

Like I said in the previous email, refer to US Patent 5149618
and look at the compound I. There is only one compound in
question in this patent, and it is called nitrilomonopropionic
diacetic acid in this patent, although this compound can be
called by other names as well. German company BASF owns a
patent to manufacture this compound from iminodiacetic acid
and acrylic acid. Iron (III) complex of this compound is a
good b&w bleach, although this compound is not likely used in
practice of color processing as suggested in that patent. (If
used in C-41 processing, it causes magenta fog.)

Another possibility is iron (III) complex of iminodiacetic
acid itself, or alkylated iminodiacetic acid (the latter is
more preferred). This was a subject of a Kodak
patent. However, these chelating compounds are more irritating
to human than nitrilomonopropionic diacetic acid, the subject
of the AGFA patent above.

In reality, any rehalogenating bleaching agent that can be
used in C-41 or RA-4, like iron (III) complex of EDTA or that
of DTPA is better than ferricyanide. Most modern C-41 bleach
bath uses iron (III) complex of
1,3-propylenediaminetetraacetic acid, and this is a very good
b&w bleach agent (although it requires a bit of a trick to
make it work cleanly). However, none of these compounds is
biodegradable and wouldn't be allowed to dump to the sewer in
some parts of Europe (Germany for sure) and some other parts
of the world.

These compounds are very much usable as b&w bleach in place of
rehalogenating ferricyanide bleach. The bath should contain
the bleaching agent, potassium bromide, and pH buffer. The
amount of bleaching agent and KBr is comparable to the amounts
of ferricyanide and KBr used in ferricyanide bleach. None of
these compounds is suitable for direct bleach like dichromate
or permanganate bleach.

> It would be nice if they were readily available chemicals.

The compounds named above are all common but not common enough
for small photographic chemical retail business like
Photographer's Formulary to carry, perhaps except for
iron(III) EDTA, which has been used in color processing
chemicals for some time. Ideally those companies should be
aware of the issue and they ultimately phase out ferricyanide
(except for where it is absolutely needed) and introduce
better alternatives.

Ryuji Suzuki