Re: I have a question
From: Joseph Smigiel <firstname.lastname@example.org>
Subject: Re: I have a question
Date: Sat, 20 Oct 2007 15:30:40 -0400
> Why do you consider potassium ferricyanide as "the worst
> chemical used in conventional b&w processing" and deserving
> of special disposal other than flushing down the drain?
1. it is environmentally harmful
2. discharge of this compound is very often subject to
3. there are alternative compounds that work just as well and
continuing to use ferricyanide is nothing more than our
failure to choose more eco-friendly option.
4. use of ferricyanide and ferrocyanide was discontinued in
the photographic industry (both emulsion makers and
photofinisher processing plants) decades ago due to all of
the above reasons.
> I understand it may release hydrogen cyanide if heated,
> mixed with strong acids, or subjected to UV, but other than
> huffing cyanotypes at close range as they come out of the
> contact frame, common photographic practices don't call for
> anything that would make the ferricyanide break down into
That's one consideration in terms of the safety to the direct
user but not the environment or accidental exposure to others.
> This is an excerpt from a data sheet from a supplier of the
The concern is that a lot of darkroom safety and disposal
guidelines were written way before water pollution became
a major social issue. Many of those guidelines were copied and
pasted from old literatures without even reflecting the changes
adapted by the industry.
> Consult with local sewer and water authorities regarding
> proper disposal of darkroom chemicals in your area.
If you consult them they'll tell you that maximum allowable
concentration is 1ppm or so in most US cities.
> Generally, to dispose of excess potassium ferricyanide
> (solid or in solution) wash the material down the drain with
> excessive amounts of water."
Most ferricyanide bleach solution contains 10-50g/L of
ferricyanide. Say 20g/L on average. Then, you'd have to dilute
ferricyanide bleach 20000x or more before disposing to the
sewer just to meet the standard (and whether this is advisable
or not is an additional concern). That is, in order to dispose
of one liter of bleach, you'll need to waste 20kl of
water. That is enough water to take shower 500 times (that is,
more than one year), and more if you use a water-efficient
shower head and/or stop water when you don't need it.
I am not sure if "they" did the math to figure out how much is
the "excessive amount of water" in this particular case.
If I were to dispose ferricyanide, I'd react them with a mild
reducing agent to ferrocyanide form, and then react it with
ferric chloride or something to precipitate out. Discard the
sediment as solid waste and discard the liquid phase to the
drain. That's a lot of work that is unnecessary with
> As for the fixer and selenium toner, I'm assuming your
> concern with these is the heavy metal content.
Silver in spent fixer should be recycled whenever possible
because silver is rather precious resource. In small quantity
used in individual artist's darkrooms, the silver in fixer is
not expected to cause a major problem in sewer system, but
that is my personal opinion and not what the regulatory body
sets as their standard. They set a very strict and difficult
standard for silver, and the only practical and effective way
to remove silver to that extent is to use electrolytic silver
recovery system. (many "steel wool" system is inefficient and
cannot meet the regulatory standards.)
Selenium is also regulated in many areas. Selenium, once
entered in the sewer system, can go through the treatment
plant and go to the nearby stream or ocean. (Same thing
applies to borax, boric acid, EDTA, NTA, DTPA, etc.) Selenium
is naturally present in stream water in coastal area, and sea
water, but not so inland.
> I would think something like thiocarbamide/thiourea commonly
> used in toners would be more of a concern due to its
> carcinogenic nature.
Thiourea is also of concern. It is very poorly biodegradable
and toxic to environment, aquatic organisms as well as
users. However, there are very limited alternative compounds
for odorless indirect toner, and none safer than
thiourea. Thiourea toner is almost always used with
ferricyanide bleach, for which there is an effective
alternative and therefore can be replaced without