On Oct 20, 2007, at 2:27 PM, Ryuji Suzuki wrote:
For most standard b&w silver gelatin processing, waste
chemicals other than used fixer, ferricyanide bleach and
selenium toner can go down the drain to the sewer system (not
septic tank), in small quantity, although they still contain
Ferricyanide is the worst chemical used in conventional b&w
processing. Alternative compounds are available but no one
(but me) seems to advocate phasing out ferricyanide. I don't
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? 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 HCN.
This is an excerpt from a data sheet from a supplier of the compound:
"POTASSIUM FERRICYANIDE: In spite of the fact that this compound contains cyanide, it is not particularly toxic.
The reason is that the cyanide groups are bound to the iron atom and are not free to act as a poison. The cyanide
groups can be released as hydrogen cyanide as if the potassium ferricyanide is placed in a strong acid solution:
however the copper toning process does not call for acid.
Consult with local sewer and water authorities regarding proper disposal of darkroom chemicals in your area.
Generally, to dispose of excess potassium ferricyanide (solid or in solution) wash the material down the drain with
excessive amounts of water."
As for the fixer and selenium toner, I'm assuming your concern with these is the heavy metal content. Once the silver has been recovered from fixer, isn't it a quite benign chemical that is actually used in aquarium systems (and as an antidote for cyanide poisoning btw)?
I would think something like thiocarbamide/thiourea commonly used in toners would be more of a concern due to its carcinogenic nature.