Last week, I posted a formula for a chromium intensifier along
with the usual cautions about handling a dichromate. Someone,
Ty Monson I think, asked about the safe disposal of dichromate
solutions. Good question.
As fate would have it, I was cleaning out a bookshelf last night
and came across a copy of the Kodak booklet, "Disposal of
Photographic-Processing Effluents and Solutions" (it is dated
1973, I seem to recall purchasing it in the late 1970's, so no
doubt there are more current editions available.) Here is what
it says about disposing dichromates:
"Dichromate bleaches (which are used to a certain extent in
some black-and-white and color processes) offer potential
toxicity to bacteria in a waste-treatment plant. Thus,
destruction of dichromate ion may be warrented before such
ions leave the processing laboratory. Dichromate ion in
bleach solutions contains hexavalent chromium (that is having a
valence of six), and can be converted into trivalent chromium
(and thus precipitated) by the addition of any of several
chemical reducing agents such as thiosulfate or sulfite. Any
alkaline material (including waste developer) can be added to
neutralize the acidity and cause the chromium to precipitate as
the hydroxide.
The mixture can be filtered so that the solid waste can be
disposed of without discharging soluble dichromate or chromate
ions into the sewer. Note that any silver bearing dichromate
bleach can be desilvered by adding sodium chloride after treatment
with hydroxide [??? I assume this refers to using something like
sodium hydroxide to neutralize acidity]. The silver chloride
can be filtered off along with the chromium hydroxide and sent
out for refining."
I've never tried this and don't currently work with any process
requiring a dichromate, though I am interested in it should
I start to work with dichromate solutions again. I assume that
all that is necessary is dump used developer or fixer (along with
an alkaline like baking soda) into the dichromate solution and
then filter the sludge (will a coffee filter do?). Adding salt
(to taste? ;-) will also separate any dissolved silver from the
chromate percipitate.
Does anyone one have enough background in chemistry to tell me
if these assumptions are true? I am confused by the references
to "hydroxide." It seems as if the writer is talking about two
different things, the chromium hydroxide percipitate and another
unidentified hydroxide. Of course, it could just be bad writing.
Anyway, hope this is interesting, and I hope someone can
add some details.
Carson Graves
carson@ileaf.com