>Has anyone ever had an experience with bad ferric ammonium citrate?
I know just how it feels, Judy. Getting a Prussian Blue precipitate (or
even colour - sorry J - color) immediately on mixing ammmonium ferric
citrate with potassium ferricyanide is a sure sign that one or the other
substance (or even both) is contaminated with iron(II) (or "ferrous" iron).
The Prussian Blue reaction is so sensitive that this is not uncommon. From
what you say, it looks like the amfecit may be the cause, in your case. You
should be able to prevent this by the addition of a small amount of
ammonium (or potassium) dichromate to the amfecit *before* mixing them. It
might even slowly cure the problem retrospectively in a blue mix. Try
adding enough to give a final concentration of about 0.1% (one part in a
thousand) of dichromate: this can be conveniently achieved by adding one
drop of a 20% dichromate solution to every 10 (ten) mls (ccs) of your
sensitizer. Note, however, that added dichromate can affect the contrast of
your sensitizer. Crawford says this much dichromate could increase contrast
by half to one stop (one to two steps on your 21-er tablet).
There may well be other oxidising agents, besides the infamous dichromate,
that will do this trick: hydrogen peroxide or potassium chlorate are
possibilities.
Unfortunately, ammonium ferric citrate is a very unreliable and variable
chemical (like ferric oxalate) - what chemists in the trade call
"ill-characterised". You can tell this from the fact that the solid comes
in non-crystalline brown and green forms for which the Manufacturer's
catalogues and bottle labels never quote a chemical formula; at best they
might give an iron content.
Mike