>Here is my Ferric Ammonium Citrate puzzle:
>
>Early in the semester we mixed cyanotype from a partly-used bottle of
>ferric ammonium citrate, which I shall call lot #1. I supervised mixing at
>first in the morning class, then let students mix on their own. We made
>many prints, all fine.
<snip - snip -snip>
Ammonium Ferric Citrate is not a specific formulation. While both forms
(brown and green) are prepared by the addition of Ferric hydroxide to an
aqueous solution of citric acid and ammonia, the product has an undetermined
structure (according to the Merck Manual). The brown form contains 9%
ammonia, 15 to 18% iron, and about 65% hydrated citric acid. The green form
contains approx 7.5% ammonia, 14 to 16% iron, and 75% hydrated citric acid.
The green form is more readily reduced to the ferrous salt by light than the
brown form; if the green is not kept in the dark, parts of the pearls,
flakes, or whatever physical form it's in will become brown. This will make
it less sensitive to light, and could interfere with its reaction with
ferricyanide.
A question was raised concerning the use of this chemical (there certainly
isn't enough demand for blueprints these days!). Pharmaceutically, it is
called "Soluble Citrate of Iron" so you can readily infer that it has been
(and may still be) used for iron deficiency anemia, though there are better
materials. Cheaper iron for anemia, available in drug and health stores, is
ferric sulfate, which is useless for cyanotypes. Ferric salts react with
ferrocyanides (note the "ferro") to create Prussian Blue pigment. Likewise,
ferrous salts react with ferricyanides for the same purpose. In the
cyanotype reaction, light reduces the ferric salt to ferrous, which then
reacts with the ferricyanide. (The ferricyanide is critical in this reaction
- if it contains any ferrocyanides, blue staining will result.) Just as a
side issue: for those of you who worry about the "cyanide" in the name of
this compound, stop worrying. The CN radical is a very common one (one
personal example because I like them so much, is that it is present in
quantity in peaches!) and when it is bound up with iron as in the
ferricyanide, it has very low toxicity; in fact, the ferrocyanide is used to
prevent caking of rock salt, and is added to pickle baths.
So there.
Sil Horwitz, FPSA
Technical Editor, PSA Journal
silh@iag.net