>How do you make it ?
According to the Merck Index, the composition of "ammonium ferric citrate"
(same thing as fac) is of "undetermined structure," thus all the chemicals
we know by that name are probably just different mixtures, rather than a
definite, structured compound. They are prepared by treating ferric
hydroxide with citric acid and ammonia. The brown contains 9% ammonia, 16 to
18 percent iron, and about 65% hydrated citric acid. The green has 7.5%
ammonia, 14 to 16% iron, and about 75% hydrated citric acid. So,
theoretically (always theoretically with organic compounds because they
never react exactly as expected) the green is made by adding more citric
acid. I've never tried that, but it might be an interesting experiment
(though as I say, organics don't react reliably!).
The Merck Index also notes that the green form is more readily reduced to
the ferrous salt by light. Which is also what most cyanotypists have learned
empirically, or by following the advice of others!
Sil Horwitz, FPSA
Technical Editor, PSA Journal
silh@iag.net