Darlington Media Group (postmaster@mediaworkshop.demon.co.uk)
Sat, 16 Jan 1999 21:35:44 +0000
>On Thu, 14 Jan 1999, Joao Ribeiro wrote:
>
>> Hi,
>> I saw somewhere that this salt exists in a brown and green form and
>> that the green one is better for photographic use. Why are they
>> different and in what way the green one is better?
>> TIA
>> Joao
>>
This was discussed in a thread in Feb 1997. The most complete answer to
your question was given by Sil Horowitz from which I quote:....Tony McLean.
>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
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