[alt-photo] Re: Making ammonium iron(III) citrate and ammonium iron(III) oxalate

[Peter Friedrichsen]

Hi Loris,

I was going to post this earlier but time was 
unavailable to transcribe this from my notes: 
Here is what I do for the production of making 
ferric ammonium citrate(green) FAC(green):

Firstly, you need to produce a precipitate of 
ferric hydroxide from a ferric salt. This is the 
messy part of the process. I find that the salt 
can take the form of a sulfate, or chloride, and 
even a double salt such as ferric ammonium 
sulfate. Iron salts of citrates do not seem to 
precipitate well so I do not use them. I use 
Ferric chloride because it is for me, a readily 
available reasonable priced copper etchant which 
is at a strength of 42 degree baume and can be 
bought through various sources such as electronic 
and etchant supply houses. Of course the iron 
salt chosen must be of the ferric form, and not 
ferrous, unless you wish to go through an additional oxidation step.

The ferric is neutralized with sodium hydroxide 
but you can use ammonia, or even sodium 
carbonate. The bostick and sullivan web site has 
details on this messy process of precipitating 
and washing for the production of ferric oxalate 
-they use a different iron salt I believe.

Basically, I start out with the following quantities:

100ml 43 deg baume Ferric chloride etchant solution
45g pre-dissolved sodium hydroxide in about 2 
litres water (ice cubes added during dissolution)
50g Ammonium bicarbonate
100g Citric acid
(Makes about 700 ml of a 20% solution of green FAC:)


The ferric chloride is added to a 10 litre 
container, mixed well, and the sodium hydroxide 
solution stirred in gradually. Once all is mixed, 
it is left to rest for an hour or so and 
decanted. You should do this several times. After 
all of this, strain through a thin but strong 
thin cloth or sheet -polyester is good for this. Squeeze out any excess water.

After the messy precipitation and washing 
process, I end up with roughly 650 ml of a ferric 
precipitate with the consistency of thick yogurt 
(somewhat clumpy)I add all of the ammonium 
bicarbonate to the ferric mud and mix it well to 
dissolve. It is important to add this ammonium 
bicarbonate before the citric acid if you want 
the green variety of FAC. It well get cold so you 
may need to warm it, then add about half (slowly) 
of the citric acid and stir. I use a tall 
container for this as it will bubble up quite a 
bit and produce some mist. I now transfer this 
muddy mix to a tall SS pot and warm it on a stove 
burner, slowly adding the remaining citric acid. 
Once all of the fizz has ended, I slowly heat it 
up to about 70 deg C with constant stirring. 
Every few minutes I dab a drop from the solution 
on to a blotting paper or paper towel and check 
for the amount of hydroxide since the hydroxide 
will form a brown spot in the center of this drop 
with a transparent ring that will slowly go from 
yellow to a light green. Over the course of about 
10-15 minutes, the liquid will transform to a 
dark emerald green as seen through several 
centimeters of depth. It is about 20% w/v 
strength but can vary a little depending on how 
much water was squeezed out of the ferric mud.

I preserve the liquid with crushed aspirin 
(500mg) (ASA) added warm and filtered out after a 
cool down to room temperature to a final clear 
solution. Leaving it to decant is another more 
patient approach. It will keep without mould 
formation. The ASA is not very soluble in water 
so it does not become a significant impurity and 
I have not noticed any measurable negative effect 
on performance. The salicylate in the aspirin is 
a phenol and mould will avoid it. Store in brown 
glass or other non metallic container. It will keep for a very long time.

Precautions: Careful if using the Sodium 
hydroxide and dissolve it into water with ice 
cubes to absorb heat. Further, some acidic fumes 
will evaporate off during heating up to 70 C but 
since citric acid is a weak acid, the fumes are 
not that acrid, of course good ventilation is good practice.

The final pH typically ends up around 4, and the 
FAC(green) solution when spread on cleaned glass 
will dry to a glassy film and will not crystallize and this is to be expected.


Peter Friedrichsen







At 05:31 PM 05/08/2010, you wrote:
>Peter, (and others) What is the method of making 
>ammonium iron(III) citrate? (I think this is a 
>little bit like cooking, since you get something 
>depending on method - I mean structure / 
>composition changes with method. ???) Also, how 
>one can compound ammonium iron(III) oxalate? 
>Anyone w/ information regarding those two 
>issues? Thanks in advance, Loris. P.S. I can no 
>longer get ammonium iron(III) citrate locally, 
>chemical suppliers say it is not produced 
>anymore - by their sources. (That is 
>Sigma-Aldrich and/or Fluka I presume. Merck 
>wasn't listing it already...) 2010/5/5 Loris 
>Medici <mail at loris.medici.name>: > Peter, this 
>is very interesting. Can you please elaborate? 
>It would be > nice to have a home recipe for 
>making green FAC... How much of each do > you 
>use? > > TIA, > Loris. > > > 2010/5/4 Peter 
>Friedrichsen <pfriedrichsen at sympatico.ca>: >> 
>Trevor, >> >> As far as altering it to change it 
>to the green variety, I had no success >> with 
>this by adding additional ammonia and citric 
>acid. It appears that >> above and beyond the 
>ratios of iron, ammonia, and citrate, the way in 
>which >> it is made also affects its final 
>properties. I know this because I make my >> own 
>green variety from scratch. >> >> Essentially it 
>is made by adding ammonia (or use ammonium 
>bicarbonate) and >> citric acid to iron 
>hydroxide ( a messy red mud) Â If you add the 
>citric acid >> first, Â to make ferric citrate, 
>then add ammonium citrate to make FAC, it >> 
>will make the brown variety. Now, if you add 
>ammonium citrate + citric acid, >> both together 
>to this mud, then the green variety forms. So it 
>seems that as >> the ferric hydroxide dissolves, 
>it must have the ammonia component >> available, 
>or else you will just get the brown variety, and 
>it will stay >> brown even if you attempt to 
>re-balance the ratios. >> >> At least this is my 
>experience. >> >> Peter Friedrichsen > 
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