[Alt-photo] Cyanotype chemistry question

Keith Schreiber keith at jkschreiber.com
Sat Jan 25 18:49:42 UTC 2020

Hi Chris,

I'm not a chemist, but I'll attempt to answer your question anyhow. Hopefully someone with better chemistry background will correct my errors and offer a better explanation.

Ammonium Iron(III) Oxalate, Iron(III) Ammonium Oxalate, Ammonium Ferric Oxalate, Ferric Ammonium Oxalate, Ammonium Ferrioxalate, etc., are all synonyms. 

The difference in molecular structure is simply that (NH4)3[Fe(C2O4)3]*3H2O includes 3 molecules of water, while C6H12FeN3O12 does not. If you do the math on the first formula (and ignore the water) you get N3H12FeC6O12 which can be rearranged to C6H12FeN3O12.

That said, (NH4)3[Fe(C2O4)3]*3H2O has CAS #13268-42-3, while C6H12FeN3O12 has CAS #14221-47-7, and the molecular weights are different which I think is due to the water in the former but not the latter.

There is no practical difference for us. As far as I can tell, what we can obtain from the usual suppliers such as B&S is (NH4)3[Fe(C2O4)3]*3H2O. It think this is because it is a very hygroscopic compound, which means that maintaining it without the water molecules is challenging since it absorbs moisture from the air.

Hopefully, this is at least somewhat correct and helps to answer your question. ;)


Keith Schreiber

> On Jan 25, 2020, at 9:27 AM, Chris Seiberling via Alt-photo-process-list <alt-photo-process-list at lists.altphotolist.org> wrote:
> Hello photographic chemists (and others):
> Mike Ware's 'New' Cyanotype calls for Ammonium iron(III) oxalate-- (NH4)3[Fe(C2O4)3].3H2O. He says this is also known as ferric ammonium oxalate. But I understand ferric ammonium oxalate's formula to be C6H12FeN3O12 ? Is there a practical difference? I’m not enough of a chemist to understand the formulaic implications. 
> Thanks, Chris Seiberling
> _______________________________________________
> Alt-photo-process-list | altphotolist.org

More information about the Alt-photo-process-list mailing list