The molecular formula of ammonium iron(III) citrate is variable. It can be prepared by adding Fe(OH)3 to an aqueous solution of citric acid and ammonia. The brown form is approximately 9% NH3, 16.5–18.5% Fe, and 65% hydrated citric acid; the green form is approximately 7.5% NH3, 14.5–16% Fe, and 75% hydrated citric acid. The green type is more readily reduced by light than the brown.
Other uses for ammonium ferric citrate include water purification and printing (cyanotype). It is used as a reducing agent of metal salts of low activity like gold and silver and is also in a commonly used recipe with potassium ferricyanide to make cyanotype prints. Ammonium ferric citrate is also used in Kligler iron deeps to determine hydrogen sulfide production in microbial metabolism.
Ammonium ferric citrate is present in Scottish carbonated soft drink Irn-Bru.