Ammonium ferric citrate

Ammonium ferric citrate has the formula (NH4)5[Fe(C6H4O7)2]. A distinguishing feature of this compound is that it is very soluble in water, in contrast to ferric citrate which is not very soluble.

In its crystal structure each moiety of citric acid has lost four protons. The deprotonated hydroxyl group and two of the carboxylate groups ligate to the ferric center, while the third carboxylate group coordinates with the ammonium.[1]

Ammonium ferric citrate has a range of uses, including:

As a food ingredient, it has an INS number, and is used as an acidity regulator[citation needed]. Most notably used in the Scottish beverage Irn-Bru.
Water purification.
As a reducing agent of metal salts of low activity like gold and silver.
With potassium ferricyanide as part of the cyanotype photographic process.
Used in Kligler iron deeps to determine hydrogen sulfide production in microbial metabolism.
In medical imaging, ammonium ferric citrate is used as a contrast medium.
As a hematinic.[3]