In medicine, ammonium ferric citrate is used as a contrast medium.

The aesculetin forms dark brown or black complexes with ferric citrate, allowing the test to be read.

The paper is impregnated with a solution of ammonium ferric citrate and dried.

The best known is probably a process using ammonium ferric citrate and potassium ferricyanide.

Excess ammonium ferric citrate and potassium ferricyanide are then washed away.

Other uses for ammonium ferric citrate include water purification and printing (cyanotype).

Ammonium ferric citrate is a food additive with E number E381 used as an acidity regulator.

Ammonium ferric citrate is present in Scottish carbonated soft drink Irn-Bru.

Sodium thiosulfate also serves as a sulfur source and, in combination with ferric citrate, detects hydrogen sulfide production.

Peptone iron agar contains the amino acid cysteine and a chemical indicator, ferric citrate.

For analysis of Biolog PM3 plates 150 μl ferric citrate was also added to each tube of inoculating fluid.

Ammonium ferric citrate is also used in Kligler iron deeps to determine hydrogen sulfide production in microbial metabolism.

The degradation of cysteine releases hydrogen sulfide gas that reacts with the ferric citrate to produce ferrous sulfide.

Aesculin is incorporated into agar with ferric citrate and bile salts (bile aesculin agar).

Enterococcus hydrolyze esculin to products that react with ferric citrate in the medium to produce insoluble iron salts, resulting in the blackening of the medium.

Herschel's system involved coating paper with ferric citrate, exposing it to the sun in contact with an etching used as mask, then developing the print with a chloroaurate solution.

Irn-Bru's advertising slogans used to be 'Scotland's other National Drink', referring to whisky, and 'Made in Scotland from girders', a reference to the rusty colour of the drink; though the closest one can come to substantiating this claim is the 0.002% ammonium ferric citrate listed in the ingredients.

This is because the hydrogen sulphide produced from thiosulphate (it acts as a source of sulphur besides creating a reduced atmosphere in which Vibrio can grow due to its facultative anaerobic nature) combines with ferric ions from ferric citrate to produce ferric sulphide, which is black in colour.