Ferric citrate or iron citrate describes any of several complexes of trivalent iron and a trivalent and any of several conjugate basesderived from citric acid. Most of these complexes are orange or red-brown solid, slightly soluble in hot water yielding an orange or orange-yellow solution at pH < 2.0. The solution becomes green at pH > 2.0. Ferric citrates play important roles in the metabolism of iron by living organisms. Citrate anions are released by plant roots and some microorganisms in order to extract iron from insoluble compounds in the soil, such as ferric hydroxide, and form soluble ferric citrates that can be absorbed by the organism. Ferric citrate is used in medicine to regulate the blood levels of iron in patients with chronic kidney disease on dialysis. It acts by forming an insoluble compound with phosphate present in the diet and thus reducing its uptake by the digestive system.
Structure
Ferric citrate easily forms coordination complexes, oligomers, and polymers. Thus it is not a single well-defined compound, but a family of compounds, all with the same basic formula and many different structures. This complexity extends to solutions, where various forms can coexist in equilibrium. At physiological pH, ferric citrate forms an insoluble red polymer. In other conditions, it forms anionic complexes like 22]2−. In the present of excess citrate anions, the iron forms negatively charged complexes like 5− and 7−.
The ion in ferric citrate is reduced by exposure to light, especially blue and ultraviolet, to ion with concomitant oxidation of the carboxyl group adjacent to the hydroxyl, yielding carbon dioxide and acetonedicarboxylate: where -R represents the group -. This reaction is not observed at pH < 1.5 or pH > 5.0, or in aprotic water-free solvents such as acetonitrile. In water, it has maximum quantum efficiency around pH = 2.9. It can be observed even in the solid state. This reaction plays an important role in plant metabolism: iron is carried up from the roots as ferric citrate dissolved in the sap, and photoreduced in the leaves to iron that can be transported into the cells.
