Ferric citrate

Ferric citrate or iron(III) citrate is an ionic chemical compound with formula FeC6H5O7, consisting of a trivalent iron cation (Fe3+) and a trivalent citrate anion (C6H5O3−7). It is a dark orangeish or reddish 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 citrate or iron(III) citrate is an ionic chemical compound with formula FeC6H5O7, consisting of a trivalent iron cation (Fe3+) and a trivalent citrate anion (C6H5O3−7). It is a dark orangeish or reddish 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. Sometimes the term is also used for the salt tetraferric tricitrate Fe4(C6H4O7)3, where the citrate anions are fully deprotonated (C6H4O4−7). 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. 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 2(H2O)2]2−. In the present of excess citrate anions, the iron forms negatively charged complexes like 5− and 7−. Ferric citrate can be prepared by adding a stoichiometric amount of iron(III) sulfate tetrahydrate to a solution of citric acid and sodium hydroxide at pH 3.0. The salt can be precipitated by adding ethanol to the solution. The Fe3+ ion in ferric citrate (as in many iron(III) carboxylates) is reduced by exposure to light, especially blue and ultraviolet, to Fe2+ (ferrous) ion with concomitant oxidation of the carboxyl group adjacent to the hydroxyl, yielding carbon dioxide and acetonedicarboxylate: where -R represents the group -CH2CO−2. 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.