Ferric refers to iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

-containing materials or compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...

s. In chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....

 the term is reserved for iron with an oxidation number
Oxidation number
In coordination chemistry, the oxidation number of a central atom in a coordination compound is the charge that it would have if all the ligands were removed along with the electron pairs that were shared with the central atom. Oxidation numbers are often confused with oxidation states.The...

 of +3, also denoted iron(III) or Fe3+. On the other hand, ferrous
Ferrous , in chemistry, indicates a divalent iron compound , as opposed to ferric, which indicates a trivalent iron compound ....

 refers to iron with oxidation number of +2, denoted iron(II) or Fe2+. Iron(III) is usually the most stable form of iron in air, as illustrated by the pervasiveness of rust
Rust is a general term for a series of iron oxides. In colloquial usage, the term is applied to red oxides, formed by the reaction of iron and oxygen in the presence of water or air moisture...

, an insoluble iron(III)-containing material.

Ferric iron and life

The bioavailability of iron is of great interest because (i) all known forms of life require iron and (ii) ordinary iron (III) compounds are insoluble in an aerobic environment. Iron-deficiency Anemia
Anemia is a decrease in number of red blood cells or less than the normal quantity of hemoglobin in the blood. However, it can include decreased oxygen-binding ability of each hemoglobin molecule due to deformity or lack in numerical development as in some other types of hemoglobin...

 illustrates the problems resulting from low iron intake. Many foods contain soluble iron compounds and are therefore necessary for good nutrition.

The low bioavailability of iron affects all forms of life. Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 secrete iron-attracting agents called siderophore
Siderophores are small, high-affinity iron chelating compounds secreted by grasses and microorganisms such as bacteria and fungi...

s that form soluble compounds of iron that can be reabsorbed into the cell and used in building iron-containing metalloprotein
Metalloprotein is a generic term for a protein that contains a metal ion cofactor. Metalloproteins have many different functions in cells, such as enzymes, transport and storage proteins, and signal transduction proteins. Indeed, about one quarter to one third of all proteins require metals to...

s. The impact of increasing the bioavailability of iron was famously demonstrated by an experiment where a large area of the ocean surface was sprayed with iron(III) salts. After several days, the phytoplankton
Phytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...

 within the treated area bloomed to such an extent that the effect was visible from outer space. This fertilizing process has been proposed as a means to mitigate the carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 content of the atmosphere.

Ferric iron mitigates the eutrophication
Eutrophication or more precisely hypertrophication, is the movement of a body of water′s trophic status in the direction of increasing plant biomass, by the addition of artificial or natural substances, such as nitrates and phosphates, through fertilizers or sewage, to an aquatic system...

 of lakes by reducing the bioavailability of phosphorus (as phosphate) in the water. Mitigation arises because ferric phosphate is insoluble. Like iron, phosphate is often a limiting nutrient, and its removal from solution limits the growth of algae, which in turn cause eutrophication.

Hydrolysis of iron(III) and rust

In water, ferric iron forms compounds that are often insoluble, at least near neutral pH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

. A salt of ferric iron hydrolyzes water and produces iron(III) oxide-hydroxide
Iron(III) oxide-hydroxide
A number of species are dubbed iron oxide-hydroxide. These chemicals are oxide-hydroxides of iron, and may occur in anhydrous or hydrated forms...

s while contributing hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 ions to the solution, lowering the pH. In contrast, typical Na+ salts (e.g. NaCl) dissolve in water without lowering the pH. The differing behavior of Na+ vs Fe3+ ions reflects the effect of charge: water when bound to Fe3+ is highly acidic, inducing the formation of iron(III) hydroxides, which polymerize via the process called olation
In inorganic chemistry, olation is the process by which metal ions form polymeric oxides in aqueous solution. The phenomenon is important to understand the relationship between metal ions in aqueous solution and metal oxides, which are represented by many minerals.At low pH, many metal ions exist...

. Aluminium(III) (Al3+) behaves similarly to ferric ion.

Rust, a mixture of ferric hydroxide
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and a hydrogen atom held together by a covalent bond, and carrying a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, as a ligand, a nucleophile, and a...

 compounds, illustrates the low solubility of ferric ions in water. Various reagent
A reagent is a "substance or compound that is added to a system in order to bring about a chemical reaction, or added to see if a reaction occurs." Although the terms reactant and reagent are often used interchangeably, a reactant is less specifically a "substance that is consumed in the course of...

s cause rust to dissolve even at neutral pH. These ligands include EDTA
Ethylenediaminetetraacetic acid, widely abbreviated as EDTA , is a polyamino carboxylic acid and a colourless, water-soluble solid. Its conjugate base is named ethylenediaminetetraacetate. It is widely used to dissolve limescale. Its usefulness arises because of its role as a hexadentate ligand...

, which forms a chelate complex with the ion, displacing the hydroxide and oxide ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...

s that comprise rust. For this reason, EDTA is often used to dissolve iron deposits or to deliver soluble iron in fertilizers. Citrate
A citrate can refer either to the conjugate base of citric acid, , or to the esters of citric acid. An example of the former, a salt is trisodium citrate; an ester is triethyl citrate.-Other citric acid ions:...

 also solubilizes ferric ion at neutral pH, although its complexes are less stable than those of EDTA.

Inorganic chemistry

Ferric iron is a d5 center, meaning that the metal has five "valence" electrons" in the 3d orbital shell. The magnetism of ferric compounds is mainly determined by these five electrons, and their behavior depends on the number and type of ligands attached to iron, as described by ligand field theory
Ligand field theory
Ligand field theory describes the bonding, orbital arrangement, and other characteristics of coordination complexes. It represents an application of molecular orbital theory to transition metal complexes. A transition metal ion has nine valence atomic orbitals, five d, one s, and three p orbitals...

. Usually ferric ions are surrounded by six ligands arranged in octahedron
Octahedral molecular geometry
In chemistry, octahedral molecular geometry describes the shape of compounds where in six atoms or groups of atoms or ligands are symmetrically arranged around a central atom, defining the vertices of an octahedron...

. Sometimes three and sometimes as many as seven ligands are observed. A common ferric compound is ferric chloride (FeCl3).


Most iron-containing proteins contain ferric ions, at least transiently. Well studied examples include iron-sulfur cluster
Iron-sulfur cluster
For biological Fe-S clusters, see iron-sulfur proteins.Iron-sulfur clusters are ensembles of iron and sulfide centres. Fe-S clusters are most often discussed in the context of the biological role for iron-sulfur proteins. Many Fe-S clusters are known in the area of organometallic chemistry and as...

s, oxyhemoglobin, ferritin
Ferritin is a ubiquitous intracellular protein that stores iron and releases it in a controlled fashion. The amount of ferritin stored reflects the amount of iron stored. The protein is produced by almost all living organisms, including bacteria, algae and higher plants, and animals...

, and the cytochrome
Cytochromes are, in general, membrane-bound hemoproteins that contain heme groups and carry out electron transport.They are found either as monomeric proteins or as subunits of bigger enzymatic complexes that catalyze redox reactions....


See also

  • Ferric chloride (iron(III) chloride
    Iron(III) chloride
    Iron chloride, also called ferric chloride, is an industrial scale commodity chemical compound, with the formula FeCl3. The colour of iron chloride crystals depends on the viewing angle: by reflected light the crystals appear dark green, but by transmitted light they appear purple-red...

  • Ferric oxide (iron(III) oxide
    Iron(III) oxide
    Iron oxide or ferric oxide is the inorganic compound with the formula Fe2O3. It is one of the three main oxides of iron, the other two being iron oxide , which is rare, and iron oxide , which also occurs naturally as the mineral magnetite. As the mineral known as hematite, Fe2O3 is the main...

  • Iron(III) fluoride
    Iron(III) fluoride
    Iron fluoride, also known as ferric fluoride, is an inorganic compound. It exists in an anhydrous form as well as two hydrated forms . It is produced commercially mostly for laboratory work and is not as useful as the related compound iron chloride...

    (ferric fluoride)
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