9 julio, 2024

Iron(III) hydroxide: structure, properties and uses

He iron(III) hydroxide is an inorganic compound whose strictly formula is Fe(OH)3, in which the ratio of Fe3+ and OH– ions is 3:1. However, the chemistry of iron can be quite convoluted; so this solid is not composed only of the mentioned ions.

In fact, Fe(OH)3 contains the anion O2-; therefore, it is an iron hydroxide oxide monohydrate: FeOOH·H2O. If the number of atoms for this last compound is added, it will be verified that it coincides with that of Fe(OH)3. Both formulas are valid to refer to this metal hydroxide.

In teaching or research chemistry laboratories, Fe(OH)3 is observed as an orange-brown precipitate; similar to the sediment in the upper image. When this gelatinous, rusty sand is heated, it releases excess water, turning its orange-yellowish color (yellow pigment 42).

This yellow pigment 42 is the same FeOOH·H2O without the additional presence of water coordinated to Fe3+. When this is dehydrated, it is transformed into FeOOH, which can exist in the form of different polymorphs (goethite, akaganeite, lepidocrocite, feroxyhite, among others).

The mineral bernalite, on the other hand, exhibits green crystals with a base composition Fe(OH)3·nH2O; mineralogical source of this hydroxide.

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Structure of iron(III) hydroxide

The crystal structures of iron oxides and hydroxides are a bit complicated. But, from a simple point of view, they can be considered as ordered repetitions of FeO6 octahedral units. Thus, these iron-oxygen octahedrons intertwine through their corners (Fe-O-Fe), or their faces, establishing all kinds of polymer chains.

If such chains appear ordered in space, the solid is said to be crystalline; otherwise it is amorphous. This factor, together with the way in which the octahedrons are joined, determine the energetic stability of the crystal and, therefore, its colors.

For example, orthorhombic crystals of bernalite, Fe(OH)3·nH2O, are greenish in color because their FeO6 octahedrons only join through their corners; unlike other iron hydroxides, which have reddish, yellow or brown colors, depending on the degree of hydration.

It should be noted that the oxygens of FeO6 come from either OH– or O2-; the exact description corresponds to results of crystallographic analysis. Although it is not addressed as such, the nature of the Fe-O bond is ionic with a certain covalent character; which for other transition metals becomes even more covalent, such as silver.

Properties

Although Fe(OH)3 is a solid that is easily recognized when iron salts are added to an alkaline medium, its properties are not fully understood.

It is known, however, that it is responsible for modifying the organoleptic properties (taste and color, especially) of drinking water; which is very insoluble in water (Ksp=2.79·10-39); and also that its molar mass and density are 106.867 g/mol and 4.25 g/mL.

This hydroxide (like its derivatives) cannot have a defined melting or boiling point because when it is heated it releases water vapor, thus becoming its anhydrous form FeOOH (together with all its polymorphs). Therefore, if heating is continued, the FeOOH will melt and not the FeOOH·H2O.

To study its properties further it would be necessary to subject the yellow pigment 42 to numerous studies; but it is more than likely that in the process it changes color to reddish, indicative of the formation of FeOOH; or, conversely, it dissolves in the complex aqueous Fe(OH)63+ (acid medium), or in the anion Fe(OH)4– (very basic medium).

Applications

Absorbent

In the previous section it was mentioned that Fe(OH)3 is very insoluble in water, and can even precipitate at a pH close to 4.5 (if there are no interfering chemical species). When precipitating, you can drag (coprecipitate) some impurities from the medium that are harmful to health; for example, the chromium or arsenic salts (Cr3+, Cr6+, and As3+, As5+).

Then, this hydroxide allows these metals and other heavier ones to be occluded, acting as an absorbent.

The technique consists not so much in precipitating the Fe(OH)3 (alkalizing the medium), but instead adding it directly to contaminated water or soil, using commercially purchased powders or grains.

therapeutic uses

Iron is an essential element for the human body. Anemia is one of the most outstanding diseases resulting from its deficiency. For this reason, it is always a matter of research to devise different alternatives to incorporate this metal into our diet so that no side effects are generated.

One of the Fe(OH)3-based supplements is based on its complex with polymaltose (polymaltose iron), which has a lower degree of interaction with food than FeSO4; that is, more iron is biologically available to the organism and is not coordinated with other matrices or solids.

The other supplement is composed of Fe(OH)3 nanoparticles suspended in a medium made up mainly of adipates and tartrates (and other organic salts). This proved to be less toxic than FeSO4, in addition to increasing hemoglobin, not accumulating in the intestinal mucosa, and promoting the growth of beneficial microbes.

Pigment

The Yellow 42 pigment is used in paints and cosmetics, and as such does not represent a potential health risk; unless ingested by accident.

iron battery

Although Fe(OH)3 is not formally used in this application, it could serve as a starting material for FeOOH; compound with which one of the electrodes of a cheap and simple iron battery is manufactured, which also works at a neutral pH.

The half-cell reactions for this battery are expressed below with the following chemical equations:

½ Fe ⇋ ½ Fe2+ + e–

FeIIIOOH + e– + 3H+ ⇋ Fe2+ + 2H2O

The anode becomes an iron electrode, which releases an electron that later, after going through the external circuit, enters the cathode; electrode made of FeOOH, reducing to Fe2+. The electrolytic medium for this battery is composed of soluble Fe2+ salts.

References

Shiver & Atkins. (2008). Inorganic chemistry. (Fourth edition). Mc Graw Hill.
National Center for Biotechnology Information. (2019). Ferric hydroxide. PubChem Database. CID=73964. Retrieved from: pubchem.ncbi.nlm.nih.gov
Wikipedia. (2019). Iron(III) oxide-hydroxide. Retrieved from: en.wikipedia.org
N.Pal. (nd). Granular Ferric Hydroxide for Elimination of Arsenic from Drinking Water. [PDF]. Retrieved from: archive.unu.edu
RM Cornell and U. Schwertmann. (nd). The iron oxides: structure, properties, reactions, occurrences and uses. [PDF]. ****://epsc511.wustl.edu/IronOxide_reading.pdf
Birch, W. D., Pring, A., Reller, A. et al. Naturwissenschaften. (1992). Bernalite: a new ferric hydroxide with perovskite structure. 79:509. doi.org/10.1007/BF01135768
Environmental Geochemistry of Ferric Polymers in Aqueous Solutions and Precipitates. Retrieved from: geoweb.princeton.edu
Giessen, van der, AA (1968). Chemical and physical properties of iron(III)-oxide hydrate Eindhoven: Technische Hogeschool Eindhoven DOI: 10.6100/IR23239
Funk F, Canclini C and Geisser P. (2007). Interactions between iron(III)-hydroxide polymaltose complex and commonly used medications / laboratory studies in rats. DOI: 10.1055/s-0031-1296685
Pereira, DI, Bruggraber, SF, Faria, N., Poots, LK, Tagmount, MA, Aslam, MF, Powell, JJ (2014). Nanoparticulate iron(III) oxo-hydroxide delivers safe iron that is well absorbed and used in humans. Nanomedicine : nanotechnology, biology, and medicine, 10(8), 1877–1886. doi:10.1016/j.nano.2014.06.012
Gutsche, S. Berling, T. Plaggenborg, J. Parisi, & M. Knipper. (2019). Proof of Concept of an Iron-Iron(III) oxide hydroxide Battery Working at Neutral pH. Int. J. Electrochem. Sci., Vol. 14, 2019 1579. doi: 10.20964/2019.02.37

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