IRON (III) HYDROXIDE: STRUCTURE, PROPERTIES AND USES - CHEMISTRY - 2024

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

In fact, Fe (OH) ₃ contains the anion O ^2-; therefore, it is a monohydrated iron hydroxide oxide: FeOOH · H ₂ O. 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.

Iron (III) hydroxide in a frog pond. Source: Clint Budd (https://www.flickr.com/photos//13016864125)

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

This yellow pigment 42 is the same FeOOH · H ₂ O without the additional presence of water coordinated with Fe ³⁺. When this is dehydrated, it is transformed into FeOOH, which can exist in the form of different polymorphs (goethite, akaganeite, lepidocrocite, feroxihita, among others).

The mineral bernalite, on the other hand, exhibits green crystals with a base composition Fe (OH) ₃ · nH ₂ O; mineralogical source of this hydroxide.

Structure of iron (III) hydroxide

The crystal structures of iron oxides and hydroxides are a bit complicated. But, from a simple point of view, it can be considered as ordered repeats of octahedral units FeO ₆. Thus, these iron-oxygen octahedra intertwine through their corners (Fe-O-Fe), or their faces, establishing all kinds of polymeric chains.

If such chains look 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 energy stability of the crystal and, therefore, its colors.

For example, the orthorhombic crystals of bernalite, Fe (OH) ₃ · nH ₂ O, have a greenish color due to the fact that their FeO ₆ octahedra only bind through their corners; unlike other iron hydroxides, which appear reddish, yellow or brown, depending on the degree of hydration.

It should be noted that the oxygens of FeO ₆ come from either OH ^- or O ^2-; the exact description corresponds to the results of crystallographic analysis. Although 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, as with 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 entirely clear.

However, it is known that it is responsible for modifying the organoleptic properties (taste and color, especially) of drinking water; which is very insoluble in water (K _sp = 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 heated it releases water vapor, thus converting it into its anhydrous form FeOOH (along with all its polymorphs). Therefore, if heating continues, the FeOOH will melt and not the FeOOH · H ₂ O.

To study its properties more thoroughly, it would be necessary to subject the yellow pigment 42 to numerous studies; but it is more than probable that in the process it changes color to reddish, indicative of the formation of FeOOH; or on the contrary, it dissolves in the complex aqueous Fe (OH) ₆ ³⁺ (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 is no intervening chemical species). By precipitating, it can carry away (co-precipitate) some impurities from the environment that are harmful to health; for example, the salts of chromium or arsenic (Cr ³⁺, Cr ⁶⁺, and As ³⁺, As ⁵⁺).

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

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

Therapeutic uses

Iron is an essential element for the human body. Anemia is one of the most prominent diseases due to 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 collateral effects are not generated.

One of the supplements based on Fe (OH) ₃ is based on its complex with polymaltose (polymaltose iron), which has a lower degree of interaction with food than FeSO ₄; that is, more iron is biologically available to the body and is not coordinated with other matrices or solids.

The other supplement is composed of nanoparticles of Fe (OH) ₃ suspended in a medium consisting mainly of adipates and tartrates (and other organic salts). This proved to be less toxic than FeSO ₄, in addition to increasing hemoglobin, it does not accumulate in the intestinal mucosa, and it promotes the growth of beneficial microbes.

Pigment

Pigment Yellow 42 is used in paints and cosmetics, and as such does not pose 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 Fe ²⁺ + e ^-

Fe ^III OOH + e ^- + 3H ⁺ ⇋ Fe ²⁺ + 2H ₂ O

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 Fe ²⁺. The electrolytic medium for this battery is composed of soluble salts of Fe ²⁺.

References

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