The iron (III) hydroxide is an inorganic compound whose formula is strictly Fe (OH)3, in which the proportion of Fe ions3+ and OH- is 3: 1. However, the chemistry of iron can be quite convoluted; so that this solid is not only composed of the ions mentioned.
In fact, Fe (OH)3 contains the anion Otwo-; therefore, it is an oxide of iron hydroxide monohydrate: FeOOH · HtwoO. 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 it is observed as an orange-brown precipitate; similar to the sediment in the image above. When this rusty and gelatinous sand is heated, it releases excess water, turning its color orange-yellow (pigment yellow 42)..
This yellow pigment 42 is the same FeOOHHtwoOr 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, feroxihita, among others).
The mineral bernalite, on the other hand, exhibits green crystals with a base composition Fe (OH)3NHtwoOR; mineralogical source of this hydroxide.
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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 repetitions of octahedral units FeO6. Thus, these iron-oxygen octahedra intertwine through their corners (Fe-O-Fe), or their faces, establishing all kinds of polymer 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)3NHtwoOr, they are greenish in color because their FeO octahedra6 they only join 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 FeO6 come from either the OH- or of the Otwo-; 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.
While the Fe (OH)3 it 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, in particular) 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 heated it releases water vapor, thus converting it into its anhydrous form FeOOH (together with all its polymorphs). Therefore, if it continues to heat, the FeOOH will melt and not the FeOOH · HtwoOR.
To study its properties more thoroughly, it would be necessary to subject yellow pigment 42 to numerous studies; but it is more than probable that in the process it changes its color to reddish, indicative of the formation of FeOOH; or on the contrary, it dissolves in the aqueous complex Fe (OH)63+ (acid medium), or in the anion Fe (OH)4- (very basic medium).
In the previous section it was mentioned that Fe (OH)3 it is very insoluble in water, and can even precipitate at a pH close to 4.5 (if no chemical species interferes). By precipitating, it can carry away (co-precipitate) some impurities from the environment that are harmful to health; for example, salts of chromium or arsenic (Cr3+, Cr6+, and As3+, Ace5+).
Then, this hydroxide allows to occlude these metals and other heavier ones, acting as an absorbent..
The technique consists not so much in precipitating the Fe (OH)3 (alkalizing the medium), but instead is added directly to contaminated water or soil, using commercially purchased powders or grains.
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 It 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 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 FeSO4, in addition to increasing hemoglobin, it does not accumulate in the intestinal mucosa, and it promotes the growth of beneficial microbes.
Pigment Yellow 42 is used in paints and cosmetics, and as such does not pose a potential health risk; unless accidentally ingested.
Although Fe (OH) is not formally used in this application3, this 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 Fetwo+ + and-
FaithIIIOOH + e- + 3H+ ⇋ Faithtwo+ + 2HtwoOR
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 Fetwo+. The electrolytic medium for this battery is composed of soluble salts of Fetwo+.
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