Black Iron Oxide

 

Black Iron Oxide

Black Iron Oxide is the chemical compound with formula Fe₃O₄. It occurs in nature as the mineral magnetite. It is one of a number of iron oxides, the others being iron(II) oxide (FeO), which is rare, and iron(III) oxide (Fe₂O₃) also known as hematite. It contains both Fe²⁺ and Fe³⁺ ions and is sometimes formulated as FeO ∙ Fe₂O₃. This iron oxide is encountered in the laboratory as a black powder. It exhibits permanent magnetism and is ferrimagnetic, but is sometimes incorrectly described as ferromagnetic.^[3] Its most extensive use is as a black pigment. For this purpose, it is synthesised rather than being extracted from the naturally occurring mineral as the particle size and shape can be varied by the method of production.^[4]

Black Iron Oxide Preparation

Under anaerobic conditions, ferrous hydroxide (Fe(OH)₂) can be oxidized by water to form magnetite and molecular hydrogen. This process is described by the Schikorr reaction:

${\displaystyle {\ce {{\underset {ferrous\ hydroxide}{3Fe(OH)2}}->{\underset {magnetite}{Fe3O4}}+{\underset {hydrogen}{H2}}+{\underset {water}{2H2O}}}}}$

The well-crystallized magnetite (Fe₃O₄) is thermodynamically more stable than the ferrous hydroxide (Fe(OH)₂ ).^[5]

Black Iron Oxide

 
Magnetite can be prepared in the laboratory as a ferrofluid in the Massart method by mixing iron(II) chloride and iron(III) chloride in the presence of sodium hydroxide.^[6] Magnetite can also be prepared by the chemical co-precipitation in presence of ammonia, which consist in a mixture of a solution 0.1 M of FeCl₃·6H₂O and FeCl₂·4H₂O with mechanic agitation of about 2000 rpm. The molar ratio of FeCl₃:FeCl₂ can be 2:1; heating this solution at 70 °C, and immediately the speed is elevated to 7500 rpm and adding quickly a solution of NH₄OH (10 volume %), immediately a dark precipitate will be formed, which consists of nanoparticles of magnetite.^[7] In both cases, the precipitation reaction rely on a quick transformation of acidic hydrolyzed iron ions into the spinel iron oxide structure, by hydrolysis at elevated pH values (above ca. 10).
Considerable efforts has been devoted towards controlling the particle formation process of magnetite nanoparticles due to the challenging and complex chemistry reactions involved in the phase transformations prior to the formation of the magnetite spinel structure.^[8] Magnetite particles are of interests in bioscience applications such as in magnetic resonance imaging (MRI) since iron oxide magnetite nanoparticles represent a non-toxic alternative to currently employed gadolinium-based contrast agents. However, due to lack of control over the specific transformations involved in the formation of the particles, truly superparamagnetic particles have not yet been prepared from magnetite, i.e. magnetite nanoparticles that completely lose their permanent magnetic characteristic in the absence of an external magnetic field (which by definition show a coercivity of 0 A/m). The smallest values currently reported for nanosized magnetite particles is Hc = 8.5 A m⁻¹,^[9] whereas the largest reported magnetization value is 87 Am² kg⁻¹ for synthetic magnetite.^[10][11]
Pigment quality Fe₃O₄, so called synthetic magnetite, can be prepared using processes that use industrial wastes, scrap iron or solutions containing iron salts (e.g. those produced as by-products in industrial processes such as the acid vat treatment (pickling) of steel):

• Oxidation of Fe metal in the Laux process where nitrobenzene is treated with iron metal using FeCl₂ as a catalyst to produce aniline:^[4]

C₆H₅NO₂ + 3 Fe + 2 H₂O → C₆H₅NH₂ + Fe₃O₄

• Oxidation of Fe^II compounds, e.g. the precipitation of iron(II) salts as hydroxides followed by oxidation by aeration where careful control of the pH determines the oxide produced.^[4]

Reduction of Fe₂O₃ with hydrogen:^[12][13]

3Fe₂O₃ + H₂ → 2Fe₃O₄ +H₂O

Reduction of Fe₂O₃ with CO:^[14]

3Fe₂O₃ + CO → 2Fe₃O₄ + CO₂

Production of nano-particles can be performed chemically by taking for example mixtures of Fe^II and Fe^III salts and mixing them with alkali to precipitate colloidal Fe₃O₄. The reaction conditions are critical to the process and determine the particle size.^[15]

 

Black Iron Oxide

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