Editing Iron(II,III) oxide (section)

==Preparation==
Heated iron metal interacts with steam to form iron oxide and hydrogen gas.

:<chem>3Fe + 4H2O->Fe3O4 + 4H2 </chem>

Under [[Hypoxia (environmental)|anaerobic]] conditions, [[Iron(II) hydroxide|ferrous hydroxide]] (Fe(OH)<sub>2</sub>) can be oxidized by water to form magnetite and molecular [[hydrogen]]. This process is described by the [[Schikorr reaction]]:
:<chem>\underset{ferrous\ hydroxide}{3Fe(OH)2} -> \underset{magnetite}{Fe3O4} + \underset{hydrogen}{H2} + \underset{water}{2H2O}</chem>
This works because crystalline magnetite (Fe<sub>3</sub>O<sub>4</sub>) is thermodynamically more stable than amorphous ferrous hydroxide (Fe(OH)<sub>2</sub> ).<ref>{{cite journal | vauthors = Ma M, Zhang Y, Guo Z, Gu N | title = Facile synthesis of ultrathin magnetic iron oxide nanoplates by Schikorr reaction | journal = Nanoscale Research Letters | volume = 8 | issue = 1 | pages = 16 | date = January 2013 | pmid = 23294626 | pmc = 3598988 | doi = 10.1186/1556-276X-8-16 | bibcode = 2013NRL.....8...16M | doi-access = free }}</ref>

The [[Massart method]] of preparation of magnetite as a [[ferrofluid]], is convenient in the laboratory: mix [[iron(II) chloride]] and [[iron(III) chloride]] in the presence of [[sodium hydroxide]].<ref>{{cite journal| vauthors = Massart R |title=Preparation of aqueous magnetic liquids in alkaline and acidic media|journal=IEEE Transactions on Magnetics|date=1981|volume=17|issue=2|pages=1247–1248|doi=10.1109/TMAG.1981.1061188|bibcode=1981ITM....17.1247M}}</ref>

A more efficient method of preparing magnetite without troublesome residues of sodium, is to use ammonia to promote chemical co-precipitation from the iron chlorides: first mix solutions of 0.1 M FeCl<sub>3</sub>·6H<sub>2</sub>O and FeCl<sub>2</sub>·4H<sub>2</sub>O with vigorous stirring at about 2000 rpm. The molar ratio of the FeCl<sub>3</sub>:FeCl<sub>2</sub> should be about 2:1. Heat the mix to 70&nbsp;°C, then raise the speed of stirring to about 7500 rpm and quickly add a solution of NH<sub>4</sub>OH (10 volume %). A dark precipitate  of nanoparticles of magnetite forms immediately.<ref>{{cite journal| vauthors = Keshavarz S, Xu Y, Hrdy S, Lemley C, Mewes T, Bao Y |title=Relaxation of Polymer Coated Fe<sub>3</sub>O<sub>4</sub> Magnetic Nanoparticles in Aqueous Solution|journal=IEEE Transactions on Magnetics|date=2010|volume=46|issue=6|pages=1541–1543|doi=10.1109/TMAG.2010.2040588|s2cid=35129018}}</ref>

In both methods, the precipitation reaction relies on rapid transformation of acidic iron ions into the spinel iron oxide structure at pH 10 or higher.

Controlling the formation of magnetite nanoparticles presents challenges: the reactions and phase transformations necessary for the creation of the magnetite spinel structure are complex.<ref>{{cite journal | vauthors = Jolivet JP, Chanéac C, Tronc E | title = Iron oxide chemistry. From molecular clusters to extended solid networks | journal = Chemical Communications | issue = 5 | pages = 481–7 | date = March 2004 | pmid = 14973569 | doi = 10.1039/B304532N }}</ref> The subject is of practical importance because magnetite particles are of interest in bioscience applications such as [[magnetic resonance imaging]] (MRI), in which iron oxide magnetite nanoparticles potentially present a non-toxic alternative to the gadolinium-based [[contrast agents]] currently in use. However, difficulties in controlling the formation of the particles, still frustrate the preparation of superparamagnetic magnetite particles, that is to say: magnetite nanoparticles with a coercivity of 0 A/m, meaning that they completely lose their permanent magnetisation in the absence of an external magnetic field. The smallest values currently reported for nanosized magnetite particles is ''Hc'' = 8.5 A m<sup>−1</sup>,<ref>{{cite journal| vauthors = Ström V, Olsson RT, Rao KV |title=Real-time monitoring of the evolution of magnetism during precipitation of superparamagnetic nanoparticles for bioscience applications|journal=Journal of Materials Chemistry|date=2010|volume=20|issue=20|pages=4168|doi=10.1039/C0JM00043D}}</ref> whereas the largest reported magnetization value is 87 Am<sup>2</sup> kg<sup>−1</sup> for synthetic magnetite.<ref>{{cite journal| vauthors = Fang M, Ström V, Olsson RT, Belova L, Rao KV |title=Rapid mixing: A route to synthesize magnetite nanoparticles with high moment|journal=Applied Physics Letters |date=2011 |volume=99 |issue=22 |pages=222501 |doi=10.1063/1.3662965 |bibcode=2011ApPhL..99v2501F }}</ref><ref>{{cite journal | vauthors = Fang M, Ström V, Olsson RT, Belova L, Rao KV | title = Particle size and magnetic properties dependence on growth temperature for rapid mixed co-precipitated magnetite nanoparticles | journal = Nanotechnology | volume = 23 | issue = 14 | pages = 145601 | date = April 2012 | pmid = 22433909 | doi = 10.1088/0957-4484/23/14/145601 | bibcode = 2012Nanot..23n5601F | s2cid = 34153665 }}</ref>

Pigment quality Fe<sub>3</sub>O<sub>4</sub>, 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 (metal)|pickling]]) of steel):

*Oxidation of Fe metal in the Laux process where [[nitrobenzene]] is treated with iron metal using FeCl<sub>2</sub> as a catalyst to produce [[aniline]]:<ref name = "Cornell"/>
:C<sub>6</sub>H<sub>5</sub>NO<sub>2</sub> + 3 Fe + 2 H<sub>2</sub>O   →   C<sub>6</sub>H<sub>5</sub>NH<sub>2</sub> + Fe<sub>3</sub>O<sub>4</sub>
*Oxidation of Fe<sup>II</sup> 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.<ref name = "Cornell"/>

Reduction of Fe<sub>2</sub>O<sub>3</sub> with hydrogen:<ref>{{cite patent | country = US | number = 2596954 | gdate = 13 May 1952 | title = Process for reduction of iron ore to magnetite | inventor = Heath TD | assign1 = Dorr Company }}</ref><ref>{{cite journal | title = Kinetics of reduction of iron oxides by H2 Part I: Low temperature reduction of hematite | vauthors = Pineau A, Kanari N, Gaballah I | journal = Thermochimica Acta | volume = 447 | issue = 1 | pages = 89–100 | year = 2006 | doi=10.1016/j.tca.2005.10.004}}</ref>
:3Fe<sub>2</sub>O<sub>3</sub> + H<sub>2</sub> → 2Fe<sub>3</sub>O<sub>4</sub> +H<sub>2</sub>O
Reduction of Fe<sub>2</sub>O<sub>3</sub> with CO:<ref>{{cite journal | title = The effects of nucleation and growth on the reduction of Fe<sub>2</sub>O<sub>3</sub> to Fe<sub>3</sub>O<sub>4</sub> | vauthors = Hayes PC, Grieveson P | journal = Metallurgical and Materials Transactions B | year = 1981 | volume = 12 | issue = 2 | pages = 319–326 | doi = 10.1007/BF02654465|bibcode=1981MTB....12..319H |s2cid=94274056 }}</ref>
:3Fe<sub>2</sub>O<sub>3</sub> + CO → 2Fe<sub>3</sub>O<sub>4</sub> + CO<sub>2</sub>

Production of nano-particles can be performed chemically by taking for example mixtures of Fe<sup>II</sup> and Fe<sup>III</sup> salts and mixing them with alkali to precipitate colloidal Fe<sub>3</sub>O<sub>4</sub>. The reaction conditions are critical to the process and determine the particle size.<ref>Arthur T. Hubbard (2002) ''Encyclopedia of Surface and Colloid Science'' CRC Press, {{ISBN|0-8247-0796-6}}</ref>

[[Iron(II) carbonate]] can also be thermally decomposed into Iron(II,III):<ref>{{Cite web |title=FeCO3 = Fe3O4 + CO2 + CO {{!}} The thermal decomposition of iron(II) carbonate |url=https://chemiday.com/en/reaction/3-1-0-5222 |access-date=2022-10-14 |website=chemiday.com}}</ref>

: {{Chem2|3FeCO3 → Fe3O4 + 2CO2 + CO}}