chemistry

Iron(III) oxide-hydroxide

Iron(III) oxide-hydroxide is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Iron(III) oxide-hydroxide rather than just read about it. In short: Iron(III) oxide-hydroxide or ferric oxyhydroxide is the chemical compound of iron, oxygen, and hydrogen with formula FeO(OH). The compound is often encountered as its hydrates - FeO(OH)·nH2O, e.g. in rust.

Key takeaways

Iron(III) oxide-hydroxide belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Iron(III) oxide-hydroxide to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Iron(III) oxide-hydroxide from memory before moving on to harder problems.

Reference excerpt

Iron(III) oxide-hydroxide or ferric oxyhydroxide is the chemical compound of iron, oxygen, and hydrogen with formula FeO(OH). The compound is often encountered as its hydrates - FeO(OH)·nH2O, e.g. in rust. The monohydrate FeO(OH)·H2O (sometimes incorrectly labelled Fe(OH)3) is often referred to as iron(III) hydroxide, hydrated iron oxide, yellow iron oxide, or Pigment Yellow 42.

Natural occurrences

Minerals Anhydrous ferric hydroxide occurs in the nature as the exceedingly rare mineral bernalite, Fe(OH)3·nH2O (n = 0.0–0.25). Iron oxyhydroxides, FeO(OH), are much more common and occur naturally as structurally different minerals (polymorphs) denoted by the Greek letters α, β, γ and δ.

Goethite, α-FeO(OH), has been used as an ochre pigment since prehistoric times. Akaganeite is the β polymorph, formed by weathering and noted for its presence in some meteorites and the lunar surface. However, recently it has been determined that it must contain some chloride ions to stabilize its structure, so that its more accurate formula is FeO0.833(OH)1.167Cl0.167 or Fe6O5(OH)7Cl. Lepidocrocite, the γ polymorph, is commonly encountered as rust on the inside of steel water pipes and tanks. Feroxyhyte (δ) is formed under the high pressure conditions of sea and ocean floors, being thermodynamically unstable with respect to the α polymorph (goethite) at surface conditions.

Non-mineral Siderogel is a naturally occurring colloidal form of iron(III) oxide-hydroxide. Goethite and lepidocrocite, both crystallizing in orthorhombic system, are the most common forms of iron(III) oxyhydroxide and the most important mineral carriers of iron in soils.

Mineraloids Iron(III) oxyhydroxide is the main component of other minerals and mineraloids:

Limonite is a commonly occurring mixture of mainly goethite, lepidocrocite, quartz and clay minerals. Ferrihydrite is an amorphous or nanocrystalline hydrated mineral, officially FeO(OH)·1.8H2O but with widely variable hydration.

Properties The color of iron(III) oxyhydroxide ranges from yellow through dark-brown to black, depending on the degree of hydration, particle size and shape, and crystal structure.

Structure The crystal structure of β-FeO(OH) (akaganeite) is that of hollandite or BaMn8O16. The unit cell is tetragonal with a = 1.048 and c = 0.3023 nm, and contains eight formula units of FeO(OH). Its dimensions are about 500 × 50 × 50 nm. Twinning often produces particles with the shape of hexagonal stars.

Chemistry On heating, β-FeOOH decomposes and recrystallizes as α-Fe2O3 (hematite).

Uses Limonite, a mixture of various hydrates and polymorphs of ferric oxyhydroxide, is one of the three major iron ores, having been used since at least 2500 BC. Yellow iron oxide, or Pigment Yellow 42, is Food and Drug Administration (FDA) approved for use in cosmetics and is used in some tattoo inks. Iron oxide-hydroxide is also used in aquarium water treatment as a phosphate binder. Iron oxide-hydroxide nanoparticles have been studied as possible adsorbents for lead removal from aquatic media.

Medication Iron polymaltose is used in treatment of iron-deficiency anemia.

Production Iron(III) oxyhydroxide precipitates from solutions of iron(III) salts at pH between 6.5 and 8. Thus the oxyhydroxide can be obtained in the lab by reacting an iron(III) salt, such as ferric chloride or ferric nitrate, with sodium hydroxide:

FeCl3 + 3 NaOH → FeO(OH)·H2O + 3 NaCl Fe(NO3)3 + 3 NaOH → FeO(OH)·H2O + 3 NaNO3 In fact, when dissolved in water, pure FeCl3 will hydrolyze to some extent, yielding the oxyhydroxide and making the solution acidic:

FeCl3 + 2 H2O ⇌ FeO(OH) + 3 HCl Therefore, the compound can also be obtained by the decomposition of acidic solutions of iron(III) chloride held near the boiling point for days or weeks:

FeCl3 + 2 H2O → FeO(OH)(s) + 3 HCl(g) (The same process applied to iron(III) nitrate Fe(NO3)3 or perchlorate Fe(ClO4)3 solutions yields instead particles of α-Fe2O3.) Another similar route is the decomposition of iron(III) nitrate dissolved in stearic acid at about 120 °C. The oxyhydroxide prepared from ferric chloride is usually the β polymorph (akaganeite), often in the form of thin needles. The oxyhydroxide can also be produced by a solid-state transformation from iron(II) chloride tetrahydrate FeCl2·4H2O. The compound also readily forms when iron(II) hydroxide is exposed to air:

4Fe(OH)2 + O2 → 4 FeO(OH) + 2 H2O The iron(II) hydroxide can also be oxidized by hydrogen peroxide in the presence of an acid:

2Fe(OH)2 + H2O2 → 2 FeO(OH)·H2O

See also Rust Iron oxide Yellow boy, a yellow precipitate when acidic runoff such as mine waste, is then neutralised

References

Illustrations

Worked examples

Example 1 — a first encounter with Iron(III) oxide-hydroxide

Start with the simplest possible case. Write down what Iron(III) oxide-hydroxide claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Iron(III) oxide-hydroxide before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Iron(III) oxide-hydroxide ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Iron(III) oxide-hydroxide

In research: Iron(III) oxide-hydroxide appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Iron(III) oxide-hydroxide in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Iron(III) oxide-hydroxide is common in secondary-school and first-year university syllabi. It links to neighbouring topics Hydroxides, Iron(III) compounds, Transition metal oxides, so understanding it makes those chapters shorter.
In everyday life: Look for Iron(III) oxide-hydroxide outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Iron(III) oxide-hydroxide in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Iron(III) oxide-hydroxide means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
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Frequently asked questions

What is Iron(III) oxide-hydroxide in simple terms?

Why does Iron(III) oxide-hydroxide matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Iron(III) oxide-hydroxide?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Iron(III) oxide-hydroxide.