# Two Ions of Iron

Laplace’s Mini World · Plate 90 · Matter, Light & Sound · Matter

> Iron dissolved in water is usually one of two ions: iron(II), pale green, or iron(III), yellow-⁠brown. One drop of thiocyanate turns the iron(III) tube blood-⁠red; the iron(II) tube barely changes.

- Interactive model: https://world.lapla.si/en/iron/
- Embed: https://world.lapla.si/embed/en/iron/
- 中文: https://world.lapla.si/zh/iron/
- Updated: 2026-11-03

## 01 Two Charges, Two Colours

Iron in water is there as ions. With two electrons lost it is Fe²⁺, iron(II); with three lost, Fe³⁺, iron(III). Each ion is held in a ring of six water molecules: \[Fe(H₂O)₆\]²⁺ and \[Fe(H₂O)₆\]³⁺.

The pale green tube is iron(II) sulfate at 0.1 mol/L, faintly coloured; the yellow-⁠brown one is iron(III) chloride at the same strength. Both hold a little acid; school iron solutions are made up in dilute acid so they keep.

The colours belong to the ions in the solution, not to iron itself: iron filings are dark grey. The same iron, with a different charge and different molecules round it, takes different light away.

*FeSO₄, 0.1 mol/L: pale green · FeCl₃, 0.1 mol/L: yellow-⁠brown · Fe → Fe²⁺ + 2e⁻ · Fe²⁺ → Fe³⁺ + e⁻*

## 02 Where the Yellow Comes From

The six water molecules round Fe²⁺ let its 3d electrons take up a little red light, so what comes through leans blue-⁠green. The absorption is weak, and the solution only pale green.

Iron(III)’s own hexaaqua ion, \[Fe(H₂O)₆\]³⁺, is almost colourless, a very pale lilac. The yellow-⁠brown of iron(III) chloride comes from other ions: the hexaaqua ion hands one H⁺ from a water molecule to the solution and becomes \[Fe(H₂O)₅(OH)\]²⁺, written FeOH²⁺; or a chloride ion takes a water molecule’s place, giving FeCl²⁺.

In Coppel’s 1957 measurements, at 400 nm in the violet, FeOH²⁺ absorbs 108 times as strongly as the hexaaqua ion, and FeCl²⁺ 255 times. With under 1% of the iron as FeOH²⁺, it takes as much violet light as the other 99%. Stronger solutions look more orange; more acid pushes the iron back to the hexaaqua ion and the yellow pales.

*At 400 nm: Fe³⁺(aq) 0.499 · FeOH²⁺ 53.8 · FeCl²⁺ 127 L/(mol·cm) · 0.94% as FeOH²⁺ matches the other 99%*

## 03 One Drop, Blood-⁠Red

A drop of potassium thiocyanate, KSCN, in the yellow-⁠brown iron(III) turns blood-⁠red where it lands. The thiocyanate ion, SCN⁻, takes a water molecule’s place and gives \[Fe(SCN)(H₂O)₅\]²⁺, written \[Fe(SCN)\]²⁺. It is an exchange of ligands; the iron is still iron(III).

The new ion absorbs blue-⁠green light, about 450⁠–⁠480 nm, and lets mostly red through. Its colour is so deep that very little Fe³⁺ shows, which is why this drop is the usual test for Fe³⁺.

The pale green tube had a drop too, and barely changed. Thiocyanate tests for Fe³⁺, not Fe²⁺. If an iron(II) solution does turn faintly red, some Fe³⁺ is already in it, as there usually is unless it is pure and fresh.

*Fe³⁺ + SCN⁻ ⇌ \[Fe(SCN)\]²⁺ · absorbs blue-⁠green, about 450⁠–⁠480 nm · a test for Fe³⁺, not Fe²⁺*

## 04 How Deep the Red Goes

The reaction runs both ways: the red ion also comes apart again. Its constant K₁ depends on the conditions it is measured in; published values run from 138 (at ionic strength 0.5) to 890.

By this page’s calculation, one drop of 0.10 mol/L thiocyanate (a drop taken as 0.05 mL) in 2.0 mL of 0.10 mol/L Fe³⁺ finds iron for 93⁠–⁠99% of its SCN⁻. With iron in excess, the depth of the red follows the thiocyanate added; with thiocyanate in excess, nearly every Fe³⁺ is bound and the red follows the iron, which is how iron is measured by colour.

With little Fe³⁺, say 1% of the iron in a 0.10 mol/L iron(II) solution, the same drop finds iron for only 23⁠–⁠62% of its SCN⁻, and the red is far paler: the faint red of an iron(II) tube. Left standing, the red slowly fades, so colour measurements are read at a set time.

*Fe³⁺ + SCN⁻ ⇌ \[Fe(SCN)\]²⁺ · K₁: 138⁠–⁠890 · one drop in 2.0 mL: 93⁠–⁠99% bound at 0.10 mol/L Fe³⁺ · 23⁠–⁠62% at 0.001 mol/L · this page’s calculation*

## 05 Prussian Blue

Another reagent: potassium hexacyanoferrate(II), K₄\[Fe(CN)₆\], also called potassium ferrocyanide. Dropped into iron(III), it throws down a deep blue solid at once: Prussian blue. Into iron(II), it gives a white solid that turns blue in air.

Potassium hexacyanoferrate(III), K₃\[Fe(CN)₆\], works the other way round: with iron(II) a deep blue solid, long called Turnbull’s blue; with iron(III) only a brown colour and no solid. Both blues are the same kind of compound, iron(III) bound through cyanide to hexacyanoferrate(II): when Fe²⁺ meets ferricyanide, an electron changes places. Whether the two are quite identical is still argued in the literature.

The blue comes from an electron hopping between neighbouring Fe(II) and Fe(III) in the solid, which takes up red light near 1.7 eV, about 730 nm. The solid is fine particles held in suspension, not dissolved, even in the kind called “soluble”. The particles sink slowly to the foot of the tube, and the liquid above them clears.

*K₄\[Fe(CN)₆\] + Fe³⁺ → Prussian blue · K₃\[Fe(CN)₆\] + Fe²⁺ → Turnbull’s blue · absorbs red light near 730 nm*

## 06 Air Browns It, Slowly

Oxygen from the air turns Fe²⁺ into Fe³⁺, one O₂ molecule taking an electron from each of four Fe²⁺. From the standard potentials the reaction has a drive of +0.458 V and goes by itself; how fast is another matter.

Near neutral the rate goes with the square of the OH⁻ concentration: one pH unit lower, a hundred times slower. With the rate constant Stumm and Lee measured, in water open to the air, half the Fe²⁺ is gone in about 4 minutes at pH 7 and about 7 hours at pH 6; below pH 4 another term takes over, the rate no longer depends on pH, and it takes about 7.5 years. That is why iron(II) solutions are made up in dilute sulfuric acid, and fresh.

Add sodium hydroxide and Fe²⁺ comes down first as green iron(II) hydroxide. In the alkali it oxidises much faster, and the layer near the mouth of the tube, open to the air, turns orange-⁠brown first. Rusting iron takes the same road: iron loses electrons to Fe²⁺, oxygen turns that to Fe³⁺, and the end is a hydrated iron oxide, rust; the first step, on the iron’s surface, is a small cell (see The Galvanic Cell).

*4Fe²⁺ + O₂ + 4H⁺ → 4Fe³⁺ + 2H₂O · half the Fe²⁺ gone: about 4 min at pH 7 · 7 h at pH 6 · 7.5 years below pH 4 · this page’s model*

## 07 Iron Filings Turn It Back

Drop iron filings into the yellow-⁠brown iron(III), and the iron reduces Fe³⁺ to Fe²⁺ and becomes Fe²⁺ itself: one iron atom gives up two electrons, and three Fe²⁺ come of it. The solution fades slowly to pale green.

The standard potentials are +0.771 V for Fe³⁺/Fe²⁺ and −0.447 V for Fe²⁺/Fe, 1.218 V apart; the equilibrium constant is about 10⁴¹, so while any filings remain, almost no Fe³⁺ is left. How fast depends on the filings’ surface; the page gives no time.

Very little iron is needed: 2.79 mg for each millilitre of 0.10 mol/L Fe³⁺, 25 mg for this tube’s 9 mL, far less than a tip of filings. One Fe²⁺ in three comes from the filings, so the Fe²⁺ ends at 1.5 times the Fe³⁺ there was. Take the filings out and air slowly oxidises it back; round and round, what is used up is iron and oxygen.

*Fe + 2Fe³⁺ → 3Fe²⁺ · +1.218 V · 2.79 mg of iron per mL of 0.10 mol/L Fe³⁺ · Fe²⁺ ends at 0.15 mol/L*

## 08 Iron(II) in the Blood

Haemoglobin is four protein chains, each with a haem, and at the centre of each haem is an Fe²⁺. Each Fe²⁺ can bind one O₂, so one haemoglobin carries up to four. The binding runs both ways: taken up in the lungs, let go in the tissues.

A litre of blood holds about 150 g of haemoglobin; at 64,⁠500 g/mol that carries about 0.52 g of iron, 0.42⁠–⁠0.60 g across the normal range. An adult has 3⁠–⁠4 g of iron in all, about two-⁠thirds of it in red cells.

The haem iron must be Fe²⁺ to bind oxygen. It can turn into Fe³⁺ on its own, and haemoglobin with such an iron, methaemoglobin, no longer binds O₂ there. An enzyme in red cells, cytochrome b5 reductase, keeps turning it back, so normally it is under 1⁠–⁠2%. Some drugs and nitrites speed the change; above about 10% the skin turns bluish and the blood dark brown. Myoglobin stores oxygen in muscle with a single haem; when raw meat browns at the surface, the iron there has become Fe³⁺ too.

*One haemoglobin: 4 haems, 4 Fe²⁺, up to 4 O₂ · methaemoglobin: Fe³⁺, binds no O₂ · normally under 1⁠–⁠2%*

## 09 A Blue from Berlin

Berlin, around 1706. The colour maker Johann Jacob Diesbach, working in Johann Konrad Dippel’s laboratory, was making a red pigment from a recipe that used iron sulfate and potash. The potash he borrowed had been heated with animal blood in Dippel’s work and carried hexacyanoferrate, and what settled out was blue. The year is given differently; on Kraft’s reading of the original sources, 1706 is the most likely.

In 1710 the Prussian academy’s journal announced the new “Berlin blue”, without a recipe; the recipe appeared only in 1724, published by John Woodward in the Royal Society’s Philosophical Transactions. Often called the first modern synthetic pigment, it was cheaper than ultramarine. In the 19th century it was shipped in bulk to China and Japan, and Hokusai used it for the skies and water of his Thirty-⁠six Views of Mount Fuji.

The blueprint Herschel invented in 1842 also comes out in Prussian blue, and analytical chemists have long used it as a spot test for iron. Today it is also a prescription drug: insoluble Prussian blue taken by mouth binds caesium and thallium in the gut, so the body passes them out.

*Berlin, around 1706 · first announced 1710 · recipe published 1724 · Fe₄\[Fe(CN)₆\]₃*

## 10 Eleven Things on the Tile

Five tubes: iron(II) hydroxide, iron(II) sulfate, iron(III) chloride with a drop of thiocyanate, iron(III) chloride, Prussian blue; and the rack, the white tile, two dropper bottles (thiocyanate and hexacyanoferrate(II)), a watch glass of green vitriol and a spatula with iron filings. The tubes are 16 × 150 mm borosilicate glass, 9 mL in each.

*Iron(II) hydroxide · iron(II) sulfate · thiocyanate · iron(III) chloride · Prussian blue · rack · tile · dropper bottles · watch glass · spatula*

## 11 Mix a Tube

This is the page’s model, computed in the browser from the reactions and numbers above. The tube holds 9 mL of 0.10 mol/L iron(II) or iron(III), made up in acid; a drop is 0.05 mL and each reagent is 0.10 mol/L.

Add a drop of thiocyanate or of hexacyanoferrate(II), put in iron filings, or leave the tube in the air for a year: at pH 2, by the rate above, about 9% of the Fe²⁺ oxidises. The colour and the share of each kind of iron change together. The red is computed with K₁ = 890; Prussian blue is counted as one Fe³⁺ for each hexacyanoferrate(II), the white solid turns blue in the air, and filings reduce all the Fe³⁺ as soon as they go in.

*This page’s model · K₁ = 890 (OpenStax) · air: the PHREEQC rate, pH 2 · a drop of 0.05 mL*

**Try this**: In the lab, start with iron(II) and add a drop of thiocyanate: does it turn red? Leave it a year in the air: what colour now? Then add iron filings?

## What the model leaves out

The bench is a model of a school bench: 16 × 150 mm borosilicate test tubes (in the size of Corning’s PYREX 9800-16), 9 mL in each, with a rack, a white tile, dropper bottles and a spatula of common kinds, not a copy of any one set. The colours are drawn from how much of each colour of light each solution takes away; those amounts are set by the page to look as the solutions do, not measured spectra. The only measured values found are Coppel’s below 400 nm and one for \[Fe(SCN)\]²⁺ at 610 nm. How the red spreads from the drop, how the solid settles and how the top browns are the page’s drawing, not a fluid calculation. The air-⁠oxidation rate holds for dilute solutions and uses a rate constant measured at 20.5 °C; ionic strength, catalysis by iron(III) solids and bacteria are left out. In the lab, filings react completely at once, Prussian blue is counted as KFe\[Fe(CN)₆\], and only the first thiocyanate complex is counted.

## Easy to get wrong

The colours belong to ions and complexes, not to iron: filings are dark grey, \[Fe(H₂O)₆\]³⁺ itself is almost colourless, and iron(III) chloride’s yellow-⁠brown comes from ions that have lost an H⁺ or taken on chloride. Fe²⁺ solutions are pale green, not bright green. Thiocyanate tests for Fe³⁺; a faint red in an iron(II) solution means some Fe³⁺ is already there. Air does not oxidise Fe²⁺ quickly: in acid it takes years, minutes only near neutral. Turnbull’s blue and Prussian blue are the same kind of compound, and whether they are identical is still argued; the electron’s change of place happens in the solid and is not explained by potentials measured in solution. In methaemoglobin the haem that has become Fe³⁺ binds no oxygen, while the Fe²⁺ left in the same molecule holds its oxygen tighter and gives less to the tissues.

## Common questions

### What colour is the hexaaqua iron(III) ion itself?

Iron(III)’s own hexaaqua ion, \[Fe(H₂O)₆\]³⁺, is almost colourless, a very pale lilac.

See: [02 Where the Yellow Comes From](https://world.lapla.si/en/iron/#colour)

### What does thiocyanate do to iron(III)?

A drop of potassium thiocyanate, KSCN, in the yellow-⁠brown iron(III) turns blood-⁠red where it lands.

See: [03 One Drop, Blood-⁠Red](https://world.lapla.si/en/iron/#red)

### How fast does air oxidise Fe²⁺?

With the rate constant Stumm and Lee measured, in water open to the air, half the Fe²⁺ is gone in about 4 minutes at pH 7 and about 7 hours at pH 6; below pH 4 another term takes over, the rate no longer depends on pH, and it takes about 7.5 years.

See: [06 Air Browns It, Slowly](https://world.lapla.si/en/iron/#air)

### Is the iron in haemoglobin iron(II) or iron(III)?

The haem iron must be Fe²⁺ to bind oxygen. It can turn into Fe³⁺ on its own, and haemoglobin with such an iron, methaemoglobin, no longer binds O₂ there.

See: [08 Iron(II) in the Blood](https://world.lapla.si/en/iron/#blood)

## Sources

1. [OpenStax: Chemistry 2e, 19.3 (\[Fe(H₂O)₆\]²⁺ absorbs red light and looks blue-⁠green; high and low spin)](https://openstax.org/books/chemistry-2e/pages/19-3-spectroscopic-and-magnetic-properties-of-coordination-compounds)
2. [OpenStax: Chemistry 2e, 19.1 (the common oxidation states of iron; Fe³⁺ forms coloured hydrated ions)](https://openstax.org/books/chemistry-2e/pages/19-1-occurrence-preparation-and-properties-of-transition-metals-and-their-compounds)
3. [OpenStax: Chemistry 2e, 17.6 (corrosion: the electrode reactions of rusting and rust, Fe₂O₃·xH₂O)](https://openstax.org/books/chemistry-2e/pages/17-6-corrosion)
4. [OpenStax: Chemistry 2e, appendix L (standard electrode potentials: Fe³⁺/Fe²⁺ +0.771 V, Fe²⁺/Fe −0.447 V, O₂/H₂O +1.229 V)](https://openstax.org/books/chemistry-2e/pages/l-standard-electrode-half-cell-potentials)
5. [OpenStax: Chemistry 2e, appendix K (the formation constant of \[Fe(SCN)\]²⁺, 8.9 × 10²)](https://openstax.org/books/chemistry-2e/pages/k-formation-constants-for-complex-ions)
6. [Jim Clark: chemguide, Iron (thiocyanate as a test for Fe³⁺, “intense blood red”)](https://www.chemguide.co.uk/inorganic/transition/iron.html)
7. [Jim Clark: chemguide, The acidity of the hexaaqua ions (\[Fe(H₂O)₆\]³⁺ very pale lilac; the yellow from the ion that has lost an H⁺; acid pales it)](https://www.chemguide.co.uk/inorganic/complexions/acidity.html)
8. [Jim Clark: chemguide, hexaaqua ions and hydroxide ions (the green iron(II) hydroxide, turning orange around the top of the tube)](https://www.chemguide.co.uk/inorganic/complexions/aquaoh.html)
9. [AQA: A-⁠level Chemistry 7405, Reactions of metal ions in aqueous solution (the colours and precipitates of Fe²⁺ and Fe³⁺)](https://filestore.aqa.org.uk/resources/chemistry/AQA-7405-REACTIONS-OF-METAL-IONS.PDF)
10. [Royal Society of Chemistry: Oxidation states of iron (microscale; thiocyanate gives virtually no colour with iron(II), a faint red unless it is pure)](https://edu.rsc.org/experiments/oxidation-states-of-iron/517.article)
11. [Royal Society of Chemistry: Iron by thiocyanate assay (\[Fe(SCN)\]²⁺ absorbs most at 480 nm; used to measure iron by colour)](https://edu.rsc.org/download?ac=12734)
12. [CLEAPSS: Student Safety Sheet 38, Iron and its compounds (iron salt solutions are made up in dilute acid to slow oxidation)](https://science.cleapss.org.uk/Resource/SSS038-Iron-and-its-compounds.pdf)
13. [J. P. Birk: Characteristic Reactions of Iron (LibreTexts; potassium ferrocyanide and ferricyanide with the two ions)](https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Supplemental_Modules_%28Analytical_Chemistry%29/Qualitative_Analysis/Characteristic_Reactions_of_Select_Metal_Ions/Characteristic_Reactions_of_Iron__%28Fe%29)
14. [LibreTexts: Determination of Kc for a Complex Ion Formation (\[Fe(SCN)\]²⁺ measured at 447 nm; reddish-⁠orange when dilute)](https://chem.libretexts.org/Courses/University_of_California_Davis/Chem_4B_Lab%3A_General_Chemistry_for_Majors_II/6%3A_Determination_of_Kc_for_a_Complex_Ion_Formation_(Experiment))
15. [C. P. Coppel (1957): The Kinetics of the Complexing of Ferric Ion by Thiocyanate and Chloride Ions, UCRL-⁠8059 (measured K₁; molar absorption of Fe³⁺, FeOH²⁺ and FeCl²⁺; the slow decomposition of the red)](https://www.osti.gov/servlets/purl/4310665)
16. [M. W. Lister, D. E. Rivington (1955): Some measurements on the iron(III)–thiocyanate system, Can. J. Chem. 33 (precautions against fading)](https://doi.org/10.1139/v55-192)
17. [E. Reguera et al. (1999): The existence of ferrous ferricyanide, Transition Met. Chem. 24 (Turnbull’s blue similar to Prussian blue but not identical)](https://repositoriodigital.ipn.mx/bitstream/123456789/11711/1/1999TMCh_TB%201.pdf)
18. [M. Li et al. (2023): Identification of the Chromophores in Prussian blue, arXiv:2307.03883 (red light absorbed near 1.7 eV, 730 nm; particles suspended, not dissolved)](https://arxiv.org/abs/2307.03883)
19. [W. Sung, J. J. Morgan (1980): Kinetics and Product of Ferrous Iron Oxygenation in Aqueous Systems, Environ. Sci. Technol. 14 (the rate law; Stumm and Lee’s rate constant)](https://semspub.epa.gov/work/09/1117508.pdf)
20. [USGS: PHREEQC 3, example 9 (the rate of oxidation of Fe²⁺ by oxygen, with its acid-⁠range term)](https://water.usgs.gov/water-resources/software/PHREEQC/documentation/phreeqc3-html/phreeqc3-71.htm)
21. [NIST: CODATA 2022 values (the Faraday constant F and the molar gas constant R)](https://physics.nist.gov/cgi-bin/cuu/Value?f)
22. [OpenStax: Anatomy and Physiology 2e, 18.3 (Fe²⁺ in haem; one haemoglobin carries four O₂)](https://openstax.org/books/anatomy-and-physiology-2e/pages/18-3-erythrocytes)
23. [OpenStax: Anatomy and Physiology 2e, 22.5 (oxygen transport in the blood, a reversible binding)](https://openstax.org/books/anatomy-and-physiology-2e/pages/22-5-transport-of-gases)
24. [US National Library of Medicine: MedlinePlus Genetics, CYB5R3 gene (haemoglobin with Fe³⁺ cannot deliver oxygen; the reductase; normally under 2 percent)](https://medlineplus.gov/genetics/gene/cyb5r3/)
25. [A. Iolascon et al. (2021): Recommendations for diagnosis and treatment of methemoglobinemia, Am. J. Hematol. 96 (under 1%; cyanosis and dark brown blood at 10⁠–⁠30%; drugs and nitrites)](https://europepmc.org/article/PMC/PMC9291883)
26. [Y. Kohgo et al. (2008): Body iron metabolism and pathophysiology of iron overload, Int. J. Hematol. 88 (3⁠–⁠4 g of iron in the body, two-⁠thirds in red cells)](https://europepmc.org/article/PMC/PMC2516548)
27. [Oregon Medical Laser Center: Optical Absorption of Hemoglobin (64,⁠500 g/mol; 150 g of haemoglobin per litre of blood)](https://omlc.org/spectra/hemoglobin/)
28. [RCSB PDB-⁠101: Molecule of the Month, Myoglobin (myoglobin stores oxygen in muscle)](https://pdb101.rcsb.org/motm/1)
29. [S. H. J. M. Dobbelstein (2005): Relationship between beef colour and myoglobin, Wageningen UR (the iron of metmyoglobin is ferric; meat turns brown)](https://edepot.wur.nl/346180)
30. [A. Kraft (2008): On the Discovery and History of Prussian Blue, Bull. Hist. Chem. 33 (Diesbach and Dippel, Berlin, most probably 1706; 1710; Woodward 1724; the blueprint)](https://acshist.scs.illinois.edu/bulletin_open_access/bhc2008v033/bhc2008v033p061.pdf)
31. [A. Kraft (2009): On Two Letters from Caspar Neumann to John Woodward, Bull. Hist. Chem. 34 (where the 1724 recipe came from)](https://acshist.scs.illinois.edu/bulletin_open_access/FullIssues/bhc2009v034f2.pdf)
32. [A. Kraft (2016): Addendum to “On the Discovery and History of Prussian Blue”, Bull. Hist. Chem. 41 (Dippel’s own account)](https://acshist.scs.illinois.edu/bulletin_open_access/FullIssues/bhc2016v041f.pdf)
33. [Swiss National Museum: The Swiss alchemist who invented a new shade of blue (exported in bulk to China and Japan in the 19th century)](https://blog.nationalmuseum.ch/en/2026/03/the-swiss-alchemist-who-invented-a-new-shade-of-blue/)
34. [Art Institute of Chicago: Katsushika Hokusai, Under the Wave off Kanagawa (Berlin blue in the series’ skies and water)](https://www.artic.edu/artworks/24645)
35. [US FDA: Radiogardase (Prussian blue insoluble) prescribing information (internal contamination with caesium and thallium)](https://www.accessdata.fda.gov/drugsatfda_docs/label/2014/021626s010lbl.pdf)
36. [Corning: PYREX 9800-16 test tube (16 × 150 mm)](https://ecatalog.corning.com/life-sciences/b2c/US/en/Glassware/Tubes,-Glass/PYREX%C2%AE-Test-Tube-with-Beaded-Rim/p/9800-16)

## Data

The chemistry after OpenStax Chemistry 2e (Flowers, Theopold, Langley, Robinson; Rice University, 2019; access for free at openstax.org: 17.6, 19.1, 19.3, appendices K and L), Jim Clark’s chemguide, the AQA A-⁠level data sheet on metal ions, the Royal Society of Chemistry’s Education pages, CLEAPSS Student Safety Sheet 38 and J. P. Birk’s LibreTexts pages; molar absorption of the iron(III) species from C. P. Coppel’s 1957 thesis (UCRL-⁠8059, OSTI); Prussian blue after M. Li et al. (arXiv:2307.03883) and E. Reguera et al. (1999); the oxidation of iron(II) by air after W. Sung and J. J. Morgan (1980) and the USGS PHREEQC documentation (example 9). Every number the page computes (potentials, the share of thiocyanate bound, half-⁠lives, the iron in the filings and in blood) is computed here from those values. Haemoglobin, methaemoglobin and myoglobin after OpenStax Anatomy and Physiology 2e, MedlinePlus (US National Library of Medicine), the EHA recommendations (Iolascon et al. 2021), Y. Kohgo et al. (2008), the Oregon Medical Laser Center, RCSB PDB-⁠101 and S. H. J. M. Dobbelstein (2005). The history of Prussian blue after Alexander Kraft’s papers in the Bulletin for the History of Chemistry (2008, 2009, 2016), the Swiss National Museum and the Art Institute of Chicago; its use as a medicine after the US FDA label of Radiogardase. The bench is this page’s model of a school bench, in the size of a Corning PYREX 9800-16 test tube (16 × 150 mm), with a beech rack, a glazed tile, two 30 mL dropper bottles, a watch glass and a micro spatula; not a copy of any one set. The colours, the drop’s red spreading, the settling and the browning are the page’s own drawing. Material presets and the model kit as on the site’s other model plates (public/looks/CREDITS.md).

## Licence CC BY-NC 4.0

The text, pictures and models on this page are licensed CC BY-NC 4.0: free to use and adapt for teaching and other non-commercial purposes, with credit to world.lapla.si. Third-party material listed above keeps its own licence.

https://creativecommons.org/licenses/by-nc/4.0/deed.en

## Cite this plate

Laplace’s Mini World, Plate 90, “Two Ions of Iron”, https://world.lapla.si/en/iron/, CC BY-NC 4.0, updated 2026-11-03.