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Prussian blue
A sample of Prussian blue
IUPAC name Ferric hexacyanoferrate
Other names ferric ferrocyanide, iron(III) ferrocyanide, ferric ferrocyanide, iron(III) hexacyanoferrate(II), ferric hexacyanoferrate (German: Preußischblau and Berliner Blau, Berlin blue
CAS number 14038-43-8
RTECS number V03AB31
Molecular formula Fe7(CN)18(H2O)x
Molar mass 859.23 g/mol
Appearance blue solid
Solubility in water insoluble
Related compounds
Other anions Potassium ferrocyanide
Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)

Infobox references

Prussian blue is a very dark blue, colorfast, non-toxic pigment – one of the first synthetic dyes – which was discovered accidentally in Berlin in 1704. Its name comes from the fact that it was first extensively used to dye the dark blue uniforms of the Prussian army.[1]

It is an inorganic compound with the idealized formula Fe7(CN)18, containing also variable amounts of water and other ions. With several other names (see table to right), this dark blue solid is commonly abbreviated "PB."[2] PB is a common pigment, the object of instructional experiments, and an antidote for certain kinds of heavy metal poisoning. Because it is easily synthesized in impure form, it also has a complicated chemistry that has led to extensive speculation on its structure. It is used in paints and is the "blue" in blueprints.


Prussian blue was discovered accidentally[3] by the chemist and paint maker Heinrich Diesbach and the alchemist Johann Konrad Dippel in Berlin in 1704–05 (which is why it has the alternative name of Berlin blue). The pair were attempting to create a red lake pigment but obtained the blue instead as a result of the potash they were using having come from a contaminated source.[4]

The pigment is significant as the first stable and lightfast blue to be widely used. European painters had previously used a number of pigments such as indigo and smalt, which tended to fade, and the extremely expensive ultramarine. Japanese painters and woodblock print artists likewise did not have access to a long-lasting blue pigment until they began to import Prussian blue from Europe, though cobalt blue had been used extensively by Chinese artists in blue and white porcelains for centuries.[citation needed]


Prussian Blue
About these coordinatesAbout these coordinates
— Color coordinates —
Hex triplet #003153
RGBB (r, g, b) (0, 49, 83)
HSV (h, s, v) (205°, 100%, 43%)
Source BF2S Color Guide
B: Normalized to [0–255] (byte)

Despite being one of the oldest known synthetic compounds the composition of Prussian blue (PB) was uncertain until recently. The precise identification of PB was complicated by three factors: (i) PB is extremely insoluble but also tends to form colloids, (ii) traditional syntheses tend to afford impure compositions, and (iii) even pure PB is structurally complex, defying routine crystallographic analysis.

The chemical formula of PB is Fe7(CN)18(H2O)x where 14 ≤ x ≤ 16. The assignment of the structure and the formula resulted from decades of study using IR spectroscopy, Moessbauer spectroscopy, and X-ray and neutron crystallography. Parallel studies were conducted on related materials such as Mn3[Co(CN)6]2 and Co3[Co(CN)6]2 (i.e., Co5(CN)12). Since X-ray diffraction cannot distinguish C from N, the locations of these lighter elements are deduced by spectroscopic means as well as distances from the Fe centers. By growing crystals slowly from 10 mol/L HCl, Ludi obtained crystals wherein the defects were ordered. These workers concluded that the framework consists of Fe(II)-CN-Fe(III) linkages, with Fe(II)-C distances of 1.92 Å (192 pm) and Fe(III)-N distances of 2.03 Å (203 pm). The Fe(II) centers, which are low spin, are surrounded by six carbon ligands. The Fe(III) centers, which are high spin, are surrounded on average by 4.5 N centers and 1.5 O centers, the latter from water. Again, the composition is notoriously variable due to the presence of lattice defects, allowing it to be hydrated to various degrees as water molecules are incorporated into the structure to occupy four cation vacancies. The variability of PB's composition is attributable to its low solubility, which leads to its rapid precipitation vs. growth of a single phase.

Turnbull's blue[]

The story of "Turnbull's Blue" (TB) illustrates the complications and pitfalls associated with the characterization of a composition obtained by rapid precipitation. One obtains PB by the addition of Fe(III) salts to a solution of [Fe(CN)6]4−. TB supposedly arises by the related reaction where the valences are switched on the iron precursors, i.e. the addition of a Fe(II) salt to a solution of [Fe(CN)6]3-. One obtains an intensely blue colored material, whose hue was claimed to differ from that of PB. It is now appreciated that TB and PB are the same because of the rapidity of electron exchange through a Fe-CN-Fe linkage. The differences in the colors for TB and PB reflect subtle differences in the method of precipitation, which strongly affects particle size and impurity content.

"Soluble" Prussian blue[]

PB is insoluble, but it tends to form such small crystallites that colloids are common. These colloids behave like solutions, for example they pass through fine filters. According to Dunbar and Heintz, these "soluble" forms tend toward compositions with the approximate formula KFe[Fe(CN)6].[2]

The color of PB[]

PB is strongly colored and tends towards black and dark purple when mixed with other oil paints. The exact hue depends on the method of preparation, which dictates the particle size. The intense blue color of Prussian blue is associated with the energy of the transfer of electrons from Fe(II) to Fe(III). Many such mixed valence compounds absorb visible light. Orange-red light at 680 nm is absorbed, and the transmitted light appears blue as a result.

Other properties[]

Prussian Blue has been extensively studied by inorganic chemists and solid-state physicists because of its unusual properties.

  • It undergoes intervalence charge transfer. Although intervalence charge transfer is well-understood today, PB was the subject of intense study when the phenomenon was discovered.
  • It is electrochromic—changing from blue to colorless upon reduction. This change is caused by reduction of the Fe(III) to Fe(II) eliminating the intervalence charge transfer that causes PB's blue color.
  • It undergoes spin-crossover behavior. Upon exposure to visible light the Fe(III) centers change from low spin to high spin. This spin transition also changes the magnetic coupling between the Fe atoms, making PB one of the few known classes of material that has a magnetic response to light.[citation needed]

Despite the presence of the cyanide ion, PB is not especially toxic because the cyanide groups are tightly bound. Other cyanometalates are similarly stable with low toxicity. Treatment with acids, however, can liberate hydrogen cyanide which is extremely toxic as discussed in the article on cyanide.


PB, such as that in inks, is prepared by adding a solution containing iron(III) chloride to a solution of potassium ferrocyanide. During the course of the addition the solution thickens visibly and the color changes immediately to the characteristic hue of PB.



PB is the pigment formed on cyanotypes, giving them their name blueprint.


The formation of PB is a "wet" chemical test for cyanide. This test was a key component of the Errol Morris film Mr. Death: The Rise and Fall of Fred A. Leuchter, Jr..


PB is the coloring agent used in Engineer's blue.

Clothes laundering[]

Colloids derived from PB are the basis for laundry bluing.


PB is a common stain used by pathologists to detect the presence of iron in biopsy specimens, such as on bone marrow. PB's ability to incorporate +1 cations makes it useful as a sequestering agent for certain heavy metals ions. Pharmaceutical-grade PB in particular is used for patients who have ingested radioactive caesium or thallium (also non-radioactive thallium). According to the International Atomic Energy Agency an adult male can eat 10 grams of Prussian Blue per day without serious harm. It is also occasionally used in cosmetic products. The US FDA has determined that the "500 mg Prussian blue capsules, when manufactured under the conditions of an approved New Drug Application (NDA), can be found safe and effective for the treatment of known or suspected internal contamination with radioactive caesium, radioactive thallium, or non-radioactive thallium."[5] Radiogardase (Prussian blue insoluble capsules) is a commercial product for the removal of caesium-137 from the bloodstream.[6]


PB is the pigment formed in the production of cyanotype prints.


  1. http://www.sewanee.edu/chem/Chem&Art/Detail_Pages/Pigments/Prussian_Blue Website of The University of the South, Sewanee
  2. 2,0 2,1 *Dunbar, K. R. and Heintz, R. A., "Chemistry of Transition Metal Cyanide Compounds: Modern Perspectives", Progress in Inorganic Chemistry, 1997, 45, 283-391.
  3. Prussian Blue Pigment - The Accidental Creation of Prussian Blue
  4. http://www.miniatures.de/int/preussisch-blau.html Military Miniatures Magazine: Parisian or Prussian Blue, Historical Paint for Miniatures
  5. Questions and Answers on Prussian Blue
  6. Heyltex Corporation - Toxicology

Further reading[]

  • Ludi, A., "Prussian Blue, an Inorganic Evergreen", Journal of Chemical Education 1981, 58, 1013.
  • Sharpe, A. G., "The Chemistry of Cyano Complexes of the Transition Metals," Academic Press: London, 1976

See also[]

External links[]

de:Berliner Blau es:Azul de Prusia fr:Bleu de Prusse ko:프러시안 블루 it:Blu di prussia he:כחול פרוסי la:Caeruleus Berolinus lt:Karaliaučiaus mėlis nl:Pruisisch blauw ja:紺青 no:Preussisk blå pl:Błękit pruski (pigment) pt:Azul-da-prússia ro:Albastru de Berlin ru:Берлинская лазурь sv:Berlinerblått zh:普鲁士蓝