Prussian blue « Netica Solution

Prussian blue

August 25th, 2008

For other uses, see Prussian blue (disambiguation).

Prussian blue

IUPAC name
Ferric hexacyanoferrate

Other names
ferric ferrocyanide, iron(III) ferrocyanide, iron(III) hexacyanoferrate(II), ferric hexacyanoferrate (German: Preußischblau and Berliner Blau, Berlin blue

Identifiers

CAS number

RTECS number
V03AB31

Properties

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. Another name for the color Prussian blue is Berlin blue.

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.” 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.

Contents

1 History
2 Composition
3 Other properties
4 Production
5 Uses
6 References
7 Further reading
8 See also
9 External links

History

Prussian blue was discovered accidentally

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.

Composition

Prussian Blue

— Color coordinates —

Hex triplet
#003153

B
(r, g, b)
(0, 49, 83)

HSV
(h, s, v)
(205°, 100%, 43%)

Source
BF2S Color Guide

B: Normalized to (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 Mn32 (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 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. “Soluble” forms of PB tend toward compositions with the approximate formula KFe

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.

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.

Production

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.

Uses

Chelation therapy

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 thallium or radioactive caesium. According to the International Atomic Energy Agency an adult male can eat 10 grams of Prussian Blue per day without serious harm. 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 therapy.”

As a laboratory test for iron

PB is a common stain used by pathologists to detect the presence of iron in biopsy specimens, such as on bone marrow.

As a pigment

PB is the coloring agent used in Engineer’s blue and the pigment formed on cyanotypes, giving them their name blueprint. Crayons were once colored with Prussian Blue (later relabeled Midnight Blue).

Clothes laundering

Colloids derived from PB are the basis for laundry bluing.

References

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

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