Prussian blue

Prussian blue (Preußisch Blau, Berliner Blau) is a dark blue pigment used in paints and formerly in blueprints. It has several different chemical names, these being iron(III) ferrocyanide, ferric ferrocyanide, iron(III) hexacyanoferrate(II), and ferric hexacyanoferrate. Most commonly and conveniently it is simply called PB.

What exactly is Prussian Blue?

For one of the oldest known synthetic compounds, it is curious that until recently the composition of PB was uncertain. 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. So first one must decide on a definition of PB. If it is not simply whatever "blue stuff" one gets by adding a solution of a Fe(III) salt to a solution of ^*^4-, then what is it?

As summarized in the classic review by Dunbar and Heintz, 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 Mn₃and Co₃[Co(CN)₆₂ (i.e., Co₅(CN)₁₂). Since X-ray diffraction cannot distinguish C from N, the locations of these lighter elements is deduced by spectroscopic means as well as distances from the Fe centers. By growing crystals slowly from 10M HCl, Ludi obtained crystals wherein the defects were ordered. These workers concluded that the framework consists on Fe(II)-CN-Fe(III) linkages, with Fe(II)-C distances of 1.92 Å and Fe(III)-N distances of 2.03 Å. 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 for 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 compositons obtained by rapid precipitation. One obtains PB by the addition of Fe(III) salts to a solution of 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 act like solutions, for example they pass through fine filters. According to Dunbar and Heintz, these "soluble" forms tend toward compositions with the approximate formula KFe₂Fe(CN)₆

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 for the transfer of electrons from Fe(II) to Fe(III). Many such mixed valence compounds absorb visible light. 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 color from blue to white in response to a voltage. This is caused by partial oxidation of the Fe(II) to Fe(III), eliminating the intervalence charge transfer that causes PB's blue color.

It undergoes spin-crossover behavior. Upon exposure to visible light the Fe atoms transition from low spin to high spin. This spin transition also changes the magnetic coupling between the Fe atoms, making P.B. 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, however, with acids can liberate hydrogen cyanide, which is extremely toxic as discussed in the article on cyanide.

Production

PB 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 colour changes immediately to the characteristic hue of PB.

This reaction is the first step to obtain ink. Dissolving it with water, and then filtering it, results in a blue residuum. The addition of a dilution of oxalic acid previously heated results in a useful blue ink.

Uses

Colloids derived from PB are the basis for laundry bluing.

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

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

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

PB's ability to incorporate +1 cations makes it useful as a sequestering agent for certain heavy metals ions. In particular, pharmaceutical-grade PB (not artists' pigment!) is used for patients who have ingested radioactive caesium or thallium (also non-radioactive thallium). According to the IAEA an adult man can eat 10 grams per day without serious harm. It is also occasionally used in cosmetic products.

It is also used as a generic treatment of certain types of radiation poisoning by stopping the cycle of radiation minerals from staying in the liver causing long term health effects.

References

Ludi, A., "Prussian Blue, an Inorganic Evergreen", Journal of Chemical Education 1981, 58, 1013
Dunbar, K. R. and Heintz, R. A., "Chemistry of Transition Metal Cyanide Compounds: Modern Perspectives", Progress in Inorganic Chemistry, 1997, 45, 283-391.
Sharpe, A. G., "The Chemistry of Cyano Complexes of the Transition Metals," Academic Press: London, 1976

External links

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