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Prussian blue (German: Preußischblau or Berliner Blau, in English Berlin blue) is a dark blue pigment used in paints and formerly in blueprints. Prussian blue was discovered by accident by painter Heinrich Diesbach in Berlin in 1704-5, which is why it is also known as Berlin blue. (Diesbach was attempting to create a paint with a red hue.) It has several different chemical names, these being iron(III) ferrocyanide, ferric ferrocyanide, iron(III) hexacyanoferrate(II), and ferric hexacyanoferrate. Commonly and conveniently it is simply called "PB."
Prussian Blue was the first modern dye to be synthesized and was the result of an accident. The chemist and paint maker Heinrich Diesbach had intended to prepare a red lake pigment. Due to a contaminated source of potash, he obtained the blue instead.
The pigment is significant as the first stable and lightfast blue to be widely used. European painters 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 prior.
Despite being one of the oldest known synthetic compounds, the composition of 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 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 of 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 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.
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].
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.
Prussian Blue has been extensively studied by inorganic chemists and solid-state physicists because of its unusual properties.
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. However, treatment with acids 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.
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. In particular, pharmaceutical-grade PB 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." Radiogardase® (Prussian blue insoluble capsules) is a commercial product for the removal of cesium-137 from the bloodstream.
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