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.
Prussian blue was discovered accidentally by the chemist and paint maker Heinrich Diesbach
and the alchemist Johann Konrad Dippel
in 1704–05, which is why it has the alternative name of Berlin blue
. Diesbach and Dipple 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.
This Prussian blue pigment is significant since it was the first stable and lightfast blue pigment to be widely used. European painters had previously used a number of pigments such as indigo dye, smalt, and Tyrian purple, which tend to fade, and the extremely expensive ultramarine and lapis lazuli. 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. Cobalt blue has been used extensively by Chinese artists in blue and white porcelains for centuries, and was introduced to Europe in the 18th century.
Despite being one of the oldest known synthetic compounds, the composition of Prussian blue was uncertain until recently. The precise identification of Prussian blue was complicated by three factors: (a.) Prussian blue is extremely insoluble but also tends to form colloids, (b.) traditional syntheses tend to afford impure compositions, and (c.) even pure Prussian blue is structurally complex, defying routine crystallographic analysis.
The chemical formula of Prussian blue is Fe7(CN)18(H2O)x where 14 ≤ x ≤ 16. The determination of the structure and the formula resulted from decades of study using IR spectroscopy, Moessbauer spectroscopy, X-ray crystallography, 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 carbon from nitrogen, the location of these lighter elements is deduced by spectroscopic means as well as distances from the iron atom centers. By growing crystals slowly from 10 mol/L hydrochloric acid, Ludi obtained crystals in which the defects were ordered. These workers concluded that the framework consists of Fe(II)-CN-Fe(III) linkages, with Fe(II)-carbon distances of 1.92 Å (0.192 nanometers) and Fe(III)-nitrogen distances of 2.03 Å (0.203 nanometers). 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 nitrogen atom centers and 1.5 oxygen atom 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 Prussian blue'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
. 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
. 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[Fe(CN)6
The color of Prussian blue
Prussian blue is strongly colored and tends towards black and dark purple when mixed into 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 certain wavelenghts of visible light. In this case, orange
light around 680 nanometers
in wavelength 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.
- It undergoes intervalence charge transfer. Although intervalence charge transfer is well-understood today, Prussian blue 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 Prussian blue's color.
- It undergoes "spin-crossover" behavior. Upon exposure to visible light the Fe(III) centers change from the low spin state to high spin states. This spin transition also changes the magnetic coupling between the iron atoms, making Prussian blue one of the few classes of materials that have a magnetic response to light.
Despite the presence of the cyanide ion, Prussian blue 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.
Prussian blue, 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 blue of Prussian blue.
Prussian Blue in Chelation Therapy in Medicine
Prussian blue's ability to incorporate cations that have one unit of positive charge makes it useful as a chelation sequestering agent for certain heavy-metals ions. Pharmaceutical-grade Prussian blue in particular is used for patients who have ingested thallium or radioactive cesium. According to the International Atomic Energy Agency, an adult male can eat at least 10 grams of Prussian Blue per day without any serious harm. The U.S. Food and Drug Administration (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." in certain poisoning cases. Radiogardase (Prussian blue insoluble capsules) is a commercial product for the removal of cesium-137 from the bloodstream.
As a laboratory test for iron
Prussian blue is a common stain used by pathologists
to detect the presence of iron in biopsy specimens, such as in bone marrow samples.
As a pigment
Prussian blue is the coloring agent used in engineer's blue
and the pigment formed on cyanotypes
- giving them their common name blueprints
. Certain crayons
were once colored with Prussian blue (later relabeled Midnight Blue
Colloids derived from Prussian blue are the basis for laundry bluing
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