Prussian blue
From Wikipedia, the free encyclopedia
- For other uses, see Prussian blue (disambiguation).
| Prussian Blue | ||
|---|---|---|
| — Color coordinates — | ||
| Hex triplet | #003153 | |
| RGBa | (r, g, b) | (0, 49, 83) |
| HSV | (h, s, v) | (205°, 100%, 43%) |
| a: Normalized to [0–255] (byte) | ||
Prussian blue (German: Preußisch Blau, Berliner Blau) is a dark blue pigment used in paints and formerly in blueprints. Prussian blue was discovered by accident [1] by the painter Diesbach in Berlin and therefore it is also called Berlin blue. He originally 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.[2]
Contents |
[edit] Composition
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. So first one must decide on a definition of PB.
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 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 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.
[edit] 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.
[edit] "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
[edit] 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.
[edit] 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 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. However, treatment with acids can liberate hydrogen cyanide which is extremely toxic as discussed in the article on cyanide.
[edit] 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.
[edit] 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 Mr. Death: 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 is used for patients who have ingested radioactive caesium or thallium (also non-radioactive thallium). According to the IAEA 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 cesium, radioactive thallium, or non-radioactive thallium." - [1]
[edit] References
- ^ http://painting.about.com/cs/colourtheory/a/prussianblue.htm
- ^ *Dunbar, K. R. and Heintz, R. A., "Chemistry of Transition Metal Cyanide Compounds: Modern Perspectives", Progress in Inorganic Chemistry, 1997, 45, 283-391.
[edit] 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
[edit] See also
[edit] External links
- The FDA's page on prussian blue
- The CDC's page on prussian blue
- National Pollutant Inventory - Cyanide compounds fact sheet
