Prussian blue

Prussian blue
IUPAC name	Iron(II,III) hexacyanoferrate(II,III)
Other names	ferric ferrocyanide, iron(III) ferrocyanide, iron(III) hexacyanoferrate(II), ferric hexacyanoferrate (German: Preußischblau and Berliner Blau, Berlin blue, Parisian blue
Identifiers
CAS number	14038-43-8 Y
RTECS number	V03AB31
Properties
Molecular formula	Fe4[Fe(CN)6]3
Molar mass	859.23 g/mol
Appearance	blue solid
Solubility in water	insoluble
Hazards
MSDS	MSDS prussian blue
EU Index	Not listed
Flash point	Non-flammable
Related compounds
Related compounds	Sodium ferrocyanide Potassium ferrocyanide Potassium ferricyanide
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

"The Great Wave off Kanagawa" by Hokusai, a famous artwork which makes extensive use of Prussian blue.

Prussian blue is a dark blue pigment – one of the first synthetic pigments – which was synthesized for the first time in Berlin around the year 1706.^[1][2] It was named "Preußisch blau" and "Berlinisch Blau" in 1709 by its first trader.^[3] Another name for the color Prussian blue is Berlin blue or, in painting Parisian blue.

The key colored substance in Prussian blue pigment is an insoluble inorganic compound composed of iron and cyanide ions, with water. It has the idealized formula Fe₇(CN)₁₈ with 14 to 16 H₂O. The pigment may also contain variable amounts of other ions. The color of the pigment depends partly on the size of precipitated particles formed when it precipitates from addition of iron(III) to soluble ferrocyanide. With several other names (see table to right), this dark blue solid is commonly abbreviated "PB."^[4] Because it is easily synthesized in impure form, it is the object of instructional experiments. It has a complicated chemistry, however, that has led to extensive speculation on its structure.

Prussian blue is a common pigment, used in paints, and it is the traditional "blue" in blueprints. It has been used as an antidote for certain kinds of heavy metal poisoning.

History

Prussian blue was probably synthesized for the first time by the paint maker Diesbach in Berlin around the year 1706.^[5] Most historical sources do not mention a first name of Diesbach. Only Berger refers to him as Johann Jacob Diesbach.^[6] The pigment was an important topic in the letters exchanged between Johann Leonhard Frisch and the president of the Royal Academy of Sciences, Gottfried Wilhelm Leibniz, between 1708 and 1716.^[7] It is first mentioned in a letter written by Frisch to Leibniz, from March 31, 1708. Not later than 1708, Frisch began to promote and sell the pigment across Europe. By August 1709, the pigment had been termed "Preussisch blau"; by November 1709, the German name "Berlinisch Blau" had been used for the first time by Frisch. Frisch himself is the author of the first known publication of Prussian blue in the paper Notitia Coerulei Berolinensis nuper inventi in 1710, as can be deduced from his letters. Diesbach had been working for Frisch since about 1701.

In 1731, Georg Ernst Stahl published a story dealing with the incidence of the first synthesis of Prussian blue.^[8] The story involves not only Diesbach but also Johann Konrad Dippel. Diesbach was attempting to create a red lake pigment from cochineal but obtained the blue instead as a result of the contaminated potash he was using. He borrowed the potash from Dippel, who had used it to produce his "animal oil". No other known historical source mentions Dippel in this context. It is therefore difficult to judge the reliability of this story today. In 1724, the recipe was finally published by Woodward.^[9]

To date, the painting "Entombment of Christ", dated 1709 by Pieter van der Werff (Picture Gallery, Sanssouci, Potsdam) is the oldest known painting where Prussian blue has been used. Around 1710, painters at the Prussian court were already using the pigment. At around the same time, Prussian blue arrived in Paris, where Antoine Watteau and later his successors Nicolas Lancret and Jean-Baptiste Pater used it in their paintings.^[10]

This Prussian blue pigment is significant since it was the first stable and relatively lightfast blue pigment to be widely used following the loss of knowledge regarding the synthesis of Egyptian Blue. 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 made from 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.

In 1752 the French chemist Pierre J. Macquer made the important step of showing the Prussian blue could be reduced to a salt of iron, and a new acid, previously unknown, which could be used to reconstitute the dye. The new acid, hydrogen cyanide, first isolated from Prussian blue in pure form and characterized about 1783 by the Swedish chemist Carl Wilhelm Scheele, was eventually given the name Blausäure (literally "Blue acid") because of its derivation from Prussian blue, and in English became known popularly as Prussic acid. Prussian blue would also give the name to the cyanide family of compounds, which are named from the Greek word for "blue," because they were first isolated from Prussian blue.

Composition

Prussian Blue
— Color coordinates —
Hex triplet		#003153
RGBB	(r, g, b)	(0, 49, 83)
HSV	(h, s, v)	(205°, 100%, 43%)
Source		[Unsourced]
B: Normalized to [0–255] (byte)

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:

1. Prussian blue is extremely insoluble but also tends to form colloids;
2. Traditional syntheses tend to afford impure compositions; and
3. 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.^[11] 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 without the time to achieve full equilibrium between solid and liquid.

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.

"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].^[4]

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 wavelengths of visible light. In this case, orange-red light around 680 nanometers in wavelength is absorbed, and the transmitted light appears blue as a result.

Other properties

Prussian blue and its analogs have 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.

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.

Production

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.

Uses

Prussian blue in medicine

Prussian blue's ability to incorporate cations that have one unit of positive charge makes it useful as a 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.^[12] Radiogardase (Prussian blue insoluble capsules) is a commercial product for the removal of cesium-137 from the bloodstream.^[13]

As a laboratory histopathology stain for iron

Prussian blue is a common histopathology stain used by pathologists to detect the presence of iron in biopsy specimens, such as in bone marrow samples. The original stain formula, known historically (1867) as "Perls' Prussian blue" after its inventor, German pathologist Max Perls (1843-1881), used separate solutions of potassium ferrocyanide and acid to stain tissue (these are now used combined, just before staining). Iron deposits in tissue then form the purple Prussian blue dye in place, and are visualized as blue or purple deposits.^[14] The formula is also known as Perls Prussian blue and (incorrectly) as Perl's Prussian blue.

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). It is also a popular pigment in paints.

Clothes laundering

Colloids derived from Prussian blue are the basis for laundry bluing.

By machinists and toolmakers

Prussian blue in oil paint is the traditional material used for spotting metal surfaces such as surface plates and bearings for hand scraping. A thin layer of non-drying paste is applied to a reference surface and transfers to the high spots of the workpiece. The toolmaker then scrapes, stones, or otherwise removes the high spots. Prussian blue is preferable because it will not abrade the extremely precise reference surfaces as many ground pigments may.

References

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.

See also

External links

• The FDA's page on prussian blue
• The CDC's page on prussian blue
• National Pollutant Inventory - Cyanide compounds fact sheet
• Heyltex Corporation distributors of Radiogardase (Prussian blue insoluble capsules)