Prussian blue

Berlin Blue is a lightfast, deep blue, inorganic pigment that exists as an air- and water-stable solid. It is an organometallic compound in which iron ions are present in the oxidation states +2 and +3 and are linked via the cyanide anion ([C≡N^]-). The essential structural element of Berlin blue is the sequence Fe(II)-[C≡N]-Fe(III) in a three-dimensional polymeric framework.

Berlin blue occurs in a form designated as "insoluble" and a form designated as "soluble", the structure of the two being the same. The general molecular formula is Mnm+[Fe(III)Fe(II)(CN)₆]₃ - 15 H2O. It is a face-centered cubic lattice in which alkali or ammonium ions occupy the places in the octahedral gaps of the lattice (M = Na, K, NH4, n = 3, m = 1) in the case of the soluble Berlin blue and serve to compensate the charge of the complex; in the case of the insoluble Berlin blue, iron(III) ions take over this function (M = Fe, n = 1, m = 3). Here, the soluble variants are present in aqueous solution in the form of colloidal solutions, it is not a real solubility under dissociation of the compound.

The Berlin paint manufacturer Johann Jacob Diesbach first produced Berlin blue around 1706. It immediately gained commercial importance as a pigment for oil painting and the dyeing of fabrics. With the publication of the recipe in 1724, several companies began producing Berliner Blau under many other names.

Berlin blue is considered the first synthetic coordination compound. It owes its deep blue color to metal-metal charge transfer transitions that absorb radiation in the yellow-red range and reflect blue light as a complementary color.

The names prussic acid and cyanide (from ancient Greek κυανός kyanos "dark blue") are derived from the color of Berlin blue. The name prussiate, in which one cyanoligand in the complex is replaced by another ligand, such as nitroprusside, is derived from the name Prussian blue. Berlin blue is the namesake for the so-called Berlin blue analogues, a class of microporous inorganic solids with a wide range of catalytic, electronic, optical, and magnetic properties. Even more than three hundred years after its first synthesis, Berlin blue is still a research object about which many scientific articles are published every year.

Due to its simple production from a solution of iron(III) salt and yellow blood lye salt, it is mainly used as an inexpensive colorant. Berlin blue is practically non-toxic and is used as an antidote for poisoning with radioactive cesium or thallium. The therapy takes advantage of the ion exchange properties and the high affinity of the compound for certain metal cations. It is on the World Health Organization's list of essential medicines needed in a health care system.

Nomenclature

Berlin blue was offered under many different names. The designations refer to the names of the inventors or manufacturers, the places of manufacture, the color shades, the applications or the chemical components and processes. The varieties may differ in color cast. All names refer to blue pigments based on the Fe(II)/Fe(III) cyano complex and differ only slightly in composition. The type and amount of alkali metal or ammonium ions used influences the production of certain color shades. Historically, the pigment was marketed as blue salt. In the Colour Index, Berliner Blau is listed as C.I. Pigment Blue 27 by colour and by structure as C. I. 77510 for potassium-doped iron blue and C.I. 77520 for ammonium-sodium iron blue, respectively.

Diesbach blue is named after the actual inventor. Turnbull's blue was developed in 1828 by John Turnbull Jr. and distributed through the Scottish company Turnbull & Ramsay in Glasgow. Turnbull's blue is a blue pigment formed from red blood liquor salt with excess iron(II) ions. It is obtained by reacting ferrous salts with potassium hexacyanidoferrate(III) in aqueous solution. Initially, it was assumed that the dark blue precipitate formed had a different composition from the Berlin blue obtained by reacting ferric salts with potassium hexacyanidoferrate(II) (yellow blood leach salt). However, by means of EPR and Mößbauer spectroscopy, it was found that the reaction products are largely identical, since the following equilibrium exists:

Milori blue refers to cooked varieties of the pigment, which have a somewhat warmer reddish hue and were first produced by the Milori de France company. According to Diesbach, this company had obtained a blue pigment by a different process, which is somewhat weaker in color strength compared to Prussian blue. The name Milori blue has survived to the present day. Vossenblau was named after L. Vossen & Co G.m.b.H. near Düsseldorf, which was the exclusive distributor from 1905.

French blue or Paris blue refers to the company headquarters of A. Milori. Prussian blue, also Prussian blue, as well as Zwickau blue also refer to production locations.

The name bronze blue refers to the bronzing red tint that is evident in various binders. In particular, bronze blue refers to the reddish sheen of the unground, black-blue lumps.

Chinese Blue or China Blue takes its name from decorative porcelain. This production variant of the pigment produces the purest and most brilliant shades with a green tint. It gives the best full tone and the highest opacity, however it has the hardest texture and the highest oil requirement. Saxon blue refers to the color of the uniforms of the Saxon army, which were dyed with Berlin blue. Toner blue or ink blue got this name because of its use to tint the reddish (brown) tint of carbon black.

The names iron blue, iron cyanogen blue, iron hexacyanidoferrate, steel blue, iron cyanogen cyanide, ferrocyanogen blue and ferriferrocyanide blue as well as steel blue are derived from the structure or composition of the pigment. Potash blue refers to the use of potash in its manufacture. Until World War I, the cation of the complex salt was predominantly potassium. When the price of potash rose sharply in the early 20th century, the ammonium salt was produced with equally good properties. Iron blue also refers to an ancient pigment from the mineral vivianite.

Luisenblau, Modeblau, Wasserblau are product names for modified colorants in textile dyeing and may have originated as names for fashion dyes. The reddest pigment is Milori blue, the greenest variant is Chinese blue.

In the Frenchlanguage area the designations Bleu de prusse or Bleu de Milori are common, in the English language area the designations iron blue, toning blue or Prussian blue.

Occurrence

Berlin blue is considered to be the first modern pigment that does not occur naturally in this form. One of the main components, potassium hexacyanidoferrate(II), on the other hand, occurs as the rare mineral kafehydrocyanite.