Cyanide Iron Blue Pigments

Cyanide iron blue pigments (formerly known as prussian blue, milori blue or tumbull s blue) have the composition  

They are precipitated as a white dough when ferrous salts react with complex iron(II)-cyanides. This is then converted to cyanide iron blue by oxidation with chlorates or dichromates. 

Mixtures of cyanide iron blue pigments with chrome yellow and zinc yellow are known as chrome green and zinc green respectively and are used in paints and printing inks. 

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Cyanide Iron Blues. Cyanide iron blue, also known as Pmssian blue, is one of the oldest industrially produced, inorganic pigments. Chemically, cyanide iron blues are based on the [Fe " Fe (CN) ] anion. The charge is balanced by sodium, potassium, or ammonium cations. Modem... 

Two blue pigments can be prepared in transparent form cyanide iron blue and cobalt aluminum blue. These pigments are used in achieving a blue shade of the metal effect pigments in metallic paints. Transparent cyanide iron blue is prepared by a precipitation reaction similar to the one used for the preparation of the opaque pigment, but considerably lower concentrations of solutions are used. It is produced by Degussa (Germany), Manox (U.K), and Dainichiseika (Japan). 

Iron blue pigments are produced by the precipitation of complex iron(II) cyanides by iron(II) salts in aqueous solution. The product is a whitish precipitate of iron(II) hexacyanoferrate(II) M 2Fe1I[Fe1I(CN)6] or M11Fe11[Fe11(CN)6], (Berlin white), which is aged and then oxidized to the blue pigment [3.180]. 

Blue vapor over the victims. Hydrogen cyanide is a colorless liquid and/or an invisible gas. The name Blausaure (blue acid) is due to the reaction of hydrogen cyanide with iron, forming the iron-blue pigment. There cannot, therefore, have been any blue vapor. 

Iron blue pigment is the international standard name (ISO 2495) for ferrous cyanide blue pigments of various compositions which are also known as Berlin Blue, Turnbull s Blue, Prussian Blue, Vossen Blue , Milori Blue, Parisian Blue, French Blue, China Blue, Bronze Blue, Steel Blue, Ink Blue, etc. 

Alkali or alkaline-earth salts of both complexes are soluble in water (except for Ba2[Fe(CN)g]) but are insoluble in alcohol. The salts of hexakiscyanoferrate(4—) are yellow and those of hexakiscyanoferrate(3—) are mby red. A large variety of complexes arise when one or more cations of the alkah or alkaline-earth salts is replaced by a complex cation, a representative metal, or a transition metal. Many salts have commercial appHcations, although the majority of industrial production of iron cyanide complexes is of iron blues such as Pmssian Blue, used as pigments (see Pigments, inorganic). Many transition-metal salts of [Fe(CN)g] have characteristic colors. Addition of [Fe(CN)g] to an unknown metal salt solution has been used as a quaUtative test for those transition metals. 

Iron Blue, Cl Pigment Blue 27, which has been known by various names over the years, perhaps the best known being Prassian Blue, is ferric ammonium ferricyanide, FeNH Fe(CN)g(xH20. The hrst step in its preparation involves the precipitation of complex iron(ii) cyanides, e.g. potassium hexacyanoferrates(ii) with iron(i) salts, e.g. the sulfate or chloride, in an aqueous solution in the presence of ammonium... 

The iron cyanides have long been known for their extraordinary stability, one of them having achieved particular fame as one of the most commonly used blue pigments during the last three centuries ... 

Iron Blue is an extremely acid-resistant, but base-decomposing pigment.357 Hydrogen cyanide is only released by warm, diluted sulfuric acid, while hydrochloric acid, by contrast, has no effect.358 In a clearly alkaline environment, i.e., in the presence of high concentrations of OH ions, these displace the cyanide ion from the iron(III)-ion. Fe(OH)3 is then precipitated ( rust sludge ), and the Iron Blue is destroyed.359... 

Iron Blue is considered one of the least soluble cyanide compounds, which is the precondition for its widely-varied application as a pigment.362 The literature flatly refers to Iron Blue as insoluble .363... 

As shown in chapter 6.5., the environment is only alkaline in the non-carbonated masonry. It was also established that an alkaline environment even supports the accumulation of cyanide and certain other steps in the reaction towards the formation of Iron Blue. If one assumes, as an extreme case, a complete conversion of all iron compounds contained in the masonry into pigment (1 to 2% iron content), the values found by Leuchter are even rather low. Whether the walls of the disinfestation wing were painted blue, i.e., whether a high cyanide content can only be found on the upper, i.e., the paint layer of the wall, will be discussed at a later time. 

They furthermore assume, together with J. Bailer,52 54 that the blue pigmentation of the disinfestation chamber walls could be due to a coat of paint. To exclude this pigmentation from the analysis, they decided to apply a method which is insensitive to iron cyanides. 

Blue pigmentation of the samples was not to be expected, since even if all the bound cyanide were present in the form of Iron Blue, only 0.005-0.01% of the total material would consist of the blue pigment, which would cause hardly any perceptible coloration to the naked eye. An accumulation of cyanides on the surface of the sample, finally, could not occur due to the absence of water in diffusion. In addition, the dry storage of the samples probably blocked the conversion process. 

Color Image 2 Exterior southwest wall of the Zyklon B disinfestation wing of BW 5b in the Auschwitz-Birkenau camp a deep blue discoloration, caused by cyanide compounds which penetrated the entire wall over the decades since WWII and formed the blue pigment with iron compounds contained in cement and bricks. This pigment is unaffected by 55 years of weathering. 

The relevant literature consistently describes iron blue as an extremely stable pigment. It is insoluble in water,104 resistant to acid rain105 and also surprisingly resistant to sunlight.106 When exposed to weathering, other compounds of hydrogen cyanide will even convert preferentially into iron blue. 

When not needed as a temporary morgue, Chamber III could still be used for its original purpose -hydrogen cyanide gas disinfestation. That it was so utilized, is proven by the intense blue pigmentation, i. e., the presence of high concentrations of iron blue in all the walls of this room. 

The test is done by boiling a portion of the alkaline solution from the solution fusion with iron(II) sulfate and then acidifying. Sodium cyanide reacts with iron(II) sulfate to produce ferrocyanide, which combines with iron(III) salts, inevitably formed by air oxidation in the alkaline solution, to give insoluble Prussian Blue, NaFe[Fe(CN)J. Iron(II) and iron(III) hydroxide precipitate along with the blue pigment but dissolve on acidification. 

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