Formation of Iron Blue

We are only concerned, in this connection, with how Iron Blue arises from hydrogen cyanide and iron compounds in building materials. In building materials, the iron is generally present in trivalent form (Fe3+), in the form of rust . 

330 Originally, this term was used only in organic chemistry for chainlike connected, sometimes also branched attachments of equal segments. 

331 Dispersion (lat. dispersere, distribute) are distribution of two different phases within each other. They are called colloids (gr. gluelike) if the particles are between 10 s and 10 7 m small. Such a mixture in liquids scatters the light (Tyndall effect), is thus not clear. But due to electrostatic repulsion (equally charged particles), colloids do not tend to coagulate and precipitate. 

Suspension (lat. to float) are coarsely dispersed system with particle sizes bigger than I (T m. 

New York 1979, pp. 765-771 J.A. Sistino, in Peter A. Lewis (ed.), Pigment Handbook, Vol. 1, Wiley and Sons, New York 1974, pp. 401-407 A.F. Holleman, N. Wibcrg, Lehrhuch der Anorganischen Chemie, de Gruyter, Berlin 1001985, p. 1143 

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For the formation of Iron Blue, therefore, a part of this iron must be reduced to bivalent form (Fe2+). The subsequent combination of these different iron ions with CN to Iron Blue occurs spontaneously and completely.336 The most probable mechanism337 is one in which the cyanide ion itself acts as a reducing agent. The starting point in so doing is an Fe3+ ion, largely surrounded (complexed) by CN ions [Fe(CN)4 6](1 3) A slightly alkaline environment is favorable to the final reduction of the iron(III)-ion to iron(II).338... 

Even in an alkaline environment, it must be expected that rust, in the presence of perceptible cyanide concentrations, will be quite slowly transformed into iron(III)-cyanide and finally into iron(II)-cyanide.344 The last step required for the formation of Iron Blue, however, the combination of iron(II)-cyanide with iron(III), will not occur due to the lack of dissolved iron(III)-ions. In a strongly alkaline environment, an increasing concentration of iron(II)-cyanide, which is chemically stable, can slowly accumulate. It remains in a stand-by position, waiting for the pH value to drop. 

A decisively higher water content of the solid material and the considerably better absorption and solubility properties of hydrogen cyanide in water are the reasons for the tendency of solid materials to accumulate more cyanides with lower temperatures. An increase in the reactivity of iron oxide (rust) in the solid body with relation to hydrogen cyanide with a higher water content of the solid material at lower temperatures must be anticipated, as well as with a general increase in the reactivity of all agents. A cooler, and thus moister, solid material is therefore better suited to the formation of Iron Blue than a warm, dry body.351... 

The individual parameters and their influence on the formation of Iron Blue are summarized in the following table ... 

The rust content (Fc203) of Portland cement, of particular interest to us here, the cement most frequently used for concrete and cement mortars, is usually between 1 and 5%.396 The sand added to the mortar can also exhibit a high iron content (up to 4%). As mentioned in chapter 6.5. 3., a large surface area at the solid-liquid phase limit (iron oxide-cyanide solution) is favorable to the formation of Iron Blue. This is extraordinarily large in cement and concrete mortars (microscopic inte-... 

The first step in the formation of Iron Blue in masonry is the absorption of gaseous hydrogen cyanide. A cool (10°C ) wall in a cellar with atmospheric humidity near the saturation point, due to its higher water content (by a factor of at least 10), has an increased ability (by a factor of 10) to absorb hydrogen cyanide compared to warm walls in a heated room built above ground with lower atmospheric humidity (20°C, 50% rel ). 

The second step in the formation of Iron Blue is the ionic split (disassociation) of the hydrogen cyanide, that is, its conversion into simple cyanide.405 This procedure requires an alkaline environment,... 

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. 

The only case of the formation of Iron Blue through fumigation with hydrogen cyanide, which is fairly well documented, is the case of... 

The damage to the building in Bavaria is a typical exception here, since the unheated church, notorious for its humid walls, had been plastered with cement mortar, which is known to remain alkaline for many months, only a few weeks before. These are exactly the conditions which in my view were favorable to the formation of Iron Blue. With increasing setting of the cement plaster over the course of months, the pH value of the masonry in the church finally dropped, so that the final reaction led to the formation of Iron Blue, which is stable for long periods of time. This final reaction of the adsorbed cyanide into Iron Blue was only completed after approximately two years. The prior stage of this reaction, the formation of considerably paler iron cyanides, could already have been completed or well progressed prior to this.520... 

The following is a list of individual properties which exert an influence upon the formation of Iron Blue. 

Properties that were advantageous to the formation of Iron Blue in... 

Yet there are also physical-chemical boundary conditions which can influence the analytical results. It is, for example, not inconceivable that, for whatever reason remaining unknown until the present time, the masonry of the alleged gas chambers was not, or more slightly, inclined to the formation of Iron Blue, or that possible residues were destroyed for unknown reasons. 

But the pipes fastened along the entire length of the eastern wall of Chamber III are skirted by intensely blue-pigmented plaster,44 as if they had acted, in a certain sense, as the catalyst for the formation of iron blue (ferric-ferrocyanide). In Chamber I, on the other hand, no traces of blue pigmentation are to be seen. 

Another well-studied electron transfer reaction is the oxidation of aqueous benzidine in the presence of various clays (2, 40, 43, 50, 51). An electron from the colorless benzidine molecule is abstracted by the clay with formation of a blue monovalent radical cation. Upon drying of the blue clay-benzidine system, a yellow color is produced. There is disagreement in the literature with respect to the chemical identity of the yellow product (2, 40, 52) however, in the case of hectorite, the yellow product has been suggested to be the protonated form of the radical cation (divalent radical cation) (2, 52). There is also disagreement about whether the electron-accepting sites of the clay are ferric iron at the planar surfaces, aluminum ions at the edges, or exchangeable cations <2, I). 

If Prussian blue can be used to detect cyanide, which is one of its ingredients, why not iron, another one There is no reason why it can t. When the right reagent containing cyanide is added to a sample containing ferric ions, Prussian blue forms readily. This is the basis of a laboratory test for detecting iron deposits in the human liver. The formation of Prussian blue upon addition of a reagent to the biopsy sample indicates iron overload, a potentially serious condition. 

Thus, apart from OFT (alkaline medium), there are no other ligands to be considered competing in the formation or dissolution of Iron Blue in the cases here under consideration. 

Certain wave lengths of ultraviolet radiation may set free CN from hexacyanoferrate(II) and -(III), the preliminary stages of Iron Blue. As far as hexacyanoferrate(III) is concerned, this leads to the formation of... 

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