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What is prussian blue ?

OK you good people out there, I have written a lot about Prussian blue. You might want to see what it is. Now sadly despite the marvels of modern science we do not have a clear picture of the atoms in Prussian blue.

One of the forms of Prussian blue has been crystallised, this cubic solid is very crystalline but very disordered, not good ! It has the empirical formula {Fe2(CN)6(H2O)6}n and the waters and cyanides are arranged in a random manner, which makes things complex. This form is not a true Prussian blue as it has no potassium ions but it is a good model for what Prussian blue is.

Disordered solids, they can make life very hard. A nicer solid exists, it is a 1D coordination polymer with the formula of [Me4N][(Fe(CN)6Mn(H2O)4].H2O.

This solid has linear chains of manganese and iron atoms with waters on the manganese atoms and cyanide groups bonding to the iron through carbon, some of the nitrogen lone pairs bond to the manganese thus giving us a coordination polymer.

Overall the chains have a negative charge, for every iron atom the chain has one additional atomic charge unit. Now as we need to maintain electro neutrality we need one cation for each iron. In the case of Prussian blue it is potassium while in our model it is a tetramethyl ammonium cation.

What the Prussian blue does is to act as an inorganic version of an ion exchange resin, while it is in the gut it exchanges caesium (or thallium(I)) cations for the potassium cations which it gives up.

Here is a picture of one of the repeat units of the Prussian blue; you can see the four waters on the manganese and one extra which just fills up space.

Asymmetric unit of Prussian Blue mimic with iron and manganese in it

There units link together to form long chains which make up the structure of one of the forms of Prussian blue.

1D coordination chain in the Prussian Blue mimic


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