The discovery of Prussian blue is credited to a German paint maker called Diesbach in 1706. Diesbach, who worked in Berlin, was attempting to create cochineal red lake in his laboratory.

(“Lake” was once a label for any dye-based pigment; “cochineal” was originally obtained by crushing the bodies of cochineal insects.)

The ingredients he needed were iron (Ⅲ) sulphate and alkaline potash (or better known as potassium hydroxide).

The potash had been contaminated with animal oil and was due to be thrown out. When he mixed both chemicals, instead of the strong red he was expecting, he got one that was very pale. He then attempted to concentrate it, but instead of a darker red he was expecting, he first got a purple, then a deep blue.

He’d accidentally created the first synthetic blue pigment, Prussian blue.

But how was Prussian Blue created chemically?

It was found that the caustic (or alkaline) potash crystallizes with hexacynaoferrate (Ⅱ) in animal oil to form potassium hexacyanoferrate (Ⅱ) salts:

4K+ + [Fe(CN)6]4- = K4[Fe(CN)6]

Addition of iron(Ⅲ) sulphate forms the famous Prussian Blue (PB):

K4[Fe(CN)6] + Fe3+ = KFe[Fe(CN)6] (PB) + 3K+

In today’s context, we also know that hexacynoferrate (Ⅱ) can be converted to PB through a simple controlled oxidative reaction using H2O2 (hydrogen peroxide) or sodium chlorate (Ⅰ) (NaClO).

PB can exist in both “soluble” (actually its colloidal) and insoluble forms (aggregates of colloidal PB)

K+ + Fe3+ + [Fe(CN)6]4−→KFe[Fe(CN)6] (colloidal PB)

4Fe3+ + 3[Fe(CN)6]4−→Fe[FeFe(CN)6]3 (insoluble PB)

Now, that sounds interesting, but some of us have heard of Turnbull’s Blue (TB) too. Is that different from PB?

K3[Fe(CN)6] + Fe2+ = KFe[Fe(CN)6] (TB) + 2K+

Though PB and TB might look different in terms of chemical formulae, do not forget that these are crystalline in nature, with Fe2+ and Fe3+ arranged in an orderly manner (cubic) with CNinterspersed throughout the giant lattice.  Results from X-ray and electron diffraction spectroscopy show that both TB and PB have identical crystalline structures!

So how about the crystalline structure of PB and how does its colour come about? Is it from the d-d electron transition as what we would have expected for many transition compounds which are coloured? Find out in Part 2 of this thread.

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