Simon Hibble (Lecturer in Inorganic Chemistry) writes:
These pages show the first reports in England on the synthesis from dried cattle blood of Prussian blue, an important pigment with an interesting history that has played a part in my own story. The Latin text and English commentary appeared in 1724 in the Philosophical Transactions of the Royal Society and were republished here shortly afterwards. The recipe for Prussian blue had been a trade secret and it would be many years before its composition, its chemistry, and the origin of the colour were understood.
Prussian blue was first prepared by Diesbach in Berlin around 1706, hence its other name Berliner Blau. It was an early synthetic pigment and could be used to replace ultramarine (which was then expensively prepared from lapis lazuli). Many artists would disagree with this claim, including my father, to whose eyes the two blues are quite different: a view shared by King George V when he chose ultramarine for the 2½d Silver Jubilee stamp in 1935. In error, a few copies in Prussian blue were issued to the public. As a boy, I always hoped to find an example of the Prussian blue stamp, which is currently worth about £10,000 rather than a few pence.
Years later, as an undergraduate chemist, I learnt that Prussian blue is coloured because it is a type 2 mixed valence compound in the Robin-Day classification, containing iron in two different oxidation states, FeII and FeIII. It is a metal cyanide, and a simplistic formulation would be [FeIII]4[FeII(CN)6]3. (Hydrogen cyanide is known as prussic acid because it can be prepared from Prussian blue.) In reality, Prussian blue contains water, and a more realistic formula is Fe7(CN)18.14-16H2O. Now we know the composition of Prussian blue, we can understand why the recipe for its preparation works: the blood supplies the iron, as well as the carbon and the nitrogen that form the cyanide ions.
Prussian blue is a disordered crystalline solid, in which although there is an underlying periodicity giving rise to Bragg diffraction, there are differences in local structure. There is octahedral coordination around the iron atoms, but different numbers of CN and H2O around individual iron atoms, and these variations show no long-range order. A major part of my research over the past 20 years has been the structure of disordered materials, in particular disordered crystalline materials, and especially metal cyanides. Prussian blue has a further claim to fame in that many related materials contract rather than expand when heated. Negative thermal expansion (NTE) materials have become very trendy in recent years and I have found myself by accident in a rather topical area.
Sources and further reading:
- Catalogue record on SOLO
- Woodward, John. Praeparatio caerulei Prussiaci …, Philosophical Transactions 33 15-17 (1724)
- Brown, John. Observations and experiments upon the foregoing preparation, Philosophical Transactions 33 17-24 (1724)