Prussian Blue

Prussian Blue is an Old World blue. It predates the modern Phthalo blue with similar coloring properties. In watercolor, the Winsor Blue (Winsor & Newton Colors) is a modern chemical version of the old, early industrial European color byproduct.

From: Chemistry World (http://www.rsc.org/chemistryworld/2013/04/prussian-blue-podcast)

“Prussian blue is a complicated compound and for many years, despite being able to manufacture the pigment, no-one was entirely sure what it was – apart from a blue crystalline solid that could make them money. In fact, the complex structure means that the exact composition of the pigment can vary depending on inclusions in the crystal lattice. But in general, the compound is a cubic arrangement of iron hexacyanoferrate: two different oxidations states of iron – three-plus and two-plus – in a ratio of 4:3.

The hexacyanoferrate ions are octahedral arrangements of iron two-plus ions surrounded by six cyanide groups. These octahedra are then linked by iron three-plus ions at some of the corners and water molecules sit in the spaces left over. So the iron three-plus ions are also octahedrally coordinated, but by a mix of the nitrogen atoms from the cyanide groups, and oxygen atoms from the water.

That might be difficult to visualise without me drawing it out in front of you, but the important thing to understand is that iron two-plus sits in different surroundings to the iron three-plus ions. This difference in the electronic environment means that the molecular orbitals involving the two ions are different. So when light shines on crystals of Prussian blue, it absorbs orange light and transfers an electron from iron two-plus to iron three-plus.

This is known as interatomic charge transfer – the electron jumps from one molecular orbital to one belonging to a different atom and the energy required to make that jump is equal to the wavelength of light absorbed. The unabsorbed light is reflected and that’s why we see Prussian blue as blue.

You can still paint with Prussian blue today, although other synthetic blue pigments and paints have since been developed, but the pigment has found other uses. For example, Prussian blue is also used to treat people who have been internally contaminated with radioactive caesium or highly poisonous thallium. Patients swallow capsules of the pigment and in the intestines the compound incorporates the metal cations into its lattice. By absorbing the dangerous metals, Prussian blue stops them being reabsorbed by the body and instead they are passed out in the normal way, but much quicker; for example, the biological half-life of caesium can be reduced from 110 days to about 30.

Prussian blue: from Picasso’s blue period to saving the day from nasty poisoning, this compound has seen it all. ”

Personally, I love the original Prussian blue. It streaks grayer than phthlalo blues.  Only by adding Payne’s Gray to winsor blue would I get that same repetition of a prussion blue color event on a page.