Iron Hexacyanoferrate: The Blueprint for Fun

It's Christmas break and while I don't really stop doing research I feel a little justified in taking a little time for myself. Today I decided to make blueprints and cyanotypes. What, you may ask, do blueprints and cyanotypes have in common? The answer is Prussian Blue (PB). We'll get into the chemistry of PB later, but first I'll show you what I made.


Several months ago I found this image on the web and I decided that it would be my first attempt. I chose it for two reasons:

1. It's only black and white, no grey scale, so I'm not going to need to worry about over developing it.
2. I thought that the steampunk image would be appropriate for a process that was developed in the 19th century.

To make this image I took a solution of iron ammonium citrate and potassium ferricyanide and coated a piece of paper with it. I then let it dry in the dark. The coated area looked yellow green. I then printed the image on a transparency and taped the coated paper to the back of the transparency. I then taped the paper and transparency to the window in my office. After 12 minutes I had noticed that the yellow green color had darkened to a more brown color. I then took the transparency off the window, removed the paper and ran it under deionized water to wash off all of the unreacted chemicals. It is at this point where the blue color really developed. Encouraged by the positive results I decided to make a second harder image.
The second image I made was a little more complex for a few reasons:

1. It is a grey scale image and so I needed to worry about contrast in the image.
2. I needed to make a negative of the picture I wanted to reproduce.
3. The sun went behind a cloud and so my previous time calculations wouldn't work.

As you can see this image is much more pale than the first. But over all I consider it a resounding success.
I said earlier in the post that I used a solution of iron ammonium citrate and potassium ferricyanide to make the light sensitive solution. The actual solutions are 0.95 M Fe(NH₃)C₆H₈O₇ and 0.30 M K₃[Fe(CN))₆]. If you count the electrons in the compounds you will notice that the iron in both the Fe(NH₃)C₆H₈O₇ and K₃[Fe(CN))₆] is in a +3 oxidation state (this can be indicated by superscripted roman numerals K₃[Fe^III(CN))₆]). Once mixed, the light sensitive Fe(NH₃)C₆H₈O₇ absorbs a photon causing an electron to be transfered to the iron atom reducing it to Fe⁺². This ferrous (Fe^II) ion then reacts with the hexacyanoferrate (Fe^III) to form the insoluble Prussian Blue:

K⁺(aq) + Fe⁺²(aq) + >[Fe^III(CN)₆]^-3(aq) ---> KFe^III[Fe^II(CN)₆](s)

If you look at the oxidation states you can see that there was an electron transfer between the hexacyanoferrate anion ion and the ferrous cation. Interestingly for years people thought that they were making the KFe^II[Fe^III(CN)₆] salt which they called Turnbull's blue but which recently has been shown to spontaneously transfer an electron and form Prussian Blue. Prussian Blue comes in two closely related forms that are termed "soluble" and "insoluble". Soluble Prussian Blue has the form of MFe[Fe(CN)₆] where M is a monovalent cation, while insoluble Prussian Blue has the form Fe₄[Fe(CN)₆]₃. In both cases there is an Fe^III cation bound to an [Fe^II(CN))₆]^-4 anion. These names are somewhat misleading because both materials are very insoluble in water however the soluble Prussian Blue can be suspended in water making it look like it has dissolved while it is still just a suspension.
The crystal structures for Prussian Blue is shown below where the Fe^II is shown in red and the Fe^III in blue. The potassium ions sit in four of the eight octants arranged tetrahedrally.
For more information see Day's "Molecules into materials case studies in materials chemistry -- mixed valency, magnetism and superconductivity" page 190, or "A Blueprint for Conserving Cyanotypes" by Mike Ware.

Calvin Chemistry

My reactions didn't go as I had wanted one day and so to blow off a little steam I doctored this Calvin and Hobbes strip originally published on 03/06/91. My appologies to Mr. Bill Waterson.