Prussian blue isn’t just a pigment. It is also a medicine of great importance.

Hokusai’s The Great Wave is famed for its use of Prussian blue:

You can make Prussian blue in the lab. I used to call it “ferric ferrocyanide” but the International Union of Pure and Applied Chemistry lists the stuff as iron (II, III) hexacyanoferrate (II, III).

Iron has two oxidation states, +2 and +3, and they used to be called “ferro-” and “ferric-” but the modern convention is to use Roman numerals as in Fe(II) and Fe(III). The above would be rendered to the formula: Fe₄^III[Fe^II(CN)₆]₃.

It is a very cool and very simple chemical reaction and the product is strikingly colored. I used to mix solutions of potassium ferrocyanide and ferric chloride in class. It’s a mess—the precipitate is super-fine and it sticks to everything. The individual crystals are so small they get suspended and form a colloid. You have to dry it out to get a usable powder. I once tried to make paint (mixing the stuff with linseed oil) but that was a mess of gigantic proportions. The blue stains were impervious to cleaning! Ferric ferrocyanide is not toxic as the cyanide is tightly bound to the iron.

What does this have to do with Thallium? I mentioned that Prussian blue was a medicine. It comes in 500 mg capsules. Thallium, it turns out, forms lots of compounds and all are toxic. Thallium also has radioactive isotopes. Prussian blue is used to treat heavy metal poisoning. It seems the iron in the compound will switch places with certain other metals. Prussian blue is also an antidote for poisoning by radioactive cesium.

We used to kill ants and rats with thallium sulfate. The US outlawed the practice in 1972. Although toxic, thallium is not a carcinogen. Thallium compounds have many uses in the electronics, pharmaceutical, and glass industries. World demand is about 10 million tonnes annually, and it is produced mostly as a by-product of copper and zinc mining.

Thallium sits between Mercury (Hg, #80) and Lead (Pb, #82) on the periodic table.