Prussian blue is a complex salt of iron and hexacyanoferric acids, or a double salt of iron and potassium thereof. This compound comes in several forms, though they all are similar. While the formula C18Fe7N18 is often encountered, the compound doesn't have a definite formula or IUPAC name, because it is in fact several closely related compounds usually encountered together. It is used as a pigment.
Prussian blue is a powder that can be light or dark blue (the dark variety is called Turnbull's blue). It is insoluble in water, but it is possible to prepare a colloidal solution of Prussian blue. The double salt with potassium produces colloidal solutions easily, hence it is known as "soluble Prussian blue", though it isn't truly soluble.
Prussian blue is relatively inert under normal conditions. However, if heated to 200 degrees Celsius, it will decompose and liberate cyanogen gas (dangerous!). Stronger heating causes the cyanides themselves to decompose, nitrogen is liberated, leaving iron carbide and carbon in the test tube.
In strongly basic conditions, Prussian blue decomposes into ferrocyanide and iron (III) hydroxide, and the suspension loses its characteristic blue color, turning brown.
It is still commonly used as a pigment and can be bought in art stores, though in some places it's getting hard to find.
We will list several reactions that can be used to prepare Prussian blue of different types. They are notable in that they are used to detect ions of iron, not to specifically prepare the blue pigment.
"Standard", light blue Prussian blue can be precipitated with the following reaction:
- 4 FeCl3 + 3 K4[Fe(CN)6] → Fe4[Fe(CN)6]3↓ + 12 KCl
The "soluble" kind of Prussian blue can be prepared this way:
- FeCl3 + K4[Fe(CN)6] → KFe[Fe(CN)6]3↓ + 3 KCl
The dark blue variety called Turnbull's blue can be prepared this way:
- 3 FeCl2 + FeCl3 + 3 K3[Fe(CN)6] → Fe4[Fe(CN)6]3↓ + 9 KCl
Turnbull's blue only differs from normal Prussian blue in the parameters of its crystallization, resulting in a slightly different color. In all other aspects it is identical to standard Prussian blue. The presence of FeCl3 isn't mandatory, you will still get Turnbull's blue without it. However, in solutions of Fe(II) salts Fe (III) is usually present anyway.
These reactions can be used to detect the presence of iron. Use potassium ferrocyanide to detect trivalent iron, and ferricyanide to detect divalent iron.
- Pigment in painting
- Make cyanogen gas (VERY DANGEROUS)
- Make hydrogen cyanide (VERY DANGEROUS)
- Thallium (and other heavy metal) poisoning antidote
All varieties of Prussian blue are safe under normal conditions. Contact with strong acids however should be avoided. Prussian blue may stain clothes and skin.
Prussian blue can be kept in any container, preferably a glass one, such as a jar.
Prussian blue is practically non-toxic, and can be discarded with common garbage. If you really want, you can neutralize it with bleach, though this is not necessary.