Prussian blue

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. It is used as a pigment.

Properties

Physical

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.

Chemical

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.

Availability

It is still commonly used as a pigment and can be bought in art stores.

Preparation

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:

4FeCl₃ + 3K₄[Fe(CN)₆] → Fe₄[Fe(CN)₆]₃↓ + 12KCl

The "soluble" kind of Prussian blue can be prepared this way:

FeCl₃ + K₄[Fe(CN)₆] → KFe[Fe(CN)₆]₃↓ + 3KCl

The dark blue variety called Turnbull's blue is prepared this way:

3FeCl₂ + FeCl₃ 3K₃[Fe(CN)₆] → Fe₄[Fe(CN)₆]₃↓ + 9KCl

These reactions can be used to detect the presence of iron. Use potassium ferrocyanate to detect trivalent iron, and ferriicyanate to detect divalent iron.

Projects

• Make cyanogen gas

Handling

Safety

All varieties of Prussian blue are safe under normal conditions.

Storage

Prussian blue can be kept in any jar.

Disposal

Prussian blue can be discarded with common garbage.