Difference between revisions of "Prussian blue"

Revision as of 21:13, 17 August 2015

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 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.

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 can be prepared this way:

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

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.

Projects

• Make cyanogen gas
• Make hydrogen cyanide
• Thallium (and other heavy metal) poisoning antidote

Handling

Safety

All varieties of Prussian blue are safe under normal conditions.

Storage

Prussian blue can be kept in any container, preferably a glass one, such as a jar.

Disposal

Prussian blue can be discarded with common garbage.

References

Relevant Sciencemadness threads