173
7.1.1 Prussian Blue
Prussian blue, named iron hexacyanoferrate, or ferric ferrocyanide and Milori blue,
is a mixed valence (Fe
II
and Fe
III
) polynuclear blue compound of Fe that was accidentally synthesized in 1704 by a Berlin artist (Ware 2008). The painter, named
Diesbach, was preparing a red pigment. Unfortunately, one of the precursor salt was
not available in his atelier, and he borrowed some of the alchemist Dippel, and this
was found to be the source of the cyanides as the chemicals were contaminated. Its
discovery made the pigment suddenly used in painting, flags, stamps, wallpaper,
and tea colorant and can be considered the first synthetic coordination compound.
Today, Prussian Blue is still used as a pigment, but it also has other applications.
7.1.2 Structure of Prussian Blue
The determination of the Prussian blue crystalline structure was made in the 1936
by Keggin and Miles through X-ray diffraction (Keggin et al 1936). The analysis of
the X-ray pattern revealed that Prussian blue consists of a 3D cubic network, where
alternating iron species (Fe
II
and Fe
III
) are octahedrally coordinated by carbon and
nitrogen, respectively, with the cyanide ligand forming bridges. Overall, a threedimensional network of repeating -Fe
II
-C-N-Fe
III
- units was seen. Keggin and Miles
also found that each crystallographic unit (unit cell) composes of four cubes with a
lattice parameter of about 10 Å. Compared to similar salts, Buser et al. (1977)
observed experimentally an unusual low density, a scenario related to the peculiar
crystallographic network where the presence of cavities and vacancies favor interesting physico-chemical characteristics.
A precipitate of Prussian blue can be obtained by mixing an hexacyanoferrate
(II) salt and a Fe(III) salt, such as ferric chloride FeCl 3 . Surprisingly, the same product is formed while a solution of a hexacyanoferrate(III) salt is mixed to a solution
of a Fe(II) salt, as evidenced by
57
Fe Mossbauer (Ito et al. 1968) and neutron diffraction experiments (Herren et al. 1980). The mixed valence nature of Prussian blue
was reported to be the cause of those experimental findings, where a metal-metal
electron transfer is occurring. Being the electronic transition from Fe
II
to Fe
III
(t 2g to
e g ) in the range of an electronvolt, a strong absorption in the visible spectrum
(680–730 nm) is observed (Robin 1962), therefore explaining the intense blue color
which in turn is used as pigment.
As reported in Fig. 7.1, Prussian blue structurally exists in two forms, namely,
insoluble and soluble, reported, respectively, as Fe 4 [Fe(CN) 6 ] 3 and KFeFe(CN) 6 .
Both are insoluble in water and only partially in moderated alkali solution, and both
form colloids but with a different size, thus the name above reported. In the soluble
Prussian blue structure, alkali metals are accommodated into the channels available
in the 3D network, forming very regular cubes. Unlike, the insoluble Prussian blue
is characterized by [Fe(CN) 6 ]
4−
vacancies, which are replaced by water molecules
7 Metal Hexacyanoferrate Absorbents for Heavy Metal Removal
7.1.1 Prussian Blue
Prussian blue, named iron hexacyanoferrate, or ferric ferrocyanide and Milori blue,
is a mixed valence (Fe
II
and Fe
III
) polynuclear blue compound of Fe that was accidentally synthesized in 1704 by a Berlin artist (Ware 2008). The painter, named
Diesbach, was preparing a red pigment. Unfortunately, one of the precursor salt was
not available in his atelier, and he borrowed some of the alchemist Dippel, and this
was found to be the source of the cyanides as the chemicals were contaminated. Its
discovery made the pigment suddenly used in painting, flags, stamps, wallpaper,
and tea colorant and can be considered the first synthetic coordination compound.
Today, Prussian Blue is still used as a pigment, but it also has other applications.
7.1.2 Structure of Prussian Blue
The determination of the Prussian blue crystalline structure was made in the 1936
by Keggin and Miles through X-ray diffraction (Keggin et al 1936). The analysis of
the X-ray pattern revealed that Prussian blue consists of a 3D cubic network, where
alternating iron species (Fe
II
and Fe
III
) are octahedrally coordinated by carbon and
nitrogen, respectively, with the cyanide ligand forming bridges. Overall, a threedimensional network of repeating -Fe
II
-C-N-Fe
III
- units was seen. Keggin and Miles
also found that each crystallographic unit (unit cell) composes of four cubes with a
lattice parameter of about 10 Å. Compared to similar salts, Buser et al. (1977)
observed experimentally an unusual low density, a scenario related to the peculiar
crystallographic network where the presence of cavities and vacancies favor interesting physico-chemical characteristics.
A precipitate of Prussian blue can be obtained by mixing an hexacyanoferrate
(II) salt and a Fe(III) salt, such as ferric chloride FeCl 3 . Surprisingly, the same product is formed while a solution of a hexacyanoferrate(III) salt is mixed to a solution
of a Fe(II) salt, as evidenced by
57
Fe Mossbauer (Ito et al. 1968) and neutron diffraction experiments (Herren et al. 1980). The mixed valence nature of Prussian blue
was reported to be the cause of those experimental findings, where a metal-metal
electron transfer is occurring. Being the electronic transition from Fe
II
to Fe
III
(t 2g to
e g ) in the range of an electronvolt, a strong absorption in the visible spectrum
(680–730 nm) is observed (Robin 1962), therefore explaining the intense blue color
which in turn is used as pigment.
As reported in Fig. 7.1, Prussian blue structurally exists in two forms, namely,
insoluble and soluble, reported, respectively, as Fe 4 [Fe(CN) 6 ] 3 and KFeFe(CN) 6 .
Both are insoluble in water and only partially in moderated alkali solution, and both
form colloids but with a different size, thus the name above reported. In the soluble
Prussian blue structure, alkali metals are accommodated into the channels available
in the 3D network, forming very regular cubes. Unlike, the insoluble Prussian blue
is characterized by [Fe(CN) 6 ]
4−
vacancies, which are replaced by water molecules
7 Metal Hexacyanoferrate Absorbents for Heavy Metal Removal