monitored by UV-VIS spectrometry. The key experimental tool for characterization
of solid samples of ferrates is Mössbauer spectroscopy on isotope
57 Fe. The method
is selective to the Fe element so it is a unique tool for identification/characterization
of Fe-containing phases. Importantly, Mössbauer spectroscopy is sensitive to
valence state of iron through isomer shift value as one of the hyperfine parameter.
Particularly, Fe(V) and Fe(VI) atoms show negative isomer shift values, which are
very different from common Fe(II) or Fe(III) states (see Fig. 8.3). Ferrate samples
often contain Fe(III) admixtures, which are amorphous and/or nanocrystalline. Low
temperature and external magnetic field Mössbauer spectroscopy provide very
detailed characterization of these Fe(III) phases. Even though Mössbauer spectroscopy is designed for characterization of solid samples, it is possible to analyze frozen
solutions as well. Relative areas of the spectral components correspond to relative
amounts of corresponding Fe-containing phases. It is worth mentioning that these
relative amounts are expressed in atomic percent of Fe atoms. Therefore, the values
must be recalculated when we need mass ratios of the phases (moreover, chemical
formulas of the phases are needed).
8.4 Stability of Solid Ferrates at High Temperatures
and in a Humid Air
The issue of thermal stability of ferrates is crucial for the thermal synthesis as well as
for possible applications at elevated temperatures. Generally, the instability of
ferrates at high temperatures significantly decreases yield and purity of ferrates
prepared the thermal processes. The focus will now be shifted to the mostly used
ferrate(VI), K 2 FeO 4 . A thorough study conducted by Machala et al. (2007) showed
that potassium ferrate(VI) starts to decompose above 190
C towards potassium
Fig. 8.3 A typical
Mössbauer spectrum of Fe
(VI), Fe(V), and Fe(III)
species formed together
during a transformation of
ferrate(V) in ethanol. The
sample was treated by rapidfreeze technique and
spectrum measured at 100 K
182
L. Machala et al.
of solid samples of ferrates is Mössbauer spectroscopy on isotope
57 Fe. The method
is selective to the Fe element so it is a unique tool for identification/characterization
of Fe-containing phases. Importantly, Mössbauer spectroscopy is sensitive to
valence state of iron through isomer shift value as one of the hyperfine parameter.
Particularly, Fe(V) and Fe(VI) atoms show negative isomer shift values, which are
very different from common Fe(II) or Fe(III) states (see Fig. 8.3). Ferrate samples
often contain Fe(III) admixtures, which are amorphous and/or nanocrystalline. Low
temperature and external magnetic field Mössbauer spectroscopy provide very
detailed characterization of these Fe(III) phases. Even though Mössbauer spectroscopy is designed for characterization of solid samples, it is possible to analyze frozen
solutions as well. Relative areas of the spectral components correspond to relative
amounts of corresponding Fe-containing phases. It is worth mentioning that these
relative amounts are expressed in atomic percent of Fe atoms. Therefore, the values
must be recalculated when we need mass ratios of the phases (moreover, chemical
formulas of the phases are needed).
8.4 Stability of Solid Ferrates at High Temperatures
and in a Humid Air
The issue of thermal stability of ferrates is crucial for the thermal synthesis as well as
for possible applications at elevated temperatures. Generally, the instability of
ferrates at high temperatures significantly decreases yield and purity of ferrates
prepared the thermal processes. The focus will now be shifted to the mostly used
ferrate(VI), K 2 FeO 4 . A thorough study conducted by Machala et al. (2007) showed
that potassium ferrate(VI) starts to decompose above 190
C towards potassium
Fig. 8.3 A typical
Mössbauer spectrum of Fe
(VI), Fe(V), and Fe(III)
species formed together
during a transformation of
ferrate(V) in ethanol. The
sample was treated by rapidfreeze technique and
spectrum measured at 100 K
182
L. Machala et al.