environmentally friendly process employing an iron electrode (Fe(0)), salts of Fe
(II) or Fe(III), and/or iron oxides. In the case of a melt, a suitable temperature for
ferrate(VI) synthesis is up to 200
C. Generally, in aqueous solutions, temperatures
range from 20 to 70
C (Mácová et al. 2009). Using electrochemical methods, highpurity ferrates are easily prepared. The drawback of the method lies in the overlap of
potentials of the oxidation of Fe(III) to Fe(VI) and oxygen evolution. The yield of the
synthesis depends on temperature, composition of the iron precursors, and the pH of
the alkaline solution. As an example, the electrochemical synthesis of ferrate
(VI) was studied in a eutectic NaOH-KOH molten salt and in a highly alkaline
mixed NaOH-KOH aqueous solution (Híveš et al. 2008). Moreover, electrochemical
DC (direct current) and AC (alternating current) techniques were found to be suitable
for interfacial and metal dissolution studies in both aqueous and molten systems
(Híveš et al. 2016). Transpassive iron dissolution in alkaline solutions is discussed
less frequently. It has been found that in an alkaline environment, Fe is dissolved as
ferrous(II), ferric(III) and, finally, ferrate(VI) species depending on the experimental
conditions (Híveš et al. 2008, 2014, 2016; Hrnčiariková et al. 2013; Kerekeš et al.
2014; Kubiňáková et al. 2015a, b; Mácová et al. 2009; Rai et al. 2018; Sun et al.
2016; Yu and Licht 2008).
Thermal processes were applied to synthesize sodium ferrate(IV) (Na 4 FeO 4 ),
potassium ferrate(V) (K 3 FeO 4 ), or potassium ferrate(VI) (K 2 FeO 4 ). Sodium ferrate
(IV) can be prepared by a solid-state reaction of Fe 2 O 3 and Na 2 O 2 powders mixed in
the theoretical molar ratio of Fe:Na ¼ 1:2 (nevertheless an excess of Na 2 O 2 in the
mixture is recommended to achieve complete oxidation of Fe 2 O 3 within the sample)
according to the reaction:
Fe 2 O 3 þ 3Na 2 O 2 ! 2Na 2 FeO 4 þ Na 2 O
ð8:3Þ
The homogeneous mixture is heated at 400
C for 1 h. When using a stream of
oxygen during the synthesis, the temperature of 370
C is sufficient. After annealing,
the crucible containing the sample must be rapidly cooled down. The purity of Fe
(IV) is more than 90%. It is highly recommended to carry out the synthesis in a glove
box since both the sodium peroxide precursor and the sodium ferrate(IV) product are
highly hygroscopic. Even Mössbauer spectroscopy analysis, which is typically used
for verification of the purity of the as-prepared samples, is recommended to be
conducted directly in the glove box (Fig. 8.2).
Potassium ferrate(V) (K 3 FeO 4 ) can be synthesized by a two-step solid state
reaction starting from homogenous mixture of Fe(III) hydroxide and KNO 3
(Machalová Šišková et al. 2016) with almost equal weights (e.g., several grams).
In the first reaction step, the mixture was put into a corundum annealing crucible and
the crucible was covered with a lid. The closed crucible was then put into the furnace
and annealed at 950
C for 30 min. After it was annealed, the closed crucible was
removed from the furnace and put into a desiccator where it was left to cool down
with KFeO 2 as a resulting product. Then the as-synthesized KFeO 2 was added to
KNO 3 powder. Again, the mixture was homogenized in the grinding mortar. The
homogenized mixture was then put into a corundum annealing crucible and was
180
L. Machala et al.
(II) or Fe(III), and/or iron oxides. In the case of a melt, a suitable temperature for
ferrate(VI) synthesis is up to 200
C. Generally, in aqueous solutions, temperatures
range from 20 to 70
C (Mácová et al. 2009). Using electrochemical methods, highpurity ferrates are easily prepared. The drawback of the method lies in the overlap of
potentials of the oxidation of Fe(III) to Fe(VI) and oxygen evolution. The yield of the
synthesis depends on temperature, composition of the iron precursors, and the pH of
the alkaline solution. As an example, the electrochemical synthesis of ferrate
(VI) was studied in a eutectic NaOH-KOH molten salt and in a highly alkaline
mixed NaOH-KOH aqueous solution (Híveš et al. 2008). Moreover, electrochemical
DC (direct current) and AC (alternating current) techniques were found to be suitable
for interfacial and metal dissolution studies in both aqueous and molten systems
(Híveš et al. 2016). Transpassive iron dissolution in alkaline solutions is discussed
less frequently. It has been found that in an alkaline environment, Fe is dissolved as
ferrous(II), ferric(III) and, finally, ferrate(VI) species depending on the experimental
conditions (Híveš et al. 2008, 2014, 2016; Hrnčiariková et al. 2013; Kerekeš et al.
2014; Kubiňáková et al. 2015a, b; Mácová et al. 2009; Rai et al. 2018; Sun et al.
2016; Yu and Licht 2008).
Thermal processes were applied to synthesize sodium ferrate(IV) (Na 4 FeO 4 ),
potassium ferrate(V) (K 3 FeO 4 ), or potassium ferrate(VI) (K 2 FeO 4 ). Sodium ferrate
(IV) can be prepared by a solid-state reaction of Fe 2 O 3 and Na 2 O 2 powders mixed in
the theoretical molar ratio of Fe:Na ¼ 1:2 (nevertheless an excess of Na 2 O 2 in the
mixture is recommended to achieve complete oxidation of Fe 2 O 3 within the sample)
according to the reaction:
Fe 2 O 3 þ 3Na 2 O 2 ! 2Na 2 FeO 4 þ Na 2 O
ð8:3Þ
The homogeneous mixture is heated at 400
C for 1 h. When using a stream of
oxygen during the synthesis, the temperature of 370
C is sufficient. After annealing,
the crucible containing the sample must be rapidly cooled down. The purity of Fe
(IV) is more than 90%. It is highly recommended to carry out the synthesis in a glove
box since both the sodium peroxide precursor and the sodium ferrate(IV) product are
highly hygroscopic. Even Mössbauer spectroscopy analysis, which is typically used
for verification of the purity of the as-prepared samples, is recommended to be
conducted directly in the glove box (Fig. 8.2).
Potassium ferrate(V) (K 3 FeO 4 ) can be synthesized by a two-step solid state
reaction starting from homogenous mixture of Fe(III) hydroxide and KNO 3
(Machalová Šišková et al. 2016) with almost equal weights (e.g., several grams).
In the first reaction step, the mixture was put into a corundum annealing crucible and
the crucible was covered with a lid. The closed crucible was then put into the furnace
and annealed at 950
C for 30 min. After it was annealed, the closed crucible was
removed from the furnace and put into a desiccator where it was left to cool down
with KFeO 2 as a resulting product. Then the as-synthesized KFeO 2 was added to
KNO 3 powder. Again, the mixture was homogenized in the grinding mortar. The
homogenized mixture was then put into a corundum annealing crucible and was
180
L. Machala et al.