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rate-limiting inner-sphere association between M(II) pyrophosphate and
[(P
2
O
7
)
2
Fe
IV
O]
6−
in addition to a rapid one-electron transfer [358].
6.3.1.4 Ferrate(V) and Ferrate(VI).
Salts of sodium, potassium, and rubid-
ium (Na
3
FeO
4
, K
3
FeO
4
, and Rb
3
FeO
4
) have been synthesized by heating alkali
oxides [359, 360]. In later years, black crystals of K
3
FeO
4
were synthesized by
heating well-ground mixtures of K
2
O
2
and KFeO
2
(1.78 : 1.00, Ag-tube) for
several days at 350-470°C [361]. Heating a mixture of K
2
FeO
4
and Na
2
O at
450-600°C produced a mixture of Na
3
FeO
4
and K
3
FeO
4
. The oxidation of a
mixture of RbO
x
or RbOH with Fe
2
O
3
in a stream of O
2
at 600°C produced
the salt with the composition of Rb
3
FeO
4.4-4.7
[356, 362], where subsequent
heating in a stream of N
2
at 350°C for a short time yielded Rb
3
FeO
4
. Com-
paratively, numerous salts of ferrate(VI) have been synthesized. Examples of
these salts include Na
2
FeO
4
, K
2
FeO
4
, BaFeO
4
, and SrFeO
4
. Generally, three
methods have been used to prepare ferrate(VI) compounds: (1) dry thermal
synthesis, (2) wet chemical synthesis, and (3) electrochemical synthesis
[363-366].
In solution, the reduction of ferrate(VI) by radicals generates ferrate(V).
The radicals produced in pulse radiolysis are determined using the following
scheme [367-369]:
H O H
( .
0 55
),
e
−
( .
2 65
),
OH
( .
2 75
),
H O
( .
0 72
),
H
( .
0 45
)
I
2
aq
2
2
2
N O e
+
−
+
H O OH OH N
→
+
−
+
(6.107)
2
aq
2
2
H OH
+
−
→ +
e
−
H O
(6.108)
aq
2
OH/O ROH H O/OH
−
+
→
−
+
•
ROH
(6.109)
2
Fe VI
(
)
+
•
ROH Fe V product
→
(
)
+
k
120
= ×
9 10
9
/M/s
(
14
(6.110)
).
In reaction (6.109), ROH is an alcohol (e.g., methanol, ethanol, isoproponol
or
tert
-butanol) that reacts with an
•
OH radical to form a simple carbon-
centered radical, which reduces ferrate(VI) by a diffusion-controlled rate con-
stant to produce ferrate(V). In a strong alkaline solution, ferrate(V) decays to
a longer-lived transient (
t
1/2
≈ seconds) via a first-order process. A relatively
short lifetime (
k
decay
= 4/s) was obtained in 5 M NaOH [367]. Ferrate(V)
decayed by first-order kinetics in acidic media, while second-order kinetics
followed in a moderately alkaline solution. The spectrum of ferrate(V) in
alkaline solution is shown in Figure 6.24b. The spectrum has a maximum at
380 nm (ε
380 nm
= 1460/M/cm), which undergoes a blue shift with decreasing pH.
Ferrate(V) absorbs very strongly in the uV region (ε
270 nm
≈ 5000/M/cm) (Fig.
6.24B) [368]. Ferrate(VI) also absorbs strongly in the uV region with a shoul-
der between 275 and 320 nm (ε
250 nm
= 7000/M/cm) (Fig. 6.24b) [368] in addition
to an absorbance maximum at 510 nm (ε
510 nm
= 1150/M/cm) (Fig. 6.24b). The
spectra of ferrate(V) and ferrate(VI) are strongly influenced by pH.
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