Chemistry Reference
In-Depth Information
Ta b le 6.1
Values for
a
n
a
calculated from the difference between peak and
half-wave potential for the oxidation of sodium dithionite at a
platinum-disc electrode
v
(mV s
-
1
)
E
p
(mV)
E
p/2
(mV)
a
n
a
50
-
76
-
175
0.48
100
-
56
-
159
0.46
200
-
38
-
147
0.44
400
-
26
-
141
0.42
750
-
4
-
127
0.40
1000
8
-
113
0.40
Ta b le 6.2
Values for
a
n
a
calculated from the shift of peak potential as a
function of polarisation rate for the oxidation of sodium dithionite at a
platinum-disc electrode
v
i
(mV s
-
1
)
v
j
(mV s
-
1
)
E
P,j
(mV)
E
P,i
(mV)
a
n
a
50
100
-
76
-
56
0.45
50
200
-
76
-
38
0.48
100
200
-
56
-
38
0.50
100
750
-
56
-
4
0.50
100
1000
-
56
8
0.47
200
750
-
38
-
4
0.50
200
1000
-
38
8
0.46
the determination of this order directly from the experimental currents by
variation of the sodium dithionite concentration.
In Fig. 6.9, current-potential curves are shown for different sodium
dithionite concentrations at a constant pH of 12.5 and a rotation rate of the
electrode of
N
= 6.67 Hz. In the third potential region, it is expected and
found that the limiting-current is proportional to the concentration accord-
ing to the Levich equation (1.15). When the reaction rate of the oxidation
of dithionite is controlled by electron transfer only in the first potential
region from -0.5 to -0.2 V vs. Ag|AgCl, the changes of experimental current
as a function of sodium dithionite concentration should give information
about the electrochemical reaction order in respect to sodium dithionite. A
logarithmic plot of the relation between the experimental current and
sodium dithionite concentration is shown in Fig. 6.10. A reaction order of
0.50 ± 0.02 is obtained at different applied potentials. With this knowledge,
a value of the electrochemical rate constant can be obtained by plotting 1/
I
2
as a function of 1/
I
w
1/2
with experimental currents obtained in the first and
second region of the voltammetric wave of sodium dithionite. Extrapolation
