5726
G. Karim-Nezhad et al. / Electrochimica Acta 54 (2009) 5721–5726
material with a diffusion coefficient D, the current correspond-
ing to the electrochemical reaction (under diffusional control) is
described by Cottrell equation [31,32]:
nFAD1/2
ꢂ1/2t1/2
C
i =
(11)
2
−1
where D and C are the diffusion coefficient (cm s ) and bulk con-
−
3
centration (mol cm ), respectively. From the Cottrell equation, it
−
1/2
can be seen that the plot of i versus t
is linear, and from the
slope, the value of D can be obtained. Fig. 10 shows the experi-
mental plots for different concentrations of hydrazine employed.
The slopes of the resulting straight lines were then plotted versus
hydrazine concentration (Fig. 10, inset), from which the diffusion
−
5
2
−1
.
coefficient of hydrazine was calculated as 3 × 10 cm s
4. Conclusion
Fig. 9. Chronoamperograms of copper (hydr)oxide modified copper electrode in
.1 M NaOH solution in the absence (a) and presence (b–j) of various concentrations
0
of hydrazine: 100, 200, 300, 400, 600, 800, 1000, 1400, 1800 M, respectively. Insets:
Variation of chronoamperometric currents at t = 30 s vs. concentration of hydrazine.
Copper (hydr)oxide modified copper electrode was prepared by
cyclic voltammetry in 0.1 M NaOH solution in the potential range
of −300 to 800 mV. This layer protected the electrode from cor-
rosion and thus the oxidation peaks of copper were omitted. The
modified electrode successfully used for electrocatalytic oxidation
of hydrazine. Results showed that the mechanism of the oxidation
changes with the NaOH concentration in the range of 0.01–0.1 M.
by an increase in anodic currents obtained for a potential step of
160 mV. The inset of Fig. 9 shows plots of currents sampled at fixed
time as a function of hydrazine concentration. The response is lin-
early proportional to the concentration of hydrazine in the range of
−
1
For NaOH concentration of 0.1, the Tafel slope is 0.121 V decade
100–1800 M.
that means a first electron transfer depending on the applied
potential as the rate-determining step. The charge transfer coef-
ficient, the number of electrons involved in the rate-determining
step, the overall number of electrons involved in the oxida-
tion of hydrazine and the diffusion coefficient of hydrazine were
calculated.
Chronoamperometry can be used for the estimation of the dif-
fusion coefficient of hydrazine in the solution. For an electroactive
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