Article
Qiao et al.
pH = 2.00-4.65. The reason may be that nitrogen atoms in
the HXa and Xa molecules could be intensely attracted by a
positively-charged (H+) “Hemholtz” layer as mentioned
above at higher acidity conditions. Also, the total amounts
of HXa and Xa diffusing to the reaction layer increase with
the enhancement of the solution acidity. Consequently,
their peak currents are the largest at the pH = 2.0. With the
increase in pH, the deprotonation in the “Hemholtz” layer
weakens the abilities of HXa and Xa to be adsorbed by
PAIUCPE surface, which lowers the concentration of HXa
and Xa in the reaction layer, so that their peak currents
decrease.
7.4, respectively. Also, assuming that the dissociation con-
stant (pKa) of the carboxyl (-COOH) at the PAIUCPE sur-
face is the same as the pKa (4.74) of acetic acid. According
to the calculation of acetic acid distribution coefficient, it
could be known that the carboxyl (-COOH) could be com-
pletely ionized to its anion (-COO-) on the PAIUCPE sur-
face at pH ³ 6.74. Therefore, the monovalent anion of both
Xa and HXa would be subjected to the repulsive interaction
of negative charge at the PAIUCPE surface, while the re-
pulsive interaction would impede the oxidation reactions
of Xa and HXa at the surface of PAIUCPE, this leads to the
decrease of the peak current as the pH increasing. In addi-
tion, in the alkaline solution, such the active groups as the
phenolic hydroxyl group at the PAIUCPE surface would
also be ionized into corresponding anion, which would fur-
ther weaken the interactions of hydrogen bonding between
the PAIUCPE surface and the analytes (Xa, HXa), and then
the amounts of Xa and HXa in the reaction layer would not
only reduce, but also the hydrogen bond catalysis also
would decrease. As a result, their peak currents markedly
drop with the increment of pH at pH > 6.45.
However, the peak currents increase slightly again
with pH increasing in the range of pH = 4.65-6.45, and
achieve their greater values at pH = 6.45. This might be due
to that the solutions of pH = 4.65-6.45 have all become
slightly acidic, corresponding to the degree of protonation
in the “Hemholtz” layer and the amounts of the HXa and
Xa diffused to reaction layer should have little difference
under these conditions, which make the peak currents re-
main basically unchanged. The reason that the peak cur-
rents increase slightly with increase in pH might be related
to that oxidation potentials of HXa and Xa shift negatively.
Usually, owing to the lower oxidation potential of reducing
agent, the easier it is oxidized.
The effect of scan rate on the peak current for
oxidation of Xa and HXa
The oxidation peak current (Ipa) increases gradually
with the scan rate increasing varied from 10 to 450 mV/s. In
the range of 10-100 mV/s, the oxidation peak currents of
Xa and HXa are linearly to the square root of scan rate (v1/2)
with the linear equations of Ipa(mA) = -2.04 + 34.27v1/2
(V/s) (R = 0.9924) and Ipa(mA) = -1.71 + 27.64v1/2 (V/s) (R
= 0.9930), which accord with the Randles-Sevcik equation.
As could be seen from Fig. 4, when the scan rate is greater
than 100 mV/s, the oxidation peak currents of Xa and HXa
increase with increase of v1/2 deviated from the straight to-
ward the trend of upwarp. The results suggest that their
electrochemical oxidation processes are typically con-
trolled by diffusion at lower scan rate while adsorption
characteristic at higher scan rate.25 The better signal-to-
noise ratios for the determination of Xa and HXa could be
obtained at a scan rate of 100 mVs–1, therefore, it is chosen
for the subsequent experiment.
When pH value exceeds 6.45, their peak currents ob-
viously decrease again as the raise of pH value from 6.45 to
8.60. Accordingly, the solution has turned from the nearly
neutral into weak alkaline, the degree of protonation in the
“Hemholtz” layer should also has little difference in the
range of pH = 6.45-8.60. Their peak currents should not
once more obviously be decreased with the increase in pH.
As a matter of fact, once pH value exceeds 6.45, peak cur-
rents for oxidation of HXa and Xa unexpectedly begin to
drop with the rise in pH value, which is inconsistent with
the principia of which the lower the oxidation potential of
reducing agent, the easier it to be oxidized theoretically.
The reason may be because the sizes of peak currents for
oxidation of HXa and Xa are also affected by the distribu-
tion coefficients of their different species and the charac-
ters of PAIUCPE.
The effect of concentration time and concentration
potential on the peak current for oxidation of Xa
and HXa
In the light of the pKa (5.4) of hydroxyl of UA,24 as-
suming that the dissociation constants (pKa) of hydroxyl of
the Xa and HXa are the same as pKa (5.4) of UA on the ba-
sis of their similarity molecular structure, the distribution
coefficients for the monovalent anions of Xa and HXa all
equal to 92% and are near to 100% at pH = 6.45 and pH ³
The influence of different concentration potential on
the sizes of oxidation peak currents of Xa and HXa is inves-
tigated under stirring in the range of +0.10 to +0.40 V. Re-
sults show that the change of concentration potential has
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© 2015 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
J. Chin. Chem. Soc. 2015, 62, 1011-1019