The variation of peak potential (Ep) and peak current (ip) with pH in 0.10 mol/L PBS buffer solution are shown in Figs. 4 and 5. Within the range of pH 4.00–pH 9.00, the peak potential of UA and FA shifted negatively gradually with the increasing pH, representing the participation of protons in their electrochemical oxidation process. But their peak currents increased gradually until attaining a maximum at about pH 6.00, and then decreased. The possiblereasons for this phenomenon were as follows. As seen in Fig. 3, because UA and FA were oxidized at the PAIUCPE, some species should be reduced at the platinum electrode (the auxiliary electrode) at the same time. Furthermore, only H+ or oxygen (O2) dissolved in the solution could get electrons to be reduced. But there was no hydrogen (H2) appeared at the platinum electrode in the experiments since a higher overpotential was required for hydrogen evolution. Thus, a little of oxygen that was not completely removed in the solution should get electrons to be reduced at the platinum electrode. In view of the above-mentioned facts and the literature [12,27],the reasonable reaction mechanisms at the anode and cathode were given in Scheme 1, respectively.