Several anions which are reduced at potentials more anodic than the point of zero charge (p.z.c.) show a characteristic minimum in the polarographic current at potentials just more negative than this point, provided that the supporting electrolyte is absent or else present at a low concentration 1 . This current minimum is due to a mutual compensation between the current decrease due to the increased electrostatic repulsion of the anion from the negatively charged electrode and the current increase due to the increase in the rate constant for the charge-transfer reaction proper, as we gradually proceed towards more negative potentials 1 . After the recent discovery of perbromate ion by Appelman 2 , a polarographic determination of this anion has shown that it is reduced to bromate at potentials more positive than the point of zero charge 3 . We have therefore found it interesting to ascertain whether perbromate electroreduction is affected by the diffuse layer structure to such an extent as to give rise to a current minimum in the absence of a supporting electrolyte.
The polarogram in Fig. 1 shows that this is actually the case. At potentials more cathodic than the minimum, the current rises again attaining the diffusion limiting value relative to the BrO2 + H20 + 2e-~ BROW-+ 2 OH-electrode process; subsequently, a cathodic wave with a half-wave potential of about -1.85 V/SCE is observed, due to the BROW-+ 3 H20 + 6e-~ Br-+ 6 OH-electrode reaction. It should be noted that in the potential range from +0.1 to -0.5 V/SCE, corresponding to the position of the hump in the polarogram of Fig. 1, a poorly reproducible streaming maximum is sometimes observed, depending on the characteristics of the glass capillary; however, a 10-a M concentration of a 1-1 valent supporting electrolyte is sufficient to eliminate it completely.
In order to examine the effect of a change in the potential ~2 at the outer Helmholtz plane upon the rate of BrO2 electroreduction to BROW-in the absence of specific ionic adsorption, the polarograms of 2 x 10 -4 M BrO2 + 10 -2 M NaOH in the presence of several concentrations of NaF were recorded with a three-electrode system consisting of a Metrohm E 261 Polarecord joined to a Metrohm E 446 IR-Compensator. All measurements