✍ Scribed by Maria Luisa Foresti; Danilo Cozzi; Rolando Guidelli
No coin nor oath required. For personal study only.
In a systematic study of the effect of specific ionic adsorption upon the kinetics of electrode processes carried out in this laboratory ~-3 we found it interesting to examine the influence of halide and pseudohalide adsorption upon the reduction rate of BrO4 ion. The polarographic behaviour of BrO2 is quite similar to that 4 of $402-. Thus in both cases the rising portion of the reduction wave in fluoride medium develops at -0.3 to -0.4 V/SCE and a dip in the polarographic current is observed at potentials just negative of the point of zero charge, Ez. Moreover, the electroreduction rate of BrO2 ion is independent of pH just as is that of $ 402--(ref. 5). Also, the effect of halide and pseudohalide adsorption upon BrO2 electroreduction is in many aspects analogous to that described in ref. 3 (henceforth referred to as Paper I) for $40~ -electroreduction. Hence, for brevity, we will limit ourselves to pointing out the differences between the behaviour of these two electro-active oxyanions.
Certain advantageous features of BrOg electroreduction over $402-electroreduction are: (1) the streaming maximum exhibited by the BrO;, polarogram around Ez is much less pronounced than that shown by the $402-polarogram and disappears completely upon addition of small amounts of any surface-active halide or pseudohalide ion. This permitted us to study the effect of CI-adsorption on BrOg reduction at potentials anodic of Ez; (2) the monovalency of BrOg reduces the possibility of ion-pair formation with the cation of the supporting electrolyte; (3) the high symmetry of the BrO£ ion allows a much better estimate of its cross-sectional area and hence of the distance of closest approach between BrO2 and the surrounding adsorbed halide ions.
On the other hand, as opposed to the $402-ion, BrO~ oxidizes 1-in neutral media, thus preventing us from studying the effect of I-adsorption upon the kinetics of BrOg electroreduction. Moreover, relatively small amounts of TIF shift the rising portion of the BrOw7 polarogram to potentials more positive than +0.0 V/SCE, where no data on the potential ~b~ at the outer Helmholtz plane nor on the charge density q~ due to TI + specific adsorption are available 6. Hence, even though the wave shift is in qualitative agreement with theoretical predictions, we were unable to relate the observed increase in the rate of BrO,/ electroreduction following TI ~ adsorption to the charge density qi of this cation.
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