Polarography and Voltammetry of Anions

The oxidation processes of various anions, including halide ions, have been studied by voltammetry at solid electrodes [38 a]. For example, at a platinum electrode, iodide ion (I-) in various organic solvents is oxidized in two steps, i.e. 3I - I3 + 2e and 2Ii- 3I2+2e. From the difference in the standard potentials of the two steps, the dissociation constants of I3, p/f=-log ([yin/KD- have been determined as in Table 8.4. In aqueous solutions, p/f is 3 and I3 is not stable enough to use this method. The electrode oxidation of CN has been studied in several solvents using ESR spectroscopy [38b]. From the ultimate formation of the relatively stable tricyanomethylenimine radical anion, the following steps have been considered  

Electrode Reactions of Dissolved Oxygen, Dissolved Hydrogen, Carbon Dioxide and Solvated Electrons 

The solubility of oxygen at 1 atm and 25 °C has been determined to be 1.0 mM in water, 2.1 mM in DM SO, 4.8 mM in DMF, 8.1 mM in AN, 10 mM in hydrocarbons, and 25 mM in fluorocarbons [39a]. In aqueous solutions, the first reduction step of the dissolved oxygen is either a two-electron or four-electron process, depending on the catalytic activity of the electrode material and on the solution composition  

In aprotic solvents, however, the first step is a one-electron process, producing a superoxide ion O2 [40]  

If a tetraalkylammonium salt is used as supporting electrolyte, this process is either reversible or quasi-reversible and occurs at around -0.8 V vs aqueous SCE in various aprotic solvents and with various electrode materials (Hg, Pt, GC). If a Bmisted acid is added to the solution, the first step is converted to a two-electron process 0 produced in the first step is protonated to form 02H, which is more reducible than 02. Thus, 02H is further reduced to 02H at the potential of the first step. According to detailed polarographic studies in H20-DMS0 mixtures, about 30% v/v water is needed to convert the one-electron process to the two-electron process [41]. A metal ion, M+, interacts with 02 to fonn an ion-pair M+-02 (often insoluble) and shifts the half-wave potential of the first wave in a positive direction [42]. Electrogenerated superoxide 02 can act either as a nucleophile or as an electron donor and has been used in organic syntheses [43], 

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