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Electrochemistry

Gives us information about the potential and nature of redox processes

To determine the redox chemistry of a particular compound we use the techniqes of Cyclic Voltammetry, Differential Pulse Polarography and Coulometry. A three-electrode system comprising of a working electrode, reference elecrode and counter electrode is used for all techniques. The potential across the working electrode/solution interface (working electrode e.g. Platinum wire embedded in glass) is varied with respect to the reference electrode (reference electrode e.g. Ag/AgCl in KCl) while current is allowed to flow round the working electrode/counter electrode loop (counter electrode e.g. Platinum rod/scrap Platinum). This surmounts any problems with the resisitance of the solution. An electrolyte (e.g. [ⁿBu₄N][BF₄] ) is added to eliminate migration as a mode of mass transport. All solutions are bubbled with N₂ prior to study to remove O₂.

Cyclic Voltammetry

To obtain a cyclic voltammogram, the potential across the working electrode/solution interface is varied in a cyclic manner while current is monitored.

e.g. Reduction of 4-NO₂-bpy (bpy=2,2´-bipyridine)

4-NO₂-bpy

Fig. 1a (left) - As the potential is swept from 0 V in a negative direction, a flat line (no current flow) is observed as the applied potential is not close enough to the reduction potential of 4-NO₂-py to make the process occur. When the applied potential approaches the reduction potential a negative current is observed as some of the compound around the working electrode is reduced.
Fig. 1b (right) - As the applied potential passes the reduction potential of the complex, the negative current increases as more and more compound is reduced around the working electrode.

Fig. 2a (left) - Eventually, the current peaks as all the compound around the working electrode is reduced. Now, any more compound that is going to be reduced has to diffuse towards the working electrode from the bulk solution. Thus the reduction is limited by diffusion and the current drops to the “diffusion limited current”.
Fig. 2b (right) - Reversing the potential back towards 0 V initially gives a flat line as the applied potential is too far beyond the reduction potential of the compound to induce re-oxidiation (reduction is still occurring of compound diffusing from the bulk solution. As the applied potential approaches the reduction potential of the complex, re-oxidation starts to occur.

Fig. 3a (left) - As the applied potential passes the reduction potential of the complex the current peaks as all the compound around the working electrode is re-oxidised.
Fig. 3b (right) - The current now drops back to zero as no further reduction/oxidation occurs.

This gives the cyclic voltammogram for 4-NO₂-bpy shown in Fig. 4

Fig. 4 - Cyclic voltammogram of 4-NO₂-bpy vs. Ag/AgCl in 0.1 M [TBA][BF₄]/DMF at 293 K

Thus, we can determine that 4-NO₂-bpy undergoes a reduction process at -0.72 V vs. Ag/AgCl. We can also confirm by bulk reduction (coulometry) that this is a one-electron process.

If a compound in solution is particularly dilute or has closely spaced redox processes then we may resort to the more sensitive technique of differential pulse polarography

Electrochemistry - EPR - UV-Vis

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